WO2009022758A1 - Pb-free solder compositions and pcb and electronic device using the same - Google Patents

Pb-free solder compositions and pcb and electronic device using the same Download PDF

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Publication number
WO2009022758A1
WO2009022758A1 PCT/KR2007/003892 KR2007003892W WO2009022758A1 WO 2009022758 A1 WO2009022758 A1 WO 2009022758A1 KR 2007003892 W KR2007003892 W KR 2007003892W WO 2009022758 A1 WO2009022758 A1 WO 2009022758A1
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WO
WIPO (PCT)
Prior art keywords
added
lead
amount
alloy
silicon
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Application number
PCT/KR2007/003892
Other languages
French (fr)
Inventor
Myung-Wan Ko
Sang-Bok Park
Myung-Kyu Song
Yun-Soo Park
Kwang-Yeol Lee
Original Assignee
Ecojoin
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Filing date
Publication date
Application filed by Ecojoin filed Critical Ecojoin
Priority to CN200780100259.1A priority Critical patent/CN101848787B/en
Priority to PCT/KR2007/003892 priority patent/WO2009022758A1/en
Publication of WO2009022758A1 publication Critical patent/WO2009022758A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
    • B23K35/24—Selection of soldering or welding materials proper
    • B23K35/26—Selection of soldering or welding materials proper with the principal constituent melting at less than 400 degrees C
    • B23K35/262—Sn as the principal constituent
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
    • H05K3/3457—Solder materials or compositions; Methods of application thereof
    • H05K3/3463—Solder compositions in relation to features of the printed circuit board or the mounting process

Definitions

  • the present invention also relates to a high temperature lead-free solder composition to be used at a temperature over 400 ° C and a PCB and an electronic device using the lead-free solder composition; and, particularly, to a high temperature lead-free solder composition including a binary composition of tin and copper added with silicon and cobalt, a quaternary composition of tin, copper, nickel and phosphorous added with silicon and cobalt, for improving soldering work efficiency through constant prevention against oxidation with the help of silicon added therein, preventing discoloration whereas maintaining a typical soldering temperature of a lead-free solder and a typical wettability with the help of a low fraction of silicon added therein, and preventing deterioration of the wettability and thus preventing deterioration of joinability caused by excess addition of silicon as well as preventing oxidation, preventing erosion of copper, and significantly improving a joint fracture load with the help of a very low fraction of cobalt added therein, and a PCB and an electronic device using
  • the present invention also relates to a lead-free solder composition for dilution and a PCB and an electronic device using the lead-free solder composition; and, particularly, to a lead-free solder composition for dilution, including a ternary composition of tin, nickel and phosphorous added with silicon and cobalt, a binary composition of tin and silver added with silicon and cobalt, or a ternary composition of tin, silver and phosphorous added with silicon and cobalt, for improving soldering work efficiency through constant prevention against oxidation with the help of silicon added therein, preventing discoloration with the help of a low fraction of silicon added therein, preventing deterioration of the wettability and thus deterioration of joinability caused by excess addition of silicon with the help of a very low fraction of cobalt added therein, and improving a characteristic of controlling copper content and soldering property by using them as a diluent, and a PCB and an electronic device using the lead-free solder composition
  • solders have been used as a jointing material for mounting components on printed circuit boards (PCB) for a long time.
  • Soldering is a technology for joining matters using a solder, and is being used in mounting small electronic components, such as semiconductor chips and resistors, on a printed circuit boards.
  • Such joining technologies using solders are widely used in mounting small electronic components, such as semiconductor chips and resistors, on a printed circuit boards.
  • the use of lead in the solder alloy is regulated or restricted, and thus a variety of environment friendly lead-free solders are under development.
  • the typical lead-free solder compositions have high " melting points and poor wettabilities in comparison to the lead-containing solders.
  • the typical lead-free solder compositions have poor soldering work efficiency because of severe oxidation of molten solders.
  • a PCB assey through the soldering has poor quality and poor reliability .
  • solder having sufficient soldering joinability such as wettability, and, at the same time, having a melting temperature similar to the melting temperature, which isl83 ° C , of the typical lead-containing solder (Sn37Pb).
  • the lead-free solders are now being used only with modifications in a soldering process such as replacement of ovens. This is because only limited metals, such as copper, silver, zinc (An), bismuth and indium (In), are used as alloying elements for dropping the melting temperature.
  • the typical lead-free solders do not satisfy both the sufficient soldering joinability such as the wettability and the erosion resistance and the low melting temperature near the melting temperature (183 ° C ) of the typical lead-containing solder (Sn37Pb). Accordingly, in most lead-free solders, small amount of metals, such as copper, silver, zinc, bismuth and indium, for lowering the melting temperature are added in lead to lower the melting temperature of the lead-free solders. However, there is no further development on lowering the melting temperature of the lead-free solder.
  • a silver alloy of SnAgCu and a non-silver alloy of SnCu of a melting temperature ranging from 220 ° C to 230 ° C are mainly used.
  • the developments of the solder with regard to the melting temperature are at the end of their rope, and thus the developments of the solder are focused on improving characteristics, work efficiency, quality, and reliability of the solder.
  • the addition of the element such as germanium, gallium and nickel is limited because it may increase the melting temperature and may cause cracks in the soldering part when exposed to thermal processing and thermal fatigue caused by hardening, which results in deterioration of reliability.
  • the addition of a large quantity of phosphorous may increase brittleness of the soldering part due to the hardening of the solder alloy. Phosphorous rises above the surface of the molten solder to prevent oxidation of the molten' solder.
  • phosphorous has only a short-lived effect on preventing dross (oxide) formation at a soldering temperature of approximately 260 ° C because of its high volatility.
  • the high temperature lead-free solder uses tin, which is expensive, instead of lead as a raw material and experiences a significant oxidation (dross formation) during melting (soldering) in comparison with lead- containing solders, which result in an increased economic load on users.
  • the oxides mixed in the molten solder may be incorporated in the soldering fillet (soldering part) of the PCB assey, which result in deterioration of soldering assembly set.
  • the soldering temperature of the high temperature solder is much higher than that of the solder for a general wave, resulting in a significant increase of dross formation and thus in deterioration of the work efficiency.
  • germanium, gallium and nickel are expensive materials.
  • they when they are added above a certain level, they may cause cracks in the soldering part exposed to a thermal process and a thermal fatigue, decreasing the reliability.
  • phosphorous may increase the brittleness of the soldering part due to overhardening of the solder alloy when added in a large quantity.
  • phosphorous rises over a surface of a soldering bath to prevent oxidation of the molten solder.
  • the volatilization temperature of phosphorous is 380 ° C whereas typical operations of the high temperature solder are performed at a soldering temperature of approximately 420 ° C to approximately 520 ° C , phosphorous has little effect on preventing the dross (oxide) formation.
  • a dip soldering after the bath is filled with the solders, the process is generally continued for months with adding only a small amount of the solders for compensating the consumed solders. Therefore, the dross formation can be prevented only for a short time in an initial filling of the bath, and the effect cannot last for a long time.
  • an overhardening of the solder alloy may increase the brittleness, deteriorating the soldering characteristics.
  • the high temperature solder alloy is used at a temperature of 460 ° C to 540 ° C whereas a general solder alloy is used at a temperature 260+ 10 ° C .
  • oxides are generated more and at a higher rate and thus a higher oxidation resistance is required than in the general solder alloy.
  • An embodiment of the present invention is directed to providing a lead-free solder composition including silicon and a very small amount of cobalt to constantly prevent oxidation during soldering, maintain wettability, prevent discoloration, improve mechanical properties, prevent erosion of a copper pad, and improve soldering joinability, and a PCB and an electronic device using the lead-free solder composition.
  • a lead-free solder composition including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a printed circuit board (PCB) and an electronic device using the composition.
  • PCB printed circuit board
  • a lead-free solder composition including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a lead-free solder composition including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a lead-free solder composition including from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a lead-free solder composition comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 17 wt% bismuth, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a high temperature lead-free solder composition comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a high temperature lead-free solder composition comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • a lead-free solder composition for dilution comprising from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
  • the lead- free solder composition and the PCB and the electronic device using the lead-free solder composition include silicon and cobalt at the same time to constantly prevent oxidation during soldering, maintain wettability, prevent discoloration, improve mechanical properties, prevent erosion of a copper pad, and improve soldering joinability .
  • Fig. 1 illustrates a principle of preventing oxide formation by a lead-free solder composition in accordance with an embodiment of the present invention.
  • Fig. 2 illustrates a result of an element analysis performed in an initial state of a dross experiment based on the principle of Fig. 1.
  • Fig. 3 illustrates a result of an element analysis performed after 3 hours of the dross experiment based on the principle of Fig. 1.
  • Fig. 4 illustrates a silicon peak measured by Auger electron spectroscopy (AES) after an experiment based on the principle of Fig. 1.
  • AES Auger electron spectroscopy
  • Fig. 5 illustrates SiO2 peak measured by AES after an experiment based on the principle of Fig. 1.
  • Fig. 6 illustrates a principle of preventing oxide formation by a high temperature lead-free solder composition in accordance with an embodiment of the present invention.
  • Fig. 1 illustrates a principle of preventing oxide formation by a lead-free solder composition in accordance with an embodiment of the present invention.
  • Fig. 1 illustrates a wave soldering process using a solder alloy including a SnAgCu base alloy which is added with small amounts of P and Si to prevent oxide formation. A portion of the added P and Si are consumed in deoxidization of the molten solder base alloy and another portion is dissolved in the molten solder base alloy.
  • the solubility of P and Si in a molten Sn is very small, most of the added P and Si rise towards a surface of the molten solder base alloy due to a difference in specific gravities. Then, a film of phosphorous oxides and silicon oxides formed over the surface interferes with the contact of an oxygen gas in the air and the molten solder to prevent the formation of oxides of the lead-free solder.
  • Fig. 2 illustrates a result of an element analysis by Auger electron spectroscopy (AES). This element analysis was performed by adding 0.5 mass % copper, 0.06 mass % nickel, 0.005 mass % phosphorous, 0.05 mass % silicon and 0.05 mass % cobalt to tin, melting and casting the solder alloy to an ingot, cutting the ingot, polishing the cut surface of the ingot, and analyzing the polished cut surface by AES.
  • Fig. 3 illustrates a result of an element analysis by AES, after a dross experiment (oxidation experiment) at 260 ° C for 3 hours using an ingot fabricated according to the procedure described above with reference to Fig. 2. As shown in Fig.
  • the concentration of Si (0.08 mass %) is higher than that of P (0.06 mass %) at the point 1 (upper region), and the concentration of Si (0.19 mass %) is also higher than that of P (0.13 mass %) at the point 2.
  • silicon oxides (estimated to be SiO2) are formed at the point 1.
  • Si peak is detected instead of SiO2 peak, as shown in Fig. 4.
  • Fig. 6 illustrates a principle of preventing oxide formation by a high temperature lead-free solder composition in accordance with an embodiment of the present invention. As shown in Fig.
  • Fig. 7 illustrates a result of element analysis of a high temperature solder by AES after 3 hours of a dross experiment.
  • Fig. 7 illustrates a result of element analysis of a high temperature solder by AES after 3 hours of a dross experiment at 420 ° C , for demonstrating the above described phenomenon.
  • P was not detected at all, whereas 1.09 wt% of Si was detected. This tells us that although most of P is evaporated from the surface of the molten solder, the silicon still remains even at a high temperature to form a silicon oxide film over the surface of the molten solder.
  • the silicon oxide film formed over the surface of the molten solder interferes with contact of the molten solder and an oxygen gas in the air to prevent the oxidation.
  • silicon When only silicon is added, the effect of discoloration and the dross may be improved.
  • silicon when silicon is added above a predetermined level, the melting temperature of the solder alloy increases, and wettability decreases and thus joinability decreases, which is a fundamental characteristic of the solder. Accordingly, by adding a very small amount of cobalt, it is possible to obtain a significant result of maintaining the wettability and preventing the oxidation, the discoloration and the erosion of a copper.
  • the property and the use of the components in the lead-free solder composition in accordance with embodiments will be described.
  • Tin (Sn) is an essential component of the lead-free solder, and is used as a base metal.
  • Copper (Cu) in a solder alloy for a general wave soldering slightly decreases the melting temperature of a solder alloy and a joint strength of a solder joint.
  • the amount of copper added in the solder alloy for the general wave soldering is preferably from 0.1 wt% to 2 wt%.
  • copper in a solder alloy for a high temperature dip used at least 400 ° C has no effect of increasing the melting temperature of the solder alloy when added in an amount less than 2 wt%.
  • Copper in the solder alloy for the high temperature dip increases the melting temperature excessively and causes over-hardening of the solder alloy when added in an amount greater than 5 wt%. Therefore, the preferable content of copper in the solder alloy for the high temperature dip is from 2 wt% to 5 wt%.
  • Silver (Ag) is nonpoisonous .
  • silver decreases the melting temperature of a solder alloy, increases the dispersibility of the base metal, and improves the thermal fatigue property.
  • the preferable content of silver in the solder alloy is from 0.1 wt% to 4 wt% .
  • Phosphorous (P) prevents oxide formation during a soldering, and improves the work.
  • phosphorous is added in an amount less than 0.001 wt% , there is no effect.
  • phosphorous is added in an amount greater than 0.2 wt%, the melting temperature increases. Therefore the preferable content of silver in the solder alloy is from 0.001 wt% to 0.2 wt%.
  • Germanium (Ge) improves the wettability and prevents the oxidation. When germanium is added in an amount less than 0.001 wt%, there is no effect. When germanium is added in an amount greater than 0.1 wt%, the raw material cost increases rapidly and the melting temperature increases. Therefore, the preferable content of germanium in the solder alloy is from 0.001 wt% to 0.1 wt%.
  • Nickel (Ni) prevents erosion of a copper pad, and improves the joint strength. When nickel is added in an amount less than 0.001 wt%, there is no effect. When nickel is added in an amount greater than 0.5 wt%, the melting temperature increases rapidly and over-hardening of the solder alloy occurs. Therefore, the preferable content of nickel in the solder alloy is from 0.001 wt% to 0.5 wt%.
  • Bismuth (Bi) decreases the melting temperature and improves the joint strength. When bismuth is added in an amount less than 0.1 wt%, there is no effect of decreasing the melting temperature.
  • the melting temperature of Sn alloy decreases to approximately 210 ° C , or over-hardening of the solder alloy occurs. Therefore, the preferable content of bismuth in the solder alloy is from 0.1 wt% to 17 wt%.
  • Co Co
  • Co does not have the rising characteristic.
  • by adding a small amount of cobalt together with silicon it is possible to prevent the oxide formation, the discoloration and the erosion to thus improve the soldering characteristics and the workability, while maintaining the wettability.
  • the embodiment provides a lead-free solder composition including from 0.1 wt% to 4.0 wt% silver, from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt, and the balance of Sn, and a PCB and an electronic device using the lead-free solder composition.
  • Graph 1 illustrates experimental data of the amount of oxidation measured by the following procedure. Silicon and cobalt were added in a predetermined base alloy. The resulting alloy was molten to fabricate a solder composition. The solder composition was heated to 260 ° C in an SUS crucible of 160 mm diameter, on a hot plate. Then, the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time.
  • the amount of oxides formed in Sn3AgO .5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g.
  • the amount of oxides formed in Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g. This is because P serves to reduce the amount of oxidation.
  • Si when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours . Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P.
  • the amount of oxidation decreased further more when 0.05 wt% of Si was added.
  • the amount of oxides was 215 g after 1 hour, 240 g after 2 hours, and 264 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 719 g. It can be seen from this result that the amount of oxidation was decreased in comparison to the cases of Sn3Ag0.5Cu0.005P and Sn3AgO .5CuO .005Si .
  • Graph 2 illustrates experimental results of the wettability.
  • the solder was molten at 260 ° C using a SP2 wetting tester (manufactured by MalCom Co., Ltd.).
  • a copper terminal was dipped into a depth of 1 mm from the surface of the molten solder to measure the wetting time after 5 seconds of the dipping.
  • the wetting time of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 0.48 seconds.
  • the wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds.
  • Graph 3 illustrates experimental results of the erosion rate of copper.
