JP5716332B2 - Pb-free solder alloy - Google Patents

Description

  The present invention relates to a solder alloy containing no lead (Pb), and more particularly to a high temperature solder alloy.

  In recent years, regulations on chemical substances harmful to the environment have become stricter, and this regulation is no exception for solder materials used for the purpose of joining electronic components to a substrate. Pb (lead) has been used as a main component for solder materials for a long time, but it has already been regulated by the Rohs Directive. For this reason, development of solder containing no Pb (hereinafter also referred to as Pb-free solder or lead-free solder) has been actively conducted.

  Solders used when 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 limit temperature of use. As for the solder for low temperature, Sn is a main component and Pb free is put into practical use. For example, in Patent Document 1, Sn is the main component, Ag is 1.0 to 4.0 mass%, Cu is 2.0 mass% or less, Ni is 0.5 mass% or less, and P is 0.2 mass%. The following Pb-free solder alloy composition is described. 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, development of various high-temperature Pb-free solder materials is also being conducted. For example, 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. Patent Document 4 discloses a solder alloy in which a binary eutectic alloy is added to a eutectic alloy containing Bi and an additional element is further added. This solder alloy is a quaternary or higher multi-component solder. However, it has been shown that the liquidus temperature can be adjusted and variations can be reduced.

  Further, Patent Document 5 discloses a solder alloy in which Cu—Al—Mn, Cu, or Ni is added to Bi, and these solder alloys include a power semiconductor element and an insulator substrate having a Cu layer on the surface. It is described that, when used in the above, it is difficult to form an unnecessary reaction product at the joint interface with the solder, so that occurrence of defects such as cracks can be suppressed.

  Further, in Patent Document 6, among 100% by mass of the solder composition, a first metal element composed of 94.5% by mass or more of Bi, a second metal element composed of 2.5% by mass of Ag, and Sn: 0.1-0.5% by mass, Cu: 0.1-0.3% by mass, In: 0.1-0.5% by mass, Sb: 0.1-3.0% by mass, and Zn: 0 A solder composition composed of a third metal element containing at least one selected from the group consisting of 0.1 to 3.0% by mass in a total of 0.1 to 3.0% by mass is shown.

  Patent Document 7 discloses a Pb-free solder composition containing 0.3 to 0.5% by mass of Ni in a Bi-based alloy containing at least one of Ag, Cu, Zn and Sb as subcomponents. The Pb-free solder has a solidus temperature of 250 ° C. or higher and a liquidus temperature of 300 ° C. or lower. Further, Patent Document 8 discloses a binary alloy containing Bi, and it is described that this binary alloy has an effect of suppressing the occurrence of cracks in the soldering structure.

Japanese Patent Laid-Open No. 1999-077366 JP-A-8-215880 JP 2002-160089 A JP 2006-167790 A JP 2007-281212 A Japanese Patent No. 3671815 JP 2004-025232 A JP 2007-181880 A

  As described above, with respect to high-temperature Pb-free solder materials, Bi-based solder alloys have been developed by various organizations. In this Bi-based solder, the three characteristics of (1) prevention of reaction with Ni metallized layers such as electronic parts and Ni diffusion prevention, (2) workability, and (3) wettability are all balanced at a practical level or more. Thus, making the Bi-based solder practical is a big issue. However, the actual situation is that no acceptable solder material has yet been found in terms of practicality in which these three characteristics are balanced.

  That is, Bi, which is the main component of the Bi-based solder alloy, reacts with the Ni metallized layer formed on the electronic component or the like to form a brittle layer, or Ni in the Ni metallized layer diffuses into Bi and has a bonding property. May be drastically reduced. Furthermore, since Bi is very brittle, it is difficult to process into a sheet or a wire as it is. When various elements are added in order to improve the Ni diffusion problem and brittle Bi characteristics, Bi wettability, which originally has insufficient wettability to Cu or the like, may be further reduced.

