Solder paste and solder joint

Abstract

This invention discloses a solder paste and a solder joint. The solder paste contains a first metal powder, a second metal powder, and a flux. The first metal powder contains Sn. The second metal powder 20A has a core 201 made of an alloy containing Ni and Fe and a surface layer 202 covering the core 201 and made of a Ni-containing metal. The Ni content in the metal forming the surface layer 202 of the second metal powder 20A is 50% by mass or more relative to the total mass of the metal forming the surface layer 202 of the second metal powder 20A. The thickness of the surface layer 202 of the second metal powder 20A is 0.05 μm or more and 0.30 μm or less. According to this invention, a solder paste with excellent heat resistance and the ability to further suppress void formation during solder bonding, as well as a solder joint formed using this solder paste, can be provided.

Description

Technical Field

[0001] This invention relates to solder paste and solder joints. This application claims priority based on Japanese Patent Application No. 2024-008280, filed on January 23, 2024, the contents of which are incorporated herein by reference. Background Technology

[0002] In recent years, with the increasing high operating temperatures of power semiconductor devices using silicon carbide (SiC), the temperature at solder joints sometimes reaches around 250–280°C. Therefore, high-temperature solders that do not melt under such high-temperature conditions are required.

[0003] In the fabrication of the aforementioned solder joints, various solder pastes are used as welding materials. For example, Ag paste, which can be sintered at low temperatures, and TLP (Transient Liquid Phase) paste, which complies with the EU's RoHS directive, are examples of solder pastes.

[0004] TLP paste is a paste containing various solder powders. In TLP paste, the solder powders form high-melting-point compounds upon heating. As a result, remelting is suppressed even if the solder joint is reheated, thus improving heat resistance. For example, a TLP paste in which Cu balls and Sn solder balls are dispersed in flux has been proposed (see Patent Document 1).

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2012-91223 Summary of the Invention

[0008] The problem to be solved by the present invention

[0009] However, the flux-containing solder paste described in Patent Document 1 has the following problems: the flux that vaporizes when the solder powder melts remains in the molten solder, easily generating a large number of voids during solidification. In particular, in TLP pastes that form high-melting-point compounds during solidification, the fluidity is reduced, making it difficult for the vaporized flux to be released to the outside.

[0010] The present invention was made in view of the above circumstances, and its object is to provide a solder paste with excellent heat resistance and the ability to further suppress the generation of voids during solder bonding, and a solder joint formed using the solder paste.

[0011] Problem-solving methods

[0012] The present invention includes the following methods.

[0013] [1] A solder paste comprising a first metal powder, a second metal powder, and a flux, wherein the first metal powder contains Sn, the second metal powder has a core made of an alloy containing Ni and Fe and a surface layer covering the core and made of a Ni-containing metal, wherein the Sn content in the first metal powder is 20% by mass or more and 100% by mass or less relative to the total mass of the first metal powder, and the Ni content in the metal forming the core of the second metal powder is 80% by mass or more and 99% by mass or less relative to the total mass of the metal forming the core of the second metal powder. The Fe content in the metal forming the core of the second metal powder is 1% by mass or more and 20% by mass or less relative to the total mass of the metal forming the core of the second metal powder; the Ni content in the metal forming the surface layer of the second metal powder is 50% by mass or more relative to the total mass of the metal forming the surface layer of the second metal powder; the particle size of the first metal powder is 0.1 to 1000 μm; the particle size of the second metal powder is 0.2 to 1000 μm; and the thickness of the surface layer of the second metal powder is 0.05 μm or more and 0.30 μm or less.

[0014] [2] According to the solder paste of [1], the content of the first metal powder is 30 to 99% by mass relative to the total mass of the first metal powder and the second metal powder, and the content of the second metal powder is 1 to 70% by mass relative to the total mass of the first metal powder and the second metal powder.

[0015] [3] A solder joint formed using the solder paste described in [1] or [2].

[0016] Effects of the present invention

[0017] According to the present invention, a solder paste with excellent heat resistance and the ability to further suppress void formation during solder bonding can be provided. Attached Figure Description

[0018] Figure 1 This is a schematic diagram showing the cross-section of the second metal powder 20A in the first embodiment of the solder paste.

[0019] Figure 2 This is a schematic diagram showing the cross-section of the second metal powder 20B in the second embodiment of the solder paste.

[0020] Figure 3 This is a schematic diagram showing the cross-section of the second metal powder 20C in the third embodiment of the solder paste.

[0021] Figure 4These are X-ray images of the gaps in the solder joints of Examples 1-3, 5, 6, and Comparative Examples 1-2.

[0022] Figure 5 This is a graph showing the DSC curves of Examples 1-3, 5, and Comparative Examples 1-2.

[0023] Figure 6 This is a graph showing the calorific value of Examples 1-3, 5, and Comparative Examples 1-2.

[0024] Figure 7 This is a graph showing the ratio of the calorific value of Examples 1 to 3, 5, and Comparative Example 2 to the calorific value of Comparative Example 1.

[0025] Figure 8 These are photographs showing the formation of compounds after reflow soldering in the solder pastes of Examples 1-3, 5, 6, and Comparative Examples 1-2.

[0026] Explanation of symbols

[0027] 20A, 20B, 20C Second Metal Powder

[0028] 201 core

[0029] 202 surface

[0030] 203 Intermediate Layer

[0031] 204 metal layer

[0032] Rc core diameter

[0033] Thickness of Rs surface layer

[0034] Thickness of Ri Intermediate Layer

[0035] Thickness of Rm metal layer Detailed Implementation

[0036] As used in this article, the terms “containing” and “including” refer to concepts that encompass any one of “comprise”, “consist essentially of”, and “consist of”.

[0037] In this specification, "first metal", "second metal" and "third metal" sometimes refer to "particles formed by the first metal", "particles formed by the second metal", and "particles formed by the third metal", respectively.

[0038] In this specification, "first metal powder", "second metal powder" and "third metal powder" refer to "a group of particles formed by the first metal", "a group of particles formed by the second metal", and "a group of particles formed by the third metal", respectively.

[0039] (First embodiment of solder paste)

[0040] The solder paste of the first embodiment contains a first metal powder, a second metal powder, and a flux.

[0041] The first metal powder contains Sn.

[0042] The second metal powder has a core made of an alloy containing Ni and Fe and a Ni-containing surface layer covering the core.

