Preform solder, method for manufacturing same, and method for manufacturing solder joint

Abstract

Provided is a preform solder capable of enhancing thermal conductivity of a solder connection part, a method for manufacturing the same, and a method for manufacturing a solder joint. The present invention uses a preform solder that contains: a first metal including Sn; a second metal comprising an alloy including Ni and Fe; and a third metal having an entire surface formed from a metal including Ni. Alternatively, the present invention uses a preform solder 1 that has a metal structure comprising a first phase (10) which is a continuous phase, a second phase (20) which is dispersed in the first phase (10), and a third phase (30) which is dispersed in the first phase. The first phase (10) includes Sn; the second phase (20) comprises an alloy including Ni and Fe; and the entire surface of the third phase (30) is configured from a metal formed from a metal including Ni.

Description

Preform solder, its manufacturing method, and solder joint manufacturing method

[0001] The present invention relates to a solder preform, a method for manufacturing the same, and a method for manufacturing a solder joint. This application claims priority to Japanese Patent Application No. 2023-207004, filed on December 7, 2023, the contents of which are incorporated herein by reference.

[0002] In recent years, as the operating environment of power semiconductor elements using silicon carbide (SiC) and the like has become hotter, the temperature at solder joints can reach approximately 250 to 280°C. For this reason, there is a demand for high-temperature solder that does not melt when operating under such high-temperature conditions.

[0003] To produce such high-temperature solder joints, a joining method using a preformed solder is used as the soldering material. A preformed solder is a molded product obtained by processing solder into various shapes such as a square, ribbon, or disk. For example, a preformed solder has been proposed that is produced by compressing a mixed powder of a metal powder of Sn and a metal powder made of an alloy of Ni and Fe (see Patent Document 1).

[0004] Patent No. 7014991

[0005] In order to ensure the operation of power semiconductor elements, it is necessary to improve the heat dissipation from the power semiconductor elements by increasing the thermal conductivity of the solder joints. In contrast, in the preform solder described in Patent Document 1, it is necessary to further improve the thermal conductivity of the solder joints.

[0006] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a preform solder, a method for manufacturing the same, and a method for manufacturing a solder joint, which increase the thermal conductivity of the solder joint.

[0007] The present invention includes the following aspects: [1] A metal conductor includes a first metal containing Sn, a second metal made of an alloy containing Ni and Fe, and a third metal whose entire surface is formed of a metal containing Ni, wherein the melting point of the first metal is 300°C or less, the melting point of the alloy in the second metal is greater than 300°C, the melting point of the metal containing Ni that forms the entire surface of the third metal is greater than 300°C, the content of Sn in the first metal is 20% by mass or more and 100% by mass or less with respect to the total mass of the first metal, the content of Ni in the second metal is 80% by mass or more and 99% by mass or less with respect to the total mass of the second metal, and the content of Fe in the second metal is 100% by mass or more and 100% by mass or less with respect to the total mass of the second metal. the Ni content in the Ni-containing metal forming the entire surface of the third metal is 50% by mass or more and 100% by mass or less with respect to the total mass of the metal forming the entire surface of the third metal; the particle size of the second metal is 0.1 to 1000 μm, the particle size of the third metal is 0.1 to 1000 μm, the content of the second metal is 1 to 70% by mass with respect to the total content of the first metal, the second metal, and the third metal; and the content of the third metal is 1 to 70% by mass with respect to the total content of the first metal, the second metal, and the third metal.

[0008] [2] The composition of the third metal is different from the compositions of the first metal and the second metal, and the third metal consists only of Ni or contains Ni and a metal other than Ni, the content of Ni in the third metal is 50 mass% or more with respect to the total mass of the third metal, the melting point of the third metal is greater than 300 ° C, the content of the first metal is 20 to 90 mass% with respect to the total content of the first metal, the second metal, and the third metal, and the content of the third metal is 5 to 30 mass% with respect to the total content of the first metal, the second metal, and the third metal. Preform solder according to [1].

[0009] [3] The preform solder according to [1], wherein the third metal has a structure consisting of a core portion and a surface layer covering the core portion, the composition of the metal forming the surface layer of the third metal is different from the compositions of the first metal and the second metal, the metal forming the surface layer of the third metal consists of only Ni or contains Ni and a metal other than Ni, the content of Ni in the metal forming the surface layer of the third metal is 50 mass% or more with respect to the total mass of the metal forming the surface layer of the third metal, the melting point of the metal forming the surface layer of the third metal is above 300°C, the content of the first metal is 20 to 90 mass% with respect to the total content of the first metal, the second metal, and the third metal, and the content of the third metal is 5 to 30 mass% with respect to the total content of the first metal, the second metal, and the third metal.

[0010] [4] The preform solder according to [3], which has an intermediate layer between the core portion and the surface layer covering the core portion.

[0011] [5] The preform solder according to any one of [1] to [4], further comprising a fourth metal containing Sn that coats the third metal.

[0012] [6] A metal structure including a first continuous phase, a second phase dispersed in the first phase, and a third phase dispersed in the first phase, wherein the first phase is composed of a metal containing Sn, the second phase is composed of an alloy containing Ni and Fe, and the entire surface of the third phase is composed of a metal containing Ni, the melting point of the metal constituting the first phase as a whole is 300°C or less, the melting point of the alloy constituting the second phase as a whole is greater than 300°C, the melting point of the metal containing Ni constituting the entire surface of the third phase is greater than 300°C, the content of Sn in the metal constituting the first phase is 20% by mass or more and 100% by mass or less with respect to the total mass of the metal, the content of Ni in the alloy constituting the second phase is 80% by mass or more and 99% by mass or less with respect to the total mass of the alloy, and the content of Fe in the alloy constituting the second phase is greater than 300°C. the content of Ni in the Ni-containing metal forming the entire surface of the third phase is 50% by mass or more and 100% by mass or less with respect to the total mass of the metal forming the entire surface of the third phase; the particle size of the second phase is 0.1 to 1000 μm, the particle size of the third phase is 0.1 to 1000 μm; the total content of the alloy constituting the second phase is 1 to 70% by mass with respect to the total content of the entire metal constituting the first phase, the entire alloy constituting the second phase, and the entire metal constituting the third phase; and the total content of the metal constituting the third phase is 1 to 70% by mass with respect to the total content of the entire metal constituting the first phase, the entire alloy constituting the second phase, and the entire metal constituting the third phase.

[0013] [7] The overall composition of the metal constituting the third phase is different from the overall composition of the metal constituting the first phase and the overall composition of the alloy constituting the second phase, and the third phase consists of only Ni, or contains Ni and a metal other than Ni, and the content of Ni in the metal constituting the third phase is 50% by mass or more with respect to the total mass of the metal constituting the third phase, and the melting point of the metal constituting the third phase as a whole is greater than 300 ° C., and the total content of the metal constituting the first phase is 20 to 90% by mass with respect to the total content of the metals constituting the first phase, the total alloy constituting the second phase, and the total content of the metals constituting the third phase. The preform solder according to [6] is 5 to 30% by mass with respect to the total content of the metals constituting the first phase, the total alloy constituting the second phase, and the total content of the metals constituting the third phase.

[0014] [8] The third phase has a structure consisting of a core portion and a surface layer covering the core portion, and the overall composition of the metal forming the surface layer of the third phase is different from the overall composition of the metal forming the first phase and the overall composition of the alloy forming the second phase, the surface layer of the third phase consists of only Ni or contains Ni and a metal other than Ni, the content of Ni in the surface layer of the third phase is 50 mass% or more with respect to the total mass of the surface layer of the third phase, and The melting point of the preform solder according to [6] is above 300°C, the total content of the metals constituting the first phase is 20 to 90 mass% with respect to the total content of the metals constituting the first phase, the total content of the alloys constituting the second phase, and the total content of the metals constituting the third phase, and the total content of the metals constituting the third phase is 5 to 30 mass% with respect to the total content of the metals constituting the first phase, the total content of the alloys constituting the second phase, and the total content of the metals constituting the third phase.

[0015] [9] The solder preform according to [8], which has an intermediate layer between the core portion and the surface layer covering the core portion.

[10] The solder preform according to any one of [6] to [9], which has a metal structure further including a fourth phase containing Sn that covers the third phase.

[0016]

[11] A method for manufacturing a solder preform, comprising: a mixing step of mixing a first metal powder containing Sn, a second metal powder made of an alloy containing Ni and Fe, and a third metal powder whose entire surface is formed of a metal containing Ni, to prepare a metal powder mixture; and a rolling step of rolling the metal powder mixture to manufacture a solder preform, wherein the melting point of the first metal powder is 300°C or less, the melting point of the alloy in the second metal powder is higher than 300°C, the melting point of the metal containing Ni that forms the entire surface of the third metal powder is higher than 300°C, the content of Sn in the first metal powder is 20% by mass or more and 100% by mass or less with respect to the total mass of the first metal powder, the content of Ni in the second metal powder is 80% by mass or more and 99% by mass or less with respect to the total mass of the second metal powder, a content of Fe in the metal powder is 1% by mass or more and 20% by mass or less with respect to the total mass of the second metal powder; a content of Ni in the Ni-containing metal forming the entire surface of the third metal powder is 50% by mass or more and 100% by mass or less with respect to the total mass of the metal forming the entire surface of the third metal powder; a particle size of the first metal powder is 0.1 to 1000 μm, a particle size of the second metal powder is 0.1 to 1000 μm, a particle size of the third metal powder is 0.1 to 1000 μm; and a particle size of the third metal powder is 0.1 to 1000 μm; and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 95 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 1 to 70 parts by mass of the third metal powder.