On the other hand, Patent Document 5 does not solve the problem of Ni diffusion, as can be seen from the fact that the bonding surface with the solder is not a Cu layer but a Ni layer as a comparative example. Absent. Further, it is described that a large amount of Bi ₃ Ni is formed at the joint interface in a solder alloy in which Cu—Al—Mn, Cu, or Ni is added to Bi, and a large number of voids are observed in the periphery thereof. It is described that Bi ₃ Ni has a very brittle property and it has been confirmed that it is difficult to obtain reliability with respect to a heat cycle under severe conditions, but a problem of reactivity between Ni and Bi is simply presented. It ’s just that.

  Further, in the solder composition containing 2.5% by mass of Ag as disclosed in Patent Document 6, for example, even if Sn is contained by 0.5% by mass or more and Zn is contained by 3.0% by mass or more, Bi The reaction between Ni and Ni and the diffusion of Ni into Bi cannot be suppressed, and it has been experimentally confirmed that the bonding strength is low and the solder material cannot withstand practical use.

  Further, Patent Documents 5 and 6 have no description on workability and wettability. For example, Bi—Cu, Bi—Ni, Bi alloys in which Mn is added to Patent Document 5, and Bi-2 in Patent Document 6 are further disclosed. Experiments have confirmed that a composition such as .5Ag (eutectic composition) is very brittle and difficult to process into a wire or sheet.

  That is, since Cu hardly dissolves in Bi, there is no effect of improving workability, and Ni generates a brittle phase with Bi as described above. Further, when Mn is added, discoloration occurs, and it is visually confirmed that the surface oxidation proceeds. Furthermore, even if Ag is a eutectic composition with Bi, the workability is not improved. This is presumed to be due to the non-uniform dispersion state of Ag in Bi.

Moreover, in the Pb-free solder composition disclosed in Patent Document 7, Ni forms a brittle alloy with Bi as described above. That is, as can be seen from the Bi-Ni binary phase diagram, when a large amount of Bi is present, a brittle alloy called Bi ₃ Ni alloy is formed. When Ni is contained in an amount of 0.3 to 0.5% by mass, a very brittle alloy phase is dispersed in the solder, and it is assumed that the originally brittle Bi-based solder is further embrittled. Further, in the composition within the scope of claims of Patent Document 7, it is naturally considered that the workability is deteriorated, but there is no mention of it and there is no description about wettability.

  Further, Patent Document 4 and Patent Document 8 do not mention anything about the problem of diffusion of Ni into Bi and measures for preventing the problem. In particular, Patent Document 8 discloses a Bi-Ag system, a Bi-Cu system, a Bi-Zn system, and the like. Of these, a Bi-Ag system requires a Ni diffusion countermeasure in particular. It is not described at all.

  In addition, regarding the Bi-Cu system, it is considered that a Cu phase having a high melting point precipitates due to a very small amount of solid solution of Cu in Bi, and there arises a problem in the bondability. Not. Furthermore, in Bi-Zn system, it is speculated that wettability is lowered by Zn having strong reducibility and it is difficult to join electronic parts etc., but this is not mentioned, and there is also a description about the reaction between Ni and Bi. Absent. Patent Document 8 does not describe workability and wettability.

  By the way, in the case of a high-temperature Pb-free solder alloy, in addition to the balance between the above three characteristics, it is also necessary to withstand reflow at about 260 ° C. and to be able to join at about 400 ° C. or less. . In other words, since materials having relatively low heat resistance such as thermoplastic resins and thermosetting resins are generally used for electronic parts and substrates, 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 is considered that the heat resistance temperature of the electronic component or the substrate is exceeded and good bonding cannot be performed.

  Thus, the conventional high temperature Bi-based solder alloys that do not contain lead are (1) prevention of reaction with Ni metallized layers such as electronic parts and prevention of Ni diffusion, (2) workability, and (3) wettability. Therefore, it cannot be said that the material is balanced in terms of these three characteristics, and is not satisfactory in terms of practicality. The present invention has been made in view of such conventional problems, and is a Bi-based high-temperature Pb-free solder that is excellent in wettability and workability and can suppress reaction with Ni metallized layers such as electronic parts and Ni diffusion. The aim is to provide alloys.