[0043] <First Metal Powder>

[0044] The metal constituting the first metal powder (hereinafter referred to as the first metal) is a metal containing Sn. The first metal may also contain metals other than Sn.

[0045] The first metal can contain metals other than Sn, such as Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and As. One or more of these metals can be present. The group of metals other than Sn can be arbitrarily chosen from these metals.

[0046] The first metal can contain either Sn or an elemental form of a metal other than Sn, or it can be a metal formed by alloying Sn and an elemental form of a metal other than Sn.

[0047] The first metal can be, for example, elemental Sn, a metal formed by mixing Sn with other metals, a metal formed by alloying Sn with other metals, or a metal formed by mixing an alloy containing Sn with other metals.

[0048] In addition to the metals mentioned above, the first metal may also contain unavoidable impurities. Even in the presence of unavoidable impurities, the effectiveness of the present invention will not be affected.

[0049] The first metal can be one type or two or more types.

[0050] The phrase "melting point of the metal to be measured or melting point of the metal powder to be measured" in this specification refers to the melting point determined by differential scanning calorimetry (DSC). The melting point of the metal to be measured is the temperature at which the highest heat absorption per unit time is achieved, based on the DSC measurement results. If the metal to be measured has a single peak in the DSC measurement, the melting point is the temperature of the peak's apex. If the metal to be measured has multiple peaks in the DSC measurement, the melting point is the temperature of the peak with the highest heat absorption per unit time among the multiple peaks.

[0051] That is, when the metal being measured contains multiple metals, the melting point of the metal being measured is the temperature of the peak with the highest heat absorption per unit time among the multiple peaks that the metal being measured can have.

[0052] The melting point of the first metal can be determined, for example, using a DSC7020 prepared by Hitachi High Technology Scientific. The melting point of the second metal, described later, can be determined, for example, using a DSC404-F3 Pegasus prepared by NETZSCH.

[0053] The melting point of the first metal is preferably below 300°C, but can be below 250°C or between 80°C and 200°C.

[0054] When the melting point of the first metal is below the upper limit of the preferred range, the wettability of the solder can be easily ensured.

[0055] The Sn content in the first metal is 20% by mass or more and 100% by mass or less relative to the total mass of the first metal. To fully utilize the properties of Sn, the Sn content in the first metal is preferably 90% by mass or more relative to the total mass of the first metal, more preferably 95% by mass or more, and even more preferably 100% by mass.

[0056] The term "particle size of metal or particle size of metal powder" as used in this specification refers to the average particle size measured using a laser diffraction / scattering particle size distribution measuring device based on volume.

[0057] The average particle size can be determined, for example, using a laser diffraction / scattering particle size distribution measuring device (MT3300EXII) manufactured by Microtrac Bel.

[0058] The particle size of the first metal powder is 0.1 to 1000 μm, preferably 1 to 100 μm.

[0059] When the particle size of the first metal powder is above the lower limit of the preferred range, wettability is easily ensured. When it is below the upper limit of the preferred range, intermetallic compounds are more easily formed.

[0060] <Second Metal Powder>

[0061] like Figure 1 As shown, the second metal powder 20A contains a core 201 and a surface layer 202 covering the entire core 201.

[0062] Rc refers to the core diameter of core 201.

[0063] Rs refers to the thickness of the surface layer 202.

[0064] The metal forming the core 201 is composed of an alloy containing Ni and Fe.

[0065] The metal forming the surface layer 202 is composed of a Ni-containing metal.

[0066] "surface layer"

[0067] The metal forming the surface layer of the second metal powder can be composed of only Ni, or it can contain Ni and other metals.

[0068] It can contain Ni and metals other than Ni, or it can be an alloy of metals other than Ni and Ni.

[0069] The metal that forms the surface layer is preferably elemental Ni.

[0070] Metals other than Ni that can be included as the metal forming the surface layer include, for example, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. One or more of these metals other than Ni can be included. The group of metals other than Ni can be arbitrarily selected from these metals.

[0071] In addition to the metals mentioned above, the metal forming the surface layer may also contain unavoidable impurities. Even in the presence of unavoidable impurities, the effectiveness of the present invention will not be affected.

[0072] When the metal forming the surface layer is a metal containing Ni and other metals, the Ni content in the metal forming the surface layer is 50% by mass or more and less than 100% by mass relative to the total mass of the metal forming the surface layer, preferably 70% by mass or more, more preferably 80% by mass or more, further preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 98% by mass or more.

[0073] When the metal forming the surface layer contains Fe, the Fe content in the metal forming the surface layer 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.

[0074] The thickness Rs of the surface layer is greater than 0.05 μm and less than 1.20 μm.

[0075] The thickness Rs of the surface layer is preferably 0.05μm, 0.10μm, 0.15μm, 0.20μm, 0.25μm, 0.30μm, 0.35μm, 0.40μm, 0.45μm, 0.50μm, 0.55μm, 0.60μm, 0.70μm, 0.75μm, 0.80μm, 0.90μm, 1.00μm, 1.10μm, or 1.15μm, and the upper and lower limits can be appropriately selected from these values.

[0076] The surface thickness Rs can be greater than 0.10 μm and less than 1.20 μm, greater than 0.15 μm and less than 1.20 μm, greater than 0.20 μm and less than 1.20 μm, greater than 0.25 μm and less than 1.20 μm, greater than 0.30 μm and less than 1.20 μm, greater than 0.35 μm and less than 1.20 μm, greater than 0.40 μm and less than 1.20 μm, greater than 0.45 μm and less than 1.20 μm, or greater than 0.50 μm and less than 1.2 μm. 0 μm, can be greater than 0.55 μm and less than 1.20 μm, can be greater than 0.60 μm and less than 1.20 μm, can be greater than 0.70 μm and less than 1.20 μm, can be greater than 0.75 μm and less than 1.20 μm, can be greater than 0.80 μm and less than 1.20 μm, can be greater than 0.90 μm and less than 1.20 μm, can be greater than 1.00 μm and less than 1.20 μm, can be greater than 1.10 μm and less than 1.20 μm, can be greater than 1.15 μm and less than 1.20 μm.

