JP2011194410A - COATED Pb-FREE Bi-BASED SOLDER ALLOY AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pb-free solder alloy for high temperature excellent in wettability and reliability, and also to provide a method for producing the same.SOLUTION: The coated Pb-free solder alloy has a sheet shape, a wire shape or a ball shape and is comprised of the Pb-free solder alloy containing ≥50 mass% and ≤99.5 mass% of Bi and a coating layer coated on a surface, having a thickness of ≥0.05 μm and ≤1.20 μm and comprising at least one element of Ag, Au, Cu and Ni. This Pb-free solder alloy may contain ≥0.4 mass% and ≤13.5 mass% of Zn and/or ≥0.01 mass% and ≤1.5 mass% of Sn, and further may contain ≥0.02 mass% and ≤2.5 mass% of Al and/or ≥0.001 mass% and ≤0.500 mass% of P.

Description

  The present invention relates to a coated Pb-free solder alloy and a method for producing the same, and more particularly to a Bi-based one in which the coated Pb-free solder alloy is used for high temperatures. The present invention further relates to a mounting substrate on which electronic components are bonded using the coated Pb-free solder alloy, and various devices on which the mounting substrate is mounted.

  In recent years, regulations on chemical substances that are harmful to the environment have become increasingly strict, and this is no exception for solder materials used for bonding electronic components to substrates. Pb has been used for a long time as a main component of the solder material, but has already been regulated by the Rohs Directive. For this reason, the development of solder that does not contain Pb (lead) (hereinafter referred to as Pb-free solder) has been actively conducted.

  Solders used for bonding electronic components to a substrate are roughly classified into high temperature (about 260 ° C. to 400 ° C.) and medium / low temperature (about 140 ° C. to 230 ° C.) depending on the use limit temperature. As for the solder for solder, Pb-free is practically used because it is mainly composed of Sn. For example, Patent Document 1 contains 1.0 to 4.0% by mass of Ag, 2.0% by mass or less of Cu, 0.5% by mass or less of Ni, and 0.2% by mass or less of P. A Pb-free solder alloy comprising Sn is disclosed. Patent Document 2 describes a Pb-free solder having an alloy composition containing 0.5 to 3.5% by mass of Ag, 0.5 to 2.0% by mass of Cu, and the balance being Sn.

  On the other hand, high temperature solder materials that do not contain Pb have been developed by various institutions. For example, for Bi-based solder, Patent Document 3 discloses a Bi / Ag brazing material containing 30 to 80% by mass of Bi and having a melting temperature of 350 to 500 ° C. Further, Patent Document 4 discloses a solder alloy in which a binary Kyochang alloy is added to a Kyochang alloy containing Bi, and an additive element is further added, so that the liquidus temperature can be adjusted and variations can be reduced. A method is disclosed.

  The characteristics generally required for high-temperature solder are required to have good wettability between the solder and the electronic component or the substrate, in addition to the above-mentioned appropriate liquidus temperature. For this reason, in Patent Document 5, the surface of the solder alloy powder particles is provided with one or more coating layers of one or more of the constituent components of the solder alloy, and the average composition of the solder alloy powder particles is A Pb-free solder alloy powder characterized by being equal to a predetermined alloy composition of the solder alloy is disclosed. It is described that the coating has excellent wettability and that the composition of the preliminarily prepared coating material and the initial solder material is uniformly mixed during reflow and can be soldered with the target composition. .

  Patent Document 6 discloses that 50 to 65 mass% Cu, 15 to 29 mass% Sn, 10 mass% Ag, 5 mass% Bi, 5 mass% In, 1 mass% or less Zn (Zn Alloy particles are disclosed in which the surface of particles produced by melting an alloy made of an alloy made of Sn is surface-treated with Sn. The average particle diameter of the alloy particles is 0.1 to 20 μm, and each alloy particle exhibits a plurality of melting points defined as temperatures at which endothermic peaks are observed by differential scanning calorimetry (DSC). It is described that the measurement shows at least one exothermic peak.

  The plurality of melting points include an initial lowest melting point at which the lowest endothermic peak is observed and a highest melting point at which the highest endothermic peak is observed, and each alloy particle has at least a surface portion. Each of the alloy particles after exhibiting the initial minimum melting point and heating each alloy particle at a temperature at or above the initial minimum melting point, thereby melting each alloy particle at least its surface portion exhibiting the initial minimum melting point Has been shown to solidify the molten portion of each alloy particle by cooling to room temperature. Thus, it is described that each of the alloy particles that have been heated and solidified exhibits a rising minimum melting point higher than the initial minimum melting point. That is, it is shown that surface treatment (coating) is performed with one of the main components of alloy particles (Sn), and these have a minimum melting point higher than the initial minimum melting point after heating and melting.

Japanese Patent Laid-Open No. 1999-077366 JP-A-8-215880 JP 2002-160089 A JP 2006-167790 A Japanese Patent Laid-Open No. 2004-090011 Patent No. 4342176

  Although high temperature solder materials that do not contain lead have been developed by various institutions as described above, they still have acceptable characteristics in practical use, including solders mainly composed of Bi. The fact is that no materials have been found.

