JP2005021958A - Lead-free solder paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a Sn-Zn solder paste of excellent soldering strength and wettability by solving disadvantages that the soldering strength of conventional Sn-Zn lead-free solder is degraded when the solder is placed in the environment of high humidity, and alloy of improved strength formed by adding a trace of 1B group metal or Al to the solder to cover the degradation is degraded in wettability in a drawback-and-advantage manner. <P>SOLUTION: This solder paste with alloy powder and flux kneaded therein blocks water deposited on a fillet surface of a Cu-Zn alloy layer by adding liquid paraffin and paraffin was to the flux, and prevents oxidation of Zn caused by deposited water, and also prevents voids dezincing. Therefore, the soldering strength can be maintained even under the environment of high humidity. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a solder paste used for soldering an electronic device, in particular, a Sn—Zn-based lead-free solder paste.

  As a method for soldering electronic components, there are a brazing method, a flow method, a reflow method, and the like.

  The brazing method is a method of soldering by applying a greased solder wire to a soldering portion and heating and melting the solder wire with a soldering iron. This brazing method has a problem in productivity because soldering is performed for each soldering portion, and is not suitable for mass production.

  The flow method is a soldering method in which the soldering surface of the printed circuit board is brought into contact with the molten solder and soldering is performed, and the entire printed circuit board can be soldered in one operation. However, this flow method forms a bridge in which solder adheres to electronic components with a narrow pitch, or molten solder directly contacts the electronic components for heat-sensitive electronic components. For this reason, the electronic component may be thermally damaged to cause functional deterioration. In addition, when a connection component such as a connector is mounted on the soldering surface of the printed circuit board, there is a problem that the molten solder enters the hole of the connector and cannot be used.

  In the reflow method, solder paste consisting of solder powder and flux is applied only to the necessary parts of the printed circuit board by the printing method or the discharge method, and electronic components are mounted on the application part, and then the solder paste is heated by a heating device such as a reflow furnace. In which the electronic component and the printed circuit board are soldered. This reflow method not only enables soldering at many locations in a single operation, but also does not generate bridging even for electronic components with a narrow pitch, and the productivity and reliability that solder does not adhere to unnecessary locations. Excellent soldering capability.

  By the way, the solder paste used in the conventional reflow method has a solder powder of Pb—Sn alloy. This Pb-Sn alloy has an eutectic composition (Pb-63Sn) with a melting point of 183 ° C, has little thermal effect on heat-sensitive electronic components, and has excellent solderability. It has the feature that there are few occurrences of soldering defects such as wet. When an electronic device soldered with a solder paste using this Pb-Sn alloy becomes old or breaks down, it has been disposed of without being upgraded or repaired. When the printed circuit board is discarded, it was disposed of in landfill instead of incineration. However, in the landfill disposal, the solder is attached to the copper foil of the printed circuit board, and the copper foil and the solder are separated. This is because it cannot be reused. When acid rain comes into contact with this printed circuit board that has been disposed of in landfill, Pb in the solder is eluted, which contaminates groundwater. And if people and livestock drank groundwater containing Pb for many years, there is concern about causing Pb poisoning. Therefore, the so-called “lead-free solder” not containing Pb has been strongly demanded by the electronic equipment industry.

  Lead-free solder is based on Sn, and currently used lead-free solder is Sn-3.5Ag (melting point: 221 ° C), Sn-0.7Cu (melting point: 227 ° C), Sn- In addition to binary alloys such as 9Zn (melting point: 199 ° C), Sn-58Bi (melting point: 139 ° C), Ag, Cu, Zn, Bi, In, Sb, Ni, Cr, Co, Fe, Mn, P , Ge, Ga and other third elements are appropriately added. The “system” referred to in the present invention is an alloy itself or an alloy to which one or more third elements are added based on a binary alloy. For example, the Sn-Zn system is an Sn-Zn alloy itself, or an alloy obtained by adding one or more of the above-mentioned third elements to Sn-Zn, and the Sn-Ag system is the Sn-Ag alloy itself or Sn-Ag. An alloy to which one or more third elements are added.

  Sn-Ag lead-free solder, Sn-Cu lead-free solder, etc. have a melting point of 220 ° C or higher, so when used as a solder paste in the reflow method, the peak temperature during reflow becomes 250 ° C or higher. There has been a problem that the electronic component and the printed circuit board are thermally damaged.