To achieve the above object, Pb-free solder alloys provided by the present invention, the Sn contained less 4.8 wt% 0.01 wt%, the A l 0.03 mass% to 1.0 mass% (0.5 excluding mass% or less) have free, a P containing 0.5 mass% or more 0% by mass and the residue is characterized in that it consists of Bi except inevitable impurities. In addition, the Pb-free solder alloy may contain 0.18% by mass or more and 1.8% by mass or less of Cu. In the case of Ag, 0.01% by mass or more of Ag or Zn. In the case of Zn, it may be contained in an amount of 0.01% by mass or more and less than 0.4% by mass.

  According to the present invention, it is possible to provide a high-temperature Pb-free solder alloy having strength necessary for joining an electronic component and a substrate, and having excellent wettability and workability. That is, by adding a predetermined metal element to Bi as a main component so as to have a predetermined content rate, the reflow temperature is substantially 260 ° C. or higher, and wettability and workability are excellent. Furthermore, it becomes possible to suppress the reaction between the Ni layer of the electronic component or the like and Bi in the solder alloy and the diffusion of Ni into the Bi-based solder.

  The Pb-free solder of the present invention contains Sn in an amount of 0.01% by mass or more and 10% by mass or less, and contains at least one of Al and Cu, and in the case of Al, 0.03% by mass to 1.0% by mass. In the case of Cu, 0.01 mass% or more and 1.8 mass% or less is contained, P is contained 0 mass% or more and 0.5 mass% or less, and the balance is made of Bi except for inevitable impurities. . In order to further improve the properties of the solder alloy, at least one of Ag and Zn is used. In the case of Ag, 0.01% by mass to 3.0% by mass, and in the case of Zn, 0.01% by mass. % Or more and less than 0.4% by mass.

  With this, in addition to being able to withstand reflow of about 260 ° C. and being able to perform bonding at about 400 ° C. or less, (1) prevention of reaction with Ni metallized layers such as electronic parts and prevention of diffusion of Ni into Bi , (2) Workability, and (3) A practical high-temperature solder material balanced in terms of wettability can be obtained.

  That is, the high-temperature solder material needs to withstand a reflow temperature of about 260 ° C., but in the case of Bi-based solder, the reaction between Bi and Ni and the diffusion of Ni into Bi must be further suppressed. If this is insufficient, the Ni layer provided on the electronic component or the like reacts with Bi contained in the solder to form a brittle Bi—Ni alloy and Ni diffuses into Bi to make the joint brittle. There is a fear.

  As a result, the bonding strength is reduced, and the reliability of the apparatus including the electronic substrate bonded with the solder alloy is impaired. Since Bi is a semimetal and is essentially a brittle metal, it may be possible to add various elements in order to solve these brittleness problems. In this case, Bi that is not originally excellent in wettability. There is a risk of further reducing the wettability.

  Therefore, as a result of investigating various elements regarding the reactivity between Bi and Ni, it was found that Sn reacts preferentially with the Ni layer over Bi, forms an alloy, and wettability can be secured at an unnecessarily high level depending on conditions. It was. Further, in the case of a binary alloy in which only Sn is added to Bi, the reaction between Bi and Ni and Ni diffusion can be suppressed, but the workability becomes insufficient, and the wettability becomes insufficient depending on conditions. I found out that

  Therefore, as a result of conducting an experiment for the purpose of improving the wettability, it was found that the addition of Al or Cu is effective. It was also found that Al has an effect of further improving workability. And when improving wettability further, it turned out that adding P is preferable.

  There are various manufacturing conditions and specifications for products manufactured using solder materials, such as bonding temperature, bonding time, reflow conditions, chip size, formed metallization layer, and allowable current. Further, various elements may be added. Thereby, it becomes possible to further improve the characteristics suitable for each purpose, and the application of the solder of the present invention can be further expanded.