  The problem to be overcome in the solder containing Bi as a main component is whether or not its brittleness and melting point can exceed about 260 ° C. For example, if an attempt is made to process a solder alloy containing Bi as a main component into a wire or the like, the brittleness may cause it to break immediately, making it very difficult to handle. Therefore, various elements are added for the purpose of improving the brittleness or raising the melting point.

  Although the brittle properties and melting point problems of Bi alloys are considerably improved by adding various elements, this may adversely affect the so-called wettability between solder and electronic components or substrates. It was. In other words, in order to make Bi-based solder easier to use and practical, various elements are added. However, wettability is reduced by this, and measures for that are required separately. There was a problem.

  In general, materials having relatively low heat resistance such as thermoplastic resins and thermosetting resins are often used for electronic parts and substrates, so the working temperature must be less than 400 ° C, preferably 370 ° C or less. There is. However, for example, in the Bi / Ag brazing material disclosed in Patent Document 3, since the liquidus temperature is as high as 400 to 700 ° C., it is estimated that the working temperature at the time of joining is also 400 to 700 ° C. or more and is joined. It will exceed the heat resistance temperature of electronic parts and substrates. Patent Document 3 does not mention wettability and does not describe solder workability. The solder alloy disclosed in Patent Document 4 is not described regarding wettability. Moreover, it becomes a quaternary or higher multi-component solder only by adjusting the temperature of the liquidus, and no effective measures for improving the fragile mechanical properties of Bi have been shown.

  As a means for overcoming the problem of wettability of Bi-based solder, as shown in Patent Document 5, it has been proposed to coat a metal with good wettability on the solder. However, there are many problems in the solder coating technology. For example, the surface state after coating, the temperature and time at the time of heating and melting are very important. It is presumed that it is very difficult to carry out coating in consideration of these factors.

  Patent Document 5 does not show coating conditions and reflow conditions which are such important conditions at all. Furthermore, in Example 1, it is described that Sn-10.2Zn solder as an initial solder material is coated with Sn as a coating material, and the entire solder composition after melting becomes Sn-9Zn. However, with this composition, it is considered that a considerable heating time is required until the coating material and the initial solder material become a uniform alloy. Furthermore, since there is a reaction with the metallized layer, the reaction occurs only with the initial solder material and the coating material, and it is difficult for these to have a uniform composition, and it is difficult to think of a practically feasible technology. .

  In addition, in the ternary composition as described in Patent Document 6, the composition that raises the minimum melting point is only a part, and the melting point increasing effect due to Sn having a relatively low melting point is not large. Not applicable. Furthermore, in order to improve the bondability by surface treatment of the main component of alloy particles, extremely strict work and management are required in terms of surface treatment conditions, storage and handling conditions of alloy particles, atmosphere and temperature conditions during heating and melting, etc. In terms of this, the disadvantages are quite large and the practicality is considered to be poor.

  As described above, in the high-temperature Pb-free solder, many problems such as wettability remain, and the actual situation is that they are not put into practical use. In particular, in a solder mainly composed of Bi, which is a candidate for high-temperature Pb-free solder, it is necessary to improve the wettability which is lowered by an element added to increase the workability and melting point of the Bi alloy.

  The present invention has been made in view of the above problems, and provides a high-temperature Pb-free Bi-based solder alloy excellent in wettability and reliability, a method for manufacturing the same, and a mounting bonded by the solder alloy. An object of the present invention is to provide a substrate and various devices for mounting the mounting substrate.

  The reflow temperature in the case of so-called high temperature for mounting electronic components on a substrate is about 260 ° C. or higher. Therefore, in the case of high-temperature solder used for soldering an electronic component and a substrate, the solidus wire of the solder alloy is required to be about 260 ° C. or higher. In order to realize this, it is conceivable to employ Pb-free solder containing Bi as a main component.

  However, wettability often decreases conversely due to elements added for improving the workability and melting point of Bi alloys. Therefore, as a result of extensive research, the present inventor has improved the wettability of Bi-based solder by coating Bi-based solder with at least one element of Ag, Au, Cu or Ni having good wettability. Has been found to be possible, leading to the present invention.

  That is, the coated Pb-free solder alloy provided by the present invention is a sheet-like, wire-like, or ball-like, containing Pb-free solder alloy containing 50 mass% or more and 99.5 mass% or less, and its surface And a coating film having a thickness of 0.05 μm or more and 1.20 μm or less composed of at least one element of Ag, Au, Cu, and Ni.

  The coated Pb-free solder alloy of the present invention further contains Zn in an amount of 0.4% by mass to 13.5% by mass and / or Sn in an amount of 0.01% by mass to 1.5% by mass. In addition, Al may be contained in an amount of 0.02% by mass to 2.5% by mass and / or P in an amount of 0.001% by mass to 0.500% by mass.

  The present invention also provides a method of coating the coating film of the coated Pb-free solder alloy by a plating method or a vapor deposition method. Furthermore, the present invention provides a mounting board on which electronic components are bonded using the coated Pb-free solder alloy, and a device on which the mounting board is mounted.