  • a solder alloy was molten and kept at 260 ° C .
  • a copper wire of 0.8 mm diameter was dipped into a depth of 30 mm from the surface of the molten solder to measure the weights of the copper wire before the dipping and after 60 seconds of the dipping, respectively.
  • the weights of the copper wire was compared with each other to obtain the average erosion rate of five samples.
  • the copper erosion rate of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag, and the balance of Sn was 58.3 %.
  • the copper erosion rate of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag, and the balance of Sn was 58.3 %.
  • Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.1 %.
  • Table 1 lists the experimental data of Graphs 1, 2 and 3 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 220 ° C .
  • the melting temperature of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 ° C .
  • the melting temperature of Sn3AgO.5Cu was changed to 220 ° C when added with 0.005 wt% of Si, and the melting temperature of Sn3AgO .5Cu was increased to 224 " C when added with 0.1 wt% of Si.
  • the melting temperature of Sn3AgO .5CuO .05Si was increased to 223 ° C when added with 0.01 wt% of Co.
  • Si of a high melting point above 1,400 ° C
  • the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level .
  • the discoloration data in Table 1 was obtained as follows. An oxygen-free copper of 99.99 % purity having a size of 25x31x0.3 mm was dipped at 260 °C for 3 seconds to prepare a specimen. The specimen was heated to 250 ° C for 10 minutes. Then, the discoloration level of yellow was measured using Minolta CM3700B chromatic aberration meter.
  • the discoloration level of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 16.47
  • the discoloration level of Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 19.31.
  • the discoloration of the Sn-Ag-Cu base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein .
  • the joint fracture load data in Table 1 was obtained as follows. A 2 mm diameter wire coated with tin-bismuth was vertically inserted in a hole of a PCB substrate and wave soldering was performed thereon to prepare a specimen. Then, the joint fracture load (kgf) thereof was measured using a tension tester.
  • the joint fracture load of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf
  • the joint fracture load of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf.
  • the joint fracture load of the Sn-Ag-Cu base alloy can be improved by adding a small amount of Si therein, and can also be significantly improved by adding a very small amount of Si and Co therein.
  • the lead-free solder composition in accordance with the embodiment including the Sn-Ag-Cu base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • Cu added in the Sn-Ag-Cu-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint.
  • the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%.
  • Ag added in the Sn-Ag-Cu-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous .
  • the preferable content of Ag is from 0.1 wt% to 4.0 wt%.
  • Sn is an essential component of the lead-free solder, and is used as a base metal .
  • the lead-free composition in accordance with the embodiment can be used in a form of cream, bar or wire.
  • the lead-free composition can be implemented in electronic devices including a plurality of electronic components fixed thereto through the lead-free soldering alloy.
  • the electronic devices can be applied to various electronic instruments such as computers, digital camcorders, digital televisions, digital cameras, and mobile communication terminals.
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy.
  • the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
  • the lead-free composition, and thus the PCB and the electronic device using the lead-free solder composition can have a mechanical characteristic and workability similar to those of Sn-Pb alloy and Sn-Ag-Cu alloy.
  • they can prevent the oxidation of the molten solder constantly and effectively by adding Si and Co in the ternary composition system of tin, silver and copper. Furthermore, they can maintain the soldering temperature and the wettabiltiy of the typical lead-free solder, prevent the discoloration after the soldering and the erosion of the copper pad in the PCB, and improve the soldering joinability, by adding only a very small amount of alloying elements.
  • the lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), silver (Ag), phosphorous (P), silicon (Si) and cobalt (Co).
  • Graph 4 illustrates data obtained from oxide formation experiments .
  • the amount of oxides formed in Sn3AgO.5CuO.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g.
  • the amount of oxides formed in Sn3AgO .5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g. This is because P serves to reduce the amount of oxidation.
  • Si when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours . Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, when Si was added in an amount of 0.05 wt%, the oxidation was decreased further more.
  • the wetting time of Sn3AgO.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds.
  • the wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.48 seconds .
  • the erosion rate of Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.1 %.
  • the erosion rate of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 58.3 %.
  • Table 2 lists the experimental data of Graphs 4, 5 and 6 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 ° C .
  • the melting temperature of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 220 ° C .
  • the melting temperature of Sn3AgO .5CuO .005P was 221 ° C when added with 0.005 wt% of Si, and the melting temperature of Sn3AgO.5CuO.005P was increased to 225 ° C when added with 0.1 wt% of Si.
  • the melting temperature of Sn3AgO .5CuO .005P0.05Si was increased to 223 ° C when added with 0.01 wt% of Co.
  • Si of a high melting point above 1,400 " C
  • the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level.
  • the discoloration level of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 19.31, and the discoloration level of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 16.47.
  • the discoloration level was significantly decreased to 7.02, and when 0.01 wt% of Co was added in Sn3AgO .5CuO .005P0.005Si , the discoloration was significantly decreased to 4.7. Therefore, it can be concluded that the discoloration of the Sn-Ag-Cu-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu-P base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
  • the joint fracture load of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf
  • the joint fracture load of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf.
  • the joint fracture load was 110 kgf, which is greater than that of Sn3AgO.5Cu.
  • the joint fracture load was significantly increased to 115 kgf.
  • the joint fracture load of the Sn-Ag-Cu-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
  • the lead-free solder composition in accordance with the embodiment including the Sn-Ag- Cu-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
  • Cu added in the Sn-Ag-Cu-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint.
  • the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%.
  • Ag added in the Sn-Ag-Cu-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous .
  • the preferable content of Ag is from 0.1 wt% to 4.0 wt% .
  • P added in the Sn-Ag-Cu-P base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability.
  • the preferable content of P is from 0.001 wt% to 0.2 wt%.
  • Sn is an essential component of the lead-free solder, and is used as a base metal .
  • the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from
  • the lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), silver (Ag), nickel (Ni), germanium (Ge), silicon (Si) and cobalt (Co).
  • Graph 7 illustrates data obtained from oxide formation experiments .
  • the amount of oxides formed in Sn3Ag0.5Cu0.06Ni0.01Ge including 0.5 wt% Cu, 3 wt% Ag, 0.06 wt% Ni, 0.01 wt% Ge and the balance of Sn was 147 g after 1 hour, 151 g after 2 hours, and 160 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 458 g.
  • the amount of oxides formed in Sn3AgO .5CuO .05P including 0.5 wt% Cu, 3 wt% Ag, 0.05 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g.
  • the amount of oxides formed in Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g.
  • Ni serves to reduce the amount of oxidation.
  • the amount of oxides was 105 g after 1 hour, 115 g after 2 hours, and 121 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 341 g, which is smaller than the amount of oxides formed in Sn3AgO .5CuO.06NiO.01Ge without Si.
  • the wetting time of Sn3AgO.5CuO.06NiO.01Ge including 0.5 wt% Cu, 3 wt% Ag, 0.06 wt% Ni, 0.01 wt% Ge and the balance of Sn was 0.38 seconds.
  • the wetting time of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 0.48 seconds.
  • the wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds .
  • Table 3 lists the experimental data of Graphs 7, 8 and 9 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature of Sn3AgO.5CuO. O ⁇ NiO.OlGe including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% Ni, 0.01 wt% Ge and the balance of Sn was 222 ° C .
  • the melting temperature of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 ° C .
  • the melting temperature of Sn3AgO.5Cu was 220 ° C when added with 0.005 wt% of Si.
  • the melting temperature of Sn3AgO .5CuO .06NiO .01Ge was increased to 225 ° C when added with 0.1 wt% of Si.
  • the melting temperature of Sn3Ag0.5Cu0.06Ni0.01Ge0.05Si was increased to 224 ° C when added with 0.01 wt% of Co.
  • a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 " C) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level.
  • Table 3 the discoloration level of Sn3AgO.5CuO.
  • O ⁇ NiO.OlGe was 15.6, the discoloration level of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 16.47, and the discoloration level of Sn3Ag0.5Cu0.005P was 19.31.
  • the discoloration level was 13.5, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 9.47, which is lower than those of Sn3AgO.5Cu and Sn3Ag0.5Cu0.005P.
  • the joint fracture load of Sn3AgO.5CuO.06NiO.01Ge was 100 kgf
  • the joint fracture load of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf
  • the joint fracture load of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf.
  • the joint fracture load of the Sn-Ag-Cu-Ni-Ge base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
  • the lead-free solder composition in accordance with the embodiment including the Sn-Ag-
  • Cu-Ni-Ge base alloy added with a small amount of Si could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • a very small amount of Co was added in the Sn-Ag-Cu-Ni-Ge base alloy together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
  • the preferable content of Ag is from 0.1 wt% to 4.0 wt%.
  • Ni added in the Sn-Ag-Cu- Ni-Ge base alloy serves to prevent the erosion of the copper pad and improve the joint strength.
  • the preferable content of Ag is from 0.001 wt% to 0.5 wt%.
  • the preferable content of Ge is from 0.001 wt% to 0.1 wt% .
  • Sn is an essential component of the lead-free solder, and is used as a base metal.
  • P added in the Sn-Ag-Cu-Ni-Ge base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability.
  • the preferable content of P is from 0.001 wt% to 0.2 wt% .
  • Si and Co added in the Sn-Ag-Cu- Ni-Ge base alloy each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together .
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy.
  • the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
  • the lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), phosphorous (P), nickel (Ni), silicon (Si) and cobalt (Co).
  • Graph 10 illustrates data obtained from oxide formation experiments.
  • the amount of oxides formed in Sn0.5Cu0.06Ni0.005P including 0.5 wt% Cu, 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 134 g after 1 hour, 2686 g after 2 hours, and 384 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 786 g.
  • the amount of oxides formed in SnO .5Cu was 371 g after 1 hour, 395 g after 2 hours, and 460 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,226 g.
  • the amount of oxides formed in SnO .5CuO .005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 251 g after
  • the wetting time of Sn0.5Cu0.06Ni0.005P including 0.5 wt% Cu, 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 2.07 seconds.
  • the wetting time of SnO .5CuO .005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 1.88 seconds.
  • the wetting time of SnO .5Cu including 0.5 wt% Cu and the balance of Sn was 1.67 seconds.
  • 0.005 wt% of Si was added in SnO .5CuO.06Ni0.005P, the wetting time was shorten to 1.85 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 2.09 seconds.
  • the copper erosion rate of SnO.5CuO.06NiO.005P was 47.1 %
  • the copper erosion rate of SnO.5Cu was 53.6 %
  • the copper erosion rate of Sn0.5Cu0.005P was 52.7 %.
  • Table 4 lists the experimental data of Graphs 16, 17 and 18 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature of SnO.5CuO.06Ni0.005P was 233 ° C .
  • the melting temperature of SnO.5CuO.005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 231 ° C .
  • the melting temperature of SnO.5Cu including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 230 ° C .
  • the melting temperature of SnO .5CuO .06NiO .005P was increased to 237 ° C when added with 0.1 wt% of Si.
  • the melting temperature of SnO .5CuO .06NiO .005P0.05Si was increased to 236 ° C when added with 0.01 wt% of Co.
  • Si a high melting point (above 1,400 " C) in a small amount above a predetermined level
  • the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level .
  • the discoloration level of Sn0.5Cu0.006Ni0.005P was 13.5
  • the discoloration level of SnO.5Cu0.005P was 14.71
  • the discoloration level of SnO .5Cu was 15.81.
  • the discoloration of the Sn-Cu-P-Ni base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu-P-Ni base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
  • the joint fracture load was 90 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 96 kgf, which is greater than those of SnO .5Cu and SnO .5CuO .005P .
  • the joint fracture load of the Sn-Cu-P-Ni base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
  • the lead-free solder composition in accordance with the embodiment including the Sn-Cu-P- Ni base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
  • Cu added in the Sn-Cu-P-Ni base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%.
  • Ni added in the Sn-Cu-P-Ni base alloy serves to increase the strength and the toughness of the solder, and prevent the erosion, the formation of oxides and the growth of intermetallic compounds at the joining surface. The preferable content of Ni in the solder alloy is from 0.001 wt% to 1.0 wt%.
  • Sn-Cu-P-Ni base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability.
  • the preferable content of P is from 0.001 wt% to 0.2 wt% .
  • Sn is an essential component of the lead-free solder, and is used as a base metal.
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy.
  • the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt, from 0.001 wt% to 0.2 wt% phosphorous and the balance of tin.
  • the lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), phosphorous (P), bismuth (Bi), silicon (Si) and cobalt (Co).
  • the above mentioned solder composition is a non- silver alloy and a low temperature lead-free solder alloy having a melting temperature of approximately 210 ° C .
  • the amount of oxides formed in Sn0.5Cul7Bi0.005P including 0.5 wt% Cu, 17 wt% Bi, 0.005 wt% P and the balance of Sn was 330 g after 1 hour, 360 g after 2 hours, and 450 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,140 g.
  • the amount of oxides was 201 g after 1 hour, 241 g after 2 hours, and 273 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 715 g, which is smaller than the amount of oxides formed in Sn0.5Cul7Bi0.005P.
  • the wetting time of Sn0.5Cul7Bi0.005P was 0.24 seconds.
  • the wetting time of Sn0.5Cul7Bi0.005P was shorten to 0.22 sec when 0.005 wt% of Si was added therein.
  • the wetting time of SnO .5Cul7BiO .005P was rather increased when 0.05 wt% of Si was added therein.
  • the wetting time was shorten to 0.28 seconds.
  • 0.005 wt% of Co was added in SnO .5Cul7BiO .005P0.05Si
  • the wetting time was 0.23 seconds.
  • Graph 15 illustrates experimental results of the copper erosion rate.
  • Si was added in SnO .5Cul7BiO .005P , the copper erosion rate was 31.7 %, which is smaller than that of Sn0.5Cul7Bi0.005P.
  • Table 5 lists the experimental data of Graphs 13, 14 and 15 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature was increased to 214 ° C .
  • Si of a high melting point above 1,400 ° C
  • the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level.
  • the discoloration level of Sn0.5Cul7Bi0.005P was 16.78.
  • the discoloration level was 11.87, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 8.97, which is lower than that of Sn0.5Cul7Bi0.005P.
  • the joint fracture load of the Sn-Cu-Bi-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • silicon and cobalt each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be further improved by adding them together.
  • the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 0.2 wt% phosphorus, from 0.1 wt% to 8 wt% bismuth, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
  • a high temperature lead-free solder composition in accordance with the embodiment may include tin (Sn), copper (Cu), silicon (Si) and cobalt (Co).
  • Graph 16 illustrates experimental data of the amount of oxidation measured by the following procedure.
  • a base alloy of tin-copper was prepared, and it is molten in a high-frequency melting furnace together with both silicon and cobalt or one of silicon and cobalt to form a solder composition.
  • melting temperatures thereof were measured using DSC analysis at 5 ° C/min.
  • 3 kg of the solder was heated to 420 ° C in an SUS crucible of 160 mm diameter, on a hot plate.
  • the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time.
  • the amount of oxides formed in Sn4Cu including 4 wt% Cu and the balance of Sn was 687 g after 1 hour, 743 g after 2 hours, and 805 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 2,235 g.
  • the amount of oxides formed in Sn4CuO . INiO .005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,981 g.
  • the reason why the amount of oxides formed in Sn4CuO . INiO .005P is smaller than that formed in Sn4Cu is that P serves to reduce the amount of oxidation .
  • the amount of oxides was 461 g after 1 hour, 532 g after 2 hours, and 596 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,589 g, which is smaller than the amount of oxides formed in Sn4Cu and Sn4Cu0.005P without Si.
  • the amount of oxidation decreased further more when 0.05 wt% of Si was added therein.
  • Graph 17 illustrates experimental results of the erosion rate of copper.
  • Table 6 lists the experimental data of Graphs 22 and 23 together with the experxmental data of the melting temperature, the discoloration and the joint strength.
  • the melting temperature of Sn4Cu including 4 wt% Cu and the balance of Sn was 357 ° C
  • INiO .005P was 362 ° C.
  • the joint fracture load was 100 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 130 kgf, which is greater than those of
  • the joint fracture load of the Sn-Cu base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
  • the lead-free solder composition in accordance with the embodiment, including the Sn-Cu base alloy added with a small amount of Si could constantly and significantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
  • Cu added in the Sn-Cu base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint.
