JP2012024803A - Pb-FREE SOLDER ALLOY HAVING EXCELLENT STRESS RELAXATION PROPERTY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Pb-free solder alloy which has excellent thermal stress relaxation properties, and can suppress the reaction of Ni-Bi and Ni diffusion on joining of Ni-containing electronic parts and a substrate.SOLUTION: The Pb-free solder alloy essentially consists of Bi, comprises Sn by 1.6 to 10 mass%, does not comprise Zn by >13.5 mass% and does not comprise P by >0.5 mass%, and in which a liquid phase is present at ≥150°C. The Pb-free solder alloy may comprise at least either Zn or P, by ≥0.4 mass% in the case of Zn and by ≥0.001 mass% in the case of P.

Description

  The present invention relates to a solder alloy containing no 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. With regard to low-temperature solder, Pb-free solder alloys containing Sn as a main component have 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%. Pb-free solder alloy compositions containing up to 10% are 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

  Although the Pb-free solder material for high temperature has been developed by various organizations as described above, the actual situation is that a solder material having an acceptable characteristic in terms of practical use has not yet been found.

  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 will exceed the heat resistance temperature of electronic parts and substrates.

  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 properties, and good wetting spread. In the case where the main component of the solder alloy is Bi, it is necessary to solve the problem concerning the thermal stress relaxation characteristics and the problem concerning the reaction between Ni and Bi, which is a problem peculiar to the Bi-based solder. There is.

  First, problems related to thermal stress relaxation characteristics will be described. When high-temperature solder is used for joining high-power electronic components such as power transistors and power ICs, the amount of heat generated at the solder joints also increases, so it is always used in an environment where heating and cooling are repeated. Become. In general, Cu is used for a substrate of an electronic component, and its thermal expansion coefficient is about five times larger than the thermal expansion coefficient of Si used for a semiconductor element of the electronic component.

  Therefore, a thermal stress due to the difference in the thermal expansion coefficient is generated every time the heating and cooling are performed, and the solder material plays a role of relaxing the thermal stress. Since Bi is a semi-metal and very brittle and is not a material having excellent stress relaxation properties, it is important for Bi-based solders to improve this thermal stress relaxation property. However, Patent Documents 3, 4, 7, and 8 are silent about the thermal stress relaxation characteristics.

  Patent Document 5 discloses a technology in which a power semiconductor module is formed by joining two components with a Bi-based solder material, and a Cu layer is provided on a surface to be bonded to the Bi-based solder material of each component. These two parts are specifically a combination of a semiconductor element and an insulating part, or a combination of an insulating part and a heat sink. As an example of the heat sink, a Cu layer / Mo having Cu layers on both sides of the Mo layer. A layer / Cu layer stack is shown.

  Furthermore, this Cu / Mo / Cu laminate has a high thermal conductivity, effectively exerts its function as a heat sink, and the Cu / Mo / Cu laminate has a thermal expansion coefficient of about 4 ppm / K. It is described that as a result of being close to the value of the coefficient of thermal expansion of the power semiconductor element, no significant thermal stress occurs during the heating / cooling cycle, and no defects such as cracks and peeling occur.

  In other words, the technique of Patent Document 5 does not solve the problem of thermal stress with a solder material, but the thermal expansion of the heat dissipation plate by making the structure of the heat dissipation plate a Cu layer / Mo layer / Cu layer laminate. The coefficient is brought close to the thermal expansion coefficient of the power semiconductor element, so that no thermal stress is generated. However, a heat sink made of such a laminate is expensive because it has a complicated structure, and is a very disadvantageous technology in terms of economy.

  Patent Document 6 describes a stress relaxation during solidification. Specifically, in order to suppress damage to the substrate during soldering, it is effective to relieve stress during solder solidification, and as a measure to relieve stress during solder solidification, an alloy composition that does not shrink during solidification is used. To choose. In addition, a metal element that expands in volume during solidification is selected as an alloy composition that does not shrink during solidification, and Bi and Ga are listed as elements that expand in volume during solidification.