[0017]

[12] The composition of the third metal powder is different from the compositions of the first metal powder and the second metal powder, and the third metal powder consists only of Ni or contains Ni and a metal other than Ni, the content of Ni in the third metal powder is 50 mass% or more relative to the total mass of the third metal powder, and the melting point of the third metal powder is greater than 300°C, and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 90 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 5 to 30 parts by mass of the third metal powder.

[11] The method for producing preformed solder.

[0018]

[13] The third metal powder has a structure consisting of a core portion and a surface layer covering the core portion, and the composition of the metal forming the surface layer of the third metal powder is different from the compositions of the first metal powder and the second metal powder, and the metal forming the surface layer of the third metal powder consists of only Ni or contains Ni and a metal other than Ni, and the content of Ni in the metal forming the surface layer of the third metal powder is 50 mass% or more with respect to the total mass of the metal forming the surface layer of the third metal powder, and the melting point of the metal forming the surface layer of the third metal powder is above 300°C, and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 90 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 5 to 30 parts by mass of the third metal powder.

[11] The method for producing preform solder according to

[11] .

[0019]

[14] The method for producing a preform solder according to

[13] , wherein the third metal powder has an intermediate layer between the core portion and the surface layer covering the core portion.

[15] The method for producing a preform solder according to

[11] to

[14] , further comprising a step of coating the third metal powder with a fourth metal.

[16] A method for producing a solder joint, forming a joint between objects using a preform solder produced by the method for producing a preform solder according to any one of

[11] to

[14] .

[17] A method for producing a solder joint, forming a joint between objects using a preform solder produced by the method for producing a preform solder according to

[15] .

[0020] According to the present invention, it is possible to provide a solder preform, a method for manufacturing the same, and a method for manufacturing a soldered joint, which can increase the thermal conductivity of a soldered joint.

[0021] Fig. 1 is a perspective view showing an embodiment of a preform solder; Fig. 2 is a schematic view showing a cross section in the thickness direction of an embodiment of the preform solder; Fig. 3 is a schematic view showing a cross section of a third metal 30A in an embodiment of the preform solder; Fig. 4 is a schematic view showing a cross section of a third metal 30B in an embodiment of the preform solder; Fig. 5 is an SEM image showing a cross section in the thickness direction of the preform solders of Example 1 and Comparative Example 1.

[0022] In this specification, the terms "comprise" and "contain" are concepts that encompass all of "comprise," "consist essentially of," and "consist only of."

[0023] In this specification, the terms "first metal," "second metal," "third metal," "fourth metal," and "fifth metal" may refer to "particles formed of the first metal," "particles formed of the second metal," "particles formed of the third metal," "particles formed of the fourth metal," and "particles formed of the fifth metal," respectively. In this specification, the terms "first metal powder," "second metal powder," "third metal powder," "fourth metal powder," and "fifth metal powder" may refer to "particle groups formed of the first metal," "particle groups formed of the second metal," "particle groups formed of the third metal," "particle groups formed of the fourth metal," and "particle groups formed of the fifth metal," respectively.

[0024] (Preform solder: first embodiment) Fig. 1 shows one embodiment of a preform solder according to the present invention. Preform solder 1 is square-shaped and contains a first metal containing Sn, a second metal made of an alloy containing Ni and Fe, and a third metal whose entire surface is formed from a metal containing Ni. Preform solder 1 is characterized by further containing a third metal whose entire surface is formed from a metal containing Ni.

[0025] <First Metal> The first metal contains Sn. Since Sn has excellent ductility, the first metal containing Sn can eliminate voids between the first metals by plastic deformation. Furthermore, the first metal containing Sn can ensure general performance such as wettability as a soldering material.

[0026] The first metal may contain a metal other than Sn. Examples of metals other than Sn that the first metal may contain include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, and Zr. These metals other than Sn may contain one type or two or more types. The group of metals other than Sn can be arbitrarily selected from these metals.

[0027] The metal that may be contained in the first metal may be a simple substance such as Sn or a metal other than Sn, or may be an alloy of two or more simple substances selected from Sn and simple metals other than Sn.

[0028] The first metal may be, for example, Sn alone, a mixture of Sn and a metal other than Sn, an alloy of Sn and a metal other than Sn, or a mixture of an alloy containing Sn and a metal other than Sn.

[0029] The first metal may contain unavoidable impurities in addition to the above-mentioned metals. Even if the first metal contains unavoidable impurities, the effects of the present invention are not affected. The first metal may be one type or two or more types.

[0030] The melting point of the first metal is preferably 300° C. or lower, and may be 250° C. or lower, or may be 116 to 200° C. When the melting point of the first metal is equal to or lower than the upper limit of the above-mentioned preferred range, it becomes easier to ensure the wettability of the solder.

[0031] As used herein, the "melting point of the metal to be measured, or the melting point of the metal powder to be measured" refers to the melting point measured by differential scanning calorimetry (DSC). The melting point of the metal to be measured refers to the temperature at which the amount of heat absorbed per unit time is highest, based on the results of DSC measurement of the metal to be measured. When the metal to be measured has one peak in DSC measurement, the melting point of the metal to be measured refers to the temperature at the top of that peak. When the metal to be measured has multiple peaks in DSC measurement, the melting point of the metal to be measured refers to the temperature at the peak top with the highest amount of heat absorbed per unit time among the multiple peak tops. In other words, when the metal to be measured contains multiple types of metals, the melting point of the metal to be measured is taken as the temperature at the peak top with the highest amount of heat absorbed per unit time among the multiple peak tops that the metal to be measured may have. The same applies to the melting point of the metal powder to be measured. The melting points of the first metal and the fourth metal can be measured, for example, using a DSC7020 manufactured by Hitachi High-Tech Science Corporation. The melting points of the second metal and the third metal can be measured using, for example, a DSC404-F3 Pegasus manufactured by NETZSCH.

[0032] The content of Sn in the first metal is preferably 20% by mass or more and 100% by mass or less, based on the total mass of the first metal. In order to fully exhibit the properties of Sn, the content of Sn in the first metal is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass, based on the total mass of the first metal.

[0033] <Second Metal> The second metal is an alloy containing Ni and Fe. The alloy in the second metal contains Ni and Fe, and preferably has a higher melting point than the first metal and is dispersed within the preform solder. The melting point of the alloy in the second metal is preferably above 300°C, more preferably above 500°C, and even more preferably 600 to 1600°C. If the melting point of the alloy in the second metal is above the lower limit of the above-mentioned preferred range, the shear strength of the solder joint is likely to be increased even in a high-temperature operating environment.

[0034] The alloy of the second metal may contain a metal other than Ni and Fe. That is, the second metal may be an alloy of Ni and Fe, or an alloy of Ni, Fe, and a metal other than these, and among these, an alloy of Ni and Fe is preferable.

[0035] Examples of metals other than Ni and Fe that may be included in the second metal include Sn, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Mn, and Zr. These metals other than Ni and Fe may include one type or two or more types. The group of metals other than Ni and Fe can be arbitrarily selected from these metals.

[0036] The second metal may contain unavoidable impurities in addition to the above-mentioned metals. Even if the second metal contains unavoidable impurities, the effects of the present invention are not affected. The second metal may be one type or two or more types.

[0037] The Ni content in the second metal is preferably 80% by mass or more and 99% by mass or less, more preferably 85% by mass or more and 95% by mass or less, based on the total mass of the second metal. The Fe content in the second metal is preferably 1% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 15% by mass or less, based on the total mass of the second metal. When the Ni and Fe contents in the second metal are within the above-mentioned preferred ranges, intermetallic compounds are formed at an earlier stage, and the generation of voids can be suppressed.

[0038] The term "particle size of a metal or particle size of a metal powder" as used herein refers to the average particle size measured on a volume basis using a laser diffraction / scattering particle size analyzer. The average particle size can be measured using, for example, a Microtrac-Bell laser diffraction / scattering particle size analyzer (MT3300EXII).

[0039] In the preform solder according to the first embodiment, the particle size of the second metal is preferably 0.1 to 1000 μm, more preferably 1 to 100 μm, and even more preferably 5 to 50 μm. When the particle size of the second metal is equal to or greater than the lower limit of the above-mentioned preferred range, it becomes easier to ensure wettability, and when it is equal to or less than the upper limit of the above-mentioned preferred range, it becomes easier to form an intermetallic compound.

[0040] <Third Metal> The entire surface of the third metal is formed of a metal containing Ni. That is, in the third metal, Ni is exposed on the surface. The Ni content in the metal forming the entire surface of the third metal is 50 mass % or more and 100 mass % or less with respect to the total mass of the metal forming the entire surface of the third metal. The melting point of the metal forming the entire surface of the third metal is above 300°C, preferably 500°C or more, and more preferably 600 to 1600°C. The third metal is preferably dispersed in the preform solder.

