JP2016026884A - Bi-Sn-Al BASED SOLDER ALLOY FOR MEDIUM TO LOW TEMPERATURES AND SOLDER PASTE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Bi-Sn-Al based solder alloy low in residual stress caused by expansion during solidification or low in residual stress that may be generated at a time of reducing a temperature from a solidification temperature to a use temperature, capable of reducing a junction temperature as compared with temperatures for conventional medium to low temperature solder alloys, excellent in junction properties, stress relaxation properties, junction reliability, etc., and suited for medium to lower temperatures.SOLUTION: A Pb-free Bi-Sn-Al based solder alloy and a solder paste obtained by mixing the solder alloy with flux are provided. The solder alloy comprises not less than 38.0 mass% and less than 58.0 mass% of Sn, preferably not less than 41.0 mass% and less than 55.0 mass% of Sn; more than 0.1 mass% and not more than 1.2 mass% of Al, preferably more than 0.5 mass% of Al; and the balance Bi except for elements inevitably contained in a manufacturing process. This solder alloy may further comprises one or more elements selected from a group consisting of Ag, Ni, Sb, and P.SELECTED DRAWING: None

Description

  The present invention relates to a Pb-free Bi—Sn—Al solder alloy suitable for medium and low temperatures and a solder paste including the same.

  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 and the like to substrates. Pb (lead) has been used as a main component for solder materials for a long time, but lead has already been a regulated substance under the RoHS Directive. For this reason, development of solder containing no lead (hereinafter also referred to as Pb-free solder) has been actively conducted.

  Solders used when joining electronic components to a substrate are roughly classified into high temperature (about 260 ° C. to 400 ° C.) and medium to low temperature (about 140 ° C. to 230 ° C.) depending on the use limit temperature. As for the solder for medium and low temperature, lead-free solder is put into practical use, which contains Sn as a main component. For example, as a Pb-free solder having a melting point of about 210 to 220 ° C., Patent Document 1 includes Sn as a main component, Ag of 1.0 to 4.0 mass%, Cu of 2.0 mass% or less, and Ni of 0. Pb-free solder alloy composition containing 0.5% by mass or less and P by 0.2% by mass or less is described. Patent Document 2 describes that Ag is 0.5 to 3.5% by mass and Cu is 0.5 to 0.5% by mass. A Pb-free solder having an alloy composition containing 2.0% by mass and the balance being Sn is described.

  Furthermore, as a low-temperature Pb-free solder, a Bi-42Sn solder (meaning that it is composed of 58 mass% Bi and 42 mass% Sn, and so on) is also known. For example, Patent Document 3 states that “in an electronic device in which an MCM substrate is held on another substrate, a maximum interval between I / O pins is 50 mm or more and a pin pitch (lattice pattern) is 1.3 mm or less, for example, a large computer. In the case of MCM for industrial use, misalignment due to the difference in thermal expansion coefficient between the MCM substrate (aluminum nitride) and the motherboard substrate (glass polyimide), or warpage of the MCM substrate becomes a serious problem.Use the current Sn-37Pb solder alloy In order to reflow the solder alloy, it is necessary to heat to a temperature of about 180 ° C. However, when heated to such a high temperature, the heat Misalignment due to the difference in expansion coefficient becomes significant, and joining becomes almost impossible, or permanent stress strain occurs with respect to the solder alloy at the joint after solidification of the solder alloy, and Sn-37Pb, which has a problem in creep characteristics, has a thermal problem. In order to solve the problem “joint reliability such as fatigue characteristics arises”, the following technology is disclosed.

  That is, as a composition of the solder alloy, “a solder alloy containing about 42 to 59% by weight of Sn and about 41 to 58% by weight of Bi” or “about 42% by weight of Sn, about 57% by weight of Bi, and about 1% of Ag. By using “a solder alloy containing wt%”, the action and effect is as follows: “By lowering the solidus temperature of the solder alloy used for joining to about 150 ° C. or less, the solder alloy is completely solidified to room temperature. The temperature range until the temperature atmosphere is lowered can be narrowed, and as a result, the stress strain applied to the solidified solder alloy at the joint can be reduced due to the difference in thermal shrinkage of each substrate. By using a solder alloy mainly composed of Sn and Bi, which has a high Young's modulus and is hard, deformation of the solder alloy due to stress strain generated in the joint can be suppressed. Thereby improving the reliability. Is described as ".

  In Patent Document 4, as a subject of Sn—Bi solder, “Sn—Bi solder is easier to oxidize than Sn—Pb alloy, Sn—Cu alloy, Sn—Ag—Cu alloy, and its oxide. Solderability may be poor due to difficulty in removing solder, for example, there may be a non-wetting part, and black residue adheres and mixes on the solder surface and flux residue after soldering. This black residue is considered to be an oxide of Sn and Bi. ”In order to solve this problem,“ 20 to 60% by mass of Bi and Al is included. A lead-free solder alloy including 0.001 to 0.01% by mass and the remaining main component being Sn is disclosed.

  Patent Document 5 discloses a solder alloy containing Sn as a main component, Bi of 22.0 to 42.0% by weight, and Al of 0.01 to 0.50% by weight. While keeping the Pb content at 1.0% by weight or less, the liquidus temperature can be set to about 170 to 205 ° C. close to that of the Sn—Pb eutectic solder alloy, and further, the strength or oxidation resistance is higher than that of the conventional lead-free solder. It is described that an improved solder alloy is obtained.