  However, even if it is expanded at the time of solidification, it is meaningless if the residual stress cannot be reduced, so it cannot be said that the problem of thermal stress is solved. That is, using a material that does not change in volume during solidification, for example, two kinds of metal elements that expand and contract, respectively, unless the volume expansion and volume contraction during the solidification are approximately the same, Stress remains after solidification due to shrinkage and expansion during solidification. As described above, by realizing a material that does not change its volume during solidification, it is possible to obtain a solder material that has low residual stress, excellent thermal stress relaxation by repeated cooling and heating, and high reliability. .

  Next, problems related to the reaction between Ni and Bi will be described. When a Ni layer is formed on the surface of the electronic component in order to enhance the bondability with the solder, this Ni layer reacts rapidly with Bi contained in the solder to produce a brittle alloy of Ni and Bi, The Ni layer may be broken or peeled and diffused into Bi, and the bonding strength may be significantly reduced. Therefore, Bi-based solder cannot be used as a practical material unless the problem of the reaction between Ni and Bi is solved. A layer of Ag, Au, or the like may be provided on the Ni layer. In this case, Ag or Au is intended to prevent oxidation of the Ni layer and improve wettability, so it quickly diffuses into the solder. There is almost no effect of suppressing Ni diffusion.

Also in Patent Document 5, the case where the bonding surface with the solder is not the Cu layer but the Ni layer is taken as a comparative example, and in the case of a solder alloy in which Cu—Al—Mn, Cu, or Ni is added to Bi, the bonding interface is used. A large amount of Bi ₃ Ni is formed, and a large number of voids are observed around it. Further, it is 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 a heat cycle under severe conditions.

  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.

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 Bi ₃ Ni alloy is produced. 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.

  In addition, Patent Document 4 and Patent Document 8 do not mention anything about the problem of Ni diffusion into Bi and measures to prevent it. In particular, Patent Document 8 discloses Bi-Ag system, Bi-Cu system, Bi-Zn system, etc., but the Bi-Ag system, in particular, requires measures against Ni diffusion. There is nothing to say about.

  Regarding Bi-Cu system, since the amount of Cu dissolved in Bi is very small, it has been confirmed that a Cu phase with a high melting point is precipitated and there is a problem in the bonding property. It is not done. Furthermore, in the Bi—Zn system, it is speculated that wettability decreases due to highly reducible Zn, and it is difficult to bond electronic parts and the like, but this is not mentioned, and description of the reaction between Ni and Bi. Nor.

  As described above, when an electronic component and a substrate are joined using a high-temperature Bi-based solder alloy that does not contain Pb, if Ni is present in the electronic component or the substrate, Bi and Ni react to form a brittle alloy. At the same time, Ni diffuses into the Bi solder. For this reason, suppressing the reaction between Bi and Ni and diffusion of Ni into Bi is an important issue that must be solved in high-temperature Pb-free Bi-based solder.

  The present invention not only has excellent characteristics such as high solidus temperature, good wettability, and good workability as a Bi-based solder, but also has excellent thermal stress relaxation properties, and further includes an electron containing Ni. An object of the present invention is to provide a Pb-free solder alloy capable of suppressing Ni-Bi reaction and Ni diffusion when joining components and substrates.

  In order to achieve the above object, the Pb-free solder alloy of the present invention is a Pb-free solder alloy containing Bi as a main component, containing 1.6% by mass to 10% by mass of Sn, and 13.5% of Zn. It does not contain more than mass%, P does not contain more than 0.5 mass%, and is characterized by the presence of a liquid phase at 150 ° C. or higher. In the Pb-free solder alloy of the present invention, at least one of Zn and P may be contained in an amount of 0.4% by mass or more in the case of Zn, and 0.001% by mass or more in the case of P.