[0041] The metal forming the entire surface of the third metal may consist of only Ni, or may contain a metal other than Ni. Examples of the metal forming the entire surface of the third metal include simple Ni, an alloy of Ni with a metal other than Ni, and a mixture of an alloy containing Ni and another metal, with simple Ni being preferred.

[0042] Examples of the metal other than Ni in the alloy of Ni and a metal other than Ni include Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn. These metals other than Ni may include one type or two or more types. The group of metals other than Ni can be arbitrarily selected from these metals.

[0043] The metal forming the entire surface of the third metal may contain unavoidable impurities in addition to the above-mentioned metals. Even if unavoidable impurities are contained, the effects of the present invention are not affected. The metal forming the entire surface of the third metal may be one type, or two or more types.

[0044] When the metal forming the entire surface of the third metal is a metal containing Ni and a metal other than Ni, the content of Ni in the metal forming the entire surface is 50 mass% or more, preferably 70 mass% or more, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 98 mass% or more, relative to the total mass of the metal forming the entire surface. When the metal forming the entire surface of the third metal contains Fe, the content of Fe in the metal forming the entire surface of the third metal is preferably 0 mass% or more and less than 5 mass% relative to the total mass of the metal forming the entire surface of the third metal.

[0045] In the solder preform according to the first embodiment, the particle size of the third metal is preferably 0.1 to 1000 μm, more preferably 1 to 300 μm, and even more preferably 10 to 200 μm. When the particle size of the third metal is equal to or greater than the lower limit of the above-mentioned preferred range, the thermal conductivity of the solder joint is easily increased.

[0046] The specific structure of the third metal will be described below with reference to examples. As described in (1) below, the third metal may have a uniform composition throughout. Alternatively, as described in (2) below, the third metal may have a structure with a plurality of different compositions.

[0047] When the third metal has a uniform composition throughout, the third metal may be a metal consisting only of Ni, as described in (1-1) below, or may be a metal containing Ni and a metal other than Ni, as described in (1-2) below. Alternatively, when the third metal has a structure having a plurality of different compositions, the third metal may have a core portion and a surface layer, as described in (2) below. These cases will be described below.

[0048] (1) When the entire third metal has a uniform composition (1-1) When the third metal consists only of Ni The composition of the third metal is different from the compositions of the first metal and the second metal. In this case, the content of Ni in the metal forming the entire surface of the third metal is 100 mass% relative to the total mass of the metal forming the entire surface of the third metal. The proportion of Ni on the surface of the third metal is 100% relative to the entire area (100%) of the surface of the third metal. The third metal may contain inevitable impurities in addition to Ni. Even if the third metal contains inevitable impurities, the effects of the present invention are not affected.

[0049] (1-2) When the third metal contains Ni and a metal other than Ni The composition of the third metal is different from the compositions of the first metal and the second metal. The third metal may be a mixture of Ni and a metal other than Ni, an alloy of Ni and a metal other than Ni, or a mixture of an alloy containing Ni and a metal other than Ni.

[0050] Examples of metals other than Ni that may be included in the third metal include Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn. These metals other than Ni may be included alone or in combination of two or more. The group of metals other than Ni can be arbitrarily selected from these metals.

[0051] The third metal may contain inevitable impurities in addition to the above-mentioned metals. Even if inevitable impurities are contained, the effects of the present invention are not affected. The third metal (1-2) may be one type or two or more types.

[0052] When the third metal is a metal containing Ni and a metal other than Ni, the content of Ni in the third metal is 50% by mass or more and 100% by mass or less, preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 98% by mass or more, based on the total mass of the third metal. When the third metal contains Fe, the content of Fe in the third metal is preferably 0% by mass or more and less than 5% by mass, based on the total mass of the third metal.

[0053] In the case of (1), the melting point of the third metal is more than 300°C, preferably 500°C or higher, and more preferably 600 to 1600°C.

[0054] In the case of (1), the particle size of the third metal is preferably 0.1 to 1000 μm, more preferably 1 to 300 μm, and even more preferably 10 to 200 μm.

[0055] (2) Case where the third metal has a structure consisting of a core portion and a surface layer covering the core portion As illustrated in FIG. 3, the third metal 30A has a core portion 301 and a surface layer 302 covering the core portion 301. Rc means the diameter of the core portion 301. Rs means the thickness of the surface layer 302. The "surface layer covering the core portion" can also be rephrased as "a covering layer covering the core portion." The composition of the metal forming the surface layer 302 is different from the composition of the metal forming the core portion 301. The composition of the third metal is different from the compositions of the first metal and the second metal. The composition of the metal forming the surface layer of the third metal is different from the compositions of the first metal and the second metal.

[0056] Surface Layer The metal forming the surface layer of the third metal may consist of only Ni, or may be a metal containing Ni and a metal other than Ni. That is, the metal forming the surface layer of the third metal may be Ni alone, or may be an alloy of Ni and a metal other than Ni. The metal forming the surface layer of the third metal is preferably Ni alone.

[0057] Examples of metals other than Ni that may be contained in the metal forming the surface layer of the third metal include Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn. These metals other than Ni may include one type or two or more types. The group of metals other than Ni can be arbitrarily selected from these metals.

[0058] The metal forming the surface layer of the third metal may contain unavoidable impurities in addition to the above-mentioned metals. Even if unavoidable impurities are contained, the effects of the present invention are not affected. The metal forming the surface layer of the third metal may be one type or two or more types.

[0059] When the metal forming the surface layer of the third metal is a metal containing Ni and a metal other than Ni, the content of Ni in the metal forming the surface layer of the third metal is 50% by mass or more and less than 100% by mass, preferably 70% by mass or more, more preferably 80% by mass or more, even more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 98% by mass or more, relative to the total mass of the metal forming the surface layer of the third metal. When the metal forming the surface layer of the third metal contains Fe, the content of Fe in the metal forming the surface layer of the third metal is preferably 0% by mass or more and less than 5% by mass, relative to the total mass of the metal forming the surface layer of the third metal.

[0060] At least a portion of the surface of the core part is covered with a surface layer. In Fig. 1, the entire surface of the core part is covered with the surface layer. The proportion of the surface area of ​​the core part that is covered with the surface layer is preferably 50% to 100% of the total surface area (100%) of the core part, more preferably 70% to 100%, even more preferably 90% to 100%, particularly preferably 95% to 100%, and most preferably 100%.

[0061] The thickness Rs of the surface layer of the third metal may be, for example, 0.01 μm or more and 100 μm or less. The thickness of the surface layer of the third metal may be 0.1 μm or more, 0.3 μm or more, 0.5 μm or more, 0.75 μm or more, 1 μm or more, or 2 μm or more. The average thickness of the surface layer of the third metal may be 50 μm or less, 30 μm or less, 10 μm or less, 5 μm or less, 3 μm or less, or 2 μm or less.

[0062] In this specification, the thickness Rs of the surface layer of the third metal, the diameter Rc of the core portion of the third metal described below, the thickness Ri of the intermediate layer, and the thickness of the layer of the fourth metal containing Sn can be measured from the cross-sectional structure of the third metal and the layer of the fourth metal containing Sn using an optical microscope, SEM, transmission electron microscope (TEM), etc. The thickness Rs of the surface layer and the thickness Ri of the intermediate layer can each be calculated by measuring the thickness at three or more cross sections and averaging the thicknesses. The diameter Rc of the core portion can be calculated by measuring the diameters of three or more third metals and averaging the diameters.

[0063] Alternatively, the diameter Rc of the core portion of the third metal can be measured as follows: When producing the third metal powder, the particle size of the metal powder prepared to be used as the core portion can be taken as the diameter Rc.

[0064] The surface layer of the third metal may be a plated layer formed by plating, for example, known electroplating or electroless plating.

[0065] In the case of (2), the melting point of the metal forming the surface layer of the third metal is more than 300°C, preferably 500°C or higher, and more preferably 600 to 1600°C.

[0066] Core portion The metal forming the core portion of the third metal may be one type of elemental metal, a mixture of two or more types of elemental metal, an alloy formed from two or more types of metal elements, a mixture of alloys formed from two or more types of metal elements, or a mixture of an alloy formed from two or more types of metal elements and an elemental metal.

[0067] Examples of metals that may be contained in the core portion of the third metal include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn. These metals may be contained alone or in combination of two or more. The metal group that may be contained in the core portion can be arbitrarily selected from these metals.

[0068] The core portion of the third metal may contain inevitable impurities in addition to the above-mentioned metals. Even if inevitable impurities are contained, the effects of the present invention are not affected. The metal forming the core portion of the third metal may be one type or two or more types.

[0069] The diameter Rc of the core portion of the third metal is preferably 0.1 to 1000 μm, more preferably 3 to 300 μm, even more preferably 5 to 200 μm, particularly preferably 20 to 200 μm, and most preferably 70 to 200 μm. The diameter Rc may be 150 μm or less, 100 μm or less, 75 μm or less, 50 μm or less, 30 μm or less, 20 μm or less, or 15 μm or less.