Japanese Patent Application Laid-Open No. 11-077366 JP-A-8-215880 Japanese Patent No. 3715438 JP 2013-248664 A JP 2001-347394 A

  However, since the solder alloy of Patent Document 3 is a semi-metal and contains a large amount of hard and brittle Bi, cracks and other defects are likely to occur, and stress is higher than that of commonly used Pb solder and Sn solder. It has the problem of poor relaxation and bonding reliability. In order for Bi-Sn based solder alloys to be used more widely, it is an essential condition to improve this hard and brittle nature.

Patent Document 3 describes that the Bi—Sn solder alloy contains about 1% by weight of Ag. However, since Ag forms Sn and many intermetallic compounds, it contains both Sn and Ag. The solder alloy produces an Ag ₃ Sn intermetallic compound like Pb—Sn—Ag solder. Therefore, 57 mass% Bi-42 mass% Sn-1 mass% Ag solder is considered to promote hardness and brittleness in view of its composition, and is a material with poor stress relaxation and low joint reliability. Inferred.

  In addition, the Bi—Sn solder disclosed in Patent Document 4 contains 0.001 to 0.01% by mass of Al in order to solve poor wetting caused by oxidation and poor visual inspection due to flux residue. There is no mention of improving the brittle nature of Bi-Sn solder. Patent Document 5 states that “If the Bi content exceeds 42.0% by weight, the liquidus temperature becomes too low, and the same bonding treatment temperature as that of the Sn—Pb eutectic solder alloy can be adopted. It is not suitable for low temperature solder alloys, as it is described. Moreover, it is described that “when Bi is contained too much, there is a problem that the bonding strength is lowered”, and it is assumed that there is a problem in using it as a low-temperature solder alloy.

  Further, Patent Document 5 discloses a solder alloy containing Sn as a main component and containing 22.0 to 42.0% by weight of Bi and 0.01 to 0.50% by weight of Al. Such a wide composition range is disclosed. In addition, Patent Document 5 aims to make the liquidus temperature 170-205 ° C, but the liquidus temperature is 170 ° C-205 ° C. Even if the solidus temperature is different, it may not be suitable for use. In other words, if the solder alloy is not selected so that both the liquidus temperature and the solidus temperature are appropriate, there is a possibility that the phenomenon of melting and the generation of a low melting point phase may be caused, and the bonding reliability may be significantly lowered.

  In addition, if the Sn-37 mass% Pb solder is replaced with Pb-free Sn-3 mass% Ag-0.5 mass% Cu-based solder in accordance with the Pb regulations, the melting point of the solder increases from 183 ° C to about 210-220 ° C. It is necessary to increase the soldering temperature of electronic components and the like, and accordingly, it is necessary to increase the heat-resistant temperature of electronic components and the like, and the running cost such as electricity costs at the time of soldering is increased. These cost increases are unlikely to be accepted in the highly competitive semiconductor industry.

  As described above, although the Pb-free solder material for medium and low temperatures has been developed by various organizations, there are still many problems and it cannot be a large market. The present invention has been made in view of such a situation, and the residual stress that can be generated by expansion during solidification of solder and the residual stress that can be generated when the temperature is lowered from the solidification temperature to the use temperature are small, and various members to be joined by soldering. Compared to conventional Sn-Ag-Cu solder alloys and Sn-Pb solder alloys such as Sn-Pg solder alloys, the bonding temperature, which increases the heat-resistant temperature, etc. In addition, a Bi-Sn-Al solder alloy particularly suitable for medium and low temperatures having excellent bonding properties, stress relaxation properties, bonding reliability, and the like, and a solder paste including the same are provided. It is aimed.

  In order to achieve the above object, the Pb-free Bi—Sn—Al solder alloy provided by the present invention contains Sn in a range of 38.0% by mass to less than 58.0% by mass, and Al exceeds 0.1% by mass. 1.2% by mass or less, and the balance is made of Bi except for elements inevitably contained in production. Further, the Pb-free Bi—Sn—Al solder paste of the present invention is characterized by being produced by mixing the above solder alloy powder and a flux.

  According to the present invention, it is particularly excellent as a medium / low temperature solder used for joining an electronic component and a substrate. That is, the Bi-Sn ternary eutectic alloy of the present invention in which the Bi-Sn binary eutectic alloy contains soft and highly ductile Al greatly improves the brittle nature of Bi-Sn. In addition, since the melting point is relatively low, the residual stress is small, and the bonding temperature that causes the increase in the heat-resistant temperature of the electronic parts to be bonded and the cost increase such as the electricity cost at the time of bonding is the conventional Sn-Ag-Cu type. It can be lowered as compared with solder alloys for medium and low temperatures such as solder alloys and Sn—Pb solder alloys, and in addition, the bondability, the stress relaxation property, the bonding reliability, etc. are extremely excellent.

It is the typical top view which looked at the joined object obtained by joining a solder alloy to Cu board plated with Ni from the upper part by the side of a solder alloy. It is a typical longitudinal cross-sectional view of the joined body obtained by joining Si chip | tip via a solder alloy on the Ni plating surface of Cu board | substrate with which Ni plating was carried out. It is a longitudinal cross-sectional view which shows typically the state which soldered the solder alloy on the Ni plating surface of Cu board | substrate with which Ni plating was carried out. FIG. 4 is a schematic plan view of the solder joined body of FIG. 3 viewed from the solder side.