  • the preferable content of copper in the solder alloy is from 2 wt% to 5 wt%.
  • Sn is an essential component of the lead-free solder, and is used as a base metal.
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the high temperature lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the high temperature lead- free solder alloy.
  • the high temperature lead-free solder alloy includes from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
  • the high temperature lead-free composition and thus the PCB and the electronic device using the high temperature lead-free solder composition can have a mechanical characteristic and workability similar to those of Sn-Pb alloy and Sn-Cu alloy. Further, by adding both silicon and cobalt in a binary composition system of tin (Sn) and copper (Cu) without the addition of lead (Pb), they can prevent the oxidation of the molten solder constantly and effectively at a high temperature above 400 ° C .
  • a high temperature lead-free solder composition in accordance with the embodiment may include tin (Sn), copper (Cu), nickel (Ni), silicon (Si) and cobalt (Co).
  • the amount of oxides formed in Sn4CuO. INiO.005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was
  • the amount of oxides formed in Sn4Cu was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,981 g.
  • the copper erosion rate of Sn4CuO. INiO.005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 53.4 %.
  • the copper erosion rate of Sn4Cu was 58.9 %.
  • the melting temperature of Sn4CuO. INiO.005P was 362 ° C
  • the melting temperature of Sn4Cu including 4 wt% Cu and the balance of Sn was 357 TC.
  • the discoloration of the Sn-Cu-Ni-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu-Ni-P base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein .
  • Table 7 the joint fracture load of
  • the joint fracture load was significantly increased to 115 kgf, which is greater than those of Sn4Cu and Sn4CuO .
  • INiO .005P When 0.01 wt% of Co was added in Sn4CuO. INiO.005P0.005Si, the joint fracture load was significantly increased to 122 kgf.
  • the joint fracture load of the Sn-Cu-Ni-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
  • the lead-free solder composition in accordance with the embodiment including the Sn-Cu- Ni-P base alloy added with a small amount of Si, could constantly and significantly reduce the oxidation, maintain the wettability, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
  • Cu added in the Sn-Cu-Ni-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint.
  • the preferable content of copper in the solder alloy is from 2 wt% to 5 wt%.
  • the preferable content of nickel in the solder alloy is from 0.001 wt% to 1.0 wt% .
  • Sn is an essential component of the lead-free solder, and is used as a base metal.
  • Si and Co added in the Sn-Cu base alloy each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together.
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the high temperature lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the high temperature lead- free solder alloy.
  • the high temperature lead-free solder alloy includes from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
  • a lead-free solder composition for dilution in accordance with the embodiment may include nickel (Ni), silicon (Si), phosphorous (P), cobalt (Co) and tin (Sn).
  • Ni nickel
  • Si silicon
  • P phosphorous
  • Co cobalt
  • Sn tin
  • Graph 20 illustrates experimental data of the amount of oxidation measured by the following procedure. Silicon and cobalt were added in a Sn-Ni-P base alloy. The resulting alloy was molten to fabricate a solder composition. The solder composition was heated to 260 ° C in an SUS crucible of 160 mm diameter, on a hot plate. Then, the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time.
  • solder was solidified, and then solder samples were cut from the solder.
  • the cutting surfaces of solder samples were polished.
  • the contents of the alloying elements in the solder samples were analyzed using Auger electron spectroscopy (PHI700, manufactured by Ulvac PHI), which is an instrument for analyzing elements on a metal surface using X-ray, to determine whether the alloying elements remain therein.
  • PHI700 Auger electron spectroscopy
  • the amount of oxides formed in SnO.06NiO.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 125 g after 1 hour, 195 g after 2 hours, and 320 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 640 g.
  • Graph 21 illustrates experimental results of the wettability.
  • the solder was molten at 260 ° C using a SP2 wetting tester (manufactured by MalCom Co., Ltd. ) .
  • a copper terminal was dipped into a depth of 1 mm from the surface of the molten solder to measure the wetting time after 5 seconds of the dipping.
  • the wetting time of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 2.15 seconds.
  • the acid resistance of the molten solder alloy can be improved and at the same time, the decrease of wettability due to the addition of silicon in an amount over the limit can be minimized by adding a very small amount (less than 0.01 wt%) of cobalt therein .
  • the copper erosion rate of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 48.8 %.
  • Table 8 lists the experimental data of Graphs 26, 27 and 28 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the content of Si in the Sn alloy needs to be limited below a certain level.
  • the discoloration level of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 21.03.
  • the discoloration of the Sn-Ni-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
  • the joint fracture load of SnO.06NiO.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 83 kgf.
  • the lead-free solder composition in accordance with the embodiment including the Sn-Ni-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the acid resistance of the molten solder alloy can be improved and at the same time, the decrease of wettability due to the addition of silicon in an amount over the limit can be minimized by adding a very small amount (less than 0.01 wt%) of cobalt therein.
  • a very small amount less than 0.01 wt% of cobalt therein.
  • the copper erosion rate of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.8 %.
  • Table 9 lists the experimental data of Graphs 23, 24 and 25 together with the experimental data of the melting temperature, the discoloration and the joint strength.
  • the discoloration of the Sn-Ag-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein .
  • the joint fracture load data listed in Table 9 was obtained as follows. A 2 mm diameter wire coated with tin-bismuth was vertically inserted in a hole of a PCB substrate and wave soldering was performed thereon to prepare a specimen. Then, the joint fracture load (kgf) thereof was measured using a tension tester. As shown in Table 9, the joint fracture load of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 87 kgf.
  • the joint fracture load was 94 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 101 kgf, which is greater than that of Sn3Ag0.005P.
  • the joint fracture load of the Sn-Ag-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
  • the lead-free solder composition in accordance with the embodiment including the Sn-Ag-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
  • the Sn-Ag-P base alloy including a small amount of Si it is possible to decrease the amount of oxides significantly and constantly, decrease the erosion rate of the copper and the discoloration, and increase the joint fracture load while maintaining the wettability, in comparison to the base alloy added with only Si.
  • the preferable content of Si in the solder alloy is from
  • the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt% .
  • Cu added in the Sn-Ag-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt% .
  • Ag added in the Sn-Ag-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous .
  • the preferable content of Ag is from 0.1 wt% to 4.0 wt%.
  • P added in the Sn-Cu-P-Ni base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability.
  • the preferable content of P is from 0.001 wt% to 0.2 wt%.
  • Sn is an essential component of the lead-free solder, and is used as a base metal.
  • Si and Co each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together.
  • the present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy for dilution, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy for dilution.
  • the lead-free solder alloy for dilution includes from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.

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Abstract

Provided is a lead-free solder composition, including at least two of tin (Sn), silver (Ag), phosphorous (P), nickel (Ni), copper (Cu), bismuth (Bi) and germanium (Ge), and including both silicon (Si) and cobalt (Co), for preventing the deterioration of workability caused by the oxidation of the solder alloy and eliminating the need of replacing the whole lead bath caused by the increase of the copper erosion during soldering, and a printed circuit board (PCB) and an electronic device using the lead-free solder. Through the lead-free solder composition including both silicon and cobalt, and the printed circuit board (PCB) and the electronic device using the lead-free solder, it is possible to constantly prevent the formation of the oxides, prevent the discoloration and the erosion of the copper pad, improve the mechanical properties and the soldering joinability while maintaining the wettability during the soldering.

Description

DESCRIPTION
PB-FREE SOLDER COMPOSITIONS AND PCB AND ELECTRONIC DEVICE
USING THE SAME
TECHNICAL FIELD
The present invention relates to a lead-free solder composition and a printed circuit board (PCB) and an electronic device using the lead-free solder composition; and, more particularly, to a lead-free solder composition including, without lead (Pb) which is hazardous to human body, a ternary composition of tin (Sn), copper (Cu) and silver (Ag) added with silicon (Si) and cobalt (Co), a tertiary composition of tin, copper, silver and nickel (Ni) added to silicon and cobalt, a senary composition of tin, copper, silver, nickel, germanium (Ge) and phosphorous (P) added to silicon and cobalt, or a quaternary composition of tin, copper, phosphorous and bismuth (Bi) added with silicon and cobalt, for improving soldering work efficiency through constant prevention against oxidation with the help of silicon added therein, preventing discoloration whereas maintaining a typical soldering temperature of a lead-free solder and a typical wettability with the help of a low fraction of silicon added therein, and preventing deterioration of the wettability and thus preventing deterioration of joinability caused by excess addition of silicon as well as preventing oxidation, preventing erosion of copper, and significantly improving a joint fracture load with the help of a very low fraction of cobalt added therein, and a PCB and an electronic device using the lead-free solder composition.
The present invention also relates to a high temperature lead-free solder composition to be used at a temperature over 400 °C and a PCB and an electronic device using the lead-free solder composition; and, particularly, to a high temperature lead-free solder composition including a binary composition of tin and copper added with silicon and cobalt, a quaternary composition of tin, copper, nickel and phosphorous added with silicon and cobalt, for improving soldering work efficiency through constant prevention against oxidation with the help of silicon added therein, preventing discoloration whereas maintaining a typical soldering temperature of a lead-free solder and a typical wettability with the help of a low fraction of silicon added therein, and preventing deterioration of the wettability and thus preventing deterioration of joinability caused by excess addition of silicon as well as preventing oxidation, preventing erosion of copper, and significantly improving a joint fracture load with the help of a very low fraction of cobalt added therein, and a PCB and an electronic device using the high temperature lead-free solder composition.
The present invention also relates to a lead-free solder composition for dilution and a PCB and an electronic device using the lead-free solder composition; and, particularly, to a lead-free solder composition for dilution, including a ternary composition of tin, nickel and phosphorous added with silicon and cobalt, a binary composition of tin and silver added with silicon and cobalt, or a ternary composition of tin, silver and phosphorous added with silicon and cobalt, for improving soldering work efficiency through constant prevention against oxidation with the help of silicon added therein, preventing discoloration with the help of a low fraction of silicon added therein, preventing deterioration of the wettability and thus deterioration of joinability caused by excess addition of silicon with the help of a very low fraction of cobalt added therein, and improving a characteristic of controlling copper content and soldering property by using them as a diluent, and a PCB and an electronic device using the lead-free solder composition .
BACKGROUND ART
Lead-containing alloys have been widely used since early times. Particularly, tin-lead solders have been used as a jointing material for mounting components on printed circuit boards (PCB) for a long time. Soldering is a technology for joining matters using a solder, and is being used in mounting small electronic components, such as semiconductor chips and resistors, on a printed circuit boards. Such joining technologies using solders are widely used in mounting small electronic components, such as semiconductor chips and resistors, on a printed circuit boards.
Recently, as electronics become smaller and more light-weighted, and have higher functions, denser mounting of the components are in need, and thus more advanced joining technologies using solders are in need. In addition, although a binary eutectic alloy of tin and lead has been generally used as a material for the soldering, it is a cause of environmental pollution, and thus the use of lead is now being regulated. This is because lead not only causes the environmental pollution, but also has a bad influence upon a human body.
As a result, nowadays, the use of lead in the solder alloy is regulated or restricted, and thus a variety of environment friendly lead-free solders are under development. However, the typical lead-free solder compositions have high " melting points and poor wettabilities in comparison to the lead-containing solders. Further, the typical lead-free solder compositions have poor soldering work efficiency because of severe oxidation of molten solders. Furthermore, a PCB assey through the soldering has poor quality and poor reliability .
In addition, there is no lead-free solder having sufficient soldering joinability such as wettability, and, at the same time, having a melting temperature similar to the melting temperature, which isl83 °C , of the typical lead-containing solder (Sn37Pb). Hence, the lead-free solders are now being used only with modifications in a soldering process such as replacement of ovens. This is because only limited metals, such as copper, silver, zinc (An), bismuth and indium (In), are used as alloying elements for dropping the melting temperature.
Furthermore, the typical lead-free solders do not satisfy both the sufficient soldering joinability such as the wettability and the erosion resistance and the low melting temperature near the melting temperature (183 °C ) of the typical lead-containing solder (Sn37Pb). Accordingly, in most lead-free solders, small amount of metals, such as copper, silver, zinc, bismuth and indium, for lowering the melting temperature are added in lead to lower the melting temperature of the lead-free solders. However, there is no further development on lowering the melting temperature of the lead-free solder.
Therefore, nowadays, a silver alloy of SnAgCu and a non-silver alloy of SnCu of a melting temperature ranging from 220 °C to 230 °C are mainly used. As described above, the developments of the solder with regard to the melting temperature are at the end of their rope, and thus the developments of the solder are focused on improving characteristics, work efficiency, quality, and reliability of the solder.
The lead-free solder has a high melting temperature, needs expensive raw materials, and generates a large quantity of dross in comparison to the lead-containing solder, increasing production costs. In addition, for the lead-free solder, oxides in the molten solder are incorporated in a soldering fillet (soldering part) of a PCB assey, resulting in deterioration of quality and reliability of soldering assemblages. To overcome these, KR 10-0327767, JP 3622788 and JP 3296289 describe a SnAgCu based solder alloy and a SnCu based solder alloy added with elements such as phosphorous, nickel, germanium and gallium for preventing oxidation. However, the addition of the element such as germanium, gallium and nickel is limited because it may increase the melting temperature and may cause cracks in the soldering part when exposed to thermal processing and thermal fatigue caused by hardening, which results in deterioration of reliability. Particularly, the addition of a large quantity of phosphorous may increase brittleness of the soldering part due to the hardening of the solder alloy. Phosphorous rises above the surface of the molten solder to prevent oxidation of the molten' solder. However, phosphorous has only a short-lived effect on preventing dross (oxide) formation at a soldering temperature of approximately 260 °C because of its high volatility. Particularly, as for a wave soldering and a dip soldering, after a bath is filled with hundreds of kilograms of solders, the processes are generally continued for months with adding only a small amount of the solders for compensating the consumed solders. Hence, in such processes, the addition of phosphorous has only a shortlived effect on preventing the dross formation, and the effect cannot last for a long time.
The high temperature lead-free solder uses tin, which is expensive, instead of lead as a raw material and experiences a significant oxidation (dross formation) during melting (soldering) in comparison with lead- containing solders, which result in an increased economic load on users. In addition, the oxides mixed in the molten solder may be incorporated in the soldering fillet (soldering part) of the PCB assey, which result in deterioration of soldering assembly set. Particularly, the soldering temperature of the high temperature solder is much higher than that of the solder for a general wave, resulting in a significant increase of dross formation and thus in deterioration of the work efficiency. To resolve this problem, JP2004-154864A and JP2004-181458A_describe a SnCu-based solder alloy with an element such as phosphorous, nickel, germanium and gallium added thereto for preventing oxidation.
However, germanium, gallium and nickel are expensive materials. In addition, when they are added above a certain level, they may cause cracks in the soldering part exposed to a thermal process and a thermal fatigue, decreasing the reliability. Particularly, phosphorous may increase the brittleness of the soldering part due to overhardening of the solder alloy when added in a large quantity.
In addition, as for a high temperature solder alloy, phosphorous rises over a surface of a soldering bath to prevent oxidation of the molten solder. However, regarding that the volatilization temperature of phosphorous is 380 °C whereas typical operations of the high temperature solder are performed at a soldering temperature of approximately 420 °C to approximately 520 °C , phosphorous has little effect on preventing the dross (oxide) formation. In addition, as for a dip soldering, after the bath is filled with the solders, the process is generally continued for months with adding only a small amount of the solders for compensating the consumed solders. Therefore, the dross formation can be prevented only for a short time in an initial filling of the bath, and the effect cannot last for a long time.
In addition, if the content of nickel is increased above 0.1 % for increasing the temperature, an overhardening of the solder alloy may increase the brittleness, deteriorating the soldering characteristics.
In addition, the high temperature solder alloy is used at a temperature of 460 °C to 540 °C whereas a general solder alloy is used at a temperature 260+ 10 °C .
Accordingly, in the high temperature solder alloy, oxides are generated more and at a higher rate and thus a higher oxidation resistance is required than in the general solder alloy.