[0070] The ratio of Rc to Rs, expressed as Rc / Rs, may be 0.1 to 1000, 1 to 300, or 5 to 200.

[0071] (2) The third metal 30A may be one type or two or more types.

[0072] (3) Other Forms As illustrated in FIG. 4 , the third metal 30B may have an intermediate layer 303 between the core portion 301 and the surface layer 302 covering the core portion 301. The surface layer 302 covers the core portion 301 via the intermediate layer 303. The intermediate layer 303 is adjacent to both the core portion 301 and the surface layer 302. Ri denotes the thickness of the intermediate layer 303. The intermediate layer 303 may cover a portion of the core portion 301 or the entire core portion 301, and preferably covers the entire core portion 301. The third metal 30B is similar to the third metal 30A except for having the intermediate layer 303. The third metal 30B having the intermediate layer 303 makes it easier to provide the surface layer 302 of the third metal 30B.

[0073] The composition of the metal forming the intermediate layer 303 is different from the metal forming the core portion 301 and the metal forming the surface layer 302. The intermediate layer may be one layer or two or more layers.

[0074] The metal forming the intermediate layer may be a single elemental metal or an alloy formed of two or more metal elements.

[0075] Examples of metals that may be contained in the intermediate layer include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn. These metals may be contained alone or in combination of two or more. The metal group that may be contained in the intermediate layer can be arbitrarily selected from these metals.

[0076] The intermediate layer may contain unavoidable impurities in addition to the above-mentioned metals. Even if unavoidable impurities are contained, the effects of the present invention are not affected. The metal forming the intermediate layer may be one type or two or more types.

[0077] The thickness Ri of the intermediate layer may be, for example, 0.01 μm or more and 100 μm or less, 1 μm or more and 50 μm or less, or 2 μm or more and 10 μm or less.

[0078] The intermediate layer may be formed by plating, for example, known electroplating or electroless plating.

[0079] Examples of metals forming the core portion 301 and the surface layer 302 of the third metal 30B include those similar to those described above for the third metal 30A. The third metal 30B of (3) may be one type or two or more types.

[0080] <Fourth Metal Containing Sn> The preform solder according to this embodiment may further contain a fourth metal containing Sn that coats the third metal. The fourth metal may coat a portion of the third metal, or may coat the entire third metal, and preferably coats the entire third metal. The third metal may be coated with a layer formed of the fourth metal. In this case, the fourth metal layer that coats the third metal may be one layer, or two or more layers.

[0081] The fourth metal may contain a metal other than Sn. That is, the fourth metal may be Sn alone, a mixture of Sn and a metal other than Sn, an alloy of Sn and a metal other than Sn, or a mixture of an alloy containing Sn and a metal other than Sn. The fourth metal is preferably Sn alone.

[0082] Examples of metals other than Sn that may be included in the fourth metal include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, and Zr. The metal other than Sn may include one type or two or more types. The group of metals other than Sn can be arbitrarily selected from these metals.

[0083] The fourth metal may contain inevitable impurities in addition to the above-mentioned metals. Even if inevitable impurities are contained, the effects of the present invention are not affected. The fourth metal may be formed of one type of metal or two or more types of metals.

[0084] When the fourth metal is a metal containing Sn and a metal other than Sn, the content of Sn in the fourth metal is preferably 20 mass% or more and 100 mass% or less, more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 100 mass% relative to the total mass of the fourth metal.

[0085] The average thickness of the fourth metal layer may be, for example, 0.01 μm or more and 100 μm or less, 1 μm or more and 50 μm or less, or 2 μm or more and 10 μm or less.

[0086] The fourth metal layer may be a plated layer formed by plating, for example, known electroplating or electroless plating.

[0087] The melting point of the fourth metal is preferably 300° C. or lower, and may be 250° C. or lower, or may be 116 to 200° C. When the melting point of the fourth metal is equal to or lower than the upper limit of the above-mentioned preferred range, the wettability of the preform solder is improved when melted, and further, a reduction in voids can be expected.

[0088] The fourth metal contains Sn, which improves wettability during melting of the preform solder and is expected to reduce voids. The metal layer melts after heating begins to melt the preform solder, exposing the surface layer of the third metal, so the effect of the present invention is not impaired.

[0089] <Relationship between the Contents of First Metal, Second Metal, and Third Metal> In the preform solder according to the first embodiment, the mixing ratio of the first metal, the second metal, and the third metal is such that the content of the first metal relative to the total content of the first metal, the second metal, and the third metal is preferably 20 to 95 mass%, more preferably 20 to 90 mass%, even more preferably 40 to 90 mass%, particularly preferably 50 to 85 mass%, and most preferably 60 to 75 mass%.

[0090] In the preform solder according to the first embodiment, the mixing ratio of the first metal, the second metal, and the third metal is, from the viewpoint of achieving both bondability and shear strength, preferably 1 to 70 mass% of the second metal relative to the total content of the first metal, the second metal, and the third metal, more preferably 3 to 50 mass%, even more preferably 5 to 40 mass%, particularly preferably 5 to 30 mass%, and most preferably 5 to 25 mass%. When the content of the second metal is equal to or greater than the lower limit of the preferred range, the generation of voids during Sn melting is easily suppressed. Furthermore, the heat resistance of the solder joint is further improved. When the content is equal to or less than the upper limit of the preferred range, the generation of porous structures due to intermetallic compound formation is suppressed, thereby suppressing the generation of voids and making it easier to maintain shear strength. In particular, the generation of microvoids within the solder joint is easily suppressed.

[0091] In the preform solder according to the first embodiment, the mixing ratio of the first metal, the second metal, and the third metal is preferably 1 to 70 mass %, more preferably 3 to 50 mass %, even more preferably 5 to 40 mass %, particularly preferably 5 to 30 mass %, and most preferably 20 to 30 mass % of the third metal relative to the total content of the first metal, the second metal, and the third metal. When the content of the third metal is equal to or greater than the lower limit of the preferred range, the thermal conductivity of the solder joint is easily increased, and when the content is equal to or less than the upper limit of the preferred range, the generation of voids is suppressed.

[0092] In the preform solder according to the first embodiment, the total content of the first metal, the second metal, and the third metal does not exceed 100 mass %.

[0093] In the preform solder according to the first embodiment, when the third metal is coated with a fourth metal containing Sn, the mixing ratio of the content of the fourth metal to the content of the third metal may be 0.01 to 10 as a mass ratio expressed as fourth metal / third metal.

[0094] In the preform solder according to the first embodiment, the ratio of the content of the first metal to the content of the second metal is preferably 1 or more and 50 or less, more preferably 2 or more and 9 or less, as a mass ratio expressed as the content of the first metal / the content of the second metal.

[0095] In the preform solder according to the first embodiment, the ratio of the content of the second metal to the content of the third metal is preferably 0.1 or more and 10 or less as a mass ratio expressed as the content of the second metal / the content of the third metal.

[0096] The preform solder according to the first embodiment is not limited to the above-described embodiment and may contain a metal other than the first metal, the second metal, the third metal, and the fourth metal (hereinafter also referred to as a "fifth metal"). The fifth metal is not particularly limited in composition as long as it is different in composition from the first metal, the second metal, the third metal, and the fourth metal. For example, powders of Ni, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and Sn, or alloys formed from two or more of these elemental metals, are preferred. The particle size of the fifth metal is preferably 0.1 to 1000 μm, more preferably 1 to 100 μm, and even more preferably 5 to 50 μm. The fifth metal may contain one type of metal or two or more types of metals. The fifth metal is not limited to one type, and metal powders of two or more different compositions may be used.

[0097] (Preform solder: second embodiment) Fig. 2 is a schematic diagram showing a cross section in the thickness direction of one embodiment of preform solder. The preform solder 1 shown in Fig. 2 has a metal structure including a first phase 10 that is a continuous phase, a second phase 20 dispersed in the first phase, and a third phase 30 dispersed in the first phase. R ２０ means the diameter of the second phase 20, and R ３０ means the diameter of the third phase 30. The first phase 10 contains Sn. Metal grain boundaries may be present in the first phase 10. The second phase 20 is made of an alloy containing Ni and Fe. The third phase 30 is made of a metal whose entire surface is formed of a metal containing Ni.

[0098] In the preform solder 1, the first phase 10 is a continuous phase and is composed of a metal containing Sn. The description of the metal containing Sn and its content is the same as that of the first metal described above. Furthermore, in the first phase 10, crystal grain boundaries may exist between the metal crystals containing Sn.

[0099] The melting point of the metals constituting the first phase as a whole can be measured in the same manner as the melting point of the first metal. When the first phase contains multiple types of metals, the melting point of the metals constituting the first phase as a whole is determined by the temperature of the peak top with the highest heat absorption per unit time among the multiple peak tops that the multiple types of metals constituting the first phase may have. The melting point of the metals constituting the second phase as a whole is defined in the same manner. The melting point of the metals constituting the entire surface of the third phase is also defined in the same manner. When the entire third phase has a uniform composition, the melting point of the metals constituting the third phase as a whole is also defined in the same manner. When the third phase has a structure consisting of a core portion and a surface layer covering the core portion, the melting point of the metals constituting the surface layer of the third phase is also defined in the same manner. The melting point of the metals constituting the fourth phase as a whole is also defined in the same manner.