  ADVANTAGE OF THE INVENTION According to this invention, the solder for high temperature for joining an electronic component and a board | substrate can be provided, without using Pb which is harmful to an environment and has received various restrictions. In this solder, a Bi-based solder alloy produced to ensure desired strength and reliability is coated with at least one of Ag, Au, Cu, and Ni. Thereby, it is possible to obtain a solder material that can sufficiently withstand a reflow temperature of about 260 ° C. or more and that has excellent wettability and reliability. As a result, since soldering at a high temperature, which has been impossible in the past, becomes possible, the industrial contribution according to the present invention is extremely high.

  The coated Pb-free solder alloy of the present invention has a sheet shape, a wire shape or a ball shape, and is coated on the surface thereof with a Pb-free solder alloy containing Bi of 50 mass% or more and 99.5 mass% or less. It is characterized by comprising a coating film composed of at least one element of Ag, Au, Cu and Ni and having a thickness of 0.05 μm or more and 1.20 μm or less.

  More specifically, the Pb-free solder alloy to be coated has a sheet shape, a wire shape, or a ball shape. Sheet-like Pb-free solder alloys are mainly used for chip joining applications in power modules and are generally formed to a width of about 3 to 30 mm, a length of about 3 to 30 mm, and a thickness of about 0.05 to 0.5 mm. Is done.

  The wire-like Pb-free solder alloy is mainly used for chip bonding of transistors, and is generally formed to have a wire diameter of about 0.3 to 1.0 mm. Ball-shaped Pb-free solder alloys are mainly used for bump formation in leadless packages such as BGA (Ball Grid Array), CSP (Chip Scale Package), and flip chip, and generally have a particle size of about 0.2 to 0.2. Molded to about 1.0 mm.

  In the present invention, a coating film made of at least one element of Ag, Au, Cu, and Ni is coated on the surface of the Pb-free solder alloy formed into a sheet shape, a wire shape, or a ball shape. The reason why the material of the coating film is thus at least one element of Ag, Au, Cu, and Ni is that these have a great effect of improving the wettability, bonding property, reliability, etc. of the solder material. . That is, Ag and Au are materials that are often formed as a protective layer on the top layer of the Si chip in order to improve wettability and bondability. Ni is a material formed on the substrate for the purpose of improving chip bondability, and Cu is a material for the substrate. Therefore, by coating these same metals on the surface of the solder alloy, the bondability and reliability can be improved. Etc. can be improved.

  The coating film has a thickness of 0.05 μm or more and 1.20 μm or less. By limiting the thickness of the coating film to this range, good wettability can be obtained without impairing the characteristics of the Pb-free solder alloy. If it is less than 0.05 μm, the film thickness is too thin to form a uniform film, resulting in insufficient wettability. On the other hand, if the film thickness exceeds 1.20 μm, the film becomes too thick and processing becomes difficult. In addition, the ratio of the coating film to the solder alloy becomes too high, and the composition of the solder after joining may deviate from the intended composition of the solder, and desired characteristics may not be obtained. Furthermore, since the coating film becomes thick, the material cost increases and the film formation takes time, leading to an increase in cost.

  The Pb-free solder alloy of the present invention is mainly composed of Bi. The reason is that Bi has a melting point of 271 ° C., which exceeds the reflow temperature of about 260 ° C., which is the use condition of the high-temperature solder. However, Bi belongs to group Va elements (N, P, As, Sb, Bi), and its crystal structure is a trigonal crystal (rhombohedral crystal) with a low symmetry and a very brittle metal. It can be easily seen that the fracture surface is a brittle fracture surface. That is, pure Bi is a metal that has poor ductility and is difficult to process into a wire or the like.

  Therefore, the brittleness of Bi was overcome by adding various additive elements. The kind and amount of the element to be added differ depending on which characteristic is aimed for improvement among characteristics such as brittleness of Bi. The Bi content inevitably changes depending on the type and amount of the element to be added. Therefore, in the present invention, the Bi content is set to 50 mass% or more and 99.5 mass% or less. If the Bi content is less than 50% by mass, it is difficult to satisfy the properties such as the melting point, workability, and wettability in a well-balanced manner. On the other hand, if the Bi content exceeds 99.5% by mass, the Bi component becomes too large to improve the brittleness or to obtain sufficient bondability.

  Examples of elements added to the Pb-free solder alloy of the present invention include Zn, Sn, Al, and P. When Zn is added to a Pb-free solder alloy containing Bi as a main component, brittleness can be overcome by the effect of crystal refining, and workability is also improved by the solid solution of Zn in Bi. To do. When Zn is added more than the eutectic point with Bi, a Zn-rich phase is expressed, and the workability is further improved.

  Further, the addition of Zn makes it possible to suppress the reaction between Bi and the Ni layer constituting the substrate and to suppress the diffusion of the Ni layer into the Bi-based solder. This is because Zn is more reactive than Bi in the reaction with Ni, and a thin Zn—Ni layer is formed on the upper surface of the Ni layer, which acts as a barrier to suppress the reaction between Ni and Bi. As a result, a brittle Bi—Ni alloy is not generated, and Ni is not diffused into Bi, so that strong bondability can be realized.