[0077] The surface thickness Rs can be greater than 0.05 μm and less than 1.15 μm, greater than 0.05 μm and less than 1.10 μm, greater than 0.05 μm and less than 1.00 μm, greater than 0.05 μm and less than 0.90 μm, greater than 0.05 μm and less than 0.80 μm, greater than 0.05 μm and less than 0.75 μm, greater than 0.05 μm and less than 0.70 μm, greater than 0.05 μm and less than 0.60 μm, or greater than 0.05 μm and less than 0.55 μm. Below m, can be 0.05μm and above and below 0.50μm, can be 0.05μm and above and below 0.45μm, can be 0.05μm and above and below 0.40μm, can be 0.05μm and above and below 0.35μm, can be 0.05μm and above and below 0.30μm, can be 0.05μm and above and below 0.25μm, can be 0.05μm and above and below 0.20μm, can be 0.05μm and above and below 0.15μm, can be 0.05μm and above and below 0.10μm.

[0078] From the viewpoint of ensuring sufficient heat resistance and further improving the ability to suppress voids, the thickness Rs of the surface layer is preferably 0.10 μm or more and less than 1.20 μm.

[0079] From the viewpoint of maximizing void suppression capability and further improving heat resistance, the surface layer thickness Rs is preferably 0.05 μm or more and 1.0 μm or less, more preferably 0.05 μm or more and 0.75 μm or less, even more preferably 0.05 μm or more and 0.30 μm or less, particularly preferably 0.05 μm or more and 0.25 μm or less, most preferably 0.05 μm or more and 0.20 μm or less, and can be 0.05 μm or more and 0.15 μm or less, or 0.05 μm or more and 0.10 μm or less.

[0080] By making the surface layer thickness Rs above the lower limit value, the void suppression capability can be further improved. By making the surface layer thickness Rs below the upper limit value, the heat resistance can be further improved.

[0081] In this specification, the thickness Rs of the surface layer of the second metal powder and the core diameter Rc of the core, as well as the thickness Ri of the intermediate layer of the second metal powder in the second embodiment described later and the thickness Rm of the metal layer of the second metal powder in the third embodiment, can be measured by means of the cross-sectional structure of the second metal powder using an optical microscope, SEM, transmission electron microscope (TEM), etc.

[0082] Alternatively, the thickness Rs of the surface layer of the second metal can be determined using Auger electron spectroscopy analysis equipment.

[0083] Alternatively, the core diameter Rc of the core of the second metal powder can be determined as follows: When preparing the second metal powder, the particle size of the metal powder prepared for use as the core can be set as the core diameter Rc.

[0084] The melting point of the metal forming the surface layer is preferably above 300°C, more preferably above 500°C, and even more preferably 600 to 1600°C.

[0085] The surface layer can be a coating formed through a plating process.

[0086] As a plating process, well-known methods such as electroplating and electroless plating can be cited.

[0087] At least a portion of the core's surface is covered by a surface layer. Figure 1 In the middle, the entire surface of the core is covered by the surface layer.

[0088] The ratio of the area of ​​the core surface covered by the outer layer to the total surface area of ​​the core (100%) is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, further preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100%.

[0089] Core

[0090] The core of the second metal powder is made of an alloy containing Ni and Fe.

[0091] The alloy in the core contains Ni and Fe, and has a higher melting point than the first metal powder.

[0092] The melting point of the alloy in the core is preferably above 300°C, more preferably above 500°C, and even more preferably 600 to 1600°C.

[0093] When the melting point of the alloy in the core exceeds the lower limit of the preferred range, it can improve heat resistance and easily improve the shear strength of the solder joint.

[0094] The alloy in the core may also contain metals 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 other metals, with an alloy of Ni and Fe being preferred.

[0095] The alloy used in the core may contain metals other than Ni and Fe, such as Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Mn, Zr, Sn, and As. One or more of these metals other than Ni and Fe may be included. The group of metals other than Ni and Fe can be arbitrarily selected from these metals.

[0096] In addition to the metals mentioned above, the metal forming the core may also contain unavoidable impurities. Even with unavoidable impurities, the effectiveness of the invention will not be affected. The second metal may be one type or two or more types.

[0097] The Ni content in the metal forming the core is 80% by mass or more and 99% by mass or less relative to the total mass of the metal forming the core, preferably 85% by mass or more and 95% by mass or less.

[0098] The Fe content in the metal forming the core is 1% by mass or more and 20% by mass or less relative to the total mass of the metal forming the core, preferably 5% by mass or more and 15% by mass or less.

[0099] When the content of Ni and Fe in the metal forming the core is within the preferred range, intermetallic compounds are formed at an earlier stage, thereby improving heat resistance.

[0100] The core diameter Rc of the core 201 of the second metal powder 20A is preferably 0.1 to 1000 μm, more preferably 3 to 300 μm, even more preferably 5 to 100 μm, and particularly preferably 10 to 50 μm.

[0101] For the second metal powder 20A, the ratio of the core diameter Rc of the core 201 to the thickness Rs of the surface layer 202, expressed as Rc / Rs, is preferably 0.2 to 20000, more preferably 2 to 2000, further preferably 10 to 1000, particularly preferably 20 to 500, most preferably 30 to 500, and may also be 30 to 452.

[0102] The preferred ratio, expressed as Rc / Rs, is 452, 400, 350, 300, 250, 226, 65, 45, or 30, and the upper and lower limits can be appropriately selected from these values.

[0103] By keeping Rc / Rs within the specified range, heat resistance can be easily improved and voids can be easily suppressed.

[0104] The particle size of the second metal powder is 0.2 to 1000 μm, preferably 1 to 100 μm, and more preferably 5 to 50 μm.

[0105] When the particle size of the second metal is above the lower limit of the preferred range, wettability is easily ensured; when it is below the upper limit of the preferred range, intermetallic compounds are more easily formed.

[0106] The second metal powder 20A can be one type or two or more types.

[0107] <Relationship between the contents of the first metal powder and the second metal powder>

[0108] In the solder paste of the first embodiment, regarding the mixing ratio of the first metal powder and the second metal powder, the content of the first metal powder is preferably 30 to 99% by mass, more preferably 60 to 95% by mass, further preferably 70 to 95% by mass, and particularly preferably 80 to 95% by mass, relative to the total content (100% by mass) of the first and second metal powders.

[0109] Regarding the mixing ratio of the first metal powder and the second metal powder, the content of the second metal powder is preferably 1 to 70% by mass, more preferably 5 to 40% by mass, further preferably 5 to 30% by mass, and particularly preferably 5 to 20% by mass, relative to the total content (100% by mass) of the first metal powder and the second metal powder.

