JP2013022638A - Lead-free solder alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pb-free solder alloy which includes a high solidus temperature and an appropriate liquidus temperature as a solder material, has excellent wettability and solves specific problems such as Ni-Bi excessive response and Ni diffusion in Bi-based solder.SOLUTION: The lead-free solder alloy contains 0.02 mass% or more and 3.00 mass% or less of Al, 0.001 mass% or more and 1.000 mass% or less of Ge and the balance of Bi with unavoidable impurities. The lead-free solder alloy may be further includes at least one of Zn, P, Ag and Cu.

Description

  The present invention relates to a Bi-based lead-free solder alloy containing Al and Ge.

In recent years, regulations on chemical substances that are harmful to the environment have become more and more stringent. These regulations can be applied to solder materials used for bonding semiconductor packages to printed circuit boards and bonding semiconductor elements to substrates such as lead frames. It is no exception for the solder material to be used.
For example, Pb has been used as a main component for solder materials for a long time, but Pb has already been a regulated substance under the Rohs Directive. For this reason, development of solder containing no Pb (Pb-free solder or lead-free solder) has been actively conducted.
Solder used when bonding a semiconductor element to a substrate is 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 solder, Pb-free solder mainly composed of Sn has been 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 lead-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, various proposals have been made regarding high-temperature solder materials that do not contain Pb. For example, Patent Document 3 describes 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 describes a lead-free solder alloy in which a binary eutectic alloy is added to a eutectic alloy containing Bi and an additional element is further added. Although it is a solder, it has been shown that the liquidus temperature can be adjusted and variations can be reduced. Furthermore, Patent Document 5 describes a lead-free solder alloy in which Cu, Al, and Mn are added to Bi or Cu or Ni is added. When these lead-free solder alloys are used to mount a power semiconductor element having a Cu layer on the surface thereof on an insulating substrate having the Cu layer formed on the surface, unnecessary reaction products are formed at the joint interface with the solder. It is described that the occurrence of defects such as cracks can be suppressed.

Further, in Patent Document 6, in 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 mass%, Cu: 0.1-0.3 mass%, In: 0.1-0.5 mass%, Sb: 0.1-3.0 mass%, and Zn: 0 A lead-free solder composition comprising 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 an accessory component. It is disclosed. Further, it is described that this 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.
Patent Document 9 describes a bonding material containing Bi as a main component and containing 0.2 to 0.8% by weight of Cu and 0.02 to 0.2% by weight of Ge. Since this bonding material has a melting temperature of 270 ° C. or more, it is suitable for use in a small semiconductor package such as a chip inductor, and the low wettability of the Bi—Cu alloy, that is, oxidation of the bonding material is Ge. There is a description that it is suppressed by.

Japanese Patent Application Laid-Open No. 11-077366 Japanese Patent Laid-Open No. 08-215880 JP 2002-160089 A JP 2006-167790 A JP 2007-281212 A Japanese Patent No. 3671815 JP 2004-025232 A JP 2007-181880 A Japanese Patent No. 3886144

Although the development of high-temperature solder materials that do not contain Pb has progressed, no actual solder material having acceptable characteristics in terms of practical use has yet been found. That is, since a material having a relatively low heat-resistant temperature such as a thermoplastic resin or a thermosetting resin is generally used for a printed circuit board, the working temperature needs to be less than 400 ° C., preferably 370 ° C. or less. . However, for example, in the Bi / Ag brazing material described 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 400 to 700 ° C. or more, and the joining is performed. This will exceed the heat resistance temperature of the printed circuit board.
In addition, the characteristics generally required for high-temperature solder include high solidus temperature, moderate liquidus temperature, high durability against low and high temperature heat cycles, good thermal stress relaxation characteristics, and good wettability. and so on. However, in the case where the main component of the solder is Bi, in addition to these various characteristics, it is necessary to solve a problem peculiar to the Bi-based solder such as an excessive reaction between Bi and Ni described below.