  Sn-Bi eutectic lead-free solder has a melting point of around 139 ° C, and even if it is used as a solder paste in the reflow method, the peak temperature is 200 ° C or less. There is no. However, this lead-free solder containing a large amount of Bi has a problem in heat resistance because its melting point is too low. In other words, if the inside of an electronic device case becomes hot due to heat generated by a coil or power transistor during use, the soldered portion of the printed circuit board soldered with the lead-free solder may be peeled off due to a decrease in bonding strength. It was. In addition, Pb-free solder containing a large amount of brittle Bi has a problem that it can be easily peeled off even if a slight impact is applied to the soldered portion.

  Sn-9Zn eutectic lead-free solder has a melting point of 199 ° C, which is close to the melting point of conventional Pb-Sn eutectic solder. The heat effect on electronic parts and printed circuit boards is low because it is below ℃. However, Sn-9Zn eutectic solder paste has poor solderability with respect to the soldered part of copper, “unsoldered” where no solder adheres, Soldering defects such as “wet” may occur. Such poor soldering not only weakens the bonding strength but also deteriorates the appearance. In addition, the solder paste of Sn-9Zn eutectic has been discolored from the interface of copper foil and copper land after many years have passed since the solder paste of the copper land printed circuit board and copper lead was soldered. There was something to do. The peeling of the soldered portion causes a failure in the electronic device. In addition, Sn-9Zn eutectic solder paste also had a chip standing that caused small chip components to rise during soldering. When a chip stands on a printed circuit board, an electronic device incorporating the chip cannot function at all.

  Therefore, various Sn-Zn lead-free solders added with a third element have been proposed in order to improve the problems in Sn-9Zn eutectic solder paste. For example, a solder paste with improved solderability is a solder paste using Sn-8Zn-3Bi lead-free solder in which Bi is added to an alloy near the Sn—Zn eutectic, and this solder paste is currently widely used. Further, the applicants of the present application have proposed lead-free solder in which Ag is added to an alloy near Sn-9Zn eutectic to improve corrosion resistance.

  Sn-Zn-based lead-free solder has a melting point close to that of conventional Sn-Pb solder, and because it uses Zn, an essential ingredient for humans, it is not harmful to the human body compared to other lead-free solders Zn has superior properties compared to other lead-free solders, such as In, Ag, Bi, etc., which have more reserves on the earth and lower unit prices. For this reason, Sn-Ag-based lead-free solder is used for printed circuit boards that cannot be used because it has poor heat resistance, even though solderability is poor.

  However, Sn-Zn lead-free solder has a problem that the soldering joint strength decreases when it is left at high temperature after soldering on a printed board of Cu land such as FR-4 which is generally used. . This is because the reactivity between Zn and Cu is high, and when a Cu land substrate is used, Zn in the Sn-Zn lead-free solder passes through the alloy layer and penetrates into the Cu if the high temperature state is continued for a long time. Many voids are generated between the intermetallic compound called cannendal voids and the solder (see Non-Patent Document 1). In other words, the generation of voids decreases the soldering joint strength and impairs reliability. Therefore, when using Sn-Zn lead-free solder, it is necessary to apply Ni plating to the Cu land to create a Ni barrier, but since Ni has poor wettability, it is necessary to apply Au plating on Ni. I can not use it. Therefore, when Sn—Zn lead-free solder is used, since Au plating is essential, there has been a problem that the manufacturing cost of the electronic device increases.

  Further, when Sn—Zn-based lead-free solder is soldered to a Cu land printed circuit board, humidity is a factor that decreases the joint strength with temperature. When the humidity is high, Zn easily oxidizes to Zn2 + ions, and Zn in the Sn-Zn lead-free solder easily passes through the alloy layer and enters the Cu, generating a large number of voids. This phenomenon is prominent even when the temperature is 100 ° C. or lower when the humidity is 80% or higher. In addition, voids are likely to occur even with condensation.

    As a method to improve the soldering strength of Sn-Zn lead-free solder and Cu under high temperature, a method of adding 1B group metal powder to Sn-Zn lead-free solder powder of solder paste has been proposed. (Patent Document 1). However, more than 10% of the 1B group metal powder added to Sn-Zn lead-free solder powder is present compared to Sn-Zn lead-free solder powder. The advantage of Sn-Zn lead-free solder that it does not melt and can be reflowed with almost the same temperature profile as conventional Sn-Pb solder is lost.