[0110] In the solder paste of the first embodiment, the ratio of the content of the first metal powder to the content of the second metal powder, expressed as a mass ratio of the content of the first metal powder to the content of the second metal powder, is preferably 4, 8, or 9, and the upper and lower limits can be appropriately selected from these values. The mass ratio expressed as the content of the first metal powder to the content of the second metal powder can, for example, be 1 to 20, 3 to 15, 6 to 10, or 4 to 9.

[0111] <Fluoride>

[0112] The solder paste of the first embodiment contains flux.

[0113] As for the components contained in the flux, there are no particular limitations as long as they achieve the effect of the present invention. For example, resin components such as rosin, activators, thixotropic agents, solvents, metal passivators, surfactants, silane coupling agents, antioxidants, colorants, etc. can be included.

[0114] As for rosin, examples include natural resins containing a mixture of arisin acid and its isomers, with arisin acid as the main component, as well as substances obtained by chemically modifying natural resins.

[0115] Examples of active agents include, for example, organic acids, amines, and halogen compounds.

[0116] Examples of thixotropic agents include ester-based thixotropic agents, amide-based thixotropic agents, and sorbitol-based thixotropic agents.

[0117] In the solder paste, the flux content is preferably 5 to 30% by mass relative to the total mass of the solder paste, more preferably 8 to 15% by mass.

[0118] The solder paste of the first embodiment described above contains a first metal powder, a second metal powder, and a flux. The first metal powder contains Sn. The second metal powder has a core made of an alloy containing Ni and Fe and a Ni-containing surface layer covering the core.

[0119] In the solder paste of the first embodiment, Sn forms a high-melting-point compound with an alloy containing Ni and Fe during soldering, thus suppressing remelting even if the solder joint is reheated.

[0120] In the solder paste of the first embodiment, the second metal powder has a core and a Ni-containing surface layer covering the core. The Ni content in the metal forming the surface layer is 50% by mass or more relative to the total mass of the metal forming the surface layer. The thickness of the surface layer is 0.05 μm or more and less than 1.20 μm.

[0121] The solder paste of the first embodiment has a surface layer as described above due to the second metal powder, exhibiting excellent heat resistance and further suppressing the formation of voids during solder bonding. Although the reason for achieving such an effect is not yet certain, it is speculated as follows.

[0122] Before reflow soldering begins and a certain time has elapsed, the surface layer of the second metal powder serves to protect the core of the second metal powder. The Ni and Sn in the surface layer of the second metal powder are less reactive than those in alloys containing Ni and Fe, thus inhibiting the rapid formation of compounds between Ni-Fe alloys and Sn. As a result, the vaporized flux becomes easier to expel from the solder paste, further suppressing void formation during solder bonding.

[0123] By making the surface layer thickness greater than 0.05 μm, the formation of abrupt compounds between Ni and Fe alloys and Sn can be suppressed, and the generation of voids can be further suppressed.

[0124] During the initial reflow soldering process and over a certain period, the surface Ni gradually diffuses into the molten Sn. As the surface Ni disappears, the Ni and Fe alloy in the core reacts with Sn to form a high-melting-point compound.

[0125] By making the thickness of the surface layer less than 1.20 μm, the Ni in the surface layer can easily diffuse into the molten Sn, thus forming a high-melting-point compound and improving the heat resistance of the solder joint.

[0126] In the first embodiment of the solder paste, during reflow soldering, the core of the second metal powder, which is composed of an alloy containing Ni and Fe, forms a high-melting-point compound with the Sn of the first metal powder, thereby improving heat resistance.

[0127] The reaction that forms a high-melting-point compound is an exothermic reaction. The heat of generation of the first and second metal powders contained in the solder paste of this embodiment is preferably within the following range.

[0128] First, prepare the solder paste of this embodiment containing a mixed powder composed of a first metal powder and a second metal powder (hereinafter, the mixed powder is referred to as the "sample material").

[0129] The mixing ratio (mass ratio) of the first metal powder and the second metal powder in the sample is set to be the same as the mixing ratio (mass ratio) of the first metal powder and the second metal powder in the solder paste of this embodiment of the object.

[0130] The preparation of the target sample involves a mixture of powders identical to the metal powder in the core of the second metal powder, except that the second metal powder is replaced with the metal powder in the core of the second metal powder (hereinafter, this mixture is referred to as the "reference sample").

[0131] In the reference sample, the mixing ratio (mass ratio) of the metal powder in the core of the first metal powder and the second metal powder is set to be the same as the mixing ratio (mass ratio) of the first metal powder and the second metal powder in the solder paste of this embodiment of the object.

[0132] First, for the reference sample, the calorific value is determined by differential scanning calorimetry (DSC) in the following order.

[0133] During the measurement, the EXSTAR DSC7020 (manufactured by Hitachi High Technology Scientific Co., Ltd.) can be used as the measuring device. The sample is placed in an aluminum pan for measurement. The measurement program is set to 100–350 °C with a heating rate of 5 °C / min.

[0134] The results of differential scanning calorimetry are plotted with time t on the horizontal axis and heat flux (in W or J / s) on the vertical axis. -1 It is obtained from the DSC curve of temperature T on the horizontal axis and heat flow on the vertical axis.

[0135] The resulting DSC curves show large negative values ​​(i.e., large endothermic temperatures) near 235°C when the first metal powder containing Sn melts.

[0136] Based on the DSC curve, the integral value Q0' (in J) of the heat flux per unit mass [mg] of the reference sample from time t0 to time t is calculated by integrating the heat flux per unit mass [mg] of the reference sample over time. Here, time t0 is the moment when the temperature of the reference sample reaches 200℃.

[0137] Within the temperature range of 200℃ to 350℃, the moment t when the integral value Q0' of the heat flux of the reference sample is minimized is set as t. in0 Time t in0 This is the moment when the reference sample begins to heat up. From time t... in0 The calorific value at time t is defined as the calorific value of the reference sample (hereinafter referred to as "reference calorific value Q0"). The reference calorific value Q0 is the calorific value per unit mass (mg) of the reference sample.

[0138] The baseline calorific value Q0 can also be said to be derived from temperature T. in0 The amount of heat generated up to temperature T.

[0139] Here, the reference sample at time t in0 The temperature is T in0 The temperature of the reference sample at time T is T.