That is, the reaction between Bi and Ni means that when a Ni layer is formed on the surface of the semiconductor element in order to improve the bondability with the solder, this Ni layer reacts abruptly with Bi contained in the solder and Ni and Bi. This is a problem that a brittle alloy is generated and the Ni layer is broken or peeled off and diffuses into Bi, so that the bonding strength is remarkably lowered. A layer such as Ag or Au may be provided on the Ni layer, but Ag and Au are intended to prevent oxidation of the Ni layer and improve wettability, so that it diffuses into the solder and the reaction between Ni and Bi. There is almost no effect of suppressing.
Furthermore, Bi is difficult to be alloyed with other metals, so that the wettability is poor. Ni also adds an element for suppressing diffusion, so that the wettability may be further reduced, and improvement of this wettability is also a major issue.

As described above, there are two major problems in the high-temperature Bi-based solder alloy containing no Pb. That is, the first problem is that when a Ni layer is present on a semiconductor element or a substrate when the semiconductor element and the substrate are bonded, Bi in the solder alloy reacts with Ni to form a brittle alloy and Ni is a Bi solder. It is necessary to suppress the excessive reaction between Bi and Ni and the diffusion of Ni into Bi, and the second problem is caused by the difficulty of alloying with other elements of Bi. It is to improve the poor wettability.
In the above-described high-temperature solder alloys containing Bi as a main component in Patent Documents 4 to 9, it has been difficult to solve the above problems. For example, in patent document 5, the case where the joining surface with a solder is a Ni layer is described as a comparative example, and a solder alloy in which Cu—Al—Mn, Cu, or Ni is added to Bi has a large amount at the joining interface. It is described that Bi ₃ Ni is formed and numerous voids are observed around it. It is also described that this Bi ₃ Ni has a very brittle property, and it has been confirmed that it is difficult to obtain reliability with respect to heat cycles under severe conditions.

In the solder composition described in Patent Document 6, for example, even if Sn is contained in an amount of 0.5 mass% or more and Zn is contained in an amount of 3.0 mass% or more, the reaction between Bi and Ni and the diffusion of Ni into Bi are not caused. It was not possible to suppress it, and it was confirmed by experiments that the bonding strength was low and could not be put into practical use.
In the Pb-free solder composition described in Patent Document 7, a brittle alloy of NiBi ₃ is formed when a large amount of Bi is present, as can be seen from the Bi-Ni binary phase diagram. 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.
Note that Patent Document 4, Patent Document 8, and Patent Document 9 do not mention anything about the problem of Ni diffusion into Bi and its prevention measures. In particular, Patent Document 8 discloses Bi—Ag, Bi—Cu, Bi—Zn, etc., but the Bi—Ag system is described in spite of the need for Ni diffusion countermeasures. Not. In addition, regarding the Bi—Cu system, the present inventor has confirmed that a Cu phase having a high melting point precipitates and a problem arises in the bonding property because the amount of Cu dissolved in Bi is very small. No countermeasures are described for this. Furthermore, in Bi—Zn system, wettability decreases due to highly reducible Zn, and it can be presumed that it is difficult to join a semiconductor package or the like. However, this is not also mentioned, and description of excessive reaction between Ni and Bi. Nor.

Thus, the present situation is that the excessive reaction of Ni and Bi cannot be suppressed first. Further, it is necessary to improve the poor wettability of Bi, but when various elements are added to Bi, the wettability may be further reduced. For example, in a Bi—Zn system or the like, Zn is easily oxidized, and depending on the Zn content, the wettability is drastically reduced and bonding may not be possible.
In Patent Document 9, a Bi—Cu alloy containing 0.2 to 0.8 wt% Cu is an excellent bonding material in that it does not melt at a temperature of less than 270 ° C., but it has low wettability. As a countermeasure, Ge was added as an element preferentially oxidized over Bi based on the idea that it can be suppressed by adding a small amount of an element preferentially oxidized over Bi to the Bi-Cu alloy. It is described that the formation of oxide is suppressed in the sample. Thus, depending on the elements contained in order to improve various characteristics, there are cases where wettability can be improved by adding Ge which is more easily oxidized than Bi.
The present invention intends to provide an excellent solder material by selecting so as to satisfy the necessary characteristics of an element for suppressing excessive reaction of Ni—Bi and an element for improving wettability. In the case of the alloy described in Patent Document 9, it is difficult to suppress Ni diffusion as described above, and the current situation is that the problem of excessive reaction between Ni and Bi cannot be solved.