  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. 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 a liquid phase is present at 150 ° C. or higher. A Bi-based solder alloy that has excellent thermal stress relaxation properties and can suppress reaction between the Ni layer of electronic components and Bi in the solder alloy and Ni diffusion into the solder alloy. Can be provided. As a result, the reliability of Pb-free soldering at a high temperature can be remarkably improved, and the industrial contribution is extremely high.

It is a schematic diagram which shows the state which soldered the solder alloy of each sample on Cu board | substrate which has Ni film | membrane in the case of EPMA line analysis.

  The Pb-free solder alloy composition of the present invention is a Pb-free solder alloy containing Bi as a main component, containing 1.6 mass% to 10 mass% of Sn, and Zn exceeding 13.5 mass%. And P does not contain more than 0.5% by mass. Thereby, a liquid phase comes to exist at 150 degreeC or more. In order to further improve the characteristics of this Pb-free solder alloy, at least one of Zn and P should be contained in an amount of 0.4% by mass or more in the case of Zn and 0.001% by mass or more in the case of P. preferable.

  There are two major problems to be solved in the practical application of a solder alloy containing Bi as a main component. The first problem is the problem that the thermal stress relaxation characteristics deteriorate due to the brittleness of Bi, and the second problem is the bondability and reliability at the solder joint due to the reaction between Bi and Ni. It is a problem that decreases.

  First, a problem on thermal stress relaxation characteristics due to brittleness, which is the first problem, will be described. In solder joining of electronic parts or the like, as described above, the volume of solder generally changes when the molten solder is cooled and solidified. Due to the volume change during solidification, residual stress is generated in solder, electronic components, and the like. Furthermore, when this electronic component is used as a device that uses a large amount of power, such as a power transistor or power IC, the amount of heat generated at the solder joints also increases. Will be used.

  When the material of the substrate constituting the electronic component is Cu, the thermal expansion coefficient is about five times larger than that of Si, which is the material of the semiconductor element of the electronic component. As a result, each time the heating and cooling are repeated, a thermal stress due to the difference between these thermal expansion coefficients occurs. The solder material plays a role of relieving the thermal stress. Therefore, unless the solder alloy has excellent relaxation properties against this thermal stress, it cannot withstand the thermal stress, the solder cracks, the electronic component peels off at the joint surface, or the electronic component cracks. Such a phenomenon will occur.

  Since Bi is very brittle, it is not a material having excellent stress relaxation properties. Therefore, improving the thermal stress relaxation characteristics is an important issue for Bi-based solders. The inventor has found that the problem of improving the thermal stress relaxation characteristics of the Bi-based solder can be solved by reducing the residual stress generated during solder solidification.

  In other words, by making a difference between the liquid phase temperature and the solid phase temperature during solidification after solder melting, unlike the case where a solid phase is generated instantaneously, a solid is gradually generated from a state where the entire solder is in a liquid state. Process is obtained. As a result, it has been found that the stress can be relaxed in the liquid phase portion, the residual stress is relaxed, and therefore, the material can exhibit a thermal stress relaxation characteristic that can sufficiently withstand heating and cooling. Moreover, it discovered that Sn was suitable as an element which produces | generates solid gradually from a liquid phase.

  That is, by adding Sn with a predetermined content to Bi, the liquidus temperature at which the solid begins to precipitate from the state where all the solder material is in the liquid state and the solid phase temperature at which the entire solder material becomes solid (Bi-Sn binary alloy) The solid phase temperature of 139 ° C.) can be made 90 ° C. or more, for example, and it has been confirmed that the residual stress during solidification can be absorbed by the liquid phase and the residual stress can be greatly reduced.

  By the way, if the difference between the liquid phase temperature and the solid phase temperature is made as described above, a liquid phase is generated at a temperature lower than that of the conventional high-temperature solder. There is a concern that it cannot be fixed and will be displaced. However, in this respect, by limiting the content of the liquid phase generated at the time of reflow by limiting the Sn content to 10% by mass or less, the electronic component does not move even at the time of reflow. Confirm that the problem does not occur.