  The Pb-free solder alloy of one specific example of the present invention contains Sn in a range of 38.0% to less than 58.0% by mass, preferably 41.0% to less than 55.0% by mass, and 0.1% in Al. More than 1.2% by mass and more than 1.2% by mass, preferably more than 0.5% by mass and less than 1.2% by mass, and the balance is made of Bi except for elements inevitably contained in production. . The Pb-free solder alloy of one specific example of the present invention may contain one or more of Ag, Ni, Sb, and P in order to appropriately adjust and / or improve various properties of the solder alloy. The tissue may be a lamellar tissue. Furthermore, the Pb-free Bi—Sn—Al solder paste provided by the present invention can be produced by mixing the solder alloy powder and the flux.

  An electronic component mounting board having a general heat-resistant temperature is used for an electronic component mounting board in which solder bonding is performed using a Pb-free Bi—Sn—Al solder alloy having the above composition or a solder paste containing the same. In addition, it is possible to lower the bonding temperature at the time of production as compared with conventional solder alloys for medium and low temperatures such as Sn-Ag-Cu solder alloys and Sn-Pb solder alloys. The manufacturing cost can be suppressed without lowering the quality compared to the above. Therefore, the device on which the electronic component mounting board is mounted can reduce the manufacturing cost even though it is a highly reliable device having a quality equivalent to or higher than the conventional one.

  Hereinafter, for the Pb-free solder alloy according to one specific example of the present invention and a solder paste including the same, an essential element contained in the solder alloy, an element contained as necessary, a flux contained in the solder paste, and the like Each of the various components will be described in detail.

<Bi>
Bi is an essential element which forms a main component together with Sn described later in the Pb-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 low symmetry, so it is a very brittle metal and is subjected to a tensile test or the like. It can be easily seen that the fracture surface is a brittle fracture surface. In other words, pure Bi is poor in ductile properties, and according to the experiment results of the present inventors, the elongation rate of Bi was less than 1.0%. Bi is a metal that expands during solidification, and the contraction rate (− means expansion and + means contraction) during solidification is −3.2% to −3.4%. Residual stress is generated by this expansion, and bonding strength and reliability are lowered.

  One reason for adding Sn and Al to the solder alloy containing Bi in the present invention is to improve the problem that Bi expands during solidification. That is, by containing Sn or Al that acts to counteract the volume expansion of Bi that occurs during solidification, the expansion of Bi is reduced by the shrinkage of Sn or Al, and the volume change of the entire solder is reduced. This makes it possible to reduce the residual stress of the solder alloy.

  Furthermore, by using the composition of the eutectic point in the ternary alloy of Bi—Sn—Al as a basis, the melting point can be lowered as compared with Sn—Ag—Cu based solder and Sn—Pb eutectic solder. In addition, since the eutectic point temperature can be adjusted freely, lowering the welding temperature can lower the heat resistance temperature of various parts to be welded, reduce running costs such as electricity costs during soldering, and reduce oxidation. Progress can also be suppressed. In addition, since an extremely flexible solder alloy can be obtained, the stress relaxation property and the bonding reliability are also excellent.

<Sn>
Sn is an essential element which forms a main component together with Bi described above in the Pb-free solder alloy of the present invention. The first purpose of containing Sn is to greatly reduce the melting point of the solder alloy. In the Bi—Sn binary alloy, the composition of Bi-42% by mass Sn is the eutectic point, and the eutectic point temperature is 139 ° C. Thus, an alloy that can be used up to a low temperature range can be obtained.

  The second purpose of containing Sn is to impart flexibility to the solder alloy by making it an eutectic alloy, and to greatly improve workability, stress relaxation, joint reliability, and the like. That is, by setting the composition in the vicinity of the eutectic point, the crystal structure becomes a lamellar structure, and mechanical characteristics and the like are improved. Furthermore, since Sn is excellent in reactivity with a substrate or the like, it increases the bonding strength with a substrate or a semiconductor element, and contributes to the improvement of bonding reliability. In addition, the expansion of Bi during solidification can be offset by the shrinkage of Sn together with the shrinkage of Al, which will be described later, and the volume change during solidification of the entire solder can be reduced. As a result, the residual stress of the solder alloy is reduced, and cracks and the like are difficult to enter.

  The Sn content in the present invention is 38.0% by mass or more and less than 58.0% by mass, preferably 41.0% by mass or more and less than 55.0% by mass. In the Bi—Sn—Al based solder alloy, the eutectic point temperature is set to about 140 to about by allowing the Sn content to be appropriately selected within a wide range of 38.0% to less than 58.0% by mass in this way. It can be shaken up to about 180 ° C. One of the major features of the present invention is that a wide eutectic point temperature that substantially covers the mid-low temperature range can be realized by setting the Sn content in the above range.

  That is, the eutectic point of the Bi—Sn—Al ternary alloy is from the 42 mass% Sn eutectic point (eutectic temperature: 139 ° C.) of the Bi—Sn binary alloy to Al 0.6 mass of the Sn—Al binary alloy. It is on a line connecting% eutectic points (eutectic point temperature: 228 ° C.), and a solder alloy having a composition of each eutectic point can be obtained mainly by changing the Sn content. As described above, a solder alloy having a desired eutectic point temperature can be obtained by mainly changing the Sn content, and the obtained solder alloy has a lamellar structure and thus has flexibility. An excellent solder alloy having high joining reliability and the like can be obtained.