   In addition, a method has been proposed in which a small amount of Al is added to Sn—Zn-based lead-free solder to improve the soldering strength with Cu at high temperatures (Patent Document 2). However, since Al is a metal that easily oxidizes, it is oxidized before other components during the production of solder powder, and maintaining and managing the 0.002% to 0.008% by mass component disclosed in this document in mass production Have difficulty. Also, since the Al oxide film is thick and strong, the solder paste of lead-free solder added with Al deteriorates the wettability, and many voids are generated due to poor wetting.

  Patent Document 3 proposes a solder paste obtained by kneading paraffin wax, liquid paraffin, Au—Si alloy, Au—Sn alloy, or Au—Ge alloy. The solder paste is a solder paste in which no flux residue remains because paraffin wax or liquid paraffin is completely decomposed and evaporated in the soldering process. Also, bubbling paraffin wax and liquid paraffin with Au-Si alloy, Au-Sn alloy, Au-Ge alloy solder powder and inert gas such as Ar and He to reduce the oxygen content of the solder powder. There is also a thing (refer patent document 4).

Japanese Patent Laid-Open No. 2002-224880 US Pat. No. 6,361,626 JP 3-155493 A (page 2) JP-A-62-226488 (page 3) Welding Association Mate2000 Proceedings, February 3, 2000, P.313-318

  By the way, as mentioned above, when Sn—Zn alloy and Cu are bonded and exposed to high temperature and high humidity, there is a problem that the bonding strength of the solder is remarkably lowered. For this reason, a commonly used Cu land substrate such as FR-4 cannot be used, and an expensive Au-plated printed circuit board is indispensable, raising the manufacturing cost. In addition, there is a method to maintain the strength at high temperature by adding a small amount of additive element to the Sn-Zn alloy for bonding with Cu, but there is a problem that a small amount of additive element deteriorates the wettability in the Sn-Zn alloy. there were.

  In the Sn-Zn lead-free solder paste, the present inventors added a moisture-proof material to the solder paste flux, and the soldered Cu land printed circuit board was exposed to an environment of high temperature and high humidity. However, the present invention was completed by finding that the bonding strength was not lowered.

As a result of studying the cause of the decrease in strength of the soldered portion of Cu soldered by the Sn-Zn lead-free solder by the present inventors, an alloy layer of Cu-Zn that is easily oxidized at the interface between copper and solder is found. It was found to be generated. When conventional Sn-Pb solder, Sn-Ag lead-free solder and Sn-Bi lead-free solder are soldered with Cu, Sn and Cu in the solder react to form an Sn-Cu alloy layer To do. However, when Cu is soldered with Sn—Zn lead-free solder, an alloy layer of Cu—Zn is formed. The Cu-Zn alloy layer is not only inside the fillet made of solder, but also when water adheres to the solder that is exposed to the outside of the fillet, the water becomes an electrolyte, which selectively oxidizes Zn in the solder. Will be ionized. The oxidized Zn ions move to the Cu—Zn alloy layer to become a Cu—Zn alloy, and further pass through the alloy layer to the Cu portion. In the Sn-Zn lead-free solder after the movement of Zn ions, voids of Zn removal occur. This phenomenon is likely to occur at the interface of the Cu—Zn alloy layer (FIG. 8 of Non-Patent Document 1).
As the void gradually progresses along the interface, the strength of the soldered portion decreases. Finally, it peels off at the bonding interface.

  The phenomenon of reducing the bonding strength occurs not only in the Sn-9Zn binary alloy but also in the Sn-3Bi-8Zn composition that is often used as the composition of the Sn-Zn lead-free paste. Conventionally, as disclosed in Patent Documents 1 and 2, a method of preventing a decrease in strength by adding a trace amount of Group 1B metal or aluminum to Sn-Zn lead-free solder has been used. However, these methods assume a high temperature in an environment of 40 to 60%, which is an average humidity. Soldering parts are not exposed to high temperatures of 100 ° C or higher except for in-vehicle parts and power supply parts. In Japan, there are many environments under high humidity where the humidity is 80% or higher.

  As described above, the cause of the decrease in strength of Sn—Zn lead-free solder is the ionization of Zn. Therefore, the present invention utilizes the fact that the strength reduction of the Sn—Zn-based lead-free solder does not occur by adding a moisture-proof component to the solder paste flux. As a result of intensive studies on additive substances having a moisture-proofing effect as a flux component, the present inventors have found that liquid paraffin and paraffin wax are effective.