  Next, a problem caused by the reaction between Bi and Ni, which is the second problem, will be described. In the case of Bi-based solder, Bi and Ni may react to generate a brittle phase and Ni may diffuse into Bi. More specifically, the Ni layer provided on the electronic component reacts with Bi contained in the solder to produce a brittle Bi-Ni alloy and Ni diffuses in a bulk state in Bi to make the joint brittle. It will make it. As a result, the bonding strength is extremely reduced, and the reliability of the device including the electronic substrate bonded with the solder alloy may be impaired. Therefore, unless this phenomenon is suppressed, Bi solder is unlikely to be a practically usable material.

  Therefore, as a result of examining various elements with respect to the reactivity with Ni, it was found that Zn and Sn react with the Ni layer preferentially over Bi and alloy. In addition, in the case of a binary alloy in which only Zn is added to Bi, the workability can be secured to some extent, but the knowledge that Zn has a strong reducibility, so that wettability is deteriorated and bondability may be lowered. It was. Furthermore, it was found that Sn and P are effective for the deterioration of wettability. Hereinafter, the elements contained in the Pb-free solder alloy of the present invention that can obtain the above-described characteristic effects and elements added as necessary will be described.

<Bi>
Bi is the first element, that is, the main component of the high-temperature Pb-free solder alloy of the present invention. Bi belongs to the Va group 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 surface is a brittle fracture surface. In other words, pure Bi is poor in ductile properties and, therefore, has poor thermal stress relaxation characteristics.

  In order to overcome such problems in thermal stress relaxation characteristics caused by brittle Bi, various elements described later are added. The kind and amount of the element to be added differ depending on how much of the characteristics such as brittleness of Bi are improved. Therefore, the content of Bi in the solder alloy inevitably changes depending on the type of element to be added and its content.

  The reason for selecting Bi from the group Va elements is that the group Va elements are classified as semi-metals and non-metals except for Bi, and are more brittle than Bi. In addition, Bi has a melting point of 271 ° C. However, it exceeds the reflow temperature of about 260 ° C., which is the use condition of the high-temperature solder, and even if an element described later is added within the scope of the present invention, it can substantially withstand reflow of 260 ° C. or higher.

<Sn>
Sn is a second element and an essential element of the high-temperature Pb-free solder alloy of the present invention. As described above, Sn plays two major roles and, in addition, has an effect of improving wettability. First, the mechanism for improving thermal stress relaxation characteristics, which is the first role, will be described. By adding a certain amount of Sn to the solder containing Bi as a main component, there is a difference between the solid phase temperature and the liquid phase temperature, thereby reducing the residual stress generated during solidification.

  More specifically, the difference between the liquid phase temperature and the solid phase temperature can be secured at 90 ° C. or more by setting the Sn content to be 1.6 mass% or more and 10 mass% or less. Thereby, the residual stress generated during solidification is reduced, and as a result, the thermal stress relaxation characteristics are improved. This effect is noticeable when the main component of the solder is a particularly brittle metal such as Bi.

  Next, the second effect of suppressing Ni diffusion will be described. This role is also an important role for making Bi-based solder practical. That is, Sn has a smaller ionic radius than Zn and easily causes ternary eutectic, and therefore has high reactivity with Ni. Thereby, it reacts with the upper surface of the Ni layer to produce a Sn—Ni alloy, and the reaction between Bi and Ni and the diffusion of Ni into Bi can be suppressed.

  Moreover, a relatively large number of diffusion sites are formed even if Sn is added in a small amount. When Zn described later is added, the presence of Sn promotes Zn-Zn-Ni alloying. As a result, a Zn—Ni alloy is also formed on the Ni layer, and Ni diffusion into Bi is suppressed more than when only Sn is added.