In addition, because of erosion of a copper pad in the PCB, when the concentration of copper is increased above 1 %, the melting temperature of the solder increases rapidly, and the wettability decreases. This may require replacing the whole lead bath or may cause cracks .
In addition, when only silicon is added, the effect on the dross and the discoloration may be improved. However, if the content of the silicon is greater than a certain level, the joinability, which is a basic characteristic of the solder, may be decreased. Therefore, there is a need for additional elements.
DISCLOSURE TECHNICAL PROBLEM
An embodiment of the present invention is directed to providing a lead-free solder composition including silicon and a very small amount of cobalt to constantly prevent oxidation during soldering, maintain wettability, prevent discoloration, improve mechanical properties, prevent erosion of a copper pad, and improve soldering joinability, and a PCB and an electronic device using the lead-free solder composition. TECHNICAL SOLUTION
In accordance with an aspect of the present invention, there is provided a lead-free solder composition, including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a printed circuit board (PCB) and an electronic device using the composition. In accordance with another aspect of the present invention, there is provided a lead-free solder composition, including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
In accordance with another aspect of the present invention, there is provided a lead-free solder composition, including from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
In accordance with another aspect of the present invention, there is provided a lead-free solder composition, including from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
In accordance with another aspect of the present invention, there is provided a lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 17 wt% bismuth, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition. In accordance with another aspect of the present invention, there is provided a high temperature lead-free solder composition, comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
In accordance with another aspect of the present invention, there is provided a high temperature lead-free solder composition, comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
In accordance with another aspect of the present invention, there is provided a lead-free solder composition for dilution, comprising from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition. In accordance with another aspect of the present invention, there is provided a lead-free solder composition for dilution, comprising from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin, a PCB and an electronic device using the composition.
ADVANTAGEOUS EFFECTS
According to the exemplary embodiments, the lead- free solder composition and the PCB and the electronic device using the lead-free solder composition include silicon and cobalt at the same time to constantly prevent oxidation during soldering, maintain wettability, prevent discoloration, improve mechanical properties, prevent erosion of a copper pad, and improve soldering joinability .
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 illustrates a principle of preventing oxide formation by a lead-free solder composition in accordance with an embodiment of the present invention.
Fig. 2 illustrates a result of an element analysis performed in an initial state of a dross experiment based on the principle of Fig. 1. Fig. 3 illustrates a result of an element analysis performed after 3 hours of the dross experiment based on the principle of Fig. 1.
Fig. 4 illustrates a silicon peak measured by Auger electron spectroscopy (AES) after an experiment based on the principle of Fig. 1.
Fig. 5 illustrates SiO2 peak measured by AES after an experiment based on the principle of Fig. 1.
Fig. 6 illustrates a principle of preventing oxide formation by a high temperature lead-free solder composition in accordance with an embodiment of the present invention.
Fig. 7 illustrates a result of element analysis of a high temperature solder by AES after 3 hours of a dross experiment .
BEST MODE FOR THE INVENTION
Fig. 1 illustrates a principle of preventing oxide formation by a lead-free solder composition in accordance with an embodiment of the present invention. Fig. 1 illustrates a wave soldering process using a solder alloy including a SnAgCu base alloy which is added with small amounts of P and Si to prevent oxide formation. A portion of the added P and Si are consumed in deoxidization of the molten solder base alloy and another portion is dissolved in the molten solder base alloy. However, since the solubility of P and Si in a molten Sn is very small, most of the added P and Si rise towards a surface of the molten solder base alloy due to a difference in specific gravities. Then, a film of phosphorous oxides and silicon oxides formed over the surface interferes with the contact of an oxygen gas in the air and the molten solder to prevent the formation of oxides of the lead-free solder.
As described above, a portion of P on the surface of the molten solder base alloy forms the oxide film. However, a large portion of P on the surface of the molten solder base alloy is evaporated to the air, and thus the effect of preventing the oxide formation on the surface of the molten solder is decreased gradually. On the contrary, Si on the surface is not evaporated to the air, and thus can constantly form an oxide film on the surface of the molten solder, and thus the effect of preventing the oxide formation on the surface of the molten solder lasts for a long time. In addition, as a very small amount of cobalt is added, cobalt dissolved in the molten solder also prevents the oxide formation.
Fig. 2 illustrates a result of an element analysis by Auger electron spectroscopy (AES). This element analysis was performed by adding 0.5 mass % copper, 0.06 mass % nickel, 0.005 mass % phosphorous, 0.05 mass % silicon and 0.05 mass % cobalt to tin, melting and casting the solder alloy to an ingot, cutting the ingot, polishing the cut surface of the ingot, and analyzing the polished cut surface by AES. Fig. 3 illustrates a result of an element analysis by AES, after a dross experiment (oxidation experiment) at 260 °C for 3 hours using an ingot fabricated according to the procedure described above with reference to Fig. 2. As shown in Fig. 2, in an initial state of a dross experiment, P rises towards a surface of the ingot so that most of P is concentrated in an upper region, accounting for 1.86 % of the total mass, and silicon is distributed below P. This can be seen from the composition data of points 1, 2 and 3, each being denoted by a mark ® in Fig. 2.
As shown in Fig. 3, after the dross experiment at 260 °C for 3 hours, the concentration of Si (0.08 mass %) is higher than that of P (0.06 mass %) at the point 1 (upper region), and the concentration of Si (0.19 mass %) is also higher than that of P (0.13 mass %) at the point 2. In addition, according to the peaks detected by Auger electron spectroscopy, silicon oxides (estimated to be SiO2) are formed at the point 1. However, at the points 2 and 3, Si peak is detected instead of SiO2 peak, as shown in Fig. 4.
Therefore, the followings can be considered. In an initial state, P and then Si rise to the surface of the molten solder in the molten solder due to a specific gravity difference. Thereafter, a large portion of P on the surface of the molten solder is evaporated in a short time, and only a small portion of P contacts oxygen gas in the air to form an oxide film. On the contrary, most of the Si forms a silicon oxide film such as SiO2 film to prevent the oxidation of the molten solder. Fig. 6 illustrates a principle of preventing oxide formation by a high temperature lead-free solder composition in accordance with an embodiment of the present invention. As shown in Fig. 6, when performing dip soldering on a surface of a lead of an electronic component using a solder alloy, which is fabricated by adding P and Si in SnCu base alloy to prevent oxide formation, a portion of P and Si added in a small amount is consumed in deoxidization of molten solder base alloy, and then remaining P and Si are dissolved in the molten solder base alloy. However, because the solubility of P and Si in molten Sn is very small, most of the remaining P and Si rise towards the surface of the molten solder base alloy due to the specific gravity difference. The film of the phosphorous oxides and the silicon oxides formed over the surface interferes with the contact of oxygen gas in the air and the molten solder to prevent the formation of an oxide of the lead-free solder.
In a general soldering at 260 °C , a portion of P on the surface of the molten solder base alloy forms an oxide film. However, a large portion of P on the surface of the molten solder base alloy is evaporated to the air, and thus the effect of preventing the oxide formation on the surface of the molten solder is decreased gradually. On the contrary, Si on the surface is not evaporated to the air, and thus can constantly form an oxide film on the surface of the molten solder, and thus the effect of preventing the oxide formation on the surface of the molten solder lasts for a long time.
Fig. 7 illustrates a result of element analysis of a high temperature solder by AES after 3 hours of a dross experiment. In more detail, Fig. 7 illustrates a result of element analysis of a high temperature solder by AES after 3 hours of a dross experiment at 420 °C , for demonstrating the above described phenomenon. As shown in Fig. 7, in an upper region of the surface of the ingot, P was not detected at all, whereas 1.09 wt% of Si was detected. This tells us that although most of P is evaporated from the surface of the molten solder, the silicon still remains even at a high temperature to form a silicon oxide film over the surface of the molten solder. The silicon oxide film formed over the surface of the molten solder interferes with contact of the molten solder and an oxygen gas in the air to prevent the oxidation. When only silicon is added, the effect of discoloration and the dross may be improved. However, when silicon is added above a predetermined level, the melting temperature of the solder alloy increases, and wettability decreases and thus joinability decreases, which is a fundamental characteristic of the solder. Accordingly, by adding a very small amount of cobalt, it is possible to obtain a significant result of maintaining the wettability and preventing the oxidation, the discoloration and the erosion of a copper. Hereinafter, the property and the use of the components in the lead-free solder composition in accordance with embodiments will be described.
Tin (Sn) is an essential component of the lead-free solder, and is used as a base metal. Copper (Cu) in a solder alloy for a general wave soldering slightly decreases the melting temperature of a solder alloy and a joint strength of a solder joint. Here, when copper is added in an amount less than 0.1 wt%, the effect is not sufficient, and when copper is added in an amount greater than 2 wt%, the melting temperature of the solder alloy increases. Therefore, the amount of copper added in the solder alloy for the general wave soldering is preferably from 0.1 wt% to 2 wt%. In addition, copper in a solder alloy for a high temperature dip used at least 400 °C has no effect of increasing the melting temperature of the solder alloy when added in an amount less than 2 wt%. Copper in the solder alloy for the high temperature dip increases the melting temperature excessively and causes over-hardening of the solder alloy when added in an amount greater than 5 wt%. Therefore, the preferable content of copper in the solder alloy for the high temperature dip is from 2 wt% to 5 wt%.
Silver (Ag) is nonpoisonous . In addition, silver decreases the melting temperature of a solder alloy, increases the dispersibility of the base metal, and improves the thermal fatigue property. When silver is added in an amount less than 0.1 wt%, the effect of silver is insufficient. When silver is added in an amount greater than 4 wt%, the melting temperature increases. Therefore, the preferable content of silver in the solder alloy is from 0.1 wt% to 4 wt% .
Phosphorous (P) prevents oxide formation during a soldering, and improves the work. When phosphorous is added in an amount less than 0.001 wt% , there is no effect. When phosphorous is added in an amount greater than 0.2 wt%, the melting temperature increases. Therefore the preferable content of silver in the solder alloy is from 0.001 wt% to 0.2 wt%.
Germanium (Ge) improves the wettability and prevents the oxidation. When germanium is added in an amount less than 0.001 wt%, there is no effect. When germanium is added in an amount greater than 0.1 wt%, the raw material cost increases rapidly and the melting temperature increases. Therefore, the preferable content of germanium in the solder alloy is from 0.001 wt% to 0.1 wt%.
Nickel (Ni) prevents erosion of a copper pad, and improves the joint strength. When nickel is added in an amount less than 0.001 wt%, there is no effect. When nickel is added in an amount greater than 0.5 wt%, the melting temperature increases rapidly and over-hardening of the solder alloy occurs. Therefore, the preferable content of nickel in the solder alloy is from 0.001 wt% to 0.5 wt%. Bismuth (Bi) decreases the melting temperature and improves the joint strength. When bismuth is added in an amount less than 0.1 wt%, there is no effect of decreasing the melting temperature. When bismuth is added in an amount greater than 17 wt%, the melting temperature of Sn alloy decreases to approximately 210 °C , or over-hardening of the solder alloy occurs. Therefore, the preferable content of bismuth in the solder alloy is from 0.1 wt% to 17 wt%.
Silicon (Si) is a main alloying element according to the embodiments. Silicon has the rising characteristic and is nonvolatile. Hence, by adding a small amount of silicon in the solder base alloy, the oxide formation can be prevented and the effect maintains for a long time, which is different from phosphorous. By adding a very small amount of silicon in the solder base alloy, the discoloration of a soldering region after the soldering can be significantly prevented and the prevention of the copper erosion can be improved, while maintaining the temperature and the wettability of the typical lead-free solder.
Cobalt (Co) does not have the rising characteristic. However, by adding a small amount of cobalt together with silicon, it is possible to prevent the oxide formation, the discoloration and the erosion to thus improve the soldering characteristics and the workability, while maintaining the wettability. EMBODIMENT 1
The embodiment provides a lead-free solder composition including from 0.1 wt% to 4.0 wt% silver, from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt, and the balance of Sn, and a PCB and an electronic device using the lead-free solder composition.
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 1.
Graph 1 illustrates experimental data of the amount of oxidation measured by the following procedure. Silicon and cobalt were added in a predetermined base alloy. The resulting alloy was molten to fabricate a solder composition. The solder composition was heated to 260 °C in an SUS crucible of 160 mm diameter, on a hot plate. Then, the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time.
Graph 1
Figure imgf000018_0001
As shown in Graph 1, the amount of oxides formed in Sn3AgO .5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g. The amount of oxides formed in Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g. This is because P serves to reduce the amount of oxidation.
However, when 0.005 wt% of Si was added in Sn3AgO.5Cu, the amount of oxides was 270 g after 1 hour, 302 g after 2 hours, and 345 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 917 g, which is smaller than the amount of oxides formed in Sn3AgO .5CuO .005P without Si. In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours . Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P.
In addition, it can be seen that the amount of oxidation decreased further more when 0.05 wt% of Si was added. When 0.005 wt% of Co was added in Sn3AgO .5CuO .005Si, the amount of oxides was 215 g after 1 hour, 240 g after 2 hours, and 264 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 719 g. It can be seen from this result that the amount of oxidation was decreased in comparison to the cases of Sn3Ag0.5Cu0.005P and Sn3AgO .5CuO .005Si .
That is, by adding a very small amount (0.005 wt%) of cobalt in Sn3AgO .5CuO .005Si, the oxidation of the molten solder can be minimized to a level equivalent to that of quaternary composition system added with only 0.05 wt% Si.
When 0.005 wt% of Co was added in SnO.3AgO.7CuO.005Si, it can be seen that by adding a small amount of Si and Co, the amount of oxidation can be reduced, in comparison to the comparative example. Therefore, it can be seen that by adding a small amount of Si in Sn-Ag-Cu base alloy, the oxide formation can be prevented constantly. It can also be seen that by- further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 2.
Graph 2 illustrates experimental results of the wettability. To examine the wetting time of the soldering compositions, the solder was molten at 260 °C using a SP2 wetting tester (manufactured by MalCom Co., Ltd.). A copper terminal was dipped into a depth of 1 mm from the surface of the molten solder to measure the wetting time after 5 seconds of the dipping.
Graph 2
Temp : 2600C
CJ CU -1
0.5
Sn3A90.5Cu Si3AgO.5Cu +0.005Si +0.05Si +0.1Si +0.0Q5S) +0.05Si SnO.3AgO.7Cu U.UUuP 0.005CO 0.01CO 0.005Si 0.005Cd
Comparative example Example
As shown in Graph 2, the wetting time of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 0.48 seconds. The wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds.
When 0.005 wt% of Si was added in Sn3AgO.5Cu, the wetting time was shorten to 0.44 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 0.49 seconds.
This is probably because when a very small amount (0.005 wt% ) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
When 0.005 wt% of Co is added in Sn3AgO .5CuO .005Si, the wetting time was shorten to 0,46 seconds. From this result, it can be seen that by adding a very small amount (less than 0.01 wt%) of Co, it is possible to improve the acid resistance of the molten solder alloy and minimize the decrease of the wettability caused by the addition of Si in an amount greater than the limit.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 3.
Graph 3 illustrates experimental results of the erosion rate of copper. To examine the erosion rate of copper, a solder alloy was molten and kept at 260 °C . A copper wire of 0.8 mm diameter was dipped into a depth of 30 mm from the surface of the molten solder to measure the weights of the copper wire before the dipping and after 60 seconds of the dipping, respectively. The weights of the copper wire was compared with each other to obtain the average erosion rate of five samples. Graph 3
70
• TemD : 26Q°C ■ Time : δOsec
50 ϋ CD 40 <=> 30
S 20 10
Sπ3A|5Cu sn3AgO.5Cι +0.005Si +Q.05SΪ +(1.1Si +0.005Si +0.05Si SnO.3AgO.7CU
U. UUOr 0.005CO 0.01 Co 0.005Si 0.005Cd
Comparative example Example
As shown in Graph 3, the copper erosion rate of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag, and the balance of Sn was 58.3 %. The copper erosion rate of
Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.1 %.
When 0.005 wt% of Si was added in Sn3AgO.5Cu, the copper erosion rate was decreased to 54.3 %, and when 0.05 wt% of Si was added, the copper erosion rate was decreased to 41.7 %.