  The optimum addition amount of Zn that exhibits such an excellent effect is approximately 0.4% by mass or more and 13.5% by mass or less, although it depends on the thickness of the Ni layer, the reflow temperature, the reflow time, and the like. . If the added amount of Zn is less than 0.4% by mass, suppression of Ni diffusion is insufficient, or Zn is consumed for suppression of Ni diffusion, and good workability may not be obtained. On the other hand, when Zn is added more than 13.5% by mass, the liquidus temperature exceeds 400 ° C., and good bonding cannot be performed. Furthermore, by appropriately adjusting and adding Al described later to a solder alloy containing Zn within this composition range, it becomes possible to further improve the workability of the Zn-rich phase, and draw out the effect of Zn more greatly. Can do.

  Sn can improve the wettability of the Pb-free solder alloy itself, and can also suppress the diffusion of Ni. This is because Sn has a smaller ionic radius than Zn and is likely to cause ternary eutectic, and thus is rich in reactivity with Ni. In addition, since Sn is less reducible than Zn and difficult to oxidize, adding a small amount of Sn to replace a part of Zn improves wettability while ensuring the effect of suppressing Ni diffusion. Can be made.

  In addition, by adding a small amount of Sn, a relatively large number of diffusion sites are formed, thereby promoting Zn-Zn-Ni alloying. As a result, a Zn—Ni alloy is efficiently formed on the Ni layer, and Ni diffusion into Bi is suppressed. As a matter of course, Sn itself is alloyed on the upper surface of the Ni layer and contributes to prevention of Ni diffusion.

  As described above, the addition amount of Sn can be reduced by adding a small amount of Sn, and as a result, the wettability of the Pb-free solder alloy itself can be improved. In the case of a Bi—Sn binary alloy, Ni diffusion can be suppressed in a smaller amount than when Zn is added alone. However, the inventor has confirmed that the alloy becomes brittle when Sn is contained more than 1.5 mass%. The reason is that since the amount of Sn dissolved in Bi is small, a β-Sn phase rich in Sn appears, which is presumed to have an adverse effect. Therefore, when adding Sn, it is preferable to add together with Zn.

  The optimum amount of Sn added is 0.01% by mass or more and 1.5% by mass or less. If it is less than 0.01% by mass, it is too small, and the effect of suppressing Ni diffusion and improving wettability does not appear. On the other hand, when it is more than 1.5% by mass, it has been confirmed that the workability is lowered, and it is assumed that the reason is that the ratio of fragile β-Sn increases. In addition, considering that Sn has a low melting point, the addition amount is preferably 1.5% by mass or less.

  P can further improve the wettability and bondability of the Bi / Zn / Sn alloy by adding it as necessary. This effect is also exhibited when Al is added. The reason why the effect of improving the wettability of the Bi / Zn / Sn alloy by adding P is large is that P has strong reducibility and suppresses oxidation of the solder alloy surface by oxidizing itself. In particular, when Zn which is easily oxidized is added to the Zn-rich side from 2.7% by mass which is the eutectic point in the alloy with Bi, the effect of improving the wettability by adding P is large.

  The addition of P has an effect of further reducing the generation of voids during bonding. That is, as described above, since P is easily oxidized by itself, oxidation proceeds preferentially over Bi, which is the main component of the solder, and also Zn during bonding. As a result, it is possible to prevent the solder mother phase from being oxidized and to ensure wettability. As a result, good bonding is possible, and void formation is less likely to occur.

  As described above, P has a very strong reducibility, so that the effect of improving the wettability of the Pb-free solder alloy itself is exhibited even when a small amount is added. On the contrary, even if it is added in a certain amount or more, the effect of improving the wettability does not change. If it is excessively added, P oxide may be generated on the solder surface, or P may form a brittle phase and become brittle. Therefore, P is preferably added in a trace amount.

  Specifically, the addition amount of P is preferably 0.001% by mass or more, and the upper limit is 0.50% by mass or less. When P exceeds this upper limit, the oxide covers the solder surface, and conversely, wettability may be reduced. Furthermore, since the amount of solid solution of Bi in P is very small, if the amount is large, brittle P oxide is segregated and the reliability is lowered. In particular, it has been confirmed that it is likely to cause disconnection when processed into a wire shape. On the other hand, if the addition amount of P is less than 0.001% by mass, the expected reduction effect cannot be obtained, and there is no point in adding.

  By adding Al as necessary, the effect of improving the wettability of the Pb-free solder alloy itself, the effect of improving workability, and the effect of adjusting the melting point can be obtained. The reason why the wettability of the Pb-free solder alloy itself is improved is that Al is more reducible than Bi or Zn, so even if it is added in a small amount, it oxidizes itself and improves the wettability.

  By the way, in a high-temperature solder alloy, measures for preventing Ni diffusion and improving wettability are important issues, and naturally, being able to withstand high reflow temperatures is also an important issue. In addition, good workability is an indispensable issue for the practical application of solder materials. By adding Al, the workability of the solder is improved, and a solder material that is easier to use can be obtained. In other words, Al is particularly effective in the vicinity of the eutectic composition with Zn, which is due to the refinement of the crystal. Although the workability can be ensured to some extent even if Zn is added, the constraints imposed upon the production increase, and the workability may be insufficient in some cases. At this time, addition of Al is effective.