As described above, there are two problems with the high-temperature Bi-based solder alloy that does not contain Pb. That is, the first is a problem related to Ni-Bi excess reaction, and the second is a problem of poor wettability due to the difficulty of alloying with other elements of Bi, and these problems need to be solved.
The present invention has a high solidus temperature and an appropriate liquidus temperature as a solder material, and has not only good wettability but also a unique problem in Bi-based solder, that is, Ni-Bi excess reaction. An object of the present invention is to provide a Pb-free solder alloy that can suppress Ni and Ni diffusion and has excellent wettability.

1st invention of this invention contains 0.02 mass% or more and 3.00 mass% or less Al, and 0.001 mass% or more and 1.000 mass% or less Ge, and remainder consists of Bi and an unavoidable impurity. It is a lead-free solder alloy.
In the present invention, it is particularly preferable that the Al content is 0.03% by mass to 1.00% by mass and the Ge content is 0.003% by mass to 0.500% by mass.
Since the solder alloy of the present invention contains Al, an excessive reaction between Ni and Bi can be suppressed. Moreover, since Ge is included, it has the advantage that wettability improves.
The second invention of the present invention contains 0.03 mass% or more and 1.00 mass% or less of Al, 0.003 mass% or more and 0.500 mass% or less of Ge, and further 0.01 mass% or more and 3. It is a lead-free solder alloy that contains at least one of Ag of not more than 00 mass% or Cu of not less than 0.01 mass% and not more than 1.8 mass%, with the balance being Bi and unavoidable impurities.

According to a third aspect of the present invention, 0.02 mass% to 3.00 mass% Al, 0.2 mass% to 13.5 mass% Zn, and 0.001 mass% to 1.000 mass%. It is a lead-free solder alloy containing not more than% Ge and the balance being Bi and inevitable impurities.
Zn, like Al, further promotes the effect of suppressing excessive reaction between Ni and Bi.
In a fourth aspect of the present invention, 0.03% by mass to 1.00% by mass of Al, 0.4% by mass to 5.0% by mass of Zn, 0.003% by mass to 0.50% by mass. It contains the following Ge, and further contains at least one of 0.01 mass% or more and 3.00 mass% or less of Ag or 0.01 mass% or more and 1.8 mass% or less of Cu, with the balance being Bi and inevitable Lead-free solder alloy consisting of impurities.
By adding Ag or Cu, wettability can be improved and bondability can be improved.

Furthermore, in this invention, less than 0.500 mass% P may be included.
Inclusion of P improves wettability and further improves bondability.

  According to the present invention, it has a heat resistance temperature of about 260 ° C. or more, and can suppress excessive reaction between Ni and Bi and diffusion of Ni into Bi, which is a problem of Bi-based solder, and at the same time, improves wettability In addition, an excellent Pb-free solder alloy for high temperature can be provided.

  In general, high-temperature Pb-free solder alloys need to withstand a reflow temperature of about 260 ° C., but in the case of Bi-based solder alloys containing Bi as a main component, two more specific problems need to be solved. is there. The first problem is to suppress an excessive reaction between the Ni layer provided on the semiconductor element or the substrate and Bi, and the second problem is to improve and improve the wettability of the Bi-based solder material. In particular, as can be seen from the various Bi-based phase diagrams, it is often difficult to form an alloy with other metals, making it difficult to alloy. For this reason, since Bi is an element with very poor wettability, the second problem is an important problem to be solved.

As a result of investigating various elements added to Bi for the first and second problems, it was found that the addition of Al and Zn is effective for suppressing the excessive reaction of Ni and Bi. Regarding the improvement of the wettability of the alloy, it was found that Ge is preferentially oxidized by adding a small amount of Ge to obtain good wettability. That is, the Pb-free solder alloy of the present invention contains Bi as a main component, and contains at least one of Zn or Al, essentially containing Al, and containing Ge. Moreover, in order to further improve the wettability and the bondability, at least one of Ag, Cu and P can be further added and contained.
Next, the essential elements constituting the Pb-free solder alloy of the present invention and optional elements contained as necessary will be specifically described.