  In addition, it is one of the great features that the mechanical characteristics and the like can be adjusted mainly by changing the Sn content. That is, mechanical properties and the like are changed by changing the Sn content mainly on the line connecting the eutectic points between the Bi-42Sn eutectic point and the Sn-0.6Al eutectic point. As the content of the ductile Sn increases, the elongation rate increases and becomes soft, and the hard and brittle nature of the Bi-Sn alloy is relaxed. As described above, the solder alloy of the present invention can adjust various characteristics such as melting point, mechanical characteristics, and wettability in a very wide range, and the composition is adjusted to the desired characteristics while considering these balances. It is only necessary to adjust this.

  As described above, in the present invention, a solder alloy excellent in various characteristics can be obtained by setting the Sn content to 38.0 mass% or more and less than 58.0 mass%. If the Sn content is less than 38.0% by mass, the Bi content increases, the hard and brittle properties become remarkable, and it becomes difficult to obtain sufficient characteristics as a solder alloy. On the other hand, if the Sn content is 58.0% by mass or more, the melting point becomes too high, and the features of the solder alloy of the present invention are limited, or in some cases, it can be replaced with Sn—Ag—Cu based solder. . In addition, if Sn content is 41.0 mass% or more and less than 55.0 mass%, since the characteristic of the solder alloy of this invention appears more notably, it is preferable.

<Al>
Al is an essential element contained in the Pb-free solder alloy of the present invention. By incorporating Al into a Bi—Sn—Al ternary alloy, a wide range of melting points can be selected, and mechanical characteristics can be freely adjusted. Further, since Al is a very soft metal, flexibility can be given to a hard and brittle Bi—Sn alloy. As described above, the composition of the Bi—Sn—Al solder alloy of the present invention is based on the vicinity of the eutectic point and thus is composed of a lamellar structure, and is excellent in workability and stress relaxation properties. The fact that the solid solution is one of the constituent elements is also a factor of excellent flexibility and the like.

  In order to exhibit such an excellent effect, the content of Al exceeds 0.1% by mass and is 1.2% by mass or less. When the Al content is 0.1% by mass or less, the content is too small and the effect of inclusion is not substantially exhibited. On the other hand, if it exceeds 1.2% by mass, the content is too high, and oxidation of Al proceeds, so that the solder alloy may also deteriorate wettability and bonding reliability. If the Al content exceeds 0.5% by mass and is 1.2% by mass or less, the very soft nature of Al can provide flexibility to the hard and brittle Bi—Sn alloy, and the solder of the present invention. The features of the alloy appear more prominently and are preferable.

<Ag>
Ag is an element contained as necessary in order to improve various properties in the solder alloy of the present invention. The main effect obtained by containing Ag is to improve wettability and bondability. That is, Ag is easily alloyed with various metals and has excellent wettability, as can be seen from the fact that Ag is used in a metallized layer such as a substrate. It is particularly reactive with Cu, Ni and the like often used on the top surface of a substrate or the like, and has excellent wettability and high bonding strength. Moreover, it cannot be overemphasized that it is excellent in the reactivity with metallizing layers, such as Ag and Au, which are often used for the joint surface of a semiconductor element. By containing Ag in this manner, good bonding is possible, and thus high bonding reliability can be obtained.

In order to exhibit the above-described excellent effects of improving various properties due to the inclusion of Ag, the Ag content is preferably 0.01% by mass or more and 3.0% by mass or less. If the Ag content is less than 0.01% by mass, the content is too small and the effect of the inclusion is not substantially exhibited. On the other hand, when the Ag content exceeds 3.0% by mass, an intermetallic compound such as Ag ₃ Sn is generated exceeding the allowable amount, or an intermetallic compound such as Ag or Ag segregates to obtain a good joint. There is a risk that it will not be possible.

<Ni>
Ni is an element contained as necessary in order to improve various characteristics in the solder alloy of the present invention. The main effect obtained by the inclusion of Ni is to improve the bonding reliability and the like by crystal refining. That is, Ni dissolves slightly in Sn and Bi. And when Ni is slightly contained in the solder alloy in this way, when the solder is cooled from the molten state and solidifies, Ni of high melting point is first dispersed and formed in the solder, and crystals are formed using the Ni as a nucleus. To do. For this reason, the solder crystal becomes finer. Since cracks basically propagate along grain boundaries, it is difficult for cracks to progress with such finely crystallized solder, and thus the reliability of heat cycle tests and the like is improved.

In order to exert such excellent effects, the Ni content is preferably 0.01% by mass or more and 0.7% by mass or less. If the Ni content is less than 0.01% by mass, the content is too small to produce an effect. On the other hand, if the Ni content exceeds 0.7% by mass, the crystal grains become coarse and the reliability is lowered. Or brittle NiBi ₃ intermetallic compounds may be produced in excess of the allowable amount.

<Sb>
Sb is an element contained as necessary in order to improve various properties in the solder alloy of the present invention. The main effect obtained by the inclusion of Sb is to improve the bonding reliability and the like by crystal refining like Ni. That is, although Sb is slightly dissolved in Sn and Bi, crystal refinement occurs by the same mechanism as that of Ni, and the junction reliability and the like are improved. Since Sb is similar in nature to Bi, it can contain a relatively large amount.