When a very small amount (less than 0.01 wt% ) of Co was added in Sn3AgO .5CuO .05Si, the copper erosion rate was significantly decreased to 21.1 %. From these result, it can be seen that by adding a very small amount (less than 0.01 wt%) of Co to the base alloy including 0.005 wt% of Si, the copper erosion rate can be significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 1. Furthermore, the optimum contents of silicon and cobalt in the Sn-Ag-Cu base alloy will be estimated from these results.
Table 1 lists the experimental data of Graphs 1, 2 and 3 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Figure imgf000023_0001
As shown in Table 1, the melting temperature of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 220 °C . The melting temperature of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 °C . The melting temperature of Sn3AgO.5Cu was changed to 220 °C when added with 0.005 wt% of Si, and the melting temperature of Sn3AgO .5Cu was increased to 224 "C when added with 0.1 wt% of Si.
The melting temperature of Sn3AgO .5CuO .05Si was increased to 223 °C when added with 0.01 wt% of Co. In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level .
The discoloration data in Table 1 was obtained as follows. An oxygen-free copper of 99.99 % purity having a size of 25x31x0.3 mm was dipped at 260 °C for 3 seconds to prepare a specimen. The specimen was heated to 250 °C for 10 minutes. Then, the discoloration level of yellow was measured using Minolta CM3700B chromatic aberration meter.
As shown in Table 1, the discoloration level of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 16.47, and the discoloration level of Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 19.31.
When 0.005 wt% of Si was added in Sn3AgO.5Cu, the discoloration level became 15.66, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 6.6, which is lower than those of Sn3AgO.5Cu and Sn3AgO .5CuO .005P .
When 0.005 wt% of Co was added in Sn3AgO .5CuO .005Si, the discoloration level was significantly decreased to 3.46, and when 0.01 wt% of Co was added in Sn3AgO .5CuO .005Si , the discoloration was significantly decreased to 3.02.
Therefore, it can be concluded that the discoloration of the Sn-Ag-Cu base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein .
The joint fracture load data in Table 1 was obtained as follows. A 2 mm diameter wire coated with tin-bismuth was vertically inserted in a hole of a PCB substrate and wave soldering was performed thereon to prepare a specimen. Then, the joint fracture load (kgf) thereof was measured using a tension tester.
As shown in Table 1, the joint fracture load of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf, and the joint fracture load of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf.
When 0.005 wt% of Si was added in Sn3AgO.5Cu, the joint fracture load was 95 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 105 kgf, which is greater than those of Sn3AgO.5Cu and Sn3AgO .5CuO .005P .
When 0.005 wt% of Co was added in Sn3AgO .5CuO .005Si , the joint fracture load was 96 kgf, which is greater than that of Sn3AgO.5Cu. When 0.01 wt% of Co was added in Sn3AgO.5CuO .005Si, the joint fracture load was significantly increased to 112 kgf.
Therefore, it can be concluded that the joint fracture load of the Sn-Ag-Cu base alloy can be improved by adding a small amount of Si therein, and can also be significantly improved by adding a very small amount of Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Ag-Cu base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load. However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, when a very small amount (less than 0.01 wt%) of Co was added in the Sn-Ag-Cu base alloy- together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability . In summary, the addition of Si serves to prevent the oxidation and the discoloration. However, when Si is added in an amount above a certain level, the wettability and thus the joinability decrease. Therefore, by further adding a very small amount (less than 0.01 wt%) of Co therein, it is possible to significantly reduce the oxidation, the discoloration and the copper erosion while maintaining the wettability.
Cu added in the Sn-Ag-Cu-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%. Ag added in the Sn-Ag-Cu-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous . However, when Ag is added in an amount less than 0.1 wt%, the effect of Ag is insufficient, and when Ag is added in an amount greater than 4 wt%, the melting temperature is rather increased. Therefore, the preferable content of Ag is from 0.1 wt% to 4.0 wt%. Sn is an essential component of the lead-free solder, and is used as a base metal .
The lead-free composition in accordance with the embodiment can be used in a form of cream, bar or wire. The lead-free composition can be implemented in electronic devices including a plurality of electronic components fixed thereto through the lead-free soldering alloy. The electronic devices can be applied to various electronic instruments such as computers, digital camcorders, digital televisions, digital cameras, and mobile communication terminals.
The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy. As described above, the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin. As such, the lead-free composition, and thus the PCB and the electronic device using the lead-free solder composition can have a mechanical characteristic and workability similar to those of Sn-Pb alloy and Sn-Ag-Cu alloy. Further, they can prevent the oxidation of the molten solder constantly and effectively by adding Si and Co in the ternary composition system of tin, silver and copper. Furthermore, they can maintain the soldering temperature and the wettabiltiy of the typical lead-free solder, prevent the discoloration after the soldering and the erosion of the copper pad in the PCB, and improve the soldering joinability, by adding only a very small amount of alloying elements.
The lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), silver (Ag), phosphorous (P), silicon (Si) and cobalt (Co).
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 4.
Graph 4 illustrates data obtained from oxide formation experiments .
Graph 4
Figure imgf000028_0001
As shown in Graph 4, the amount of oxides formed in Sn3AgO.5CuO.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g. The amount of oxides formed in Sn3AgO .5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g. This is because P serves to reduce the amount of oxidation.
However, when 0.005 wt% Si is added in Sn3AgO .5CuO .005P, the amount of oxides was 135 g after 1 hour, 157 g after 2 hours, and 185 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 477 g, which is smaller than the amount of oxides formed in Sn3AgO .5CuO .005P without Si. In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours . Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, when Si was added in an amount of 0.05 wt%, the oxidation was decreased further more.
When 0.005 wt% of Co was added in Sn3Ag0.5Cu0.005P0.005Si, the amount of oxides was 105 g after 1 hour, 132 g after 2 hours, and 155 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 392 g. It can be seen from this result that the amount of oxidation was decreased in comparison to the cases of Sn3AgO .5CuO .005P and Sn3Ag0.5Cu0.005P0.005Si.
That is, by adding a very small amount (0.005 wt%) of cobalt in Sn3AgO .5CuO .005P0.005Si, the oxidation of the molten solder can be minimized to a level equivalent to the case where only 0.05 wt% of Si is added.
When 0.05 wt% of Co was added in SnO .3AgO .7CuO .05Si , it can be seen that the amount of oxidation can be significantly reduced by adding a small amount of Si and Co in comparison to the comparative example. Therefore, it can be seen that by adding a small amount of Si in Sn-Ag-Cu-P base alloy, the oxide formation can be prevented constantly. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co. Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 5.
Graph 5
Figure imgf000030_0001
As shown in Graph 5, the wetting time of Sn3AgO.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds. The wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.48 seconds .
When 0.005 wt% of Si was added in Sn3AgO .5CuO .005P, the wetting time was shorten to 0.47 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 0.52 seconds, which is longer than that of Sn3AgO.5Cu.
This is probably because when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
Therefore, by further adding a very small amount (less than 0.01 wt%) of Co, it is possible to improve the acid resistance of the molten solder alloy, and, at the same time, minimize the decrease of wettability due to silicon added in an amount above the limit.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 6.
Graph 6 illustrates experimental results of the erosion rate of copper.
Graph 6
Figure imgf000031_0001
As shown in Graph 6, the erosion rate of Sn3Ag0.5Cu0.005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.1 %. The erosion rate of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 58.3 %. When 0.005 wt% of Si was added in Sn3Ag0.5Cu0.005P0.005Si, the erosion rate of copper was significantly decreased to 19.2 %, and when a very small amount (less than 0.01 wt%) of Co was added in the base alloy including 0.05 wt% of Si, the erosion rate of copper was significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 2. Furthermore, the optimum contents of silicon and cobalt in the Sn-Ag-Cu-P base alloy will be estimated from these results.
Table 2 lists the experimental data of Graphs 4, 5 and 6 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Figure imgf000032_0001
As shown in Table 2 , the melting temperature of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 °C . The melting temperature of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 220 °C . The melting temperature of Sn3AgO .5CuO .005P was 221 °C when added with 0.005 wt% of Si, and the melting temperature of Sn3AgO.5CuO.005P was increased to 225 °C when added with 0.1 wt% of Si.
The melting temperature of Sn3AgO .5CuO .005P0.05Si was increased to 223 °C when added with 0.01 wt% of Co. In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 "C) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level. The discoloration level of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 19.31, and the discoloration level of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 16.47. When 0.005 wt% of Si was added in Sn3AgO .5CuO .005P, the discoloration level was 17.5, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 8.12, which is lower than those of Sn3AgO.5Cu and Sn3AgO .5CuO .005P . When 0.005 wt% of Co was added in
Sn3AgO .5CuO .005P0.005Si , the discoloration level was significantly decreased to 7.02, and when 0.01 wt% of Co was added in Sn3AgO .5CuO .005P0.005Si , the discoloration was significantly decreased to 4.7. Therefore, it can be concluded that the discoloration of the Sn-Ag-Cu-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu-P base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
The joint fracture load of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf, and the joint fracture load of Sn3AgO.5Cu including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf.
When 0.005 wt% of Si was added in Sn3AgO .5CuO .005P, the joint fracture load was 106 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 113 kgf, which is greater than those of Sn3AgO.5Cu and Sn3AgO .5CuO .005P .
When 0.005 wt% of Co was added in Sn3AgO .5CuO .005Si , the joint fracture load was 110 kgf, which is greater than that of Sn3AgO.5Cu. When 0.01 wt% of Co was added in Sn3Ag0.5Cu0.005Si, the joint fracture load was significantly increased to 115 kgf.
Therefore, it can be seen that the joint fracture load of the Sn-Ag-Cu-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Ag- Cu-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load. In addition, when a very small amount of Co was added in the Sn-Ag-Cu-P base alloy together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
Cu added in the Sn-Ag-Cu-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%. Ag added in the Sn-Ag-Cu-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous . However, when Ag is added in an amount less than 0.1 wt%, the effect of Ag is insufficient, and when Ag is added in an amount greater than 4 wt%, the melting temperature is rather increased. Therefore, the preferable content of Ag is from 0.1 wt% to 4.0 wt% . P added in the Sn-Ag-Cu-P base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability. However, when P is added in an amount less than 0.001 wt%, the effect of P is insufficient, and when P is added in an amount greater than 0.2 wt%, the melting temperature is rather increased. Therefore, the preferable content of P is from 0.001 wt% to 0.2 wt%. Sn is an essential component of the lead-free solder, and is used as a base metal .
The present embodiment can also be implemented in a
PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy. As described above, the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from
0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
The lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), silver (Ag), nickel (Ni), germanium (Ge), silicon (Si) and cobalt (Co).
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 7.
Graph 7 illustrates data obtained from oxide formation experiments .
Graph 7
Figure imgf000037_0001
As shown in Graph 7, the amount of oxides formed in Sn3Ag0.5Cu0.06Ni0.01Ge including 0.5 wt% Cu, 3 wt% Ag, 0.06 wt% Ni, 0.01 wt% Ge and the balance of Sn was 147 g after 1 hour, 151 g after 2 hours, and 160 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 458 g. The amount of oxides formed in Sn3AgO .5CuO .05P including 0.5 wt% Cu, 3 wt% Ag, 0.05 wt% P and the balance of Sn was 295 g after 1 hour, 325 g after 2 hours, and 390 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,010 g. The amount of oxides formed in Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 478 g after 1 hour, 540 g after 2 hours, and 564 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,582 g. This is because Ni serves to reduce the amount of oxidation. When 0.005 wt% Si is added in Sn3AgO .5CuO .06NiO .01Ge, the amount of oxides was 105 g after 1 hour, 115 g after 2 hours, and 121 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 341 g, which is smaller than the amount of oxides formed in Sn3AgO .5CuO.06NiO.01Ge without Si.
In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours. Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, when Si was added in an amount of 0.05 wt%, the oxidation was decreased further more.
When 0.005 wt% of Co was added in Sn3AgO.5CuO.06NiO .01GeO .005Si, the amount of oxides was decreased in comparison to the comparative examples . Therefore, it can be seen that by adding a small amount of Si in Sn-Ag-Cu-Ni-Ge base alloy, the oxide formation can be prevented constantly in comparison to the typical solder composition added with P. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co. Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 8.
Graph 8
1
■ Temp : 2600C
0.8
1 Jg 0.6
1 0.4
0.2
nnnδnn,pi|, Sπ3Ag0.5Cu Sπ3AgO.5Cu +0.05Si Sn4AgQ.7CuO.1NI bnangoAUi 0ι005P o.OBNiO.OiGe +0.QQ5SΪ +0.05Si +0.1Si +0.005Si
0.Q05CO 0.01CO 0.005Si 0.005Co
Comparative example Example
As shown in Graph 8, the wetting time of Sn3AgO.5CuO.06NiO.01Ge including 0.5 wt% Cu, 3 wt% Ag, 0.06 wt% Ni, 0.01 wt% Ge and the balance of Sn was 0.38 seconds. The wetting time of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 0.48 seconds. The wetting time of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.51 seconds .
When 0.005 wt% of Si was added in
Sn3Ag0.5Cu0.06Ni0.01Ge, the wetting time was shorten to
0.34 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 0.39 seconds .
This is probably because when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
Therefore, by further adding a very small amount (less than 0.01 wt%) of Co, it is possible to improve the acid resistance of the molten solder alloy, and, at the same time, minimize the decrease of wettability due to silicon added in an amount above the limit.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 9.
Graph 9 illustrates experimental results of the erosion rate of copper.
Graph 9
70
• Temp: 2600C • Time : 60sec
50
SE
M 30
1 20 10
Sn3AgO.5Cu Sn3AgO.5Cu Sn3AgQ.5Cu +0.005Si +0.05Si +0.1Si +0.0Q5SΪ +0.05Si Sn4AgO.7CuO.1Ki 0.005P 0.06NiO-OIGe 0.005CO 0.01CO 0.005Si 0.005CO
Comparative example Example
As shown in Graph 9, the erosion rate of Sn3Ag0.5Cu0.06Ni0.01Ge including 0.5 wt% Cu, 3 wt% Ag, 0.06 wt% Ni, 0.01 wt% Ge and the balance of Sn was 56.5 %. The erosion rate of Sn3AgO .5CuO .005P including 0.5 wt% Cu, 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.1 %. The erosion rate of Sn3AgO .5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 58.3 %.
When 0.005 wt% of Si was added in Sn3AgO .5CuO .06NiO .01Ge, the erosion rate of copper was decreased to 53.2 %, and when 0.05 wt% of Si was added in Sn3AgO .5CuO .06NiO .01Ge, the erosion rate of copper was decreased to 31 %, which is smaller than those of Sn3Ag0.5Cu0.06Ni0.01Ge, Sn3AgO.5Cu and Sn3AgO .5CuO .005P .
When 0.01 wt% of Co was added in Sn3AgO.5CuO.06Ni0.01Ge0.05Si, the erosion rate of copper was significantly decreased to 17.2 %, and when a very small amount (less than 0.01 wt%) of Co was added in the base alloy including 0.05 wt% of Si, the erosion rate of copper was significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 3. Furthermore, the optimum contents of silicon and cobalt in the Sn-Ag-Cu- Ni-Ge base alloy will be estimated from these results .
Table 3 lists the experimental data of Graphs 7, 8 and 9 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Figure imgf000041_0001
Figure imgf000042_0001
As shown in Table 3, the melting temperature of Sn3AgO.5CuO. OβNiO.OlGe including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% Ni, 0.01 wt% Ge and the balance of Sn was 222 °C . The melting temperature of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 221 °C . The melting temperature of Sn3AgO.5Cu was 220 °C when added with 0.005 wt% of Si.
The melting temperature of Sn3AgO .5CuO .06NiO .01Ge was increased to 225 °C when added with 0.1 wt% of Si.
The melting temperature of Sn3Ag0.5Cu0.06Ni0.01Ge0.05Si was increased to 224 °C when added with 0.01 wt% of Co. In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 "C) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level. As shown in Table 3, the discoloration level of Sn3AgO.5CuO. OβNiO.OlGe was 15.6, the discoloration level of Sn3AgO.5Cu including 0.5 wt% Cu, 3 wt% Ag and the balance of Sn was 16.47, and the discoloration level of Sn3Ag0.5Cu0.005P was 19.31. When 0.005 wt% of Si was added in Sn3AgO .5CuO.06NiO.01Ge, the discoloration level was 13.5, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 9.47, which is lower than those of Sn3AgO.5Cu and Sn3Ag0.5Cu0.005P.