<Bi>
Bi is the main component of the Pb-free solder alloy of the present invention. Bi belongs to the Va group element, and its crystal structure is a trigonal crystal (rhombohedral crystal) with a low symmetry and is a brittle metal. Therefore, when a tensile test or the like is performed, it can be easily seen that the fracture surface is a brittle fracture surface. Furthermore, Bi hardly dissolves with other metals and is difficult to alloy. For this reason, the wettability to the substrates is poor.
The reason why Bi is selected as the main component from the Va group elements is that Bi has a melting point of 271 ° C. and exceeds the reflow temperature of about 260 ° C., which is the use condition of high-temperature solder, and other than Bi. This is because the Va group element is classified as a semimetal or a nonmetal and is more brittle than Bi.
In the Pb-free solder alloy of the present invention, each element described later is included for overcoming the wettability of Bi and suppressing the reaction between Ni and Bi. However, the type and amount of each additive element vary depending on the characteristics to be improved and the degree thereof. . Therefore, the Bi content inevitably changes depending on the type and amount of the additive element.

<Al>
Al is one of the second element groups of the Pb-free solder alloy of the present invention and is an essential element. Since Al is highly reducible, when it is contained in a small amount in Bi-based solder, it oxidizes itself or reduces the Bi matrix, thereby improving the wettability of the solder. Furthermore, Al is a very soft metal and has an effect of improving workability. However, since Al has its strong reducing ability, if the content is increased, a strong oxide film is formed on the solder surface to drastically reduce wettability, and the amount of Al dissolved in Bi is very small. For this reason, it is segregated and the reliability is lowered. In order to fully exhibit the reducibility and flexibility of Al, it is necessary to give sufficient consideration to its content.
That is, the optimal Al content is 0.02 mass% or more and 3.00 mass% or less. If it is less than 0.02% by mass, the effect of Al is not exhibited. If it exceeds 3.00% by mass, good bonding becomes difficult for the above reason. In particular, if the Al content is 0.03% by mass or more and 1.00% by mass or less, the effect of reducing properties and flexibility can be exhibited in a balanced manner, which is further preferable.

<Ge>
Ge is an essential third element in the Pb-free solder alloy of the present invention. In the solder composition of the present invention, Ge exhibits the effect of improving the wettability and also plays the role of improving the brittleness of Bi. In other words, Ge hardly forms an alloy with Bi, Zn, or Al, but when cooled and solidified after the solder is melted, Ge in the molten solder first precipitates, which serves as a nucleus and contributes to the refinement of the solder crystal. And improve workability. On the other hand, when the solder contains Zn, Ge forms a eutectic alloy with Zn, and refines the crystal especially in the vicinity of the eutectic composition of Zn-Ge (Zn-6 mass% Ge) to improve workability. Has an effect.

The effect of Ge is not limited to improving workability. In the solder composition of the present invention, the effect of improving wettability is the most expected effect for Ge. By including Ge, the wettability is remarkably improved. The reason for this is as follows.
That is, since Ge is easier to oxidize than Bi, it oxidizes itself to prevent oxidation of the solder base material. In addition, Ge has a lower specific gravity than Bi (specific gravity: Ge = 5.4, Bi = 9.8). This is because it is easy to be exposed on the solder surface during melting, and a reducing effect can be exhibited with a small amount. In other words, if it is easy to be exposed on the surface of the molten solder like Ge and the effect can be exerted with a small amount of content, there is a concern that the residual amount in the solder will increase so much that other metals and brittle intermetallic compounds are generated. There is no.