  In order to exert such excellent effects, the Sb content is preferably 0.01% by mass or more and 5.0% by mass or less. When the Sb content is less than 0.01% by mass, the content is too small to produce an effect. On the other hand, when the Sb content exceeds 5.0% by mass, the strength increases so much that the solder shrinks during cooling after chip bonding. The chip may break due to the hardness of the solder.

<P>
P is an element contained as needed in the solder alloy of the present invention. By containing P, the wettability and the bondability of the Bi—Sn—Al solder alloy can be further improved. This effect is similarly exhibited even when Ag, Ni, and Sb are contained. 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.

  By containing P, the effect of reducing the generation of voids during bonding can be obtained. This is because P tends to oxidize by itself, so that oxidation proceeds preferentially over Bi, which is a main component of solder, and Al, which is not a main component but is an essential component. Thereby, oxidation of a solder mother phase is suppressed and wettability can be secured. As a result, good bonding is possible and voids are less likely to occur.

  As described above, P has a very strong reducibility, and therefore exhibits an effect of improving wettability even when added in a small amount. On the other hand, even if added over a certain content, the effect of improving wettability does not change, and if 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 upper limit value of the P content in the solder alloy is preferably 0.500% by mass. When P exceeds this upper limit, the oxide covers the solder surface, and the wettability may deteriorate. Furthermore, since P has a very small amount of solid solution in Bi, if the content exceeds the upper limit, brittle P oxide may segregate and the reliability may be lowered. It has been confirmed that wire breakage is likely to occur, especially when processing wires.

<Solder paste>
The Pb-free Bi—Sn—Al solder paste of the present invention can be produced by processing the above solder alloy into a powder and mixing the flux. The method of making the solder alloy into a powder form for paste is not particularly limited, but it is preferably produced by an atomizing method in which a molten solder is produced by spraying molten solder in a gas phase or a liquid phase. The operating conditions for selection in the gas phase or liquid phase, atomization, and the like may be appropriately determined in consideration of the particle size and particle size distribution of the target solder powder. In general, the atomizing method in the gas phase is preferable because the productivity is high and a relatively fine powder can be produced.

<Flux>
The kind of flux used for the solder paste of the present invention is not particularly limited, and for example, a resin system, an inorganic chloride system, an organic halide system, or the like may be used. Here, a description will be given of the most common flux, in which rosin is used as a base material and an activator and a solvent are added thereto.

  As an example of this flux, when the total amount of the flux is 100% by mass, the base material is 20-30% by mass of rosin, the activator is 0.2-1% by mass, and the solvent is about 70-80% by mass. It is preferable to mix | blend so that the solder paste which has favorable wettability and joining property can be obtained by this. For the rosin as the base material, natural unmodified rosin such as wood resin rosin, gum rosin and tall oil rosin may be used, or modified rosin ester, hydrogenated rosin, rosin modified resin, polymerized rosin and the like. Rosin may be used.

  Solvents include acetone, amylbenzene, n-amine alcohol, benzene, carbon tetrachloride, methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, isobutyl alcohol, methyl ethyl ketone, toluene, turpentine oil, xylene, cyclohexane, ethylene glycol mono Phenyl ether, ethylene glycol monobutyl ether, carbon tetrachloride, trichloroethane, alkanediol, alkylene glycol, butadiol, triethylene glycol, tetraethylene glycol, tetradecane and the like can be used.

  Activators include phosphoric acid, sodium chloride, ammonium chloride, zinc chloride, stannous chloride, aniline hydrochloride, hydrazine hydrochloride, cetylpyridine bromide, phenylhydrazine hydrochloride, tetrachloronaphthalene, methyl hydrazine hydrochloride, methyl Amine hydrochloride, ethylamine hydrochloride, diethylamine hydrochloride, butylamine hydrochloride, benzoic acid, stearic acid, lactic acid, citric acid, oxalic acid, succinic acid, adipic acid, hivacic acid, triethanolamine, diphenylguanidine, diphenylguanidine HBr, It is possible to use erythritol, xylitolitol, sorbitol, ribitol, sulfonic acid ester, tertiary butyl carboxylic acid ester, isobutyl carboxylic acid ester, isopropyl carboxylic acid ester, and the like.

  A suitable flux can be obtained by selecting a substance suitable for the purpose from the above-mentioned solvent and activator and adjusting the addition amount thereof appropriately. For example, when the oxide film on the joint surface of a solder alloy or a substrate is strong, it is preferable to add a large amount of rosin or activator and adjust the viscosity and fluidity with a solvent.

  In the solder paste of the present invention, a thixotropic agent may be included in the flux. Thereby, since thixotropy can be adjusted, it can become a solder paste which is easier to use. Examples of thixotropic agents include castor oil, oleic acid amide, erucic acid amide, ethylenebisstearic acid amide, stearic acid amide, N.I. N-distearyl adipic acid amide and the like can be used.

  The solder paste obtained by mixing the solder alloy and the flux described above has very good wettability due to the action of the flux, and the solder alloy needs to be processed into a sheet shape that is difficult to process. And can be used in a powder form that is easy to process.

  The solder alloy of the present invention is not particularly limited with respect to the solder shape, and can be produced in an arbitrary shape such as a wire, a ball, or a paste. In the following examples, a wire shape and a paste shape will be described, but the present invention is not limited to the shapes of the examples. First, a wire-shaped solder alloy will be described.