  The optimum Sn content is 1.6 mass% or more and 10 mass% or less. If this amount is less than 1.6% by mass, there is not much difference between the liquid phase temperature and the solid phase temperature, and it is difficult to obtain good thermal stress relaxation characteristics. On the other hand, when the content is more than 10% by mass, the ratio of the liquid phase generated at the time of reflowing at 260 ° C. is too large as described above, which causes a problem that the electronic component is shifted at the time of reflowing.

<Zn>
Zn is an element added as necessary, and the workability can be improved by adding Zn. This is because by adding Zn to Bi, a Zn-rich phase is generated, and thereby brittleness can be overcome, and Zn is dissolved in Bi to improve workability. In addition, when Zn is added more than the eutectic point with Bi, more Zn-rich phases are generated, so that the workability is further improved.

  Further, by adding Zn, it is possible to suppress the reaction between Bi and Ni, which is the most important effect, and to suppress diffusion of the Ni layer into the Bi-based solder. This is because, like Sn, 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.

  Furthermore, the liquidus temperature can be adjusted by adding Zn. For example, in a Bi-Sn binary alloy containing 10 mass% of Sn, the liquidus temperature is about 230 ° C because the difference between the liquidus temperature and the solidus temperature is 90 ° C. The temperature can be increased to 260 ° C. or higher.

  The optimum content of Zn that exhibits such excellent effects is generally 0.4 mass% or more and 13.5 mass% or less, although it depends on the thickness of the Ni layer, the reflow temperature, the reflow time, and the like. If the Zn content is less than 0.4% by mass, the suppression of Ni diffusion is insufficient, or Zn is consumed for the suppression of Ni diffusion, and good workability may not be obtained. On the other hand, if the Zn content is more than 13.5% by mass, the liquidus temperature exceeds 400 ° C., 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. This effect is also exhibited when Zn is added. 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 being oxidized by itself.

  The addition of P has the 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 a main component of 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 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. 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 P content is preferably 0.001% by mass or more, and the upper limit is 0.500% by mass. When P exceeds this upper limit, the oxide covers the solder surface, and conversely, wettability may be reduced. Furthermore, since P has a very small amount of solid solution in Bi, if the content 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 P content is less than 0.001% by mass, the expected reduction effect cannot be obtained, and there is no point in adding it.

  The apparatus obtained by joining the electronic component containing Ni and the substrate with the Pb-free solder alloy of the present invention described above is good over a long period even under severe conditions where the heat cycle is repeated. Can be used for That is, this Pb-free solder alloy is used for, for example, power semiconductor devices such as thyristors and inverters, various control devices such as automobiles, and high-temperature solders for electronic boards mounted on devices used under harsh conditions such as solar cells. As a result, the reliability of these various devices can be further enhanced.

  Bi, Zn, Sn and P having a purity of 99.9% by mass or more were prepared as raw materials. Large flakes and bulk raw materials were cut and pulverized into fine pieces of 3 mm or less while paying attention to the uniformity of the composition of the melted alloy without any variation in the sampling location. 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 thoroughly stirred with a mixing rod and mixed uniformly so as not to cause local compositional variations. After confirming that the mixture was sufficiently melted and mixed, the high frequency power supply was turned off, and the molten metal in the crucible from which the crucible was taken out was poured into the solder mother alloy mold. A mold having the same shape as that generally used in the manufacture of solder alloys was used.

  In this way, solder mother alloys of Samples 1 to 16 were produced by changing the mixing ratio of each raw material. The compositions of the solder mother alloys of Samples 1 to 16 were analyzed using an ICP emission spectroscopic analyzer (SHIMAZU S-8100). The analysis results are shown in Table 1 below.

  Next, for each of the solder mother alloys of Samples 1 to 16 shown in Table 1, the following wettability (joinability) evaluation, EPMA line analysis (evaluation of Ni diffusion prevention effect), and heat cycle test are performed. went. The evaluation of the wettability of the solder alloy ordinarily does not depend on the solder shape, so it may be evaluated in the form of a wire, a ball, a paste, etc. In this example, the evaluation was performed by forming the wire. . The wire was formed by the following processing method.