[0140] T in0 The temperature of the reference sample is when the integral value Q0' of the heat flux of the reference sample is at its minimum within the range of 200℃ to 350℃.

[0141] Next, for the target sample, the DSC curve was obtained by differential scanning calorimetry in the same manner as for the reference sample.

[0142] T compared to the reference sample in0 Similarly, the T of the object sample is calculated. in For the object sample, calculate from temperature T in The calorific value up to temperature T (hereinafter referred to as "object calorific value Q"). The object calorific value Q is the calorific value per unit mass (mg) of the object sample.

[0143] The heat generated by an object, Q, can also be expressed as a function of temperature T. in The amount of heat generated up to temperature T.

[0144] Here, the object sample at time t in The temperature is T in The temperature of the sample at time T is T.

[0145] T in It is the temperature of the object sample at which the integral value of the heat flux of the object sample is minimized within the range of 200℃ to 350℃.

[0146] The embodiments are described later, but from the temperature T in The heat generated by an object at temperature T, for example, is Q. Figure 6 That's how it's calculated. Figure 6 In the middle, from temperature T in0 The reference calorific value Q0 at temperature T is the calorific value of Comparative Example 1.

[0147] From temperature T in The ratio of the heat of calorific value Q of the object sample within the temperature range up to 300°C to the heat of calorific value from temperature T. in0 The reference heat of the reference sample, Q0 (100%), is preferably 10% or more and less than 100% in the temperature range up to 300°C.

[0148] The lower limit of the proportion of the object's heat output is preferably 10% or more, more preferably 15% or more, further preferably 20% or more, particularly preferably 25% or more, most preferably 30% or more, and can be 40% or more, 50% or more, or 60% or more.

[0149] There is no specific upper limit to the proportion of heat generated by the object; it can be less than 100% or less than 80%.

[0150] As will be described later in the embodiments, the temperature T of the object sample... in The heat of calorific value Q of the object relative to the temperature T of the reference sample in the range up to 300°C in0 The proportion of the reference calorific value Q0 (100%) in the range up to 300°C, for example, Figure 7 That's how it's calculated.

[0151] In summary, the preferred calorific values ​​of the first metal powder and the second metal powder are as follows.

[0152] In measurements using differential scanning calorimetry, the temperature T per unit mass (mg) of the sample is... in The ratio of the heat of calorific value Q of an object in the range up to 300°C to the temperature T of a reference sample per unit mass (mg) in0 The reference calorific value Q0 (100%) in the range up to 300°C is preferably 10% or more and less than 100%.

[0153] The sample material is a mixture of a first metal powder and a second metal powder. The mixing ratio (mass ratio) of the first metal powder and the second metal powder in the sample material is the same as the mixing ratio (mass ratio) of the first metal powder and the second metal powder in the solder paste.

[0154] The reference sample is a mixture of metal powder composed of the core of a first metal powder and a second metal powder. The mixing ratio (mass ratio) of the first metal powder and the core of the second metal powder in the reference sample is the same as the mixing ratio (mass ratio) of the first metal powder and the second metal powder in the solder paste.

[0155] Temperature T in It is the temperature of the object sample at which the integral value of the heat flux of the object sample is minimized within the range of 200℃ to temperature T. The temperature T of the object sample can be taken as 200℃ to 350℃.

[0156] Temperature T in0 It is the temperature of the object sample at which the integral value of the heat flux of the reference sample is minimized within the range of 200℃ to temperature T. The temperature T of the reference sample can be taken as 200℃ to 350℃.

[0157] When the main component of the surface layer of the second metal powder is not Ni but Cu, Au, Ag, or Pd, Cu, Au, Ag, and Pd diffuse more readily into the molten Sn compared to Ni. Therefore, a surface layer with Cu, Au, Ag, or Pd as the main components cannot adequately suppress the rapid formation of compounds between the Ni- and Fe-containing alloys and Sn.

[0158] When the main component of the surface layer of the second metal powder is not Ni but Bi, Bi also dissolves into Sn, thus failing to adequately suppress the rapid formation of compounds between the alloy containing Ni and Fe and Sn.

[0159] When the main component of the surface layer of the second metal powder is not Ni but a low-melting-point metal such as In, the surface layer melts quickly at the start of reflow soldering, thus failing to adequately suppress the rapid formation of compounds between the Ni- and Fe-containing alloy and Sn.

[0160] Furthermore, Cu, Au, Ag, and Pd react with Sn more readily to form compounds compared to Ni. Therefore, the formation of compounds between Sn and a surface layer containing Cu, Au, Ag, or Pd hinders the formation of high-melting-point compounds between Sn and alloys containing Ni and Fe. Consequently, the heat resistance of the solder joint cannot be adequately improved.

[0161] The embodiments of the present invention are not limited to the first embodiment, and may also be other embodiments besides the first embodiment. As other embodiments, the second to fourth embodiments will be described.

[0162] (Second Implementation)

[0163] The solder paste of the second embodiment is a solder paste obtained by replacing the second metal powder 20A with the second metal powder 20B in the solder paste of the first embodiment.

[0164] The solder paste of the second embodiment is the same as that of the first, third or fourth embodiments, except that it contains the second metal powder 20B.

[0165] As the core 201 and surface layer 202 in the second metal powder 20B, examples can be the same core and surface layer as those described above in the second metal powder 20A.

[0166] like Figure 2 As shown, the second metal powder 20B has an intermediate layer 203 covering the core 201 between the core 201 and the surface layer 202 covering the core 201.

[0167] The intermediate layer can cover a portion or the entire surface of the core, preferably the entire surface. Figure 2 In the middle, the intermediate layer 203 covers the entire surface of the core 201.

[0168] The proportion of the surface area of ​​the core covered by the intermediate layer, relative to the total surface area of ​​the core (100%), is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, further preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100%.

[0169] Ri refers to the thickness of the intermediate layer 203.

[0170] The intermediate layer can be one layer or two or more layers.

[0171] exist Figure 2 In the second metal powder 20B shown, the intermediate layer 203 is a single layer.

[0172] The composition of the metal forming the intermediate layer 203 is different from that of the metal forming the core 201 and the metal forming the surface layer 202.

[0173] The metal that forms the intermediate layer can be a single elemental metal or an alloy of two or more metallic elements.