  The content of Ge having such an effect of improving wettability and workability is preferably small, specifically 0.001% by mass or more and 1.000% by mass or less, preferably 0.003% by mass. % Or more and 0.500 mass% or less. As already stated, the role of Ge is to improve the wettability due to the property of being easily exposed on the surface of the molten solder and the reduction effect, the core of solder crystal refinement, and Zn as one of the additive elements. Or refinement in the vicinity of the eutectic composition. For this reason, it is not necessary to add a large amount, but if the Ge content is less than 0.001% by mass, the content is too small to obtain these effects. Further, when the content exceeds 1.000 mass%, the nucleus of Ge itself becomes large and does not crystallize, deviates greatly from the eutectic composition with Zn, or the Ge oxide film becomes too thick. Therefore, it is not preferable. More preferably, it is 0.003 mass% or more and 0.500 mass% or less, and if it is in this range, the excellent effect of Ge is easily exhibited.

<Zn>
Zn, together with Al, is one of the second element groups of the Pb-free solder alloy of the present invention, and is an element having the same important effect as Al that suppresses excessive reaction between Ni and Bi, and supplements the effect of Al. Therefore, add as necessary. However, Zn has a high vapor pressure, and it is easy to cause compositional variation compared to Al or the like, or the working environment at the time of manufacture is deteriorated. Therefore, it may be added in a form that assists the effect of Al. It is an element that is not essential. Furthermore, Zn is harder to oxidize than Al, but depending on the content, solder wettability is lowered. It is speculated that this is because Zn is more reactive with oxygen than Al. In any case, since improvement of wettability is one of the problems, Zn having the possibility of reducing wettability is not added if it is not necessary to add it. Naturally, Zn is more reactive with Ni than Bi and has a Bi-Ni reaction suppressing effect, so it is better to add Zn if wettability and manufacturing problems do not occur. .

The effect of Zn suppressing the excessive reaction between Bi and Ni and suppressing the diffusion of Ni into Bi will be described in more detail. This effect is due to the fact that Zn is more reactive than Bi in the reaction with Ni, and a thin Zn—Ni layer is formed on the surface of the Ni layer, which acts as a barrier to suppress excessive reaction between Ni and Bi. . As a result, a brittle Bi—Ni alloy is not generated, and further, Ni does not diffuse into Bi, so that strong bondability can be realized.
In addition, Zn can also be expected to improve the workability of Bi. That is, by adding Zn to Bi, the solder composition can be microcrystallized to overcome the brittleness of Bi, and the workability is improved by dissolving Zn in Bi. In particular, if Zn is added in a larger amount than the eutectic point with Bi, a Zn-rich phase is expressed, and the workability is further improved.

The amount in the case of containing Zn is generally 0.2% by mass or more and 13.5% by mass or less although it depends on the thickness of the Ni layer provided on the semiconductor element or the substrate, the reflow temperature, the reflow time, and the like. The reason is that when the Zn content is less than 0.2% by mass, the reaction between Ni and Bi and the diffusion of Ni into Bi are not sufficiently suppressed, the reaction between Ni and Bi, and the Ni into Bi. This is because Zn is consumed for suppressing diffusion and good workability cannot be obtained.
On the other hand, when the Zn content exceeds 13.5% by mass, the liquidus temperature exceeds 400 ° C., and thus good bonding cannot be performed. Further, in the vicinity of the eutectic composition of Bi—Zn, that is, in the range where the Zn content is 0.4 mass% or more and 5.0 mass or less, various properties such as Ni diffusion suppression effect, workability improvement effect, melting point, etc. Since it will be in the state where the balance was taken, it is still more preferable.

<P>
P is added as necessary, and has the effect of further improving the wettability and bondability of the Pb-free solder alloy of the present invention. The reason why the effect of improving wettability is increased by the addition of P is that P is highly reducible and is oxidized by itself to suppress oxidation of the solder alloy surface. Of course, if the wettability of the solder is at a level where there is no problem by containing Ge, there is no need to dare to contain P. P has a strong reducibility and has a very excellent surface that it vaporizes after being oxidized and does not remain in the solder. On the other hand, it is necessary to devise stable incorporation in the Bi-based solder. That is, since P is easily ignited, for example, even if the solder raw material is dissolved in an inert gas, it reacts with a slight amount of oxygen and is difficult to remain stably in the solder. Furthermore, P has a specific gravity smaller than that of metals such as Bi, and floating on the surface in molten metal also leads to the ease of ignition, which makes it difficult to use P in solder. Therefore, Ge that is easy to be contained in such solder or has less composition variation is easier to use. Of course, in the case of P, if it is in an appropriate amount range even if the content varies, most of it will be vaporized at the time of joining, so there is often no problem, and P is cheaper in terms of cost. It is advantageous. Therefore, depending on the situation, it may be contained when P is more beneficial.