[0100] The explanation for the melting point of the metals constituting the first phase as a whole is the same as the explanation for the melting point of the first metal.

[0101] In the preform solder 1, the second phase 20 is dispersed in the first phase 10. The second phase 20 is composed of an alloy containing Ni and Fe. The description of the alloy containing Ni and Fe, its particle size, its content, etc. is the same as that of the above-mentioned <Second Metal>.

[0102] The explanation for the melting point of the alloy as a whole that constitutes the second phase is the same as the explanation for the melting point of the second metal.

[0103] In this specification, the grain size of a phase can be measured and calculated from a cross-sectional structure containing the phase using an optical microscope, SEM, transmission electron microscope (TEM), etc. The grain size of a phase can be calculated by measuring the diameters of three or more phases and averaging these diameters.

[0104] The particle size of the second phase may be the particle size of the second metal powder prepared to form the second phase.

[0105] In the preform solder 1, the third phase 30 is dispersed in the first phase 10. The entire surface of the third phase 30 is composed of a metal containing Ni. The metal forming the entire surface of the third phase 30 is preferably simple Ni. The explanation of the metal whose entire surface contains Ni, its particle size, its content, etc. is the same as in the above <Third Metal>. The explanation of the melting point of the metal forming the entire surface of the third phase is the same as the melting point of the metal forming the entire surface of the third metal.

[0106] When the third phase has a uniform composition throughout, the overall composition of the metals constituting the third phase differs from the overall composition of the metals constituting the first phase and the overall composition of the alloy constituting the second phase. The explanation for the overall melting point of the metals constituting the third phase is the same as the explanation for the melting point of the third metal. The particle size of the third phase can be the particle size of the third metal powder prepared to form the third phase.

[0107] When the third phase has a structure consisting of a core portion and a surface layer covering the core portion, the overall composition of the metal forming the surface layer of the third phase is different from the overall composition of the metal forming the first phase and the overall composition of the alloy forming the second phase. The overall melting point of the metal forming the surface layer of the third phase is the same as the melting point of the metal forming the surface layer of the third metal. The explanations for the diameter Rc of the core portion and the thickness Rs of the surface layer of the third phase are the same as those for the diameter Rc of the core portion and the thickness Rs of the surface layer of the third metal, respectively.

[0108] The preform solder 1 may have a metal structure further comprising a fourth phase containing Sn that coats the third phase.

[0109] Hereinafter, the term "content of metals constituting the phases" means "total content of metals constituting the phases." In the preform solder 1, the mixing ratio of the metal constituting the first phase 10, the alloy constituting the second phase 20, and the metal constituting the third phase 30 is preferably such that the content of the metal constituting the first phase is 20 to 95 mass %, more preferably 20 to 90 mass %, even more preferably 40 to 90 mass %, particularly preferably 50 to 85 mass %, and most preferably 60 to 75 mass %, relative to the total content of the metal constituting the first phase, the metal constituting the second phase, and the metal constituting the third phase.

[0110] In the preform solder 1, the mixing ratio of the metal constituting the first phase 10, the alloy constituting the second phase 20, and the metal constituting the third phase 30 is, from the viewpoint of achieving both bondability and shear strength, preferably 1 to 70 mass% of the alloy constituting the second phase 20 relative to the total content of the metal constituting the first phase 10, the alloy constituting the second phase 20, and the metal constituting the third phase 30, more preferably 3 to 50 mass%, even more preferably 5 to 40 mass%, particularly preferably 5 to 30 mass%, and most preferably 5 to 25 mass%. When the content of the alloy constituting the second phase 20 is equal to or greater than the lower limit of the preferred range, the generation of voids during Sn melting is easily suppressed. Furthermore, the heat resistance of the solder joint is further improved. When the content is equal to or less than the upper limit of the preferred range, the generation of porous structures due to intermetallic compound formation is suppressed, thereby suppressing the generation of voids and making it easier to maintain shear strength.

[0111] In the preform solder 1, the mixing ratio of the metal constituting the first phase 10, the alloy constituting the second phase 20, and the metal constituting the third phase 30 is preferably 1 to 70 mass %, more preferably 3 to 50 mass %, even more preferably 5 to 40 mass %, particularly preferably 5 to 30 mass %, and most preferably 20 to 30 mass %, of the total content of the metal constituting the first phase 10, the alloy constituting the second phase 20, and the metal constituting the third phase 30. When the content of the metal constituting the third phase 30 is equal to or greater than the lower limit of the preferred range, the thermal conductivity of the solder joint is easily increased, and when it is equal to or less than the upper limit of the preferred range, the occurrence of voids is suppressed.

[0112] In the preform solder according to the second embodiment, the total content of the metal constituting the first phase 10, the content of the metal constituting the second phase 20, and the content of the metal constituting the third phase 30 does not exceed 100% by mass.

[0113] When the preform solder according to the second embodiment has a metal structure further including a fourth phase containing Sn that coats the third phase, the mixing ratio of the content of the metal constituting the fourth phase to the content of the metal constituting the third phase may be 0.01 to 10 as a mass ratio expressed as the fourth phase / third phase.

[0114] In the preform solder according to the second embodiment, the ratio of the content of the metal constituting the first phase 10 to the content of the metal constituting the second phase 20, expressed as the ratio of the content of the metal constituting the first phase 10 / the content of the metal constituting the second phase 20, is preferably 1 or more and 50 or less, and more preferably 2 or more and 9 or less.

[0115] In the preform solder according to the second embodiment, the ratio of the content of the metal constituting the second phase 30 to the content of the metal constituting the third phase 30 is preferably 0.1 or more and 10 or less, expressed as the ratio of the content of the metal constituting the second phase 20 / the content of the metal constituting the third phase 30.

[0116] The preform solder according to the second embodiment is not limited to the above-described embodiment, and may further include a fifth phase dispersed in the first phase in addition to the first, second, third, and fourth phases. The description of the fifth phase is the same as that of the above-described <fifth metal>.

[0117] The metal structure constituting the preform solder 1 comprises a first phase 10, a second phase 20, and a third phase 30, while a phase containing an intermetallic compound is absent or present in a low proportion between the first phase 10 and the second phase 20. The preform solder 1 has a low content of Sn-Ni intermetallic compounds in the metal structure, preferably 0% by mass or more and 70% by mass or less, more preferably 0% by mass or more and 30% by mass or less, and most preferably 0% by mass, relative to the total mass of the metal structure. In the preform solder 1, when the content of the Sn-Ni intermetallic compounds in the metal structure is equal to or less than the upper limit of the preferred range, the generation of voids during solder joining is more easily suppressed.

[0118] The shape of the preform solder according to the first or second embodiment described above may be a square shape, a ribbon shape, a disk shape, a washer shape, a chip shape, a wire shape, or the like.

[0119] The thermal conductivity of the preform solder according to the first or second embodiment described above is preferably 27 to 100 W / (m·K), more preferably 30 to 70 W / (m·K), and even more preferably 45 to 70 W / (m·K).

[0120] The thermal conductivity of the solder preform can be measured, for example, as follows. The prepared solder preform is cut into a size of 45 mm x 15 mm to obtain a test piece. The test piece is then sandwiched between ceramic plates and heated under pressure. The heating and pressure profile is as follows: heating at a temperature rise rate of 0.25°C / sec, followed by holding at 250°C for 120 seconds, and then cooling at a cooling rate of 2°C / sec. The pressure is 0.5 MPa. The heating atmosphere is N ２ (95%) + H ２(5%). Next, the electrical resistivity is measured by the four-probe method using a measuring device such as a Loresta-GP MCP-T610 (manufactured by MITSUBISHI CHEMICAL ANALYTECH). Two test pieces of the preform solder are prepared, and measurements are taken by applying probes to three points near the center of each test piece, away from the edge. The average value of these six measured values ​​is taken as the electrical resistivity.

[0121] Known manufacturing methods can be used to manufacture the solder preform according to the first or second embodiment described above, such as a melting method or a rolling method. Among these, it is preferable to use the rolling method to manufacture the solder preform according to this embodiment, since this method is particularly effective in suppressing the generation of intermetallic compounds between Sn and Ni, suppressing the generation of large voids, and minimizing the amount of voids that are generated.

[0122] The preform solder according to the present embodiment described above contains a first metal containing Sn, a second metal made of an alloy containing Ni and Fe, and a third metal whose entire surface is formed of a metal containing Ni. By containing the second metal, the preform solder according to the present embodiment suppresses the generation of a porous structure due to the formation of intermetallic compounds, suppresses the generation of voids during solder joining, particularly under high-temperature conditions (250°C or higher), and makes it easier to maintain shear strength.