  The reason why the workability of the solder is improved by the addition of Al is that the Bi / Zn / Sn alloy can be seen from the Bi-Zn-Sn ternary phase diagram or the Bi-Zn binary phase diagram. This is because a Zn-rich phase can be expressed by appropriately adjusting the amount of Zn added, and Al can change the workability of this Zn-rich phase. Further, Al can be dissolved in Zn to improve the workability, and the workability can be further improved by refining the crystal in the vicinity of the eutectic composition. For this reason, the addition amount of Al also takes into account the solid solution amount in Bi or the like, and the addition of about 1 to 1/20 of the mass ratio with respect to Zn is optimal, thereby improving workability. it can.

  The effect obtained by the addition of Al is not limited to the above-described improvement in wettability and workability, and further exhibits a great effect in adjusting the melting point. That is, as can be seen from the Bi-Al phase diagram, the effect of increasing the melting point of Al is very large, and the melting point can be increased by adding a small amount.

  As described above, Al is added in a small amount in consideration of three characteristics of workability, melting point and wettability. The specific addition amount of Al is preferably 0.02% by mass or more, and the upper limit is 2.5% by mass or less. If it is added more than 2.5% by mass, Al having a high melting point will segregate, resulting in problems such as deterioration of the bondability.

  On the other hand, the lower limit of 0.02% by mass is that the eutectic composition of Al and Zn is about Zn = 20 with respect to Al = 1 by weight ratio, and the lower limit of Zn addition amount is about 0.4% by mass. In addition, it is a numerical value obtained by conducting an experiment in consideration of required workability. It has been confirmed that if it is less than 0.02% by mass, the expected workability and the effect of increasing the melting point are substantially absent. If the addition amount of Al is 0.02% by mass or more and 2.5% by mass or less, it is not so much as compared with the whole solder alloy, and thus does not adversely affect other characteristics required for the solder. .

  Next, the manufacturing method of the coated Pb-free solder alloy of this invention is demonstrated. When processing sheet-like, wire-like or ball-like solder alloys, first make a solder mother alloy, and set the resulting solder mother alloy in different processing equipment for each sheet-like, wire-like or ball-like shape. To do. Thereafter, the surface of the solder alloy processed into each shape is coated. Hereafter, this manufacturing method is demonstrated for every process.

(Manufacture of solder mother alloy)
First, a method for producing a solder mother alloy will be described. The method for producing the mother alloy is not particularly limited, and examples thereof include a melting method, a continuous casting method, a reduction diffusion method, and an electrolytic method. Among these, the high frequency melting method using the electromagnetic induction action by high frequency is preferable because the apparatus is small and the metal can be heated and melted in a very short time because eddy current is generated in the metal by electromagnetic induction. Hereinafter, the manufacturing method of the metal sample by the high frequency melting method will be described.

  First, a predetermined amount of raw metal is weighed. In that case, it is preferable to process each raw metal into an appropriate size. This is because if the raw material metal is in a large lump shape, particularly when a high melting point metal is contained, it may be difficult to melt with other metals and remain undissolved. Each raw material metal weighed is put into a graphite crucible and further set in a coil of a high-frequency melting furnace. An inert gas is passed through the crucible to prevent oxidation of the molten metal. In the case of a metal that is easily oxidized, a reducing gas, for example, a mixed gas in which hydrogen is mixed with 10 vol% or more of an inert gas may be used. The gas flow rate is preferably 0.7 L / min or more per kg of the raw material. If the gas flow rate is too low, the atmosphere is mixed and the alloy is oxidized, and if it is too high, the cost increases.

  Next, the coil is heated by passing an electric current so as to obtain an appropriate temperature taking into account the amount of alloy input and the melting point of each metal. If an electric current is passed too much, the temperature becomes too high, and the metal component having a high vapor pressure evaporates and deviates from the intended composition, which is not preferable. On the other hand, when the current is small, the refractory metal or the like does not melt sufficiently and segregates. After confirming that the metal has melted, the molten metal is stirred with a glass rod. When sufficiently mixed, turn off the high-frequency power source, quickly take out the graphite crucible, and pour the molten metal in it into the mold. After sufficiently cooling and confirming that the metal has hardened, it is removed from the mold. Thereby, an ingot of a mother alloy is obtained.

(Processing of sheet-like solder alloy)
The sheet-like solder alloy is obtained by processing it to a predetermined thickness by passing it through a rolling mill several times under conditions suitable for the composition. Specifically, for example, a plate-shaped ingot having a thickness of 5 mm produced by the above-described method using a rolling mill can be rolled to 0.10 mm. At that time, if the feeding speed of the ingot is not appropriate, the sheet warps or the thickness varies, so rolling is performed while adjusting the speed as appropriate. Thereafter, the sheet is cut into a predetermined width by slitting, and a sheet-like solder alloy is completed.

(Processing of wire solder alloy)
A wire-like solder alloy is obtained by processing a solder mother alloy into a wire having a predetermined diameter using an extruder. Specifically, the extruder is heated in advance to a temperature suitable for the solder composition, the mother alloy ingot described above is set, and extruded into a wire shape.