[Example 1]
Bi, Sn, Al, Ag, Ni, Sb and P having a purity of 99.99% by mass 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. P is difficult to melt, easily oxidizes and volatilizes, and is a second kind of hazardous material. If added as it is, it will ignite, so an alloy with Sn was made in advance and then crushed and redissolved.

  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 that it was sufficiently melted, the high-frequency power supply was turned off, and the molten metal in the crucible was quickly taken out and poured into the mold of the solder mother alloy. 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 27 in which the mixing ratios of the respective raw materials were changed in various ways were produced. The compositions of the solder mother alloys of Samples 1 to 27 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 27 shown in Table 1 above, as shown below, as a wettability evaluation, the determination of the spread of the solder alloy on the substrate visually, A void ratio was measured as an evaluation of 1, a shear strength was measured as a second evaluation of bondability, and a heat cycle test was performed as an evaluation of reliability. In addition, since the evaluation of the wettability of the solder alloy does not usually depend on the solder shape, it may be evaluated by the shape of a wire, a ball, a paste, etc. In this example, it was molded into a wire and evaluated. .

<Processing into wire>
Each of the solder mother alloys of Samples 1 to 27 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. 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 1>
As the wettability evaluation 1, a wire-like solder alloy obtained by the wire processing was used. 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). Thereafter, the heater was set to a temperature higher than the melting point by 50 ° C. and heated.

  After the heater temperature was stabilized at the set temperature, a Cu substrate (plate thickness: about 0.70 mm) on which a Ni plating layer (film thickness: 5.0 μm) was formed 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, the joined body in which the solder sample 3 was joined onto the Ni layer 2 of the Cu substrate 1 as shown in FIG. 1 was taken out into the atmosphere, and the joined portion was visually 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 swelled), and when the bonding was performed with the wetting and spreading well (the solder was thin on the Cu substrate) (When spread) was evaluated as “◯”.

<Evaluation of bondability 1 (measurement of void fraction)>
With respect to the joined body as shown in FIG. 1 obtained in the same manner as in the evaluation of the wettability, the void ratio between the Ni layer 2 on the Cu substrate 1 and the solder sample 3 is measured with an X-ray transmission device (stock) The measurement was performed using Toshiba TOSMICRON-6125). Specifically, the void area and the bonding area are measured by irradiating the bonded body with X-rays perpendicularly to the bonding surface from the solder sample 3 side, and substituting them into the following calculation formula 1 to obtain the void ratio Was calculated.

[Calculation Formula 1]
Void ratio (%) = void area / (void area + solder alloy / Cu substrate bonding area) × 100

<Evaluation of bondability 2 (measurement of shear strength)>
In order to confirm the solderability, the Si chip 4 is placed on the Ni layer 2 (film thickness: 3.0 μm) on the Cu substrate 1 (plate thickness: 0.3 mm) as shown in FIG. A bonded body was bonded and the shear strength was measured. The joined body was performed using a die bonder (manufactured by Westbond, model: 7327). First, while flowing nitrogen gas through the heater part of the apparatus, the temperature is set to 50 ° C. higher than the melting point of each solder sample. Then, the substrate is placed on the heater part and heated for 15 seconds, and then the solder sample is placed thereon and heated for 20 seconds. Further, a chip was placed on the molten solder and scrubbing was applied for 3 seconds. After scrubbing, the joined body was immediately transferred to a cooling section where nitrogen gas was flowing, cooled to room temperature, and taken out into the atmosphere. The shear strength was evaluated relative to the measured value of sample 27 as 100%.

<Reliability evaluation 1 (heat cycle test 1)>
A heat cycle test 1 was conducted to evaluate the reliability of solder joints. In this test, two Cu substrates to which solder alloys were bonded were prepared for each sample in the same manner as in the wettability evaluation 1, and a cooling and heating cycle comprising cooling at −55 ° C. and heating at 125 ° C., respectively. Was repeated for 300 and 500 cycles. 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 results are shown in Table 2.

  As can be seen from the results of Table 2 above, the solder alloys of Samples 1 to 18 that satisfy the requirements of the present invention show good characteristics in each evaluation item. That is, in the wettability evaluation 1, the wettability to the Ni-plated Cu substrate is very good. In particular, the sample to which P is added spreads very quickly, and the sample wets thinly as soon as the sample contacts the Cu substrate. Spread. In the measurement of the void ratio as the bondability evaluation 1, no void was generated in all the samples. In the measurement of the shear strength as the bondability evaluation 2, all were 120% or more, and it was confirmed that high joint strength was obtained. In heat cycle test 1 as reliability evaluation 1, good results were obtained, and no defect appeared even after 500 cycles.

  On the other hand, the solder alloys of Samples 19 to 27 of Comparative Examples that did not satisfy the requirements of the present invention resulted in undesirable results in at least any of the above evaluations. That is, in the wettability evaluation 1, the wettability to the Ni-plated Cu substrate could not be joined by the samples 23 and 25. In the measurement of the void ratio as the bondability evaluation 1, at least 7% or more voids were generated. In the measurement of the shear strength as the bondability evaluation 2, it was 100% or less. In heat cycle test 1 as an evaluation of reliability, defects occurred in all samples by 500 cycles. Next, examples of solder paste will be described.