[0123] The preform solder according to this embodiment contains a third metal, which makes it possible to increase the thermal conductivity of the solder joint. The reason for this effect is unclear, but is presumed to be as follows. Compared to Cu, Ni, etc., the intermetallic compounds of Cu and Sn and the intermetallic compounds of Ni and Sn have low thermal conductivities. For example, the thermal conductivities of Cu and Ni are 401 W / m·K and 88.5 W / m·K, respectively, and the intermetallic compound Cu ６ Sn ５ (Cu ３ Sn), Ni ３ Sn ４The thermal conductivities of the third metal are 34 W / m·K and 20 W / m·K, respectively. The entire surface of the third metal is made of a metal containing Ni, and Ni has low reactivity with Sn, so Ni ３ Sn ４ In other words, the surface of the third metal is less likely to react with Sn in the preform solder to form an intermetallic compound, making it possible to increase the thermal conductivity of the solder joint. Even if the third metal is coated with a metal layer containing Sn, the effect of suppressing the formation of an intermetallic compound due to Ni of the present invention can be achieved as long as the metal layer containing Sn melts immediately when the preform solder is joined. On the other hand, if a metal powder made of Cu is used instead of a third metal whose entire surface is made of a metal containing Ni, Cu is highly reactive with Sn, and therefore the intermetallic compound Cu is not formed. ６ Sn ５ (Cu ３ As a result, when a metal powder made of Cu is used, the thermal conductivity of the solder joint is lower than when a third metal is used.

[0124] (Method for manufacturing preform solder) One embodiment of the method for manufacturing preform solder according to the present invention is a manufacturing method including a mixing step of mixing a first metal powder containing Sn, a second metal powder made of an alloy containing Ni and Fe, and a third metal powder whose entire surface is formed of a metal containing Ni to prepare a metal powder mixture, and a rolling step of rolling the metal powder mixture to produce preform solder.

[0125] First metal powder: The metal constituting the first metal powder used in this embodiment is a metal containing Sn. The description of the metal containing Sn is the same as that in the above-mentioned <First metal>. The melting point of the first metal powder is preferably 300°C or lower, and may be 250°C or lower, or may be 116 to 200°C. When the melting point of the first metal powder is equal to or lower than the upper limit of the above-mentioned preferred range, it becomes easier to ensure the wettability of the solder.

[0126] The content of Sn in the first metal powder is preferably 20% by mass or more and 100% by mass or less, based on the total mass of the first metal powder. In order to fully exhibit the properties of Sn, the content of Sn in the first metal powder is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 100% by mass, based on the total mass of the first metal powder.

[0127] The first metal powder preferably has a particle size of 0.1 to 1000 μm, more preferably 1 to 100 μm. When the particle size of the first metal powder is equal to or greater than the lower limit of the above-mentioned preferred range, wettability is easily ensured, and when the particle size is equal to or less than the upper limit of the above-mentioned preferred range, an intermetallic compound is more easily formed.

[0128] Second metal powder: The metal constituting the second metal powder used in this embodiment is an alloy containing Ni and Fe, and has a higher melting point than the first metal powder. The description of the alloy containing Ni and Fe is the same as that in the above-mentioned <Second Metal>. The melting point of the alloy in the second metal powder is preferably above 300°C, more preferably above 500°C, and even more preferably 600 to 1600°C. If the melting point of the second metal powder is above the lower limit of the above-mentioned preferred range, the shear strength of the solder joint is likely to be increased even in a high-temperature operating environment.

[0129] The Ni content in the second metal powder is preferably 80% by mass or more and 99% by mass or less, more preferably 85% by mass or more and 95% by mass or less, based on the total mass of the second metal powder. The Fe content in the second metal powder is preferably 1% by mass or more and 20% by mass or less, more preferably 5% by mass or more and 15% by mass or less, based on the total mass of the second metal powder. When the Ni and Fe contents in the second metal powder are within the above-mentioned preferred ranges, intermetallic compounds are formed at an earlier stage, and the generation of voids can be suppressed.

[0130] The second metal powder preferably has a particle size of 0.1 to 1000 μm, more preferably 1 to 100 μm, and even more preferably 5 to 50 μm. When the particle size of the second metal powder is equal to or greater than the lower limit of the above-mentioned preferred range, wettability is easily ensured, and when the particle size is equal to or less than the upper limit of the above-mentioned preferred range, an intermetallic compound is more easily formed.

[0131] Third metal powder: The entire surface of the third metal powder used in this embodiment is formed of a metal containing Ni. The metal constituting the third metal powder is the same as the above-mentioned <Third metal>.

[0132] The entire surface of the third metal powder is preferably made of simple Ni.

[0133] The particle size of the third metal is preferably 0.1 to 1000 μm, more preferably 1 to 300 μm, and even more preferably 10 to 200 μm. When the particle size of the third metal is equal to or greater than the lower limit of the preferred range, the thermal conductivity of the solder joint is easily increased.

[0134] [Mixing Step] In the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed together to prepare a metal powder mixture. The compounding ratio when mixing the two is preferably 20 to 95 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 1 to 70 parts by mass of the third metal powder, more preferably 20 to 90 parts by mass of the first metal powder, 3 to 50 parts by mass of the second metal powder, and 3 to 50 parts by mass of the third metal powder, and even more preferably 40 to 90 parts by mass of the first metal powder, 5 to 40 parts by mass of the second metal powder, and 5 to 40 parts by mass of the third metal powder, and particularly preferably 50 to 85 parts by mass of the first metal powder, 5 to 30 parts by mass of the second metal powder, and 5 to 30 parts by mass of the third metal powder, and particularly preferably 60 to 75 parts by mass of the first metal powder, 5 to 25 parts by mass of the second metal powder, and 20 to 30 parts by mass of the third metal powder. By mixing the two components at a ratio within the above-mentioned preferred range, the thermal conductivity of the soldered joint can be improved, void generation can be suppressed, shear strength can be easily maintained, and the heat resistance of the soldered joint can be further improved.

[0135] The mixing step may further include a step of coating the third metal powder with a fourth metal. The coating step may be, for example, a step of plating the third metal powder with the fourth metal. The mixing step may include a step of pre-mixing the third metal powder with a fourth metal containing Sn prior to the coating step. The fourth metal is described in the same manner as in the <Fourth Metal> described above in the first embodiment of the preform solder. In the step of coating the third metal powder with the fourth metal, the mass ratio of the fourth metal to the third metal powder may be 0.01 to 10, expressed as a mass ratio of the fourth metal to the third metal powder, expressed as the fourth metal / third metal powder.

[0136] In the metal powder mixture, the compounding ratio of the first metal powder to the second metal powder, expressed as the ratio of the mass of the first metal powder to the mass of the second metal powder, is preferably 1 or more and 50 or less, and more preferably 2 or more and 9 or less.

[0137] In the metal powder mixture, the compounding ratio of the second metal powder to the third metal powder is preferably 0.1 or more and 10 or less, expressed as the ratio expressed as the mass of the second metal powder / the mass of the third metal powder.

[0138] [Rolling Step] In the rolling step, the metal powder mixture prepared in the mixing step is rolled and formed into a desired shape to produce a solder preform. A known rolling method can be used to roll the metal powder mixture, and for example, a twin-roll rolling mill or the like can be used. The number of times of rolling and the rolling load applied to the metal powder mixture can be appropriately set depending on the desired shape and thickness of the target solder preform.

[0139] As described above, the method for producing a preform solder according to this embodiment includes a mixing step in which a first metal powder, a second metal powder, and a third metal powder are mixed to prepare a metal powder mixture, and a rolling step. In the mixing step, the third metal powder, the entire surface of which is formed of a metal containing Ni, is used, thereby enabling improved thermal conductivity of the solder joint. Furthermore, the second metal powder is an alloy containing Ni and Fe, and the metal powder mixture is processed by rolling. This suppresses the formation of intermetallic compounds between Sn and Ni in the metal structure, making it possible to easily produce a preform solder in which the generation of voids during solder joining is further suppressed. The method for producing a preform solder according to this embodiment is useful as a method for producing the preform solder according to the first or second embodiment described above.

[0140] The method for manufacturing a solder preform according to the present invention is not limited to the above-described embodiment, and may be an embodiment that further includes other steps in addition to the above-described mixing step and rolling step.

[0141] Furthermore, the method for manufacturing a preform solder according to the present invention is not limited to the above-described embodiment, and a metal powder other than the first metal powder, the second metal powder, and the third metal powder (hereinafter, also referred to as a "fifth metal powder") may be used. The metal constituting the fifth metal powder is the same as the above-described "fifth metal."

[0142] (Method for Manufacturing Solder Joint) One embodiment of the method for manufacturing a solder joint according to the present invention is a manufacturing method for forming a joint between objects using a preform solder manufactured by the above-described (method for manufacturing preform solder). The objects to be joined using this manufacturing method are not particularly limited. For example, this manufacturing method can be used to join a semiconductor element and a substrate. Examples of semiconductor elements include silicon carbide (SiC) chips and Si chips. Examples of substrates include circuit boards, ceramic substrates, metal substrates, and DCB (Direct Copper Bonding) substrates. The electrode on the substrate may be, for example, a Cu electrode, or a Cu electrode plated with Sn, Ni, Ni—Au, Ni—Pd, or Ni—Pd—Au. Note that, during joining, flux may be applied in advance to one or both surfaces of the preform solder that will serve as the joining surfaces, the joining surface of the semiconductor element, or the joining surface of the substrate.