  Specifically, it was found that when Zn diffusion suppression and bondability were to be further improved, it was effective to add Zn in addition to the above composition. In addition, it was found that adding Ag was effective in further improving wettability. Hereinafter, the elements included in the Pb-free solder alloy of the present invention that can obtain the above-described characteristic effects and the elements included as necessary will be described in detail.

<Bi>
Bi is the main component of the high temperature lead-free solder alloy of the present invention. Bi belongs to the Va group element (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 with poor ductility.

  The reason for selecting Bi from the Va group elements is that the Va group elements are classified into semi-metals and non-metals except for Bi, and are more brittle than Bi. Further, Bi has a melting point of 271 ° C., which exceeds the reflow temperature of about 260 ° C., which is the use condition of high-temperature solder. In order to overcome the brittleness of Bi, various additive elements described later are necessary. The type and amount of elements added to improve characteristics such as brittleness of Bi vary depending on the characteristics to be improved. Accordingly, the Bi content inevitably changes depending on the type and amount of the element to be added.

<Sn>
Sn is an essential element contained in the high temperature lead-free solder alloy of the present invention. Sn plays an important role for ensuring the wettability of the solder alloy. Another important role of Sn is to suppress Ni diffusion. This effect of suppressing Ni diffusion has been confirmed only with Sn and Zn, but Sn has a smaller ion radius than Zn and is likely to cause ternary eutectics, and thus is more reactive with Ni. In addition, since Sn is less reducible than Zn and difficult to oxidize, it is possible to improve wettability while ensuring the effect of suppressing Ni diffusion.

  However, in the case of a Bi—Sn binary alloy, workability may be a problem depending on manufacturing conditions and the like. That is, Bi-Sn alloy is not particularly excellent in workability, so it can be produced without any problem in the shape of a ball or the like, but when rolling into a difficult-to-process sheet, cracks may occur during rolling. There is.

  Due to the above restrictions, the optimum amount of Sn added is 0.01% by mass or more and 10% by mass or less. If this amount is less than 0.01% by mass, the Ni diffusion suppression effect and the wettability improvement effect do not appear. On the other hand, if it is more than 10% by mass, it can be processed into a ball or the like, but it may be difficult to process into a sheet, which is not preferable. In addition, Sn has a low melting point, and even if a certain amount of liquid phase is allowed during reflow, it may cause problems such as displacement of electronic components due to dissolution. preferable.

<Al>
Al is an element contained in the high-temperature lead-free solder alloy of the present invention under the requirement that at least one of Al and Cu is contained. The purpose of adding Al under such requirements is to first improve the wettability of the solder alloy. In addition, it aims at improving workability. The reason why wettability is improved is that Al is more reducible than Bi and Sn, so even if it is added in a small amount, it oxidizes itself, making it difficult to oxidize the solder base material and improving wettability. is there.

  On the other hand, the reason why the workability of solder is improved is that Al itself is a soft element, and Bi is only slightly dissolved in Al, so it can exist in the solder while maintaining the flexibility of Al. It is. Further, when Zn is added and an alloy is formed in the vicinity of the eutectic composition of Al—Zn, the workability is further improved due to the crystal refinement effect.

  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 the three characteristics of wettability, workability, and melting point. When adding Al, the addition amount is 0.03 mass% or more and 1.0 mass% or less. This is because if Al is added in an amount of more than 1.0% by mass, Al having a high melting point is segregated, resulting in a problem that the bonding property is deteriorated. On the other hand, it has been confirmed that when the added amount of Al is less than 0.03 mass%, the expected workability and the effect of increasing the melting point cannot be substantially obtained.

<Cu>
As described above, Cu is an element contained in the high-temperature lead-free solder alloy of the present invention under the requirement that at least one of Al and Cu is contained. Therefore, the purpose of Cu addition is to improve the same wettability as the first purpose of Al addition. However, the mechanism of wettability improvement by addition of Cu is different from Al. That is, Al oxidizes itself to prevent oxidation of the solder mother phase, but Cu is difficult to oxidize Cu itself, so it is dispersed in the solder mother phase to prevent oxidation of the solder mother phase.