  Since the wire-like solder alloy extruded from the exit of the extruder is still hot and easily oxidized, it is preferable to make the exit of the extruder have a sealed structure and allow an inert gas to flow inside. That is, it is preferable to reduce the oxygen concentration as much as possible at the outlet of the extruder so that the solder alloy does not oxidize. In this state, the hydraulic pressure is increased and the solder mother alloy is extruded into a wire shape. At that time, the wire extrusion speed is adjusted so that the wire is not cut or deformed, and the wire is wound at the same speed.

(Processing of ball-shaped solder alloy)
The ball-shaped solder alloy can be produced as a ball-shaped solder alloy having a substantially uniform particle size by an atomizing method in the air or in a liquid. The submerged atomization method can adjust the liquid temperature in accordance with the metal, and it is easy to obtain a high-quality ball. In particular, oil is preferable because the adjustment temperature range is wide. The heater for melting the metal is preferably a high-frequency melting type heater that can be heated and melted in a short time and does not take up space.

  On the other hand, a ball-shaped solder alloy can be manufactured by an air atomization method. For example, in the uniform droplet spraying method, a solder mother alloy ingot is melted in a crucible in a reducing atmosphere to form a molten metal, and vigorously moves into an adjacent chamber in which an inert gas is flowed while applying a constant frequency vibration to the molten metal. It is a method of making molten metal into a ball shape by supplying it. Although it does not limit according to a solder alloy composition, a particle size, a manufacturing amount, etc., what is necessary is just to select the above manufacturing methods suitably.

  The sheet-like, wire-like or ball-like solder alloy thus obtained is then coated with a coating film made of at least one element of Ag, Au, Cu and Ni. Examples of the coating method include a plating method and a vapor deposition method. The plating method can be used for coating any of sheet-like, wire-like and ball-like solder alloys, while the vapor deposition method is particularly suitable for coating sheet-like and wire-like solder alloys. Hereinafter, sheet-like and wire-like solder alloys will be described as examples of solder alloys to be coated by plating and vapor deposition.

(Plating to solder alloy)
First, pretreatment is performed so that the solder alloy thus produced is plated uniformly and with good adhesion. Specifically, the sheet-like solder alloy obtained by the above processing method is air blown to remove the adhered foreign matter. Next, this solder alloy is washed with water and further acid washed. These are repeated several times as necessary to finish the surface of the solder alloy cleanly. Thereafter, it is quickly dried to complete the pretreatment.

  Next, the pre-treated solder alloy is immersed in a cyan bath to perform strike plating composed of at least one of Ag, Au, Cu and Ni, and further on Ag, Au, Cu and Ni. The plating which consists of at least 1 of these is given. The plated solder alloy is subsequently dried immediately after washing off the adhering solution. This produces a coated solder alloy. The finished solder alloy is stored in a hermetically sealed container so that it does not oxidize or adhere to moisture.

(Vapor deposition method on solder alloy)
As a method of vapor deposition on the solder alloy, for example, a solder alloy to be coated is attached to a support material such as a glass plate, and this support material is installed in the vapor deposition machine so that the surface supporting the solder alloy faces downward. And vapor deposition. In order to deposit the coating film uniformly on the surface of the solder alloy during the deposition, it is preferable that the solder alloy to be coated is separated from the support material except for the attachment portion. As a method for skipping the metal for vapor deposition, either an electron beam method or a resistance heating method may be used.

For example, in the case of the electron beam method, a metal for vapor deposition is placed on an alumina dish (hearth liner) and set at a predetermined position. The vapor deposition machine is closed and vacuumed to 10 Pa or less with an absolute pressure by a rotary pump for roughing, then switched to a turbo molecular pump and vacuumed to a high vacuum. Here, if the air cannot be vacuumed well and the atmosphere remains, the metal is oxidized and a high-purity metal film cannot be formed. Therefore, it is desirable to raise the degree of vacuum to at least 1.0 × 10 ⁻³ Pa or less. Next, turn on the film thickness meter. The stage to which the support material is attached is rotated so that the coating film is uniformly formed. Switch on the electron beam, observe the heating condition of the deposition metal from the glass window, and gradually increase the current while monitoring the film formation rate with a film thickness meter.

  The deposition rate is preferably 5 (Å / sec) or more and less than 50 (Å / sec). If it is less than 5 (Å / sec), it takes time for coating and the oxygen concentration of the film tends to increase. If it is 50 (Å / sec) or more, the deposition rate is too high and the film becomes non-uniform. When the predetermined film thickness is reached, the electron beam power supply is turned off. The turbo molecular pump is stopped and an inert gas is introduced into the vapor deposition machine. After confirming that the pressure in the vapor deposition machine has reached atmospheric pressure, the coated solder alloy is taken out. This produces a coated solder alloy.

  Since the coated Pb-free solder alloy of the present invention described above is used for joining an electronic component and a substrate, good wettability can be obtained. Therefore, under severe conditions such as an environment in which a heat cycle is repeated. Even when used, a durable and highly reliable electronic substrate can be provided. Therefore, by mounting this electronic board on, for example, power semiconductor devices such as thyristors and inverters, various control devices mounted on automobiles, devices used under harsh conditions such as solar cells, these various devices Can be further improved in reliability.

  Hereinafter, the coated Pb-free solder alloy of the present invention will be described in more detail with reference to examples, but the present invention is not limited to the examples.