[0174] Metals that can be included in the intermediate layer include, for example, Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. One or more of these metals can be present. The group of metals that can be included in the intermediate layer can be arbitrarily chosen from these metals.

[0175] In addition to the aforementioned metals, the intermediate layer may also contain unavoidable impurities. Even in the presence of unavoidable impurities, the effectiveness of the invention will not be affected.

[0176] The metal that forms the intermediate layer can be one type or two or more types.

[0177] The thickness Ri of the intermediate layer can be, for example, greater than 0.01 μm and less than 100 μm, greater than 0.05 μm and less than 50 μm, or greater than 0.1 μm and less than 10 μm.

[0178] The intermediate layer can also be a coating formed through plating.

[0179] As a plating process, well-known methods such as electroplating and electroless plating can be cited.

[0180] The second metal powder 20B can be one type or two or more types.

[0181] The second metal powder 20B has an intermediate layer 203, which makes it easy to set the surface layer 202 of the second metal powder 20B.

[0182] (Third Embodiment)

[0183] In the solder paste of the third embodiment, the surface of the second metal powder is covered by a metal layer.

[0184] The solder paste of the third embodiment is the same as that of the first, second, or fourth embodiments, except that the second metal powder has a metal layer.

[0185] like Figure 3 As shown, the second metal powder 20C has a core 201, a surface layer 202 covering the core 201, and a metal layer 204 covering the surface layer 202.

[0186] At least a portion of the surface of the layer is covered by a metal layer. Figure 3 In the middle, the entire surface of the surface layer 202 is covered by the metal layer 204.

[0187] The proportion of the surface area covered by the metal layer to the total surface area (100%) of the surface layer is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, further preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100%.

[0188] The metal layer can be one layer or two or more layers. Figure 3 In the second metal powder 20C shown, there is one metal layer.

[0189] The composition of the metal forming the metal layer 204 is different from that of the metal forming the surface layer.

[0190] The metal that forms the metal layer can be a single elemental metal or an alloy of two or more metallic elements.

[0191] Metals that can be contained in the metal layer include, for example, Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. One or more of these metals can be contained. The core can contain any group of metals selected from these metals.

[0192] In addition to the metals mentioned above, the metal layer may also contain unavoidable impurities. Even in the presence of unavoidable impurities, the effectiveness of the present invention will not be affected.

[0193] The metal that forms the metal layer can be one type of metal or two or more types of metal.

[0194] When the metal forming the metal layer contains Fe, the Fe content in the metal forming the metal layer is preferably 0% by mass or more and less than 5% by mass relative to the total mass of the metal forming the metal layer.

[0195] The thickness Rm of the metal layer can 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.

[0196] Metallic layers can also be formed through plating processes.

[0197] As a plating process, well-known methods such as electroplating and electroless plating can be cited.

[0198] The second metal powder 20C has a metal layer 204 containing Sn, which improves wettability during the melting process of the solder paste and can be expected to reduce voids. The metal layer melts after heating begins when the solder paste is melted, exposing the surface layer of the third metal, thus not hindering the effectiveness of this application.

[0199] (Fourth Embodiment)

[0200] The solder paste of the fourth embodiment also contains metal powder other than the first metal powder and the second metal powder (hereinafter referred to as "third metal powder").

[0201] The solder paste of the fourth embodiment is the same as that of the first, second or fourth embodiments, except that it also contains a third metal powder.

[0202] The composition of the third metal powder is not particularly limited as long as it differs from that of the first and second metal powders. For example, it is preferred to be a powder composed of each of the elemental metals Ni, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As, or an alloy formed by two or more of these elemental metals.

[0203] As a third metal powder, for example, it can be a metal powder having a core and a surface layer covering the core and composed of a metal containing Ni.

[0204] The composition of the core is not particularly limited as long as it differs from that of the core in the first and second embodiments. For example, it is preferable to use powder composed of each of the elemental metals Ni, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As, or an alloy formed by two or more of these elemental metals.

[0205] The particle size of the third metal powder is preferably 0.1 to 1000 μm, more preferably 1 to 100 μm, and even more preferably 5 to 50 μm.

[0206] The metals constituting the third metal powder may contain one type or two or more types.

[0207] The third metal powder can be one type or two or more types.

[0208] By including a third metal powder, the thermal conductivity of the solder paste can be improved.

[0209] (Solder joint)

[0210] The solder joint of the present invention is formed using the solder paste described in the above embodiments.

[0211] There are no particular limitations on the objects that can be joined by solder joints. For example, semiconductor components and substrates can be joined by solder joints.

[0212] Examples of semiconductor components include silicon carbide (SiC) chips and Si chips.

[0213] Examples of substrates include circuit boards, ceramic substrates, metal substrates, and DCB (Direct Copper Bonding) substrates. Electrodes on the substrate can be, for example, Cu electrodes, or Cu electrodes that have undergone any of the following treatments: Sn plating, Ni plating, Ni-Au plating, Ni-Pd plating, or Ni-Pd-Au plating.

[0214] As a method for bonding objects using the solder paste described above, reflow soldering can be cited as an example. The reflow soldering conditions can be appropriately set with reference to known methods.

[0215] In reflow soldering, the internal pressure of the reflow oven can also be reduced. By reducing the pressure, the vaporized flux can be more easily discharged from the solder paste, thus suppressing the formation of voids.

[0216] The temperature at which semiconductor elements and substrates are joined is preferably 120°C or higher and 400°C or lower, but can be 200°C or higher and 400°C or lower, or 250°C or higher and 400°C or lower.

[0217] The atmosphere at which the objects are bonded can be atmospheric, inert, such as nitrogen, or reducing.

[0218] As explained above, the solder joint of the present invention exhibits excellent heat resistance and shear strength due to the use of the solder paste of the above-described embodiment containing a first metal powder, a second metal powder, and a flux.

[0219] The solder joints of the present invention are particularly useful in applications such as power semiconductor devices where high-temperature solder is required to not melt when operating under high-temperature conditions.

[0220] Example

[0221] The present invention will be described below through embodiments, but the present invention is not limited to the following embodiments.

[0222] <Preparation of Ointment>

[0223] (Examples 1-7, Comparative Examples 1-2)

[0224] The metal powders and fluxes shown in Table 1 were mixed to prepare the pastes for the examples and comparative examples.