[Example 2]
<Method for producing solder alloy powder>
Solder mothers of samples 31 to 57 having different mixing ratios of raw materials in the same manner as in Example 1 above, except that a mold capable of producing a columnar mother alloy with a diameter of 140 mm was used for atomization for producing powder. An alloy was made. Composition analysis was performed on each of the solder mother alloys of Samples 31 to 57 using an ICP emission spectroscopic analyzer (SHIMAZU S-8100). The obtained analysis results are shown in Table 3 below.

  Next, the solder mother alloys of the above-mentioned samples 31 to 57 are each atomized in the gas phase by using a high-frequency melting type gas-phase atomizing apparatus (manufactured by Nisshin Giken Co., Ltd.) to process into powder. I went. Specifically, each solder mother alloy sample was put into a high-frequency melting crucible, covered and sealed, and then nitrogen-flowed to make it substantially free of oxygen. Similarly, the sample discharge port and the collection container were made to flow in a nitrogen-free state.

  In this state, the high frequency power supply was turned on, the solder mother alloy was heated to 400 ° C. or higher, and the solder mother alloy melted in a state where the alloy was sufficiently melted was applied with nitrogen and atomized. Thus, the solder fine powder produced by atomization was collect | recovered in the container, and after fully cooling in this container, it took out in air | atmosphere. The reason for taking out after sufficiently cooling is that if taken out in a high temperature state, it will ignite or the solder fine powder will oxidize and reduce the effect of wettability. Each taken out powder was classified by a sieve having openings of 20 μm and 50 μm, respectively, to obtain a solder alloy powder sample (hereinafter also referred to as solder fine powder) having a particle size of 20 to 50 μm.

<Method for producing solder paste>
Next, the solder fine powder produced from the samples 31 to 57 of the solder mother alloy was mixed with a flux to produce a solder paste. In the flux, rosin was used as a base material, diethylamine hydrochloride ((C ₂ H ₅ ) ₂ NH · HCl) was used as an activator, and ethyl alcohol was used as a solvent. The respective contents were such that the flux was 100% by mass, rosin was 21% by mass, diethylamine hydrochloride was 0.4% by mass, and the balance was ethyl alcohol. This flux and the above-mentioned solder fine powder were mixed at a ratio of 9.1 mass% flux and 90.9 mass% solder fine powder, and mixed using a small blender to obtain a solder paste.

  For each of the solder pastes of Samples 31 to 57 obtained in this manner, the residual solder powder was confirmed as wettability evaluation 2 and the length / short ratio measurement was performed as wettability evaluation 3 to determine the bondability. As the evaluation 3, the void ratio was measured, as the bondability evaluation 4, the shear strength was measured, and as the reliability evaluation, a heat cycle test was performed. Hereinafter, each evaluation method will be described in detail.

<Evaluation of wettability 2 (confirmation of unmelted solder powder)>
As the wettability evaluation 2, using a mask on a Cu substrate (plate thickness: about 0.70 mm) on which a Ni layer (layer thickness: about 2.5 μm) was formed, the solder paste of each sample was 2.0 mm in diameter and thickness. The substrate having the solder paste printed thereon was heated and joined as follows to make a joined body, and an optical microscope was used to confirm whether there was any undissolved solder powder.

  More specifically, first, a wettability tester (device name: atmosphere control type wettability tester) is started, a double cover is applied to the heater part to be heated, and nitrogen is supplied from four locations around the heater part. (Nitrogen flow rate: 12 L / min each). Then, the heater set temperature was set to 50 ° C. higher than the melting point of each sample, and heated. After the heater temperature was stabilized at the set temperature, the Cu substrate coated with the solder paste of each sample was set in the heater part and heated for 25 seconds. Thereafter, the Cu substrate was picked up from the heater part, temporarily 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.

  The taken-out joined body was not dared to be washed in order to confirm the unmelted solder powder. For each joined body thus produced, the surface on which the solder was joined was observed with an optical microscope from a direction perpendicular to the solder joined surface to confirm whether there was any undissolved solder powder. The case where the solder powder remained was indicated as “X”, and the case where the solder powder did not remain and the solder powder melted and a clean metallic glossy solder spread on the substrate was indicated as “◯”.

<Evaluation of wettability 3 (measurement of long / short ratio)>
As the wettability evaluation 3, a joined body similar to the joined body produced at the time of confirming the undissolved residue of the solder powder was produced for each solder paste sample, and the joined body was washed with alcohol and then vacuum-dried. . With respect to the joined body in which the solder sample 3 was joined on the Ni layer 2 on the Cu substrate 1 as shown in FIG. Specifically, as shown in FIG. 4, the major axis X1 which is the maximum solder wetting spread length and the minor axis X2 which is the minimum solder wetting spreading length are measured, and these are substituted into the following formula 2. The long / short ratio was calculated.

[Calculation Formula 2]
Long / short ratio = major axis X1 ÷ minor axis X2

  It can be determined that the closer the length ratio in the above calculation formula 2 is to 1, the more wetting and spreading on the substrate, the better the wetting and spreading property. On the other hand, as the length-to-short ratio becomes larger than 1, the wetting and spreading shape deviates from a circle, so the movement distance of the molten solder varies and the reaction becomes non-uniform or the alloy layer thickness and component variations increase. Uniform and good bonding is not possible. Furthermore, it spreads so that a lot of solder flows in a certain direction, and a portion where the amount of solder is excessive and a portion where there is no solder are formed.