[Example 1]
(Manufacture of solder mother alloy)
First, a solder mother alloy was manufactured by the high frequency melting method shown below. Bi / 4% Zn containing slightly Sn (“%” represents mass%, “%” means the remainder, the same shall apply hereinafter), aiming at the solder composition, Bi, Zn and Sn having a purity of 99.9% by mass or more were prepared. A predetermined amount of these were weighed and put into a graphite crucible, and then set in a coil of a high-frequency melting furnace. For the graphite crucible, a mixed gas in which 10 vol% or more of hydrogen was mixed with an inert gas was used to prevent oxidation of the molten metal. This mixed gas was supplied at a flow rate of 0.8 L / min per kg of the raw material.

  Next, the coil was heated by passing an electric current so as to obtain an appropriate temperature in consideration of the amount of alloy input and the melting point of each metal. After confirming that the metal was melted, the molten metal was stirred with a glass rod. When sufficiently mixed, the high frequency power supply was turned off, the graphite crucible was quickly taken out, and the molten metal in the graphite crucible was poured into the mold. After sufficiently cooling, it was taken out from the mold after confirming that the metal had hardened.

  Thus, the solder mother alloy of Sample 1 which is a plate-like ingot having a thickness of 5 mm was produced. Further, in the same manner as Sample 1, solder mother alloys of Samples 2 to 33 were produced. The compositions of the solder mother alloys of Samples 1 to 33 were analyzed using an ICP emission spectroscopic analyzer (SHIMAZU S-8100).

(Sheet processing or wire processing)
Next, among the solder mother alloys of Samples 1 to 33 in Table 1 above, Samples 1 to 9 and 17 to 25 were processed into sheets. Specifically, the plate-shaped ingot having a thickness of 5 mm produced above was rolled to a thickness of 0.10 mm using a rolling mill. At that time, rolling was performed while adjusting the feeding speed of the ingot. Then, it cut | judged to 25 mm width by the slitter process.

  On the other hand, the solder mother alloys of Samples 10 to 16 and 26 to 33 were set in an extruder and processed into a wire shape having an outer diameter of 0.80 mm. Specifically, the extruder was heated in advance to a temperature suitable for the solder composition, and a master alloy ingot was set. The exit of the extruder was sealed, and an inert gas was allowed to flow through it, and the oxygen concentration was lowered as much as possible to prevent oxidation. In this state, the solder mother alloy was extruded into a wire shape by applying pressure using hydraulic pressure. At that time, the wire was wound while adjusting the extrusion speed and the winding speed so that the wire was not cut or deformed.

(Coating by plating or vapor deposition)
Next, coating films of various thicknesses were applied to the surfaces of the solder alloys of Samples 1 to 9 and 17 to 25 processed into a sheet by a plating method, respectively. Specifically, first, as a pretreatment, each sheet-like solder alloy was air blown to remove foreign substances adhering to the surface of the solder alloy. Next, water washing and acid washing were repeated several times. Then, it dried quickly and completed the pretreatment.

  Next, the pre-processed sheet-like solder alloy was immersed in a cyan bath and subjected to strike plating with any of Ag, Au, Cu, or Ni. Furthermore, it was plated with the same type of metal as used for strike plating. After plating, the solution adhering to the sheet was washed away and dried quickly. The finished sheet-like solder alloy was stored in a hermetically sealed container so that oxidation and adhesion of moisture did not occur.

  On the other hand, coating films of various thicknesses were applied to the surfaces of the solder alloys of Samples 10 to 16 and 26 to 33 processed into a wire shape, respectively, by vapor deposition. Specifically, first, to prepare a glass plate for attaching and vapor-depositing each sample processed into a wire shape, and to support both ends of each sample processed into a wire shape on one side thereof, respectively. Two support stands having a height of about 10 mm were provided. Each sample processed into a wire shape was cut into a predetermined length, and both end portions thereof were attached to the two support tables. In order to prevent contamination of the sample and the apparatus, protective gloves and a protective mask were worn during the work, and the sample was handled with tweezers.

  This glass plate was set in a vapor deposition machine capable of vapor deposition by the electron beam method so that the surface supporting each sample faced downward. Furthermore, Ag, Au, Cu, or Ni was placed as a metal for vapor deposition on an alumina dish (hearth liner), and this was set at a predetermined position. After the vapor deposition machine was closed and roughing was performed with a rotary pump, it was further switched to a turbo molecular pump and evacuated to a high vacuum. The stage on which the glass plate was set was rotated so that the coating film was uniformly formed.

  In this state, the electron beam was turned on, and the current was gradually increased while observing the heating condition of the deposited metal from the glass window and monitoring the film forming speed of the film thickness meter. A metal film was deposited to a predetermined film thickness at a speed of about 10 (の / sec). When the predetermined film thickness was reached, the electron beam power supply was turned off. The turbo molecular pump was stopped, an inert gas was put in the vapor deposition machine and the atmospheric pressure was reached, and then the sample was taken out.