  Furthermore, Cu is particularly excellent in wettability with a Cu substrate, and can improve wettability and bondability. When adding Cu, the addition amount is 0.01 mass% or more and 1.8 mass% or less. If this amount is less than 0.01% by mass, the addition amount is too small and the effect of improving the wettability cannot be expected. On the other hand, if it exceeds 1.8% by weight, the liquidus temperature becomes too high and good bonding cannot be performed.

<P>
P is an element added as necessary, and the addition of P can further improve the wettability and bondability of the solder alloy. The reason why the effect of improving wettability is increased by the addition of P is that P is highly reducible and suppresses oxidation of the solder alloy surface by oxidizing itself.

  Furthermore, the addition of P has the effect of reducing the generation of voids during bonding. That is, as described above, since P easily oxidizes itself, oxidation proceeds preferentially over Bi, which is the main component of solder, and further with Sn 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 wettability 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, and if it is added excessively, 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 0.5% by mass. 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. It has been confirmed that wire breakage is likely to occur, especially when processing wires and the like. 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.

<Ag>
The high temperature lead-free solder alloy of the present invention may contain Ag. As can be seen from the fact that Ag is used for the metallized layer on the uppermost surface of the electronic component, the effect of improving the wettability is very large. On the other hand, in the case of Bi-based solder, Ag has a function of promoting the reaction between Ni and Bi and diffusion of Ni into the Bi-based solder. From this viewpoint, addition of Ag is not preferable.

  However, depending on the joining condition of the electronic component, a metallized layer such as Ag may be formed on the Ni metallized layer to have a thickness of several μm, or in some cases about 10 μm. As described above, when an Ag metallized layer of about several to 10 μm is present on the Ni metallized layer, the wettability effect can be exhibited without causing the above problem by including a predetermined amount of Ag in the solder. Can do. Specifically, when the Ag metallized layer on the Ni metallized layer is thick, it is preferable to include Ag in an amount of 0.01% by mass to 3.0% by mass when wettability is given the highest priority.

  The reason why the addition amount of Ag is 3.0% by mass or less is that Ag increases the effect of promoting Ni diffusion as described above with an increase in the addition amount, so that the addition amount of Ag is 3.0. If the mass% is exceeded, there is a high possibility that the reaction between Bi and Ni proceeds or Ni diffusion occurs even when the joining time is short or the Ag metallized layer on Ni is thin. It is. On the other hand, if the amount is less than 0.01% by mass, the addition amount is too small to expect improvement in wettability.

<Zn>
The high temperature lead-free solder alloy of the present invention may contain Zn. Zn is one of the few elements that has an effect of suppressing Ni diffusion. On the other hand, since Zn is easily oxidized, it often reduces the wettability of the solder alloy. Therefore, the addition amount of Zn is appropriately determined according to the situation. For example, when the wettability is the first priority, it is preferable to keep the addition amount to a minimum. On the other hand, when the wettability is already ensured by other additive elements and it is desired to simply improve the effect of suppressing Ni diffusion, it may be positively added. Furthermore, when Al and Zn are added simultaneously, the effect of improving workability in the vicinity of the eutectic composition can be obtained.

  The optimum addition amount of Zn is approximately 0.01% by mass or more and less than 0.4% by mass although it depends on the thickness of the Ni layer, the reflow temperature, the reflow time, and the like. This is because the effect expected of Zn is an effect of suppressing Ni diffusion. In the present invention, Sn is sufficiently added, so the effect is saturated even if added in an amount of 0.4% by mass or more. This is because it does not make it possible and reduces wettability. On the other hand, if it is less than 0.01% by mass, the addition amount is too small to expect the effect of addition.