<Evaluation of bondability 3 (measurement of void fraction)>
As a bondability evaluation 3, a bonded body similar to the bonded body manufactured at the time of confirming the remaining undissolved solder powder was prepared for each solder paste sample, the bonded body was washed with alcohol, and then vacuum-dried. did. With respect to the joined body as shown in FIG. 3 thus obtained, the void ratio between the Ni layer 2 on the Cu substrate 1 and the solder sample 3 is measured by an X-ray transmission device (TOSMICRON-6125, manufactured by Toshiba Corporation). ). Specifically, as in the case of Example 1 above, the void area and the joint area were measured by irradiating the joined body with X-rays perpendicularly to the joint surface from the solder sample 3 side, The void ratio was calculated by substituting into the above-described calculation formula 1.

<Evaluation of bondability 4 (measurement of shear strength)>
In order to confirm the solderability, the Si layer 2 (layer thickness: 2.5 μm) on the Cu substrate 1 (plate thickness: 0.7 mm) as shown in FIG. A joined body to which the chip 4 was joined was produced, and the shear strength was measured.

  Specifically, after setting the temperature to be 40 ° C. higher than the melting point of each solder sample while flowing nitrogen gas through the heater part of the die bonder device (manufactured by Westbond, model: 3727C), the solder paste of each sample is used with a mask. A Cu substrate printed in the shape of 2.0 mm x 2.0 mm and 120 μm in thickness is placed on the heater and heated for 35 seconds, and a 2.0 mm x 2.0 mm Si chip is placed on the molten solder and scrubbed. For 5 seconds. After scrubbing, the obtained Si chip joined body was quickly transferred to a cooling section where nitrogen gas was flowing, cooled to room temperature, and taken out into the atmosphere. The shear strength was measured using the shear strength test for the Si chip joined body thus obtained. Specifically, the bonded body was fixed to the apparatus, and the shear strength was measured by pushing the Si chip from the lateral direction with a jig. The shear strength was evaluated relative to the value of Sample 57 using Bi-42 mass% Sn solder paste as 100%.

<Reliability evaluation 2 (heat cycle test 2)>
A heat cycle test 2 was performed to evaluate the reliability of solder joints. In this test, a bonded body similar to the bonded body manufactured at the time of confirming the undissolved residue of the solder powder was manufactured, and after washing them with alcohol, they were tested using a vacuum-dried one. Two such bonded bodies were prepared for each sample, and a cooling and heating cycle including cooling at −40 ° C. and heating at 250 ° C. was repeated 300 cycles and 500 cycles, respectively. 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 with SEM (S-4800, manufactured by Hitachi, Ltd.). 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 “◯”. Table 4 shows the above results.

  As can be seen from Table 4 above, the solder paste containing the solder alloys of Samples 31 to 48 that satisfy the requirements of the present invention shows good characteristics in each evaluation item. That is, in the wettability evaluation 2, there was no undissolved powder. In the wettability evaluation 3, all the length-to-short ratios were 1.01 or less, and the wetting spread uniformly in a circular shape. In evaluation 3 of the bondability, no void was generated. In the evaluation 4 of the joining property, all showed high values of 110% or more with respect to the shear strength 100% of the sample 57 using the Bi-42 mass% Sn solder paste. In the evaluation of reliability, no defect occurred up to 500 cycles. It can be said that the reason why such a good result was obtained was that the solder alloy of the present invention was in an appropriate composition range and the solder paste was produced under appropriate conditions.

  On the other hand, each solder paste of Samples 49 to 57, which is a comparative example, resulted in an undesirable result in at least one of the above evaluations. That is, unmelted solder powder was generated in samples 49 to 56, and the long / short ratio was 1.05 or more. Furthermore, the shear strength was as low as 70 to 100%, the void ratio was about 4 to 13%, and voids were generated at a considerable rate. In the heat cycle test, which is an evaluation of reliability, defects occurred in all samples up to 300 cycles.

1 Cu substrate 2 Ni layer 3 Solder sample 4 Si chip

Claims (7)

  Excluding elements containing 38.0% by mass or more and less than 58.0% by mass of Sn, Al exceeding 0.1% by mass and 1.2% by mass or less, and the balance is inevitably contained in production. A Pb-free Bi—Sn—Al-based solder alloy characterized in that   The Pb-free Bi-Sn-Al solder according to claim 1, wherein Sn is contained in an amount of 41.0 mass% or more and less than 55.0 mass%, and Al is contained in an amount exceeding 0.5 mass%. alloy.   It further contains one or more of Ag, Ni, Sb and P. When Ag is contained, 0.01 mass% to 3.0 mass%, and when Ni is contained, 0.01 mass% to 0 mass% 0.7 mass% or less, 0.01 to 5.0 mass% when containing Sb, and 0.5 mass% or less when containing P, A Pb-free Bi—Sn—Al solder alloy described in 1.   4. The Pb-free Bi—Sn—Al solder alloy according to claim 1, wherein the metal structure of the solder alloy includes a lamellar structure. 5.   A Bi-Sn-Al solder paste produced by mixing the Bi-Sn-Al solder alloy powder according to any one of claims 1 to 4 and a flux.   The electronic component mounting board | substrate joined by the Bi-Sn-Al type solder alloy of any one of Claims 1-4, or the solder paste of Claim 5. The apparatus which mounts the board | substrate with which the electronic component of Claim 6 is mounted.

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