When 0.005 wt% of Co was added in
Sn3AgO.5CuO.06NiO.01GeO.005Si, the discoloration level was significantly decreased to 6.72, and when 0.01 wt% of
Co was added in Sn3AgO .5CuO .06NiO .01GeO .005Si , the discoloration was significantly decreased to 3.08.
Therefore, it can be seen that the discoloration of the Sn-Ag-Cu-Ni-Ge base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu-Ni-Ge base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
As shown in Table 3, the joint fracture load of Sn3AgO.5CuO.06NiO.01Ge was 100 kgf, the joint fracture load of Sn3Ag0.5Cu0.005P including 3 wt% Ag, 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 89 kgf, and the joint fracture load of Sn3AgO .5CuO .005P including 3 wt% Ag, 0.5 wt% Cu and the balance of Sn was 84 kgf. When 0.005 wt% of Si was added in Sn3Ag0.5Cu0.06Ni0.01Ge, the joint fracture load was 107 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 115 kgf, which is greater than those of Sn3AgO.5Cu and Sn3Ag0.5Cu0.005P.
When 0.005 wt% of Co was added in Sn3Ag0.5Cu0.06Ni0.01Ge0.005Si, the joint fracture load was significantly increased to 109 kgf, which is greater than that of Sn3AgO .5CuO .06NiO .01Ge . When 0.01 wt% of Co was added in Sn3AgO .5CuO .06NiO .01GeO .005Si , the joint fracture load was significantly increased to 119 kgf.
Therefore, it can be seen that the joint fracture load of the Sn-Ag-Cu-Ni-Ge base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Ag-
Cu-Ni-Ge base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load. In addition, when a very small amount of Co was added in the Sn-Ag-Cu-Ni-Ge base alloy together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
Cu added in the Sn-Ag-Cu-Ni-Ge base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt% . Ag added in the Sn-Ag-Cu-Ni-Ge base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous . However, when Ag is added in an amount less than 0.1 wt%, the effect of Ag is insufficient, and when Ag is added in an amount greater than 4 wt%, the melting temperature is rather increased. Therefore, the preferable content of Ag is from 0.1 wt% to 4.0 wt%. Ni added in the Sn-Ag-Cu- Ni-Ge base alloy serves to prevent the erosion of the copper pad and improve the joint strength. However, when Ni is added in an amount less than 0.001 wt%, there is no effect, and when Ni is added in an amount greater than 0.5 wt%, the melting temperature is rather increased rapidly and the over-hardening occur. Therefore, the preferable content of Ag is from 0.001 wt% to 0.5 wt%. In addition, the preferable content of Ge is from 0.001 wt% to 0.1 wt% . Sn is an essential component of the lead-free solder, and is used as a base metal.
P added in the Sn-Ag-Cu-Ni-Ge base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability. The preferable content of P is from 0.001 wt% to 0.2 wt% .
As described above, Si and Co added in the Sn-Ag-Cu- Ni-Ge base alloy each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together .
The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy. As described above, the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
The lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), phosphorous (P), nickel (Ni), silicon (Si) and cobalt (Co).
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 10.
Graph 10 illustrates data obtained from oxide formation experiments.
Graph 10
Figure imgf000046_0002
Figure imgf000046_0001
As shown in Graph 10, the amount of oxides formed in Sn0.5Cu0.06Ni0.005P including 0.5 wt% Cu, 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 134 g after 1 hour, 2686 g after 2 hours, and 384 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 786 g. The amount of oxides formed in SnO .5Cu was 371 g after 1 hour, 395 g after 2 hours, and 460 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,226 g. The amount of oxides formed in SnO .5CuO .005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 251 g after
1 hour, 356 g after 2 hours, and 440 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,047 g. The reason why the amount of oxides formed in SnO .5CuO .06NiO .005P is smaller than those formed in SnO .5CuO .005P and SnO.5Cu is that Ni and P serves to reduce the oxidation. When 0.005 wt% of Si is added in SnO .5CuO .06NiO .005P, the amount of oxides was 110 g after 1 hour, 132 g after
2 hours, and 155 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 397 g, which is smaller than the amount of oxides formed in SnO.5Cu, SnO .5CuO .005P and SnO .5CuO .06NiO .005P .
In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours. Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P.
In addition, when Si was added in an amount of 0.05 wt%, the oxidation was decreased further more.
When 0.01 wt% of Co was added in SnO.5CuO.005P0.06NiO.05Si, the amount of oxides was 86 g after 1 hour, 103 g after 2 hours, and 119 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 308 g, which is smaller than the amount of oxides formed in SnO .5CuO .005P and SnO.5CuO.06NiO.005PO.005Si.
That is, by adding a very small amount (0.01 wt%) of Co in SnO.5CuO.005P0.06NiO.05Si, it is possible to minimize the amount of oxidation of the molten solder to be similar to that of the solder alloy added with only 0.05 wt% of Si.
When 0.005 wt% of Co was added in SnO.75CuO.005P0. INiO.005Si, the amount of oxides was decreased in comparison to the comparative examples.
Therefore, it can be seen that by adding a small amount of Si in Sn-Cu-P-Ni base alloy, the oxide formation can be prevented constantly in comparison to the typical solder composition added with P. It can also be seen that by further adding a very small amount (less than 0.01 wt% ) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 11.
Graph 1 1
2.5
■ Temp : 2600C
I 1.5 i 1
0.5
SnO.75CuO.1Ni ςnnrp.. SM SllO.5CU +(IfIf)1Ki +0(11Ki +(11Si +0.005Si +0.05Si bπu-ouu 0ι005p Q.Q6NiQ.005P mm "m m 0.005CO 0.01CO 0.005P 0.0Q5Si0.005CO
Comparative example Example
As shown in Graph 11, the wetting time of Sn0.5Cu0.06Ni0.005P including 0.5 wt% Cu, 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 2.07 seconds. The wetting time of SnO .5CuO .005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 1.88 seconds. The wetting time of SnO .5Cu including 0.5 wt% Cu and the balance of Sn was 1.67 seconds. When 0.005 wt% of Si was added in SnO .5CuO.06Ni0.005P, the wetting time was shorten to 1.85 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 2.09 seconds.
This is probably because when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
Therefore, by further adding a very small amount (less than 0.01 wt%) of Co, it is possible to improve the acid resistance of the molten solder alloy, and, at the same time, minimize the decrease of wettability due to silicon added in an amount above the limit.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 12.
Graph 12
Figure imgf000050_0001
As shown in Graph 12, the copper erosion rate of SnO.5CuO.06NiO.005P was 47.1 %, the copper erosion rate of SnO.5Cu was 53.6 %, and the copper erosion rate of Sn0.5Cu0.005P was 52.7 %.
When 0.005 wt% of Si was added in
SnO .5CuO .06NiO .005P, the copper erosion rate was 45 %, and when 0.05 wt% of Si was added in SnO .5CuO .06NiO .005P , the copper erosion rate was 44.7 %, which is smaller than those of SnO.5Cu, SnO .5CuO .005P and SnO .5CuO .06NiO .005P .
When 0.01 wt% of Co was added in SnO.5CuO.06NiO.005P0.05Si, the copper erosion rate was significantly decreased to 11.8 %, and when a very small amount (less than 0.01 wt%) of Co was added in the base alloy including 0.05 wt% of Si, the copper erosion rate was significantly decreased because of a synergy effect of silicon and cobalt. Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 4. Furthermore, the optimum contents of silicon and cobalt in the Sn-Cu-P-Ni base alloy will be estimated from these results.
Table 4 lists the experimental data of Graphs 16, 17 and 18 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Figure imgf000051_0001
As shown in Table 4, the melting temperature of SnO.5CuO.06Ni0.005P was 233 °C . The melting temperature of SnO.5CuO.005P including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 231 °C . The melting temperature of SnO.5Cu including 0.5 wt% Cu, 0.005 wt% P and the balance of Sn was 230 °C .
The melting temperature of SnO .5CuO .06NiO .005P was increased to 237 °C when added with 0.1 wt% of Si.
The melting temperature of SnO .5CuO .06NiO .005P0.05Si was increased to 236 °C when added with 0.01 wt% of Co. In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 "C) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level .
As shown in Table 4, the discoloration level of Sn0.5Cu0.006Ni0.005P was 13.5, the discoloration level of SnO.5Cu0.005P was 14.71, and the discoloration level of SnO .5Cu was 15.81.
When 0.005 wt% of Si was added in
SnO.5CuO.006NiO.005P, the discoloration level was 12.7, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 11.3, which is lower than those of SnO .5Cu and SnO .5CuO .005P .
When 0.005 wt% of Co was added in
SnO.5CuO.006Ni0.005P0.005Si, the discoloration level was significantly decreased to 8.33, and when 0.01 wt% of Co was added in SnO .5CuO .006NiO .005P0.05Si, the discoloration was significantly decreased to 4.
Therefore, it can be seen that the discoloration of the Sn-Cu-P-Ni base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu-P-Ni base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
As shown in Table 4, the joint fracture load of Sn0.5Cu0.006Ni0.005P was 85 kgf, the joint fracture load of SnO .5CuO.005P was 87 kgf, and the joint fracture load of SnO.5Cu was 83 kgf.
When 0.005 wt% of Si was added in
SnO.5CuO.006Ni0.005P, the joint fracture load was 90 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 96 kgf, which is greater than those of SnO .5Cu and SnO .5CuO .005P .
When 0.005 wt% of Co was added in Sn0.5Cu0.006Ni0.005P0.005Si, the joint fracture load was 93 kgf, which is greater than that of Sn0.5Cu0.006Ni0.005P. When 0.01 wt% of Co was added in SnO.5CuO.006NiO.005P0.005Si, the joint fracture load was significantly increased to 99 kgf.
Therefore, it can be seen that the joint fracture load of the Sn-Cu-P-Ni base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Cu-P- Ni base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load. In addition, when a very small amount of Co was added in the Sn-Cu-P-Ni base alloy together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
Cu added in the Sn-Cu-P-Ni base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt%. Ni added in the Sn-Cu-P-Ni base alloy serves to increase the strength and the toughness of the solder, and prevent the erosion, the formation of oxides and the growth of intermetallic compounds at the joining surface. The preferable content of Ni in the solder alloy is from 0.001 wt% to 1.0 wt%. This is because as the content of Ni increases, the melting temperature increases and the wettability and the dispersibility decreases. P added in the Sn-Cu-P-Ni base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability. The preferable content of P is from 0.001 wt% to 0.2 wt% . Sn is an essential component of the lead-free solder, and is used as a base metal.
The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy. As described above, the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt, from 0.001 wt% to 0.2 wt% phosphorous and the balance of tin.
The lead-free solder alloy in accordance with the embodiment may include tin (Sn), copper (Cu), phosphorous (P), bismuth (Bi), silicon (Si) and cobalt (Co). The above mentioned solder composition is a non- silver alloy and a low temperature lead-free solder alloy having a melting temperature of approximately 210 °C .
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 13.
Graph 13 illustrates data obtained from oxide formation experiments.
Graph 13
Figure imgf000055_0001
As shown in Graph 13, the amount of oxides formed in Sn0.5Cul7Bi0.005P including 0.5 wt% Cu, 17 wt% Bi, 0.005 wt% P and the balance of Sn was 330 g after 1 hour, 360 g after 2 hours, and 450 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,140 g.
When 0.005 wt% of Si is added in SnO .5Cul7BiO .005P , the amount of oxides was 201 g after 1 hour, 241 g after 2 hours, and 273 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 715 g, which is smaller than the amount of oxides formed in Sn0.5Cul7Bi0.005P.
In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours. Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, when Si was added in an amount of 0.05 wt%, the oxidation was decreased further more.
When 0.005 wt% of Co was added in SnO.5Cu0.005P17Bi0.005Si, the amount of oxides was 168 g after 1 hour, 181 g after 2 hours, and 205 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 554 g, which is smaller than the amount of oxides formed in SnO .5CuO .005P17Bi0.005Si .
Therefore, it can be seen that by adding a small amount of Si in Sn-Cu-Bi-P base alloy, the oxide formation can be prevented constantly in comparison to the typical solder composition added with P. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co. Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 14. Graph 14
Figure imgf000057_0001
As shown in Graph 14, the wetting time of Sn0.5Cul7Bi0.005P was 0.24 seconds. The wetting time of Sn0.5Cul7Bi0.005P was shorten to 0.22 sec when 0.005 wt% of Si was added therein. On the contrary, the wetting time of SnO .5Cul7BiO .005P was rather increased when 0.05 wt% of Si was added therein. When 0.01 wt% of Co was added in Sn0.5Cul7Bi0.005P0.05Si, the wetting time was shorten to 0.28 seconds. On the contrary, when 0.005 wt% of Co was added in SnO .5Cul7BiO .005P0.05Si, the wetting time was 0.23 seconds. It is considered that when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
Therefore, by further adding a very small amount (less than 0.01 wt% ) of Co, it is possible to improve the acid resistance of the molten solder alloy, and, at the same time, minimize the decrease of wettability due to silicon added in an amount above the limit.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 15.
Graph 15 illustrates experimental results of the copper erosion rate.
Graph 15
7D
•Temp: 26O0C 60 • Time : 60sec 50
SE 40
- 20 10
SnO.75Cu2Bi
SnO.5Cu17Bi 0.D05P +0.005Si +0.05Si +0.1Si +0.005Si +0.05Si 0.005P 0.005Co 0.01CO 0.005Si0.005Co
Comparative example Example
As shown in Graph 15, the copper erosion rate of Sn0.5Cul7Bi0.005P was 51.9 %.
When 0.005 wt% of Si was added in SnO .5Cul7BiO .005P, the copper erosion rate was 44.7 %, and when 0.05 wt% of
Si was added in SnO .5Cul7BiO .005P , the copper erosion rate was 31.7 %, which is smaller than that of Sn0.5Cul7Bi0.005P.
When 0.01 wt% of Co was added in SnO .5Cul7BiO .005P0.05Si, the copper erosion rate was significantly decreased to 12.7 %, and when a very small amount (less than 0.01 wt%) of Co was added in the base alloy including 0.005 wt% of Si, the copper erosion rate was significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 5. Furthermore, the optimum contents of silicon and cobalt in the Sn-Cu-Bi-P base alloy will be estimated from these results.
Table 5 lists the experimental data of Graphs 13, 14 and 15 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Table 5 Low temperature SnCuBiP general solders
Figure imgf000059_0001
As shown in Table 5, the melting temperature of Sn0.5Cul7Bi0.005P was 211 °C . When 0.1 wt% of Si was added in SnO .5Cul7BiO .005P, the melting temperature was increased to 214 °C .
When 0.01 wt% of Co was added in
SnO.5Cul7Bi0.005P0.05Si, the melting temperature was increased to 214 °C . In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level.
As shown in Table 5, the discoloration level of Sn0.5Cul7Bi0.005P was 16.78. When 0.005 wt% of Si was added in SnO .5Cul7BiO .005P , the discoloration level was 11.87, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 8.97, which is lower than that of Sn0.5Cul7Bi0.005P. When 0.005 wt% of Co was added in Sn0.5Cul7Bi0.005P0.005Si, the discoloration level was significantly decreased to 6.81, and when 0.01 wt% of Co was added in SnO .5Cul7BiO .005P0.05Si, the discoloration was significantly decreased to 4.99. Therefore, it can be seen that the discoloration of the Sn-Cu-Bi-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu-Bi-P base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
As shown in Table 5, the joint fracture load of SnO.5Cul7BiO.005P was 95 kgf.
When 0.005 wt% of Si was added in SnO .5Cul7BiO .005P, the joint fracture load was 117 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 127 kgf, which is greater than that of Sn0.5Cul7Bi0.005P.