  Liquid paraffin is a mixture of liquid saturated hydrocarbons obtained from crude oil having a boiling point of about 300 ° C, and is a substance with excellent moisture resistance. Sn-3Bi-8Zn lead-free solder, which is commonly used as the composition of Sn-Zn lead-free solder paste, has a solid phase temperature of 190 ° C and a liquid phase temperature of 197 ° C. It can be used with the same reflow temperature profile as Sn-Pb solder, and when this temperature profile reflows the solder paste in which liquid paraffin is contained in the flux, the liquid paraffin does not evaporate by heating in the reflow oven, and almost remains as residue after reflow Remain. The liquid paraffin remaining as a residue spreads thinly on the solder fillet and prevents the adhesion of moisture.

  Paraffin wax is a wax-like solid linear saturated aliphatic hydrocarbon, and is excellent in moisture resistance and insulation. The boiling point is higher than that of liquid paraffin, and when it is added to the solder paste flux, it almost remains in the residue after reflow. The paraffin wax remaining as a residue spreads thinly on the solder fillet in the same manner as liquid paraffin and prevents moisture from adhering.

The present invention is a solder paste in which Sn—Zn alloy powder and a flux are kneaded, wherein liquid paraffin and / or paraffin wax is added to the flux.
The Sn—Zn alloy powder in the present invention includes Sn—Zn binary alloy powder and ternary or higher alloy powder obtained by adding other elements to the binary alloy powder.

  The liquid paraffin and paraffin wax used in the present invention, unlike Patent Documents 3 and 4, etc., exhibit not only the use as a solvent but also a moisture-proof coating agent. Addition to fluxes such as silicon, epoxy resin, and acrylic resin having the same action cannot be used because these resins impair solderability. However, liquid paraffin and paraffin wax do not hinder solderability. However, since liquid paraffin and paraffin wax do not exhibit a flux action on solder, even if added to the flux in the solder paste, solderability is not improved. Therefore, if too much of these are added, the activity of the entire flux is reduced, and if the amount added in the flux is small, the moisture-proof effect is weakened, and the effect of preventing the reduction of Sn-Zn lead-free solder strength does not appear. . The amount of liquid paraffin and paraffin wax added to the flux is desirably 2 to 20% by mass. If the added amount is less than 2% by mass, the effect of preventing the strength from being lowered is not obtained. If the added amount exceeds 20% by mass, the solderability is hindered and the reflow property is deteriorated. Liquid paraffin and paraffin wax may be added singly or in combination.

  Sn-Zn-based lead-free solders have poor wettability, and liquid paraffin and paraffin wax do not have a fluxing action, so flux activators that improve solderability are also limited. When the amine hydrohalide generally used as a flux activator is used in the solder paste of the present invention, the present invention contains liquid paraffin and paraffin wax in the flux. Since it works as a polar substance, its activity is inhibited. Suitable as the activator of the present invention is an aliphatic organohalogen compound whose structure is relatively close to that of liquid paraffin and paraffin wax. Preferred examples of the aliphatic organic halogen compound include hexabromocyclododecane, trans-2,3 dibromo-2-butene-1,4-diol, 2,3-dibromo-1,4-butanediol, 2,3 -Dibromo-1-propanol, 1,3-dibromo-2-propanol. Further, although not an aliphatic organic halogen compound, tris (2,3-dibromopropyl) isocyanurate is also suitable.

  In the present invention, an organic halogen compound can be further added to the flux to which liquid paraffin or paraffin wax is added.

  Since the liquid paraffin and paraffin wax of the present invention act as nonpolar substances in the flux as described above, the action of the active agent is inhibited. However, when an aromatic amine and a heterocyclic amine are added to the flux, the action of the activator is exhibited. This is thought to be because aromatic amines and heterocyclic amines act as nucleophiles to enhance the reaction of the activator. Preferable examples of the aromatic amine and heterocyclic amine of the present invention include diphenylguanidine for aromatic amines, and 2-phenylimidazole and 2-phenyl-4-methylimidazole for heterocyclic amines.

    In the present invention, an aliphatic amine or a heterocyclic amine can also be added to a flux to which liquid paraffin or paraffin wax is added.