Furthermore, as an advantage of containing P, an effect of reducing the generation of voids at the time of soldering can be given. That is, as described above, since P easily oxidizes itself, the oxidation proceeds more preferentially than Bi, which is the main component of the solder, than Zn. As a result, it is possible to prevent the surface oxidation of the solder mother phase and to join without enclosing bubbles, and to ensure wettability, so that good joining is possible and void formation hardly occurs.
In addition, since P is very reducible as described above, the effect of improving wettability is exhibited even when a small amount is added. On the other hand, when the amount exceeds a certain amount, the effect of improving wettability does not change, and excessive addition may cause P oxide to be generated on the solder surface, or P to form a brittle phase and become brittle. is there. Therefore, when adding P, it is preferable that the addition amount is a trace amount.
Specifically, the P content in the Pb-free solder alloy of the present invention is preferably 0.500% by mass or less. If the P content exceeds 0.500% by mass, the P oxide may cover the solder surface and conversely reduce wettability. Furthermore, since P has a very small amount of solid solution in Bi, if the content is large, brittle P oxide is segregated, and reliability is lowered. In particular, it has been confirmed that wire breakage is likely to occur when processing into a wire. However, if the P content is less than 0.001% by mass, the expected reduction effect may not be obtained.

<Ag>
Ag is one of the fourth element groups of the present invention, and as with P described above, the wettability and bondability of the solder alloy can be further improved by adding as necessary. As can be seen from the fact that Ag is formed in the uppermost layer of a semiconductor package or a Cu substrate, the effect of improving wettability is great. In the present invention, addition of Ag is aimed at improving wettability. That is, Ag is difficult to oxidize and improves wettability by preventing oxidation of the solder surface. Therefore, when the wettability is insufficient, the wettability can be improved by adding Ag.
On the other hand, since Ag promotes an excessive reaction between Bi and Ni, the added amount must be sufficiently considered. Ag improves wettability and facilitates alloying of the joint surface between the solder and the semiconductor package, etc., but it is considered that excessive reaction between Bi and Ni is also facilitated. However, if it is contained in an appropriate amount, wettability can be improved at the same time while an excessive reaction between Bi and Ni is suppressed.

  Specifically, the Ag content in the Pb-free solder alloy of the present invention is preferably 3.00% by mass or less. When the Ag content exceeds 3.00% by mass, even if a large amount of Zn or Al is added, the reaction between Bi and Ni proceeds to form a brittle Bi—Ni alloy, or Ni is contained in Bi. There is a risk of spreading. However, if the Ag content is less than 0.01% by mass, the expected reduction effect may not be obtained.

<Cu>
Cu is one of the fourth element groups of the present invention together with Ag, and the effect thereof is the same as that of Ag. By adding it as necessary, the wettability and bondability of the solder alloy can be further improved. . Cu is harder to oxidize than Bi. Furthermore, since the specific gravity is smaller than Bi (specific gravity: Cu = 8.9, Bi = 9.8) and the amount of solid solution in Bi is small, it exists relatively near the solder surface when solder is melted. These factors suppress the oxidation of the solder surface and improve the wettability. The Cu content is suitably 0.01% by mass or more and 1.8% by mass or less. If the Cu content is less than 0.01% by mass, the Cu content is too small to exhibit the effect of Cu. If the Cu content exceeds 1.8% by mass, segregation or the like occurs and the bonding property is deteriorated.

Next, the present invention will be specifically described with reference to examples and comparative examples.
First, Bi, Zn, Al, Ge, Ag, Cu and P having a purity of 99.9% by mass or more were prepared as raw materials. Large flakes and bulk-shaped raw materials were cut and crushed so as to be uniform, taking into account that there was no variation in composition depending on the sampling location in the alloy after melting, and the size was reduced to 3 mm or less.