  First, Bi, Sn, Al, Cu, P, Ag and Zn having a purity of 99.9% by weight or more were prepared as raw materials. Large flakes and bulk-shaped raw materials were cut and pulverized, etc. so as to be uniform with no variation in composition depending on the sampling location in the alloy after melting, and were reduced to a size of 3 mm or less. Next, a predetermined amount of these raw materials was weighed into a graphite crucible for a high-frequency melting furnace.

  The crucible containing the raw material was placed in a high-frequency melting furnace, and nitrogen was flowed at a flow rate of 0.7 L / min or more per 1 kg of the raw material in order to suppress oxidation. In this state, the melting furnace was turned on to heat and melt the raw material. When the metal began to melt, it was stirred well with a mixing rod and mixed uniformly so as not to cause local compositional variations. After confirming sufficient melting, the high frequency power supply was turned off, the crucible was quickly removed, and the molten metal in the crucible was poured into the solder mother alloy mold. A mold having the same shape as that generally used in the production of a solder alloy was used.

  In this way, the solder mother alloy of Sample 1 was produced. Further, solder mother alloys of Samples 2 to 21 were produced in the same manner as Sample 1 except that the mixing ratio of the raw materials was changed. The compositions of the solder mother alloys of Samples 1 to 21 were analyzed using an ICP emission spectroscopic analyzer (SHIMAZU S-8100). The analysis results are shown in Table 1 below.

  Next, it processed into the wire shape with the extruder as follows with respect to each of the solder mother alloys of the samples 1-21 of the said Table 1, and the wire workability of the solder mother alloys of the samples 1-21 was evaluated. Moreover, the wettability (joinability) evaluation, the EPMA line analysis (evaluation of Ni diffusion prevention effect), and the heat cycle test were performed with respect to the solder alloy of each sample processed into the wire shape. In addition, since evaluation of solder wettability, bondability, etc. does not usually depend on the shape of the solder, it may be evaluated by the shape of a wire, a ball, a paste, etc. In this embodiment, it is molded into a wire shape. evaluated.

<Evaluation of wire workability>
The solder mother alloys of Samples 1 to 21 shown in Table 1 above were set in an extruder, and a wire having an outer diameter of 0.80 mm was processed. Specifically, the extruder was heated in advance to a temperature suitable for the solder composition, and the solder mother alloy of each sample was set. The wire-like solder extruded from the extruder outlet is still hot and easily oxidized, so the outlet of the extruder has a sealed structure and an inert gas is allowed to flow through the inside. As a result, the oxygen concentration was lowered as much as possible to prevent oxidation.

  In this state, the pressure was increased by hydraulic pressure, and the solder mother alloy was extruded into a wire shape. The wire extrusion speed was adjusted in advance so that the wire was not cut or deformed, and at the same time, the wire was wound at the same speed using an automatic winder. In this way, when processing into a wire shape and winding up 60 m with an automatic winder, “○” indicates that the wire was not disconnected once, “△” indicates that the wire was disconnected 1-3 times, “Δ”, 4 times or more The case where it was disconnected was evaluated as “×”.

<Wettability evaluation (bondability evaluation)>
This wettability evaluation was performed using the wire-shaped solder alloy obtained in the evaluation of the wire workability. First, a wettability tester (device name: atmosphere control type wettability tester) was started, a double cover was applied to the heater part to be heated, and nitrogen was flowed from four locations around the heater part (nitrogen flow rate: each 12 L / min). Then, the heater set temperature was set to 340 ° C. and heated.

  After the set heater temperature was stabilized at 340 ° C., a Cu substrate (plate thickness: about 0.70 mm) provided with a Ni plating layer (film thickness: 4.0 μm) on the surface was set in the heater part and heated for 25 seconds. . Next, the solder alloy of each sample was placed on a Cu substrate and heated for 25 seconds. After the heating was completed, the Cu substrate was picked up from the heater part, and once installed in a place where the nitrogen atmosphere next to it was maintained, it was cooled. After sufficiently cooling, the sample was taken out into the atmosphere and the joint between the solder alloy of each sample and the Cu substrate was visually confirmed. As a result of the confirmation, “X” indicates that the bonding was not possible, “△” indicates that the bonding was successful but the wetting spread was poor (the state where the solder was swelled), and “Weighted” where the bonding was possible (the solder was thinly spreading) ) Was evaluated as “◯”.