  In the present invention, the addition of an organic halogen compound and an amine such as an aromatic amine or heterocyclic amine can be expected to be effective even when added alone. Furthermore, by adding these simultaneously, the defect with respect to the solderability of liquid paraffin and paraffin wax is compensated, and a synergistic effect can be expected.

Solder pastes with the following composition were made and their characteristics were compared.
A general Sn-3Bi-8Zn composition was used as the composition of the Sn-Zn lead-free solder. The flux composition is shown in Table 1.
1. Contains solder paste
Sn-3Bi-8Zn solder powder (20-40μm) 88.5% by mass
Flux (Examples 1-7, Comparative Examples 1-2) 11.5% by mass

The results of Examples and Comparative Examples are shown in Table 1.

Table description
[Corrosion resistance]
Test piece: A test piece is prepared by immersion soldering a tough pitch copper plate of 0.3 mm × 10 mm × 15 mm in a molten solder alloy at 250 ° C. to a depth of 15 mm.
Test method: After leaving the test piece in a constant temperature and humidity chamber with a temperature of 85 ° C and a relative humidity of 85% for 1000 hours, it is fixed with an epoxy-based embedding resin and subjected to cross-section polishing. The elemental analyzer is used to observe the presence or absence of corrosion oxidation at the soldering interface.
Result: When the oxide layer formation due to corrosion oxidation is not observed at the soldering interface, or when the oxide layer formation is small, it is acceptable, or when the oxide layer formation due to corrosion oxidation is common at the soldering interface, or If interfacial peeling is observed, it is not allowed.

[Reflow] (wetting test)
Test piece: Solder paste is printed on a copper wiring printed board with a 0.4mm pitch QFP pattern using a 0.15mm thick metal screen, the solder paste is dissolved in a reflow oven, and soldered to obtain a test piece. .
The reflow conditions are preheating 160 ° C. (60 seconds) and main heating 225 ° C. (5 seconds).
Result: The fillet of the solder joint part of QFP and the wetting of the end face of the lead are observed with a stereoscopic microscope with a magnification of 30 times. Judgment criteria are as follows.
Excellent: Solder is evenly wet and glossy, and solder balls are hardly generated.
Good: Solder is evenly wet and shiny, but solder balls are generated.
Yes: Solder is melted but the solder is not glossy and is not evenly wetted on the land.
Impossible: The solder does not get wet and the solder powder remains unmelted.

[QFP joint strength]
Specimen: Solder paste is applied to a copper printed circuit board with a 0.4mm pitch QFP pattern using a 0.15mm thick metal screen, and after mounting QFP, the solder paste is dissolved by reflow heating and soldered. To make a test piece. The reflow conditions are preheating 160 ° C. (60 seconds) and main heating 225 ° C. (5 seconds).
Test method: After leaving the test piece in a constant temperature and humidity chamber with a temperature of 85 ° C and a relative humidity of 85% for 1000 hours, hook it on the QFP joint and conduct a tensile test at an angle of 45 degrees to determine the bond strength. taking measurement.
Results: Comparison was made by the ratio between the bonding strength after 1000 hours and the initial bonding strength.
In Comparative Example 3 with reference to Patent Document 3, since the solder powder remained unmelted and could not be soldered, the QFP joint strength could not be measured.

As described above, although the solder paste of the present invention is a Sn-Zn lead-free solder paste, it has good solderability, corrosion resistance, and reflow properties. There is an excellent effect not found in conventional Sn-Zn solder pastes in that defects such as dewetting do not occur and the life can be extended even in high humidity environments.

Claims (5)

A solder paste in which Sn-Zn alloy powder and a flux are kneaded, wherein liquid paraffin and / or paraffin wax is added to the flux. The solder paste according to claim 1, wherein an organic halogen compound is added to the flux. 3. The solder paste according to claim 1, wherein an aromatic amine and / or a heterocyclic amine is added to the flux. The organic halogen compounds are hexabromocyclododecane, tris (2,3-dibromopropyl) isocyanurate, trans-2,3 dibromo-2-butene-1,4-diol, 2,3-dibromo-1,4-butane. 4. The solder paste according to claim 2, wherein the solder paste is any one of diol, 2,3-dibromo-1-propanol, and 1,3-dibromo-2-propanol. 4. The solder paste according to claim 3, wherein the aromatic amine is diphenylguanidine, and the heterocyclic amine is 2-phenylimidazole or 2-phenyl-4-methylimidazole.