[0143] The temperature when joining the semiconductor element and the substrate is preferably, for example, 120°C or higher and 400°C or lower, or may be 200°C or higher and 400°C or lower, or may be 250°C or higher and 400°C or lower, and the method for manufacturing a solder joint of this embodiment is useful for joining under high temperature conditions (250°C or higher).

[0144] The atmosphere in which the objects are bonded may be air, an inert atmosphere such as a nitrogen atmosphere, or a reducing atmosphere. In the case of a nitrogen atmosphere, the pressure applied during bonding is preferably adjusted to 0.1 MPa or more and 10 MPa or less. Bonding the objects in this nitrogen atmosphere enhances the effect of suppressing the generation of voids. In the case of a reducing atmosphere, the objects can be bonded without pressure.

[0145] As described above, according to the method for manufacturing a solder joint of this embodiment, the formation of an intermetallic compound by the third metal in the solder joint after reflow is suppressed, thereby improving thermal conductivity. Furthermore, during reflow, the alloy containing Ni and Fe reacts with the first metal containing Sn to form an intermetallic compound, thereby further improving the heat resistance of the solder joint. Additionally, the generation of voids in the solder joint is further suppressed, making it possible to manufacture a solder joint with increased shear strength. The method for manufacturing a solder joint of this embodiment is particularly useful for applications requiring high-temperature solder that does not melt during operation under high-temperature conditions, such as power semiconductor devices.

[0146] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0147] In this example, the following metal powders were used. The particle size of the metal powders was measured by volume using a Microtrac-Bell laser diffraction / scattering particle size distribution analyzer (MT3300EXII). The melting points of the metal powders were determined by differential scanning calorimetry (DSC). The first metal powder was measured using a Hitachi High-Tech Science DSC7020, and the second and third metal powders were measured using a NETZSCH DSC404-F3 Pegasus.

[0148] First metal powder: Metal powder of 100% Sn by mass (100% Sn by mass powder), average particle size 65 μm (maximum particle size 75 μm), melting point 232°C. Second metal powder: Metal powder consisting of an alloy of 90% Ni and 10% Fe by mass (Ni-10% Fe by mass powder), average particle size 12.8 μm.

[0149] Third metal powder: Third metal powder (1) Metal powder containing 100% Ni by mass (100% Ni by mass powder) Average particle size: 50.7 μm

[0150] Third Metal Powder (2) Ni-Plated-Cu 100% by Mass Core Balls 100% Cu core balls were used as the cores of the third metal powder (2). The average particle size of the cores (i.e., the diameter Rc of the cores) was 11.0 μm. The surface layer of the third metal powder (2) was plated with 100% Ni by mass. The plating thickness (i.e., the thickness Rs of the surface layer) was 1 to 3 μm.

[0151] Third Metal Powder (3) Ni-Plated-Cu 100% by Mass Core Balls 100% Cu core balls were used as the cores of the third metal powder (3). The average particle size of the cores (i.e., the diameter Rc of the cores) was 55.3 μm. The surface layer of the third metal powder (3) was plated with 100% Ni by mass. The plating thickness (i.e., the thickness Rs of the surface layer) was 1 to 3 μm.

[0152] Third Metal Powder (4) Ni-Plated-Cu 100% by Mass Core Balls 100% Cu core balls were used as the cores of the third metal powder (4). The average particle size of the cores (i.e., the diameter Rc of the cores) was 104.3 μm. The surface layer of the third metal powder (4) was plated with 100% Ni by mass. The plating thickness (i.e., the thickness Rs of the surface layer) was 1 to 3 μm.

[0153] The plated core balls of the third metal powders (2) to (4) were 100% Cu core balls whose entire surfaces were coated with 100% Ni plating. The 100% Ni plating was formed by electroplating. The melting point of the 100% Ni plating was 1455°C.

[0154] Fifth metal powder: metal powder of 100% by mass of Cu (100% by mass of Cu powder) average particle size 11.0 μm

[0155] <Production of Preformed Solder> The first, second, third, and fifth metal powders were each produced, and the preformed solders of each example were produced using these metal powders.

[0156] (Example 1) Mixing step: As the first metal powder, 75 parts by mass of 100% Sn powder with an average particle size of 65 μm, 20 parts by mass of Ni-10% by mass Fe powder with an average particle size of 12.8 μm as the second metal powder, and 5 parts by mass of the third metal powder (2) as the third metal powder were stirred to prepare a metal powder mixture. Rolling step: Next, the prepared metal powder mixture was introduced into the hopper of a twin-roll rolling mill, the surface temperature of the rolling roll was set to 100 ° C, and a rolling load of about 25 kN was applied to obtain a strip-shaped rolled material. Thereafter, rolling was repeated to obtain a strip-shaped preform solder with a thickness of 100 μm.

[0157] (Examples 2 to 4) A strip-shaped preform solder having a thickness of 100 μm was obtained by sequentially carrying out the mixing step and the rolling step in the same manner as in Example 1, except that the third metal powder (1) was used as the third metal powder and the first metal powder, the second metal powder, and the third metal powder (1) were used in a predetermined mixing ratio as shown in Table 1.

[0158] (Examples 5 and 6) A strip-shaped preform solder having a thickness of 100 μm was obtained by sequentially carrying out the mixing process and the rolling process in the same manner as in Example 1, except that the third metal powder (3) was used as the third metal powder and the first metal powder, the second metal powder, and the third metal powder (3) were used in a predetermined mixing ratio as shown in Table 1.

[0159] (Examples 7 and 8) A strip-shaped preform solder having a thickness of 100 μm was obtained by sequentially carrying out the mixing process and the rolling process in the same manner as in Example 1, except that the third metal powder (4) was used as the third metal powder and the first metal powder, the second metal powder, and the third metal powder (4) were used in a predetermined mixing ratio as shown in Table 1.

[0160] (Comparative Examples 1 and 2) A strip-shaped preform solder having a thickness of 100 μm was obtained by sequentially performing the mixing process and the rolling process in the same manner as in Example 1, except that the third metal powder was not used and the first metal powder and the second metal powder were used in a predetermined mixing ratio as shown in Table 1.

[0161] (Comparative Example 3) Instead of the third metal powder, 100 mass% Cu powder with an average particle size of 11.0 μm was used as the fifth metal powder, and the first metal powder, the second metal powder, and the fifth metal powder were used in a predetermined mixing ratio as shown in Table 1. Except for this, a mixing process and a rolling process were carried out sequentially in the same manner as in Example 1, to obtain a strip-shaped preform solder having a thickness of 100 μm.

[0162] <Evaluation> The electrical resistivity of the produced solder preforms was measured and the thermal conductivity was calculated as follows.

[0163] [Measurement of Electrical Resistivity] The preform solder of each example was cut into a size of 45 mm x 15 mm to obtain a test piece of each example. Each test piece was sandwiched between ceramic plates and heated under pressure. The pressure heating profile was heating at a temperature rise rate of 0.25°C / sec, followed by holding at 250°C for 120 seconds, and then cooling at a cooling rate of 2°C / sec. The pressure was 0.5 MPa. The atmosphere during heating was N ２ (95%) + H ２ (5%). The electrical resistivity was measured by the four-probe method using a Loresta-GP MCP-T610 (manufactured by MITSUBISHI CHEMICAL ANALYTECH) as a measuring device. Two test pieces for each example were prepared, and measurements were taken by applying probes to three points near the center of each test piece, away from the edge. The average value of these six measured values ​​was taken as the electrical resistivity.

[0164] [Calculation of Thermal Conductivity] Based on the electrical resistivity, the thermal conductivity was calculated according to the Wiedemann-Franz law. According to the Wiedemann-Franz law, κ / σ=LT holds. Here, κ [W / (m·K)] is the thermal conductivity, and σ [Ω －１ m －１ ] is the electrical conductivity, and L [WΩ / K ２ ] is the Lorentz number (2.44 × 10 －８ ) If the electrical resistivity obtained by measurement is ρ, then σ = 1 / ρ. Calculation was performed using κ = LT / ρ, with the temperature T set to 293 K. The calculated thermal conductivity is shown in Table 1.

[0165]

[0166] From the results shown in Table 1, it can be confirmed that the preformed solders of Examples 1 to 8 have higher thermal conductivity than the preformed solders of Comparative Examples 1 to 3.

[0167] 5 shows SEM images (magnification: 300x) showing cross sections in the thickness direction of the solder preforms of Example 1 and Comparative Example 1. It was confirmed that the solder preform of Example 1 had a metal structure comprising a first phase that was a continuous phase, a second phase dispersed in the first phase, and a third phase dispersed in the first phase.