[0225] In Examples 1-5 and 7, the flux in the paste was 12% by mass, and the total mass of the first metal powder and the second metal powder was 88% by mass.

[0226] In the paste of Example 6, the flux is 12% by mass, and the total mass of the first metal powder, the second metal powder, and metal powder C is 88% by mass.

[0227] In the paste of Comparative Example 1, the flux was 12% by mass, and the total mass of the first metal powder and metal powder B was 88% by mass.

[0228] In the paste of Comparative Example 2, the flux was 12% by mass, and the total mass of the first metal powder and metal powder A was 88% by mass.

[0229] The values ​​in the table refer to the content of each metal powder relative to the total mass (100% by mass) of the metal powder in the paste. Empty columns in the table represent "0".

[0230] Regarding the particle size of the metal powder and the particle size Rc of the core, the average particle size was determined by volume using a laser diffraction / scattering particle size distribution measuring device (MT3300EXII) manufactured by Microtrac Bel.

[0231] The melting points of the metal powders were determined by differential scanning calorimetry (DSC). For the first metal powder, the melting point was determined using a DSC7020 prepared by Hitachi High Technology Scientific Co., Ltd., and for the second metal powder and metal powders A to C, the melting point was determined using a DSC404-F3Pegasus prepared by NETZSCH Co., Ltd.

[0232] The thickness Rs of the surface layer in the second metal powder (1) to (5), metal powder A and metal powder C is determined using Auger electron spectroscopy and an Auger electron spectroscopy analyzer.

[0233] First metal powder:

[0234] Sn100% by mass metal powder (Sn100% by mass powder)

[0235] The first metal powder is defined as having a size (particle size distribution) that meets designation 4 in the powder size classification in Table 2 of JIS Z3284-1:2004. Specifically, the content of particles with a diameter of 38 μm or less in the first metal powder is 99% by mass or more relative to the total mass of the first metal powder. The average particle size of the first metal powder is 34.54 μm.

[0236] The melting point of the first metal powder is 232℃.

[0237] Second metal powders (1) to (5):

[0238] In the second metal powder (1) to (5), the core is made of a metal powder composed of an alloy of 90% by mass Ni and 10% by mass Fe (Ni-10% by mass Fe powder).

[0239] The Ni-10% Fe powder used as the core meets the size (particle size distribution) designation 5 in the powder size classification in Table 2 of JIS Z3284-1:2004. That is, in the Ni-10% Fe powder, the content of particles with a diameter of 25 μm or less is 99% by mass or more relative to the total mass of the Ni-10% Fe powder. The particle size (i.e., core diameter Rc) of the Ni-10% Fe powder used as the core is 22.59 μm.

[0240] The core has a melting point of 1444℃.

[0241] In the second metal powders (1)-(5), metal powder A, and metal powder C, the metal forming the surface layer covering the entire surface of the core is Ni 100% by mass. This surface layer is a layer that covers the entire core with Ni plating. These Ni platings are all formed by electroplating.

[0242] The surface thicknesses Rs (i.e., the thickness of the Ni coating) are (1) 0.05 μm, (2) 0.10 μm, (3) 0.35 μm, (4) 0.50 μm, and (5) 0.75 μm, respectively.

[0243] The melting point of the Ni that forms the surface layer is 1455℃.

[0244] Metal powder AC:

[0245] As metal powder A, a metal powder having a core and a surface layer made of Ni covering the core is used. Metal powder A is the same as the second metal powder (1) except that the thickness Rs of the surface layer is 1.20 μm.

[0246] The particle size (i.e., core diameter Rc) of the core of metal powder A is 22.59 μm.

[0247] The core has a melting point of 1444℃. The Ni forming the surface layer has a melting point of 1455℃.

[0248] As metal powder B, the same metal powder (Ni-10 wt% Fe powder) used as the core of the second metal powders (1) to (5) is used. The third metal powder is a metal powder without a surface layer.

[0249] The particle size of metal powder B is 22.59 μm.

[0250] The melting point of metal powder B is 1444℃.

[0251] As the metal powder C, a metal powder having a core and a surface layer composed of Ni covering the core is used. The thickness of the surface layer of the metal powder is 1 to 3 μm.

[0252] The core is made of Cu metal powder (Cu 100% by mass powder).

[0253] The particle size (i.e., core diameter Rc) of the core of metal powder C is 55.3 μm.

[0254] The core has a melting point of 1085℃. The Ni forming the surface layer has a melting point of 1455℃.

[0255] SDC5 (manufactured by Senju Metal Industries, Ltd.) was used as the flux.

[0256] The evaluation methods described in the "Evaluation" section below were used for "Evaluation of Pore Suppression Capability," "Evaluation of Compound Formation," and "Evaluation of Heat Resistance." The evaluation results are shown in Table 1. Figure 8 .

[0257] <Evaluation>

[0258] Evaluation of gap suppression capability

[0259] (1) Evaluation Method

[0260] Using a mask (aperture: 3mm × 3mm, thickness: 0.15mm) and a squeegee, the paste for each example was printed onto a Cu substrate (50mm × 50mm, thickness: 0.3mm). After mounting a Si chip (3mm × 3mm), reflow soldering was performed. The reflow profile was set to a temperature rise rate of 3℃ / sec from 30℃ to the peak temperature (250℃). The atmosphere was set to nitrogen, the peak time was 2.5 minutes, and the pressure was reduced for the first 2 minutes of the peak.

[0261] Then, the gap was observed using an X-ray observation device.

[0262] (2) Judgment Criteria

[0263] A produced a small number of voids of the same degree as when using the ointment in Comparative Example 2.

[0264] B produced more voids than when using the ointment of Comparative Example 2, but less than when using the ointment of Comparative Example 1.

[0265] C produced a large number of voids to the same extent as when the ointment of Comparative Example 1 was used.

[0266] X-ray images of Examples 1-3, 5, 6 and Comparative Examples 1-2 are shown below. Figure 4 It can be confirmed that, compared to Comparative Example 1, gaps in Examples 1-3, 5, 6 and Comparative Example 2 were suppressed. Although no images are shown, gaps in Example 4 were suppressed to the same extent as in Examples 3 and 5. Although no images are shown, gaps in Example 7 were confirmed to be suppressed compared to Comparative Example 1.