  Next, a predetermined amount was weighed from each of these raw materials and put into a graphite crucible for a high-frequency melting furnace. The crucible was placed in a high-frequency melting furnace, and nitrogen was flowed at a flow rate of 0.7 liter / min or more per kg of 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 raw material began to melt, it was thoroughly stirred 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 a solder alloy mold. As the mold, a mold having the same shape as that generally used in the production of a solder alloy was used.

  Thus, each solder alloy of samples 1-22 was produced. The composition of each of the obtained solder alloys of Samples 1 to 22 was analyzed using an ICP emission spectroscopic analyzer (manufactured by SHIMAZU, S-8100). Furthermore, the solidus temperature of each sample was measured with a differential scanning calorimeter (manufactured by Shimadzu Corporation, model: DSC-60). The analysis results obtained are shown in Table 1 below.

Next, for each of the solder alloys of Samples 1 to 22 shown in Table 1 above, evaluation of wire processability, wettability evaluation (bondability evaluation), EPMA line analysis (evaluation of Ni diffusion prevention effect), In addition, a heat cycle test and an optimum reflow temperature were measured.
Note that the evaluation of solder wettability and bondability does not usually depend on the shape of the solder, so any shape such as a wire, a ball, or a paste may be evaluated. did.

<Evaluation of wire workability>
Each solder alloy of Samples 1 to 22 shown in Table 1 above was processed into a wire having an outer diameter of 0.70 mm by using an extruder preheated to a temperature suitable for each solder composition and increasing the pressure with hydraulic pressure. 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 by flowing an inert gas inside it, the oxygen concentration is lowered as much as possible to oxidize. I didn't go forward. The extrusion speed of the wire was set to a normal speed (17 m / min) that was adjusted in advance so that the commercially available solder wire was not cut or deformed, and at the same time, it was wound at the same speed using an automatic winder. .
In this way, when processing into a wire shape and winding 70 m with an automatic winder, “○” indicates that the wire has not been broken once, “Δ” indicates that the wire has been broken 1-3 times, “Δ”, 4 times or more The wire workability was evaluated with “×” when the wire was disconnected.

<Wettability evaluation (bondability evaluation)>
Wetability tester (Device name: Atmosphere controlled wettability tester was started, a double cover was applied to the heater part, and nitrogen was flowed from four locations around the heater part (nitrogen flow rate: 12 l / min each) After that, the heater was set at a heater set temperature of 340 ° C. After the heater temperature set at 340 ° C. was stabilized, a Cu substrate having a Ni plating layer (film thickness: 0.4 μm) on the surface (plate thickness: about 0) .70 mm) was set in the heater section and heated for 25 seconds.
Next, each solder alloy was placed on the heated Cu substrate and heated for 25 seconds. After the heating was completed, the Cu substrate was taken up from the heater part, and once installed in a place where the nitrogen atmosphere next to it was kept, it was cooled. After sufficiently cooling, it was taken out into the atmosphere and the joint portion was visually confirmed.
According to visual confirmation, “x” indicates that the solder alloy could not be bonded to the Cu substrate, “△” indicates that the solder alloy was bonded but the wet spread was poor (the solder was swelled), and was able to be bonded and spread wet ( The state where the solder was thinly spread and wet) was evaluated as “◯”, and the wettability was evaluated.