  The film thickness of the coating film provided by the above method was measured by EPMA line analysis. More specifically, each coated solder alloy was embedded in a resin, and finer ones were used in order from coarse abrasive paper using a grinder, and finally buffed. Thereafter, line analysis was performed using EPMA (device name: SHIMADZU EPMA-1600) to measure the film thickness of the coating film. The film thickness was arbitrarily measured at three places, and the thin film thickness was measured at five places, and an average value was adopted. The film thickness of the coating film of each sample thus measured is shown in Table 1 below together with the material of the coating film, the shape and composition of the solder alloy.

  Next, the wettability evaluation (bondability evaluation), the heat cycle test, and the heat resistance test in air were performed on the coated solder alloys of Samples 1 to 33 shown in Table 1 above.

<Wettability evaluation (bondability evaluation)>
A Cu substrate (plate thickness: about 0.80 mm) plated with Ni (film thickness: about 1.2 μm) and each coated solder alloy were prepared. In the wettability test (device name: atmosphere control type wettability tester), a double cover was placed on the heater part to be heated by placing the test object, and nitrogen was flowed from four locations around the heater part (nitrogen flow rate: 12 L each / Min). The heater temperature was set to 330 ° C. and held until the temperature stabilized.

  After the temperature is stabilized, a Ni-plated Cu substrate is placed and heated for 25 seconds, and then each coated solder alloy is placed on the Cu substrate and heated for 25 seconds. After the heating was completed, the Cu substrate was shifted to the place where the nitrogen atmosphere by the nitrogen flow beside the heater part was maintained and cooled. After sufficiently cooling, the Cu substrate was taken out into the atmosphere. The case where bonding was not possible was evaluated as “×”, the case where bonding was possible but the wet spread was bad was evaluated as “Δ”, and the case where bonding was possible and wet spread was evaluated as “◯”.

<Heat cycle test>
The following heat cycle test was performed using the Cu substrate to which the solder used for the wettability evaluation was bonded. That is, each of the Cu substrates to which the solder was joined was subjected to 200, 400, and 600 times, with -40 ° C. cooling and 135 ° C. heating as one cycle. Thereafter, each Cu substrate to which the solder was bonded was embedded in the resin in the same manner as in the film thickness measurement described above, cross-section polishing was performed, and the bonding surface was observed by SEM (device name: HITACHI S-4800). The case where the joint surface is peeled or the solder is cracked is judged as “bad”, and the case where there is no such “bad” and the same joint surface as the initial state is kept as “good”. .

<Atmospheric heat resistance test>
A heat resistance test at 160 ° C., 100, 300, and 500 hours was performed in an air atmosphere using an oven on a sample in which solder was bonded to a substrate in wettability evaluation. After a predetermined time, the sample was taken out and cooled. Then, the sample was embedded in resin, cross-section polishing was performed, and the bonding surface was observed by SEM (device name: HITACHI S-4800). The case where the joint surface was peeled off or the solder was cracked was indicated as “X”, and the case where there was no such defect and the same joint surface as in the initial state was maintained as “◯”.

  The results of the wettability evaluation (bondability evaluation), the heat cycle test, and the heat resistance test in air are shown in Table 2 below.

  From the results of Table 2 above, it can be seen that the coated solder alloys of Samples 1 to 16 exhibit excellent characteristics in all evaluation items. On the other hand, the coated solder alloys of Samples 17 to 33 that deviate from the requirements of the present invention with respect to the coating thickness resulted in undesirable results in any of the evaluation items.

[Example 2]
Solder mother alloys of Samples 34 to 43 were produced in the same manner as in Example 1 except that Al and P having a purity of 99.9% by mass or more were added to the raw materials and the mixing ratio of the raw materials was changed. Composition analysis and coating film thickness measurement were performed on the solder mother alloys of Samples 34 to 43 in the same manner as in Example 1. The results are shown in Table 3 below.

  The coated solder alloys of Samples 34 to 43 in Table 3 were subjected to wettability evaluation (bondability evaluation), heat cycle test, and atmospheric heat resistance test in the same manner as in Example 1. The results are shown in Table 4 below.

  From the results of Table 4 above, it can be seen that the coated solder alloys of Samples 34 to 40 show excellent characteristics in all evaluation items. On the other hand, the coated solder alloys of Samples 41 to 43 that deviate from the requirements of the present invention with respect to the Bi content resulted in undesirable results in any of the evaluation items.

Claims (6)

  A Pb-free solder alloy that is in the form of a sheet, wire, or ball and contains Bi in an amount of 50 mass% to 99.5 mass%, and at least one of Ag, Au, Cu, and Ni coated on the surface thereof A coated Pb-free solder alloy comprising a coating film made of an element and having a thickness of 0.05 μm or more and 1.20 μm or less.   The Pb-free solder alloy further contains Zn in an amount of 0.4% by mass to 13.5% by mass and / or Sn in an amount of 0.01% by mass to 1.5% by mass. Item 4. The coated Pb-free solder alloy according to item 1.   The Pb-free solder alloy further contains Al in an amount of 0.02% by mass to 2.5% by mass and / or P in an amount of 0.001% by mass to 0.500% by mass. Item 3. The coated Pb-free solder alloy according to item 2.   A coating method for a Pb-free solder alloy, wherein the coating film according to claim 1 is coated by a plating method or a vapor deposition method.   An electronic component is bonded using the coated Pb-free solder alloy according to claim 1.   6. A device on which the mounting substrate according to claim 5 is mounted.