[0267] [Table 1]

[0268]

[0269] Evaluation of heat resistance

[0270] (1) Evaluation Method

[0271] The heat resistance of the solder joint is improved by allowing the Sn contained in the solder paste to form a high-melting-point compound with an alloy containing Ni and Fe. The reaction that forms this compound is exothermic. Therefore, the heat resistance is evaluated by measuring the heat generated during compound formation.

[0272] First, a mixed powder was prepared, consisting of metal powders contained in the solder pastes of Examples 1-3, 5, and Comparative Examples 1-2, respectively. Hereinafter, these mixed powders will be referred to as the mixed powders of Examples 1-3, 5, and Comparative Examples 1-2, respectively. The mixing ratio (mass ratio) of each metal powder in each mixed powder was set to be the same as the mixing ratio (mass ratio) in the solder paste.

[0273] Each mixed powder was placed in an aluminum pan, and the exothermic peak was measured using DSC. An EXSTAR DSC7020 (manufactured by Hitachi High Technology Scientific) was used as the measuring apparatus. The measurement program was set to 100–350 °C with a heating rate of 5 °C / min.

[0274] The results of the exothermic peak (heat flow) measurement (DSC curve) for each unit mass (mg) of the mixed powders of Examples 1-3, 5, and Comparative Examples 1-2 are shown in the figure. Figure 5 . Figure 5 In the figure, the large peak near 230℃ represents the endothermic peak during Sn melting.

[0275] Next, based on the DSC curves, the integral value (in [J]) of the heat flux per unit mass (mg) was calculated for each mixed powder of Examples 1-3, 5 and Comparative Examples 1-2 by integrating the heat flux per unit mass (mg) over time.

[0276] In the mixed powder of Comparative Example 1, the temperature T at which the calorific value is minimized is set as T0 within the range of 200°C to 350°C. in0 In the mixed powders of Examples 1-3, 5, and Comparative Example 2, the temperature T at which the integral value is minimized is set as Tmin within the range of 200°C to 350°C. in .

[0277] The mixed powders of Examples 1-3, 5 and Comparative Example 2 were subjected to T in The calorific value per unit mass (mg) in the range up to 350°C and the calorific value of the mixed powder of Comparative Example 1 at T in0 The calorific value per unit mass (mg) in the range up to 350°C is shown in Figure 6 .

[0278] Next, at temperature T in The proportion of heat release of each mixed powder in Examples 1-3, 5, and Comparative Example 2 within the range of up to 300°C was used as the temperature T. in0The proportion of heat release (100%) relative to the mixed powder of Comparative Example 1 within the range up to 300°C was calculated. The calculated results of the heat release proportion are shown below. Figure 7 A higher proportion of heat generation means that the compound formation occurs more during welding, resulting in higher heat resistance.

[0279] As can be confirmed from the above, in the embodiments, the thicker the Ni coating, the slower the exothermic reaction starts, and the more the reaction to form the compound is suppressed.

[0280] (2) Judgment Criteria

[0281] The calorific value of the mixed powder A is more than 60% of that of the mixed powder in Comparative Example 1.

[0282] The calorific value of the B mixed powder is more than 20% and less than 60% of the calorific value of the mixed powder of Comparative Example 1.

[0283] The calorific value of the C mixed powder is less than 20% of that of the mixed powder in Comparative Example 1.

[0284] It can be confirmed that Examples 1-7 and Comparative Example 1 have higher heat resistance than Comparative Example 2.

[0285] Evaluation of Compound Formation

[0286] Similar to the evaluation of void suppression capability, the solder paste for each example was printed onto the substrate, and reflow soldering was performed without mounting a Si chip. The solder after reflow was photographed.

[0287] The photographic images of Examples 1-3, 5, 6 and Comparative Examples 1-2 are shown below. Figure 8 .

[0288] The white areas in the image are those with weak metallic luster, representing compounds of Ni and Fe alloys with Sn.

[0289] The black areas in the captured image are areas with a strong metallic luster, representing unreacted Sn that has not undergone compounding.

[0290] In Examples 1-3, 5, 6, and Comparative Example 1, it was confirmed that more compound was formed compared to Comparative Example 2. Although no photographic images are shown, in Example 4, the compound was formed to the same extent as in Examples 3 and 5. Although no photographic images are shown, in Example 7, more compound was formed compared to Comparative Example 2. In Comparative Example 2, because the Ni plating was too thick, the core was not fully exposed during bonding, presumably because a sufficient amount of compound was not formed to provide heat resistance in the core and the first metal.

[0291] The results confirm that Examples 1-7 and Comparative Example 1 have higher heat resistance than Comparative Example 2.

[0292] Embodiments of the present disclosure have been described in detail with reference to the accompanying drawings. However, in these drawings, for the convenience of enlarging the parts that may sometimes be features, the size ratios of the constituent elements are not limited to the size ratios shown in the drawings.

[0293] The specific structure of the embodiments of the present invention is not limited to the embodiments disclosed herein. As long as it does not depart from the spirit of the present disclosure, changes and substitutions can be made.

Claims

1. A solder paste comprising a first metal powder, a second metal powder, and flux. wherein The first metal powder contains Sn. The second metal powder has a core made of an alloy containing Ni and Fe and a surface layer covering the core made of a metal containing Ni. The Sn content in the first metal powder is 20% by mass or more and 100% by mass or less relative to the total mass of the first metal powder. The Ni content in the metal forming the core of the second metal powder is 80% by mass or more and 99% by mass or less relative to the total mass of the metal forming the core of the second metal powder. The Fe content in the metal forming the core of the second metal powder is 1% by mass or more and 20% by mass or less relative to the total mass of the metal forming the core of the second metal powder. The Ni content in the metal forming the surface layer of the second metal powder is 50% by mass or more relative to the total mass of the metal forming the surface layer of the second metal powder. The particle size of the first metal powder is 0.1~1000μm. The particle size of the second metal powder is 0.2~1000μm. The thickness of the surface layer of the second metal powder is 0.05 μm or more and 0.30 μm or less. The content of the first metal powder relative to the total mass of the first metal powder and the second metal powder is 30-99% by mass. The content of the second metal powder relative to the total mass of the first metal powder and the second metal powder is 1-70% by mass. The surface layer of the second metal powder satisfies at least one of the following (a) and (b): (a) The surface layer of the second metal powder is the outermost layer; (b) The surface layer of the second metal powder is in direct contact with the core.

2. A solder joint formed using the solder paste of claim 1.

Images