When 0.005 wt% of Co was added in Sn0.5Cul7Bi0.005P0.005Si, the joint fracture load was 119 kgf, which is greater than that of SnO .5Cul7BiO .005P . When 0.01 wt% of Co was added in SnO .5Cul7BiO .005P0.005Si, the joint fracture load was significantly increased to 131 kgf.
Therefore, it can be seen that the joint fracture load of the Sn-Cu-Bi-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Cu-
Bi-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
In addition, when a very small amount of Co was added in the Sn-Cu-Bi-P base alloy together with a small amount of Si, the formation of oxides was constantly and significantly reduced, the copper erosion and the discoloration was reduced, and the joint fracture load was increased in comparison to the solder alloy added with only Si, while having the same wettability.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
As described above, silicon and cobalt each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be further improved by adding them together.
The present embodiment can also be implemented in a
PCB including electronic components fixed thereto using the lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy. As described above, the lead-free solder alloy includes from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 0.2 wt% phosphorus, from 0.1 wt% to 8 wt% bismuth, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
EMBODIMENT 2
A high temperature lead-free solder composition in accordance with the embodiment may include tin (Sn), copper (Cu), silicon (Si) and cobalt (Co).
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 16.
Graph 16 illustrates experimental data of the amount of oxidation measured by the following procedure. A base alloy of tin-copper was prepared, and it is molten in a high-frequency melting furnace together with both silicon and cobalt or one of silicon and cobalt to form a solder composition. After sampling of the solder compositions, melting temperatures thereof were measured using DSC analysis at 5 °C/min. Thereafter, to measure the amount of oxidation, 3 kg of the solder was heated to 420 °C in an SUS crucible of 160 mm diameter, on a hot plate. Then, the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time. Graph 16
Figure imgf000063_0001
As shown in Graph 16, the amount of oxides formed in Sn4Cu including 4 wt% Cu and the balance of Sn was 687 g after 1 hour, 743 g after 2 hours, and 805 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 2,235 g. The amount of oxides formed in Sn4CuO . INiO .005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,981 g. The reason why the amount of oxides formed in Sn4CuO . INiO .005P is smaller than that formed in Sn4Cu is that P serves to reduce the amount of oxidation .
However, when 0.005 wt% of Si is added in Sn4Cu, the amount of oxides was 461 g after 1 hour, 532 g after 2 hours, and 596 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,589 g, which is smaller than the amount of oxides formed in Sn4Cu and Sn4Cu0.005P without Si. In addition, the amount of oxidation decreased further more when 0.05 wt% of Si was added therein.
When 0.005 wt% of Co was added in Sn4CuO .005Si , the amount of oxides was 445 g after 1 hour, 449 g after 2 hours, and 552 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,488 g, which is smaller than the amount of oxides formed in Sn4Cu0.005P and Sn4CuO .005Si , respectively.
By adding a very small amount (0.005 wt%) of cobalt in Sn4Cu0.05Si, the oxidation of the molten solder can be minimized to a level equivalent to that of the solder alloy added with only 0.05 wt% Si.
Therefore, it can be seen that by adding a small amount of Si in Sn-Cu base alloy, the oxide formation can be prevented constantly. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt thereto, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 17.
Graph 17 illustrates experimental results of the erosion rate of copper.
Graph 17
Figure imgf000065_0001
As shown in Graph 17, the copper erosion rate of Sn4Cu including 4 wt% Cu and the balance of Sn was 58.9 %
The copper erosion rate of Sn4CuO . INiO .005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was
53.9 %.
When 0.005 wt% of Si was added in Sn4Cu, the copper erosion rate was 57.1 %, and when 0.05 wt% of Si was added, the copper erosion rate was 52 %, which is smaller than those of Sn4Cu and Sn4CuO . INiO .005P .
When 0.01 wt% of Co was added in Sn4CuO .005Si , the copper erosion rate was significantly decreased to 10.2 %. It can be seen that by adding Co in an amount smaller than 0.01 wt% in the base alloy including 0.05 wt% of Si, the copper erosion rate can be significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 1. Furthermore, the optimum contents of silicon and cobalt in the Sn-Cu base alloy will be estimated from these results.
Table 6 lists the experimental data of Graphs 22 and 23 together with the experxmental data of the melting temperature, the discoloration and the joint strength.
Table 6 High temperature SnCu solders
Figure imgf000066_0001
As shown in Table 6, the melting temperature of Sn4Cu including 4 wt% Cu and the balance of Sn was 357 °C , and the melting temperature of Sn4CuO . INiO .005P was 362 °C.
When 0.005 wt% of Si was added in Sn4Cu, the melting temperature was 358 °C , and when 0.1 wt% of Si was added in Sn4Cu, the melting temperature was increased to 361 °C .
When 0.01 wt% of Co was added in Sn4Cu0.05Si, the melting temperature was increased to 361 °C . In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si m the Sn alloy needs to be limited below a certain level. As shown in Table 6, the discoloration level of
Sn4Cu including 4 wt% Cu and the balance of Sn was 19.2, and the discoloration level of Sn4CuO . INiO .005P including
4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 16.8.
When 0.005 wt% of Si was added in Sn4Cu, the discoloration level was 17.3, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 13.42, which is lower than those of Sn4Cu and Sn4CuO . INiO .005P .
When 0.005 wt% of Co was added in Sn4CuO .005Si , the discoloration level was significantly decreased to 9.62, and when 0.01 wt% of Co was added in Sn4CuO .005Si, the discoloration was significantly reduced to 5.3. Therefore, it can be concluded that the discoloration of the Sn-Cu base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
As shown in Table 6, the joint fracture load of Sn4Cu including 4 wt% Cu and the balance of Sn was 90 kgf, and the joint fracture load of Sn4CuO . INiO .005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 98 kgf.
When 0.005 wt% of Si was added in Sn4Cu, the joint fracture load was 100 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 130 kgf, which is greater than those of
Sn4Cu and Sn4CuO . INiO .005P .
When 0.01 wt% of Co was added in Sn4CuO .005Si , the joint fracture load was significantly increased to 139 kgf. Therefore, it can be concluded that the joint fracture load of the Sn-Cu base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein. As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Cu base alloy added with a small amount of Si, could constantly and significantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%. Cu added in the Sn-Cu base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, in the high temperature solder alloy, when Cu is added in an amount smaller than 2 wt% , the effect of Cu is insufficient, and when Cu is added in an amount greater than 5 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 2 wt% to 5 wt%. Sn is an essential component of the lead-free solder, and is used as a base metal. As described above, Si and Co added in the Sn-Cu base alloy each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together . The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the high temperature lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the high temperature lead- free solder alloy. As described above, the high temperature lead-free solder alloy includes from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
As such, the high temperature lead-free composition, and thus the PCB and the electronic device using the high temperature lead-free solder composition can have a mechanical characteristic and workability similar to those of Sn-Pb alloy and Sn-Cu alloy. Further, by adding both silicon and cobalt in a binary composition system of tin (Sn) and copper (Cu) without the addition of lead (Pb), they can prevent the oxidation of the molten solder constantly and effectively at a high temperature above 400 °C . Furthermore, they can maintain the soldering temperature and the wettabiltiy of the typical lead-free solder, prevent the discoloration after the soldering and the erosion of the copper pad in the PCB, and significantly improve the mechanical properties, by adding only a very small amount of silicon and cobalt.
A high temperature lead-free solder composition in accordance with the embodiment may include tin (Sn), copper (Cu), nickel (Ni), silicon (Si) and cobalt (Co).
Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 18. Graph 18 illustrates experimental data of the amount of oxidation. Graph 18
Figure imgf000070_0001
As shown in Graph 18, the amount of oxides formed in Sn4CuO. INiO.005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was
2,235 g. The amount of oxides formed in Sn4Cu was 567 g after 1 hour, 641 g after 2 hours, and 773 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,981 g.
The reason why the amount of oxides formed in Sn4CuO. INiO.005P is smaller than that formed in Sn4Cu is that Ni and P serves to reduce the amount of oxidation.
When 0.005 wt% of Si is added in Sn4CuO . INiO .005P, the amount of oxides was 390 g after 1 hour, 441 g after 2 hours, and 512 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,343 g, which is smaller than the amount of oxides formed in Sn4Cu and Sn4CuO . INiO .005P without Si. In addition, the amount of oxidation decreased further more when 0.05 wt% of Si was added therein.
When 0.005 wt% of Co was added in Sn4CuO. INiO.005P0.005Si, the amount of oxides was 365 g after 1 hour, 426 g after 2 hours, and 478 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 1,269 g, which is smaller than the amount of oxides formed in Sn4CuO . INiO .005P0.005Si .
By adding a very small amount (0.005 wt%) of cobalt in Sn4CuO. INiO .005P0.005Si, the oxidation of the molten solder can be minimized to a level equivalent to that of the solder alloy added with only 0.005 wt% Si.
Therefore, it can be seen that by adding a small amount of Si in Sn-Cu-Ni-P base alloy, the oxide formation can be prevented constantly. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt thereto, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 19.
Graph 19
Figure imgf000072_0001
As shown in Graph 19, the copper erosion rate of Sn4CuO. INiO.005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 53.4 %. The copper erosion rate of Sn4Cu was 58.9 %.
When 0.005 wt% of Si was added in Sn4CuO . INiO .005P, the copper erosion rate was 49.5 %, and when 0.05 wt% of Si was added, the copper erosion rate was 45.5 %, which is smaller than those of Sn4Cu and Sn4CuO . INiO .005P .
When 0.01 wt% of Co was added in Sn4CuO . INiO .005Si, the copper erosion rate was significantly decreased to 10.3 %. It can be seen that by adding a very small amount (less than 0.01 wt% ) of Co in the base alloy including 0.05 wt% of Si, the copper erosion rate can be significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 1. Furthermore, the optimum contents of silicon and cobalt in the Sn-Cu-Ni-P base alloy will be estimated from these results.
Table 6 lists the experimental data of Graphs 24 and 25 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Figure imgf000073_0001
As shown in Table 7, the melting temperature of Sn4CuO. INiO.005P was 362 °C , and the melting temperature of Sn4Cu including 4 wt% Cu and the balance of Sn was 357 TC.
When 0.005 wt% of Si was added in Sn4CuO . INiO .005P, the melting temperature was 362 °C , and when 0.1 wt% of Si was added in Sn4CuO . INiO .005P, the melting temperature was slightly increased to 365 °C .
When 0.01 wt% of Co was added in Sn4CuO. INiO.005P0.05Si, the melting temperature was slightly increased to 364 °C . In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly,
Therefore, the content of Si in the Sn alloy needs to be limited below a certain level. As shown in Table 7, the discoloration level of
Sn4CuO . INiO .005P was 16.8, and the discoloration level of
Sn4Cu was 19.2.
When 0.005 wt% of Si was added in Sn4CuO . INiO .005P , the discoloration level was 14.8, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 10.2, which is lower than those of Sn4Cu and Sn4CuO . INiO .005P .
When 0.005 wt% of Co was added in
Sn4CuO. INiO .005P0.005Si, the discoloration level was significantly decreased to 6.5, and when 0.01 wt% of Co was added therein, the discoloration was significantly reduced to 4.07.
Therefore, it can be concluded that the discoloration of the Sn-Cu-Ni-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Cu-Ni-P base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein . As shown in Table 7, the joint fracture load of
Sn4CuO. INiO.005P including 4 wt% Cu, 0.1 wt% Ni, 0.005 wt% P and the balance of Sn was 98 kgf, and the joint fracture load of Sn4Cu including 4 wt% Cu and the balance of Sn was 90 kgf. When 0.005 wt% of Si was added in Sn4CuO . INiO .005P, the joint fracture load was 106 kgf, and when 0.05 wt% of
Si was' added therein, the joint fracture load was significantly increased to 115 kgf, which is greater than those of Sn4Cu and Sn4CuO . INiO .005P . When 0.01 wt% of Co was added in Sn4CuO. INiO.005P0.005Si, the joint fracture load was significantly increased to 122 kgf.
Therefore, it can be concluded that the joint fracture load of the Sn-Cu-Ni-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Cu- Ni-P base alloy added with a small amount of Si, could constantly and significantly reduce the oxidation, maintain the wettability, reduce the copper erosion and the discoloration, and increase the joint fracture load.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased.
Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
Cu added in the Sn-Cu-Ni-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, in the high temperature solder alloy, when Cu is added in an amount smaller than 2 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 5 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 2 wt% to 5 wt%. The preferable content of nickel in the solder alloy is from 0.001 wt% to 1.0 wt% .
Sn is an essential component of the lead-free solder, and is used as a base metal. As described above, Si and Co added in the Sn-Cu base alloy each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together. The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the high temperature lead-free solder alloy, or an electronic device including a plurality of electronic components fixed thereto using the high temperature lead- free solder alloy. As described above, the high temperature lead-free solder alloy includes from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
EMBODIMENT 3
A lead-free solder composition for dilution in accordance with the embodiment may include nickel (Ni), silicon (Si), phosphorous (P), cobalt (Co) and tin (Sn). Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 20.
Graph 20 illustrates experimental data of the amount of oxidation measured by the following procedure. Silicon and cobalt were added in a Sn-Ni-P base alloy. The resulting alloy was molten to fabricate a solder composition. The solder composition was heated to 260 °C in an SUS crucible of 160 mm diameter, on a hot plate. Then, the solder composition was stirred at 60 rpm with a stirrer of 140 mm diameter for 1 hour, 2 hours, and 3 hours, respectively, and oxides were extracted therefrom and weighed at each time.
Thereafter, the solder was solidified, and then solder samples were cut from the solder. The cutting surfaces of solder samples were polished. Then, the contents of the alloying elements in the solder samples were analyzed using Auger electron spectroscopy (PHI700, manufactured by Ulvac PHI), which is an instrument for analyzing elements on a metal surface using X-ray, to determine whether the alloying elements remain therein.
Graph 20
Figure imgf000077_0001
As shown in Graph 20, the amount of oxides formed in SnO.06NiO.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 125 g after 1 hour, 195 g after 2 hours, and 320 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 640 g. When 0.005 wt% of Si was added in SnO .06NiO .005P , the amount of oxides was 105 g after 1 hour, 134 g after 2 hours, and 155 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 394 g, which is smaller than the amount of oxides formed in SnO.06Ni0.005P without Si.
In summary, when P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours. Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, it can be seen that the amount of oxidation decreased further more when 0.05 wt% of Si was added.
When 0.001 wt% of Co was added in SnO.06NiO.005P0.005Si, the amount of oxides was 81 g after 1 hour, 99 g after 2 hours, and 110 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 290 g, which is smaller than those of SnO .06NiO .005P and SnO .06NiO .005P0.05Si . When a very small amount (0.005 wt%) of Co was added in SnO.06NiO.005P0.005Si, the oxidation of the molten solder can be minimized to a level equivalent to that of the solder alloy added with only 0.005 wt% Si.
It can be seen that by adding a small amount of Si and Co, the amount of oxidation can be reduced, in comparison to the comparative example.
Therefore, it can be seen that by adding a small amount of Si in Sn-Ni-P base alloy, the formation of oxides can be reduced constantly in comparison to the typical solder composition added with P. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 21.
Graph 21 illustrates experimental results of the wettability. To examine the wetting time of the soldering compositions, the solder was molten at 260 °C using a SP2 wetting tester (manufactured by MalCom Co., Ltd. ) . A copper terminal was dipped into a depth of 1 mm from the surface of the molten solder to measure the wetting time after 5 seconds of the dipping.
Graph 21
Temp: 2700C
1.5
I=
O)
0.5
SnO.lM
SnD.βnl +0.005Si +0.05Si +0.1Si +0.005Si +0.05Si
0.005P 0.005CO 0.01Co 0.05Si 0.05CO
0.005P
Comparative example Example
As shown in Graph 21, the wetting time of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 2.15 seconds.
When 0.005 wt% of Si was added in SnO .06NiO .005P , the wetting time was shorten to 2.05 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 2.17 seconds. This is probably because when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgCu), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder. In addition, it can be seen that the acid resistance of the molten solder alloy can be improved and at the same time, the decrease of wettability due to the addition of silicon in an amount over the limit can be minimized by adding a very small amount (less than 0.01 wt%) of cobalt therein .
Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 22.