<EPMA line analysis (evaluation of Ni diffusion prevention effect)>
In order to confirm whether the Ni plating layer with a film thickness of 4.0 μm provided on the Cu substrate reacts with Bi in the solder alloy of each sample and does not become thin or Ni diffuses into Bi. Line analysis was performed. This analysis was performed using a Cu substrate to which a solder alloy of each sample obtained in the same manner as the wettability evaluation was bonded. First, a Cu substrate to which a solder alloy of each sample was bonded was embedded in a resin, and was polished using a polishing machine and then gradually polished from a polishing paper, and finally buffed. Thereafter, line analysis was performed using EPMA (device name: SHIMADZU EPMA-1600) to examine the diffusion state of Ni and the like.

  The measurement method is as follows. When a cross section of a Cu substrate provided with a Ni plating layer to which a solder alloy of each sample is bonded is viewed from the side, the bonding side of the Cu substrate and the Ni plating layer is the origin and the solder side is the positive direction of the X axis It was. In the measurement, five points were arbitrarily measured and the average one was adopted. When the Ni plating layer reacts and becomes thin or Ni diffuses in the solder, “X” indicates that the Ni plating layer has almost the same thickness as the initial state, but Ni diffuses in the solder. The case where there was not was evaluated as "(circle)".

<Heat cycle test>
A heat cycle test was conducted to evaluate the reliability of solder joints. In addition, this test was done using Cu board | substrate with which the solder alloy of each sample obtained similarly to the said wettability evaluation was joined. First, with respect to the Cu substrate to which the solder alloy of each sample was bonded, cooling at −50 ° C. and heating at 125 ° C. were taken as one cycle, and this was repeated for a predetermined cycle.

  Thereafter, the Cu substrate to which the solder alloy of each sample was bonded was embedded in the resin, cross-section polishing was performed, and the bonded surface was observed by SEM (device name: HITACHI S-4800). The case where the joint surface was peeled or cracked in the solder was indicated as “×”, 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 above evaluation are shown in Table 2 below.

  As can be seen from Table 2 above, all of the solder mother alloys of Samples 1 to 15 that satisfy the requirements of the present invention obtained good results in all evaluation items. Specifically, even if it was processed into a wire, it could be automatically wound without being cut. In addition, the wettability is also good. Particularly, the samples 9, 10 to which P is added and the samples 11, 12, and 15 to which Ag is added are very good, and the solder touches the Cu substrate at the moment when it touches the Cu substrate. It spread thinly and wet. Furthermore, it was confirmed that Ni diffusion was suppressed. Good results were also obtained in a heat cycle test, which is a test related to reliability, and no defect appeared even after 500 times.

  On the other hand, the solder mother alloys of Samples 16 to 21 of Comparative Examples not satisfying the requirements of the present invention resulted in undesirable results in any of the evaluation items. Specifically, at the time of wire processing, all samples were disconnected once or more, and samples 18 and 19 could not be bonded to the Cu substrate. Further, in sample 20, Ni diffusion was observed. In the heat cycle test, defects occurred in all samples up to 300 times.

Claims (3)

The Sn contained 4.8 wt% or less than 0.01 mass% (excluding 0.5% by mass or less) The A l 0.03 wt% to 1.0 wt% has free and 0 mass P % Pb-free solder alloy characterized by containing 0.5% to 0.5% by mass and the balance being Bi except for inevitable impurities. The Pb-free solder alloy according to claim 1, further comprising Cu in an amount of 0.18% by mass to 1.8% by mass. In the case of Ag, one or more of Ag or Zn is contained in an amount of 0.01% by mass to 3.0% by mass, and in the case of Zn, 0.01% by mass or more and less than 0.4% by mass. The Pb-free solder alloy according to claim 1 or 2 , wherein