  Each of the solder mother alloys of Samples 1 to 16 shown in Table 1 above was 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 each solder mother alloy was set in a mold. Since the wire-like solder extruded from the extruder outlet is still hot and easily oxidizes, the outlet of the extruder has a sealed structure, and an inert gas is allowed to flow therethrough. As a result, the oxygen concentration was lowered as much as possible to prevent oxidation. 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.

<Evaluation of wettability (bondability)>
Evaluation of wettability (bondability) was performed using a wire-like solder alloy obtained by the wire processing method. 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 heater temperature was stabilized at 340 ° C., a Cu substrate (plate thickness: about 0.70 mm) having a Ni film (film thickness: 4.0 μm) on the surface was set in the heater portion and heated for 25 seconds. Next, the solder alloy was placed on the Cu substrate and heated for 25 seconds. After the heating was completed, the Cu substrate was picked up from the heater part and once moved to a place where the nitrogen atmosphere next to the Cu substrate was maintained and cooled. After sufficiently cooling, it was taken out into the atmosphere and a joint portion was confirmed. “X” when the bonding was not possible, “△” when the bonding was successful but the wetting and spreading was poor (the state where the solder was raised), and when the welding was successful and spreading (the solder was thinly spread on the Cu substrate) ) Was evaluated as “◯”.

<EPMA line analysis (evaluation of Ni diffusion prevention effect)>
In order to confirm whether the Ni film provided on the Cu substrate was thinned by reaction with Bi or Ni was not diffused into Bi, line analysis by EMPA was performed. This analysis was performed using a Cu substrate to which a solder alloy obtained in the same manner as the wettability evaluation was bonded. First, a Cu substrate to which a solder alloy obtained by wettability evaluation was bonded was embedded in a resin, and was polished using a polishing machine while gradually changing from coarse abrasive paper to a fine one, 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 measuring method was such that when the cross section of the Cu substrate to which the solder alloy was bonded was viewed from the side, the bonding surface of the Cu substrate and the Ni film was the origin 0 and the solder side was the positive direction of the X axis (see FIG. 1). In the measurement, five points were arbitrarily measured and the average one was adopted. When the Ni film reacts and becomes thin or Ni diffuses in the solder, “X”, when the Ni film thickness is almost the same as the initial state and Ni does not diffuse in the solder Was evaluated as “◯”.

<Heat cycle test>
A heat cycle test was conducted to evaluate the reliability of solder joints. This test was performed using a Cu substrate to which a solder alloy obtained in the same manner as the wettability evaluation was bonded. First, with respect to the Cu board | substrate with which the solder alloy was joined, -50 degreeC cooling and 135 degreeC heating were made into 1 cycle, and this was repeated predetermined cycle. Thereafter, the Cu substrate to which the solder alloy was bonded was embedded in the resin, the cross-section was polished, and the bonded 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 evaluation and test results are shown in Table 2.

  As can be seen from Table 2 above, the solder mother alloys of Samples 1 to 11 show good characteristics in each evaluation item. That is, it was confirmed that the wettability was very good and Ni diffusion was also suppressed. Furthermore, good results were obtained in a heat cycle test, which is a test related to reliability, and no defect appeared even after 500 times. From the above, the effect of adding Sn could be confirmed. On the other hand, the solder mother alloys of Samples 12 to 16 of the comparative example had undesirable results in any of the characteristics.

Claims (2)

  A Pb-free solder alloy containing Bi as a main component, containing Sn in an amount of 1.6 mass% to 10 mass%, Zn not containing more than 13.5 mass%, and P of 0.5 A Pb-free solder alloy characterized by not containing more than% by mass and having a liquid phase at 150 ° C. or higher.   2. The Pb-free solder according to claim 1, wherein at least one of Zn and P is contained in an amount of 0.4 mass% or more in the case of Zn and 0.001 mass% or more in the case of P. 3. alloy.

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