[0168] REFERENCE SIGNS LIST 1 Preform solder 10 First phase 20 Second phase 30 Third phase 30A, 30B Third metal 301 Core portion 302 Surface layer 303 Intermediate layer

Claims

1. A metal alloy comprising a first metal containing Sn, a second metal made of an alloy containing Ni and Fe, and a third metal whose entire surface is formed of a metal containing Ni, wherein the melting point of the first metal is 300°C or less, the melting point of the alloy in the second metal is greater than 300°C, the melting point of the metal containing Ni forming the entire surface of the third metal is greater than 300°C, the content of Sn in the first metal is 20% by mass or more and 100% by mass or less with respect to the total mass of the first metal, the content of Ni in the second metal is 80% by mass or more and 99% by mass or less with respect to the total mass of the second metal, and the content of Fe in the second metal is 1% by mass or more and 20% by mass or less with respect to the total mass of the second metal, A preform solder, wherein the Ni content in the Ni-containing metal forming the entire surface of the third metal is 50 mass% or more and 100 mass% or less with respect to a total mass of the metal forming the entire surface of the third metal, the particle diameter of the second metal is 0.1 to 1000 μm, the particle diameter of the third metal is 0.1 to 1000 μm, the content of the second metal is 1 to 70 mass% with respect to a total content of the first metal, the second metal, and the third metal, and the content of the third metal is 1 to 70 mass% with respect to a total content of the first metal, the second metal, and the third metal.

2. The preform solder according to claim 1, wherein the composition of the third metal is different from the compositions of the first metal and the second metal, the third metal consists only of Ni or contains Ni and a metal other than Ni, the content of Ni in the third metal is 50 mass% or more with respect to the total mass of the third metal, the melting point of the third metal is above 300°C, the content of the first metal is 20 to 90 mass% with respect to the total content of the first metal, the second metal, and the third metal, and the content of the third metal is 5 to 30 mass% with respect to the total content of the first metal, the second metal, and the third metal.

3. The preform solder according to claim 1, wherein: the third metal has a structure consisting of a core portion and a surface layer covering the core portion; the composition of the metal forming the surface layer of the third metal is different from the compositions of the first metal and the second metal; the metal forming the surface layer of the third metal consists only of Ni or contains Ni and a metal other than Ni; the content of Ni in the metal forming the surface layer of the third metal is 50 mass% or more with respect to the total mass of the metal forming the surface layer of the third metal; the melting point of the metal forming the surface layer of the third metal is above 300°C; the content of the first metal is 20 to 90 mass% with respect to the total content of the first metal, the second metal, and the third metal; and the content of the third metal is 5 to 30 mass% with respect to the total content of the first metal, the second metal, and the third metal.

4. The solder preform according to claim 3, further comprising an intermediate layer between said core portion and said surface layer covering said core portion.

5. A solder preform according to any one of claims 1 to 4, further comprising a fourth metal containing Sn that coats the third metal.

6. A metal structure including a first continuous phase, a second phase dispersed in the first phase, and a third phase dispersed in the first phase, wherein the first phase is made of a metal containing Sn, the second phase is made of an alloy containing Ni and Fe, and the entire surface of the third phase is made of a metal containing Ni, the melting point of the metal constituting the first phase as a whole is 300°C or less, the melting point of the alloy constituting the second phase as a whole is greater than 300°C, the melting point of the metal containing Ni constituting the entire surface of the third phase as a whole is greater than 300°C, the content of Sn in the metal constituting the first phase is 20% by mass or more and 100% by mass or less with respect to the total mass of the metal, and the content of Ni in the alloy constituting the second phase is 80% by mass or more and 99% by mass or less with respect to the total mass of the alloy, The content of Fe in the alloy constituting the second phase is 1% by mass or more and 20% by mass or less with respect to the total mass of the alloy; the content of Ni in the Ni-containing metal forming the entire surface of the third phase is 50% by mass or more and 100% by mass or less with respect to the total mass of the metal forming the entire surface of the third phase; the grain size of the second phase is 0.1 to 1000 μm; the grain size of the third phase is 0.1 to 1000 μm; the total content of the alloy constituting the second phase is 1 to 70% by mass with respect to the total content of the entire metal constituting the first phase, the entire alloy constituting the second phase, and the entire metal constituting the third phase; the total content of the metal constituting the third phase is 1 to 70% by mass with respect to the total content of the entire metal constituting the first phase, the entire alloy constituting the second phase, and the entire metal constituting the third phase; 7. The preform solder according to claim 6, wherein the composition of the metal constituting the third phase as a whole is different from the composition of the metal constituting the first phase as a whole and the composition of the alloy constituting the second phase as a whole, the third phase is composed of only Ni or contains Ni and a metal other than Ni, the content of Ni in the metal constituting the third phase is 50 mass% or more with respect to the total mass of the metal constituting the third phase, the melting point of the metal constituting the third phase as a whole is over 300°C, the content of the metal constituting the first phase as a whole is 20 to 90 mass% with respect to the total content of the metal constituting the first phase, the alloy constituting the second phase, and the metal constituting the third phase, and the content of the metal constituting the third phase as a whole is 5 to 30 mass% with respect to the total content of the metal constituting the first phase, the alloy constituting the second phase, and the metal constituting the third phase.

8. The third phase has a structure consisting of a core part and a surface layer covering the core part, the overall composition of the metal forming the surface layer of the third phase is different from the overall composition of the metal forming the first phase and the overall composition of the alloy forming the second phase, the surface layer of the third phase consists of only Ni or contains Ni and a metal other than Ni, the Ni content in the surface layer of the third phase is 50 mass% or more with respect to the total mass of the surface layer of the third phase, the melting point of the metal forming the surface layer of the third phase is above 300°C, the overall content of the metal forming the first phase is 20 to 90 mass% with respect to the total content of the metal forming the first phase, the alloy forming the second phase, and the metal forming the third phase, The total content of the metal constituting the third phase is 5 to 30 mass% with respect to the total content of the metal constituting the first phase, the total content of the alloy constituting the second phase, and the total content of the metal constituting the third phase. The preform solder according to claim 6, wherein the total content of the metal constituting the first phase is 5 to 30 mass%.

9. The solder preform according to claim 8, further comprising an intermediate layer between said core portion and said surface layer covering said core portion.

10. The solder preform according to any one of claims 6 to 9, further having a metal structure including a fourth phase containing Sn, the fourth phase covering the third phase.

11. A method for manufacturing a solder preform, comprising: a mixing step of mixing a first metal powder containing Sn, a second metal powder made of an alloy containing Ni and Fe, and a third metal powder whose entire surface is formed of a metal containing Ni, to prepare a metal powder mixture; and a rolling step of rolling the metal powder mixture to produce a solder preform, wherein the melting point of the first metal powder is 300°C or less, the melting point of the alloy in the second metal powder is greater than 300°C, the melting point of the metal containing Ni that forms the entire surface of the third metal powder is greater than 300°C, the content of Sn in the first metal powder is 20% by mass or more and 100% by mass or less with respect to the total mass of the first metal powder, the content of Ni in the second metal powder is 80% by mass or more and 99% by mass or less with respect to the total mass of the second metal powder, and the content of Fe in the second metal powder is 1% by mass or more and 20% by mass or less with respect to the total mass of the second metal powder, a Ni content in the Ni-containing metal forming the entire surface of the third metal powder is 50 mass% or more and 100 mass% or less with respect to a total mass of the metal forming the entire surface of the third metal powder; the first metal powder has a particle size of 0.1 to 1000 μm; the second metal powder has a particle size of 0.1 to 1000 μm; and the third metal powder has a particle size of 0.1 to 1000 μm; and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 95 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 1 to 70 parts by mass of the third metal powder.

12. A method for producing a preform solder as set forth in claim 11, wherein: the composition of the third metal powder is different from the compositions of the first metal powder and the second metal powder; the third metal powder consists only of Ni or contains Ni and a metal other than Ni; the content of Ni in the third metal powder is 50 mass% or more relative to the total mass of the third metal powder; the melting point of the third metal powder is above 300°C; and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 90 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 5 to 30 parts by mass of the third metal powder.

13. A method for producing a preform solder as set forth in claim 11, wherein: the third metal powder has a structure consisting of a core portion and a surface layer covering the core portion; a composition of a metal forming the surface layer of the third metal powder is different from the compositions of the first metal powder and the second metal powder; the metal forming the surface layer of the third metal powder consists of only Ni or contains Ni and a metal other than Ni; a content of Ni in the metal forming the surface layer of the third metal powder is 50 mass% or more with respect to a total mass of the metal forming the surface layer of the third metal powder; a melting point of the metal forming the surface layer of the third metal powder is above 300°C; and in the mixing step, the first metal powder, the second metal powder, and the third metal powder are mixed in a ratio of 20 to 90 parts by mass of the first metal powder, 1 to 70 parts by mass of the second metal powder, and 5 to 30 parts by mass of the third metal powder.

14. The method for producing a solder preform according to claim 13, wherein the third metal powder has an intermediate layer between the core portion and the surface layer covering the core portion.

15. The method of producing a solder preform according to any one of claims 11 to 14, further comprising the step of coating the third metal powder with a fourth metal.

16. A method for manufacturing a solder joint, comprising forming a joint between objects using a solder preform manufactured by the method for manufacturing a solder preform according to any one of claims 11 to 14.

17. A method for manufacturing a solder joint, comprising forming a joint between objects using a solder preform manufactured by the method for manufacturing a solder preform according to claim 15.

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