Graph 22
70
TemD: 26Q°C 6Q Time : BOsec 50 40 30
§ 20 10
SπO.OBNi +0.005Si +0.05Si +0.1Si +0.005Si +0.05Si Sn0.1Ni0.005P
0.QQ5P 0.005Co 0.01 CO 0.05Si0.005Co
Comparative example Example
As shown in Graph 22, the copper erosion rate of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 48.8 %.
When 0.005 wt% of Si was added in SnO .06NiO .005P , the copper erosion rate was 46.5 %, and when 0.05 wt% of Si was added, the copper erosion rate was 44.5 %, which is smaller than that of SnO .06NiO .005P . When a very small amount (less than 0.01 wt%) of Co was added in SnO .06NiO .005P0.005Si , the copper erosion rate was significantly decreased to 11.9 %. It can be seen that by adding a very small amount (less than 0.01 wt% ) of Co in the base alloy including 0.05 wt% of Si, the copper erosion rate can be significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 8. Furthermore, the optimum contents of silicon and cobalt in the Sn-Ni-P base alloy will be estimated from these results.
Table 8 lists the experimental data of Graphs 26, 27 and 28 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Table 8 SnNiP solder diluents
Figure imgf000081_0001
As shown in Table 8, the melting temperature of SnO.06NiO.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 236 °C . When 0.005 wt% of Si was added in Sn0.06Ni0.005P, the melting temperature was 236 °C . When 0.1 wt% of Si was added in SnO .06NiO .005P, the melting temperature was increased to 238 °C .
When 0.01 wt% of Co was added in
SnO .06NiO .005P0.05Si, the melting temperature was increased to 237 °C . In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly.
Therefore, the content of Si in the Sn alloy needs to be limited below a certain level.
As shown in Table 8, the discoloration level of Sn0.06Ni0.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 21.03.
When 0.005 wt% of Si was added in SnO .06NiO .005P, the discoloration level was 18.76, and when 0.05 wt% of
Si was added therein, the discoloration level was significantly decreased to 12.02, which is lower than that of Sn0.06Ni0.005P.
When 0.005 wt% of Co was added in SnO.06NiO.005P0.005Si, the discoloration level was significantly decreased to 9.9, and when 0.01 wt% of Co was added in SnO .06NiO .005P0.005Si, the discoloration was significantly decreased to 4.5.
Therefore, it can be concluded that the discoloration of the Sn-Ni-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein.
As shown in Table 8, the joint fracture load of SnO.06NiO.005P including 0.06 wt% Ni, 0.005 wt% P and the balance of Sn was 83 kgf.
When 0.005 wt% of Si was added in SnO .06NiO .005P, the joint fracture load was 94 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 96 kgf, which is greater than that of Sn0.06Ni0.005P.
When 0.005 wt% of Co was added in SnO.06NiO.005P0.005Si, the joint fracture load was 95 kgf, which is greater than that of SnO .06NiO .005P . When 0.01 wt% of Co was added in SnO .06NiO .005P0.005Si , the joint fracture load was significantly increased to 100 kgf.
Therefore, it can be concluded that the joint fracture load of the Sn-Ni-P base alloy can be improved by adding a small amount of Si therein, and can also be significantly improved by adding a very small amount of
Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Ni-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load.
In addition, by adding a very small amount of Co in the Sn-Ni-P base alloy including a small amount of Si, it is possible to decrease the amount of oxides significantly and constantly, decrease the erosion rate of the copper and the discoloration, and increase the joint fracture load while maintaining the wettability, in comparison to the base alloy added with only Si.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from 0.001 wt% to 0.05 wt%.
In addition, because Co showed an excellent effect even when added in a very small amount less than 0.01 wt%, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt%.
Cu added in the Sn-Ni-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt% . Ni added in the Sn-Ni-P base alloy serves to prevent the growth of intermetallic compounds at the joining surface. The preferable content of Ni in the solder alloy is from 0.001 wt% to 1.0 wt%. This is because as the content of Ni increases, the melting temperature increases and the wettability and the dispersibility decreases. P added in the Sn-Cu-P-Ni base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability. The preferable content of P is from 0.001 wt% to 0.2 wt%. Sn is an essential component of the lead-free solder, and is used as a base metal.
The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy for dilution, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy for dilution. As described above, the lead-free solder alloy for dilution includes from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.
A lead-free solder composition for dilution in accordance with the embodiment may include tin (Sn), silver (Ag), phosphorous (P), silicon (Si) and cobalt (Co). Hereinafter, the effect of the compositions of silicon and cobalt on the prevention of oxidation will be described with reference to Graph 23.
Graph 23 illustrates experimental data of the amount of oxidation.
Graph 23
Figure imgf000085_0001
As shown in Graph 23, the amount of oxides formed in Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 240 g after 1 hour, 275 g after 2 hours, and 370 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 885 g.
When 0.005 wt% of Si was added in Sn3Ag0.005P, the amount of oxides was 131 g after 1 hour, 145 g after 2 hours, and 179 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 455 g, which is smaller than the amount of oxides formed in Sn3Ag0.005P without Si. When P was added in an amount of 0.005 wt%, the amount of dross increases significantly in a time period from 2 hours to 3 hours. However, when Si was added in an amount of 0.005 wt%, the amount of oxides formed in a time period from 2 hours to 3 hours is similar to that formed in a time period from 1 hour to 2 hours . Therefore, it can be concluded that Si is more effective in the constant prevention of oxidation than P. In addition, it can be seen that the amount of oxidation decreased further more when 0.05 wt% of Si was added.
When 0.01 wt% of Co was added in Sn3AgO .005P0.05Si , the amount of oxides was 60 g after 1 hour, 75 g after 2 hours, and 94 g after 3 hours, respectively, and thus the total amount of oxides formed for 3 hours was 229 g, which is smaller than those of Sn3Ag0.005P and Sn3Ag0.005P0.05Si.
When 0.005 wt% of Co was added in Sn3AgO .005P0.005Si , the amount of oxidation can be reduced in comparison to the comparative example by adding a small amount of Si and Co.
Therefore, it can be seen that by adding a small amount of Si in Sn-Ag-P base alloy, the formation of oxides can be reduced constantly in comparison to the typical solder composition added with P. It can also be seen that by further adding a very small amount (less than 0.01 wt%) of cobalt therein, the required content of silicon can be minimized by a synergy effect of Si and Co.
Hereinafter, the effect of the compositions of silicon and cobalt on the wettability will be described with reference to Graph 24.
Graph 24
1.5
■ TemD ■■ 26O0G
105
Sn3Ag0.005P +nnn<iSi +n n^si +ni<?i +0.005Si +0.05Si Sn0.3Ag0.005P
Wm U'ϋt)bl m 0.005CO 0.01Co 0.005Si0.005Co
Comparative example Example
As shown in Graph 24, the wetting time of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 0.7 seconds. When 0.005 wt% of Si was added in Sn3Ag0.005P, the wetting time was 0.63 seconds. On the contrary, when 0.05 wt% of Si was added, the wetting time was rather increased to 0.72 seconds. This is probably because when a very small amount (0.005 wt%) of Si is added to the solder, oxygen is removed from the molten base alloy solder (SnAgP), thereby improving the wettability. However, when Si is added in an amount greater than 0.05 wt%, the melting temperature of the solder increases and the excess Si decreases the wettability of the molten solder.
In addition, it can be seen that the acid resistance of the molten solder alloy can be improved and at the same time, the decrease of wettability due to the addition of silicon in an amount over the limit can be minimized by adding a very small amount (less than 0.01 wt%) of cobalt therein. Eirosc Hereinafter, the erosion rate of copper according to the compositions of silicon and cobalt will be described with reference to Graph 25.
Graph 25
Figure imgf000088_0001
As shown in Graph 25, the copper erosion rate of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 58.8 %.
When 0.005 wt% of Si was added in Sn3Ag0.005P, the copper erosion rate was 56.5 %, and when 0.05 wt% of Si was added, the copper erosion rate was 31.7 %, which is smaller than that of Sn3Ag0.005P. When 0.01 wt% of Co was added in Sn3AgO .005P0.005Si, the copper erosion rate was significantly decreased to 14 %. It can be seen that by adding a very small amount (less than 0.01 wt% ) of Co in the base alloy including 0.05 wt% of Si, the copper erosion rate can be significantly decreased because of a synergy effect of silicon and cobalt.
Hereinafter, the amount of oxidation, the melting temperature, the discoloration, the wetting time, the erosion rate of copper, and the joint strength according to the compositions of silicon and cobalt will be described with reference to Table 9. Furthermore, the optimum contents of silicon and cobalt in the Sn-Ag-P base alloy will be estimated from these results.
Table 9 lists the experimental data of Graphs 23, 24 and 25 together with the experimental data of the melting temperature, the discoloration and the joint strength.
Table 9 SnAgP solder diluents
Figure imgf000089_0001
As shown in Table 9, the melting temperature of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 224 °C . When 0.005 wt% of Si was added in Sn3Ag0.005P, the melting temperature was 225 °C . When 0.1 wt% of Si was added in Sn3Ag0.005P, the melting temperature was increased to 228 °C .
When 0.01 wt% of Co was added in Sn3AgO .005P0.05Si , the melting temperature was increased to 227 °C . In general, according to a metal alloy phase diagram, if a Sn alloy of a low melting point is added with Si of a high melting point (above 1,400 °C ) in a small amount above a predetermined level, the melting temperature of the Sn alloy increases rapidly. Therefore, the content of Si in the Sn alloy needs to be limited below a certain level .
As shown in Table 9, the discoloration level of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 11.8.
When 0.005 wt% of Si was added in Sn3Ag0.005P, the discoloration level was 10.11, and when 0.05 wt% of Si was added therein, the discoloration level was significantly decreased to 6.7, which is lower than that of Sn3Ag0.005P.
When 0.005 wt% of Co was added in Sn3AgO .005P0.005Si , the discoloration level was significantly decreased to 4.33 and when 0.01 wt% of Co was added in Sn3AgO .005P0.005Si, the discoloration was significantly decreased to 3.82.
Therefore, it can be concluded that the discoloration of the Sn-Ag-P base alloy can be reduced by adding a small amount of Si therein, and the discoloration of the Sn-Ag-Cu base alloy due to the oxidation of the solder alloy can also be significantly reduced by adding a very small amount of Si and Co therein .
The joint fracture load data listed in Table 9 was obtained as follows. A 2 mm diameter wire coated with tin-bismuth was vertically inserted in a hole of a PCB substrate and wave soldering was performed thereon to prepare a specimen. Then, the joint fracture load (kgf) thereof was measured using a tension tester. As shown in Table 9, the joint fracture load of Sn3Ag0.005P including 3 wt% Ag, 0.005 wt% P and the balance of Sn was 87 kgf.
When 0.005 wt% of Si was added in Sn3Ag0.005P, the joint fracture load was 94 kgf, and when 0.05 wt% of Si was added therein, the joint fracture load was significantly increased to 101 kgf, which is greater than that of Sn3Ag0.005P.
When 0.005 wt% of Co was added in Sn3AgO .005PO .005Si, the joint fracture load was 97 kgf, which is greater than that of Sn3Ag0.005P. When 0.01 wt% of Co was added in
Sn3AgO.005P0.005Si, the joint fracture load was significantly increased to 105 kgf.
Therefore, it can be concluded that the joint fracture load of the Sn-Ag-P base alloy can be improved by adding a small amount of Si therein, and can be significantly improved by adding a very small amount of
Si and Co therein.
As described above, the lead-free solder composition in accordance with the embodiment, including the Sn-Ag-P base alloy added with a small amount of Si, could maintain the soldering temperature and the wettability of the typical lead-free solder composition, constantly reduce the oxidation, reduce the copper erosion and the discoloration, and increase the joint fracture load. In addition, by adding a very small amount of Co in the Sn-Ag-P base alloy including a small amount of Si, it is possible to decrease the amount of oxides significantly and constantly, decrease the erosion rate of the copper and the discoloration, and increase the joint fracture load while maintaining the wettability, in comparison to the base alloy added with only Si.
However, when Si was added in an amount above a certain level, i.e., in an amount greater than 0.05 wt%, the melting temperature of the soldering alloy increased, and the wettability thereof decreased. Therefore, the preferable content of Si in the solder alloy is from
0.001 wt% to 0.05 wt%.
When Co was added in an amount above a predetermined level, i.e., above 0.01 wt%, the wettability decreases. In addition, because Co showed an excellent effect even when it is added in a very small amount together with a small amount (0.05 wt%) of Si, the preferable content of Co in the solder alloy is from 0.001 wt% to 0.01 wt% .
Cu added in the Sn-Ag-P base alloy serves to slightly decrease the melting temperature of the alloy and improve the joint strength of the joint. However, when Cu is added in an amount smaller than 0.1 wt%, the effect of Cu is insufficient, and when Cu is added in an amount greater than 2 wt%, the melting temperature is rather increased. Therefore, the preferable content of copper in the solder alloy is from 0.1 wt% to 2 wt% . Ag added in the Sn-Ag-P base alloy serves to decrease the melting temperature of the alloy and improve the dispersibility of the joint base metal and the thermal fatigue property. In addition, Ag is nonpoisonous . However, when Ag is added in an amount less than 0.1 wt%, the effect of Ag is insufficient, and when Ag is added in an amount greater than 4 wt%, the melting temperature is rather increased. Therefore, the preferable content of Ag is from 0.1 wt% to 4.0 wt%. P added in the Sn-Cu-P-Ni base alloy serves to reduce the oxide formation during the soldering to thus improve the soldering workability. The preferable content of P is from 0.001 wt% to 0.2 wt%. Sn is an essential component of the lead-free solder, and is used as a base metal.
As described above, Si and Co each has the effects of preventing the formation of oxides and preventing the discoloration, and the effects can be improved significantly by adding them together. The present embodiment can also be implemented in a PCB including electronic components fixed thereto using the lead-free solder alloy for dilution, or an electronic device including a plurality of electronic components fixed thereto using the lead-free solder alloy for dilution. As described above, the lead-free solder alloy for dilution includes from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and the balance of tin.

Claims

WHAT IS CLAIMED IS:
1. A lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
2. A printed circuit board (PCB) and an electronic device using a lead-free solder composition, wherein the lead-free solder composition comprises from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
3. A lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
4. A (printed circuit board (PCB) and an electronic device using a lead-free solder composition, wherein the lead-free solder composition comprises from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
5. A lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
6. A printed circuit board (PCB) and an electronic device using a lead-free solder composition, wherein the lead-free solder composition comprises from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 4.0 wt% silver, from 0.001 wt% to 0.5 wt% nickel, from 0.001 wt% to 0.1 wt% germanium, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
7. A lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
8. A printed circuit board (PCB) and an electronic device using a lead-free solder composition, wherein the lead-free solder composition comprises from 0.1 wt% to 2 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
9. A lead-free solder composition, comprising from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 17 wt% bismuth, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
10. A printed circuit board (PCB) and an electronic device using a lead-free solder composition, wherein the lead-free solder composition comprises from 0.1 wt% to 2 wt% copper, from 0.1 wt% to 17 wt% bismuth, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
11. A high temperature lead-free solder composition, comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
12. A printed circuit board (PCB) and an electronic device using a high temperature lead-free solder composition, wherein the high temperature lead-free solder composition comprises from 2 wt% to 5 wt% copper, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
13. A high temperature lead-free solder composition, comprising from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
14. A printed circuit board (PCB) and an electronic device using a high temperature lead-free solder composition, wherein the high temperature lead-free solder composition comprises from 2 wt% to 5 wt% copper, from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
15. A lead-free solder composition for dilution, comprising from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
16. A printed circuit board (PCB) and an electronic device using a lead-free solder composition for dilution, wherein the lead-free solder composition for dilution comprises from 0.001 wt% to 1.0 wt% nickel, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
17. A lead-free solder composition for dilution, comprising from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
18. A printed circuit board (PCB) and an electronic device using a lead-free solder composition for dilution, wherein the lead-free solder composition for dilution comprises from 0.1 wt% to 4 wt% silver, from 0.001 wt% to 0.2 wt% phosphorous, from 0.001 wt% to 0.05 wt% silicon, from 0.001 wt% to 0.01 wt% cobalt and a balance of tin.
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