<EPMA line analysis (evaluation of Ni diffusion prevention effect)>
In order to confirm whether the Ni plating layer provided on the Cu substrate was thinned by reacting with Bi or Ni was not diffused into Bi, line analysis by EPMA was performed. In addition, EPMA line analysis was performed using Cu board | substrate with which the solder alloy was joined among the samples 1-22 obtained similarly to the said wettability evaluation.
That is, except for samples 17 and 19 obtained in the same manner as in the case of the wettability evaluation, each Cu substrate to which the solder alloys of samples 1 to 22 are bonded is embedded in a resin, and finer ones in order from coarse abrasive paper by a polishing machine. And finally buffed. Thereafter, line analysis was performed using EPMA (manufactured by SHIMADZU, EPMA-1600) to examine the diffusion state of Ni and the like.
The measurement method was such that when the cross section of the Cu substrate to which the solder alloy was bonded was viewed, the bonding surface of the Cu substrate and the Ni layer was set to the origin 0, and the solder side was set to the plus direction. In the measurement, five points were arbitrarily measured and the average one was adopted. From this measurement result and the state of diffusion of Ni into the solder, the case where the Ni layer reacts and is clearly thinned, or Ni is diffused into the solder is “x”, the thickness of the Ni layer is the initial state The case where Ni was not diffused in the solder was evaluated as “◯”, and the Ni diffusion preventing effect was evaluated.

<Heat cycle test>
In order to evaluate the reliability of solder joints, a heat cycle test was conducted. In addition, this heat cycle test was performed using Cu board | substrate with which each solder alloy of Samples 1-21 except Sample 17, 19, 21 obtained similarly to the said wettability evaluation was joined. That is, with respect to each Cu board | substrate with which the solder alloy was joined, -50 degreeC cooling and +125 degreeC heating were made into 1 cycle, and this cycle was repeated 300 times and 500 times.
After completion of the heat cycle test, each Cu substrate to which the solder alloy was bonded was embedded in a resin, cross-section polishing was performed, and the bonded surface was observed with SEM (manufactured by HITACHI, S-4800).
The case where peeling occurred on the joint surface 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 indicated as “◯”.

  As can be seen from the above results, each of the solder alloys of Samples 1 to 15 according to the present invention exhibits good characteristics in each evaluation item. That is, even if it was processed into a wire, it could be automatically wound without being cut and had good workability. In addition, each of the solder alloys of Samples 1 to 15 had very good wettability, and thinly spread on the Cu substrate. Furthermore, there was no diffusion of Ni, and no defect appeared even after 500 times in the heat cycle test relating to reliability, and good results were obtained.

  On the other hand, each solder alloy of Samples 16 to 22 as a comparative example has undesirable results in any two or more of wire workability, wettability, EPMA line analysis (Ni diffusion prevention effect) and heat cycle test. It was. In particular, samples 18, 19, and 22 were very poor in wettability and could not be bonded to the Cu substrate, so EPMA line analysis and heat cycle tests were not performed. Also, in each of the solder alloys of Samples 16, 17, 20, and 21 that were subjected to the heat cycle test, defects occurred up to 300 times.

Claims (6)

  A lead-free solder alloy characterized by containing 0.02 mass% or more and 3.00 mass% or less of Al and 0.001 mass% or more and 1.000 mass% or less of Ge, with the balance being Bi and inevitable impurities .   The Al content is 0.03% by mass or more and 1.00% by mass or less, the Ge content is 0.003% by mass or more and 0.50% by mass or less, and the balance is composed of Bi and inevitable impurities. The lead-free solder alloy according to claim 1.   0.03 mass% or more and 1.00 mass% or less of Al, 0.003 mass% or more and 0.500 mass% or less of Ge, and 0.01 mass% or more and 3.00 mass% or less of Ag or 0.003 mass% or less. A lead-free solder alloy comprising at least one of Cu of 01 mass% or more and 1.8 mass% or less, the balance being Bi and inevitable impurities.   0.02% by mass to 3.00% by mass of Al, 0.2% by mass to 13.5% by mass of Zn, and 0.001% by mass to 1.000% by mass of Ge, with the balance being A lead-free solder alloy comprising Bi and inevitable impurities.   0.03% by mass to 1.00% by mass of Al, 0.4% by mass to 5.0% by mass of Zn, 0.003% by mass to 0.500% by mass of Ge, Lead containing at least one of Ag of not less than 01% by mass and not more than 3.00% by mass or Cu of not less than 0.01% by mass and not more than 1.8% by mass, the balance being made of Bi and inevitable impurities Free solder alloy.   The lead-free solder alloy according to any one of claims 1 to 5, characterized by containing P of less than 0.500 mass%.