Solder paste and solder joint

Abstract

To provide a solder paste that is excellent in heat resistance and can further suppress void formation during solder joining, and a solder joint which is formed by using the solder paste.SOLUTION: A solder paste containing first metal powders, second metal powders, and flux is adopted. The first metal powder contains Sn. The second metal powder 20A has a core part 201 comprising an alloy containing Ni and Fe, and a surface layer 202 comprising a metal containing Ni that covers the core part 201. A content of Ni in the metal forming the surface layer 202 of the second metal powder 20A is 50 mass% or more relative to a total mass of the metal forming the surface layer 202 of the second metal powder 20A. A thickness of the surface layer 202 of the second metal powder 20A is 0.05 μm or more and less than 1.20 μm.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to solder paste and solder joints.

Background Art

[0002] In recent years, with the increase in the operating temperature of power semiconductor devices using silicon carbide (SiC) or the like, the temperature at the solder joint site may reach about 250 to 280°C. For this reason, a high-temperature solder that does not melt is required during operation under such high-temperature conditions.

[0003] For the production of the above solder joints, various solder pastes are used as soldering materials. For example, as the solder paste, an Ag paste capable of low-temperature sintering and a TLP (Transient Liquid Phase) paste compliant with the RoHS directive of the European Union can be mentioned. The TLP paste is a paste containing a plurality of types of solder powders. In the TLP paste, high-melting-point compounds are formed between the solder powders during heating. As a result, even if the solder joint is reheated, remelting can be suppressed, and the heat resistance can be enhanced. As such a TLP paste, for example, a paste in which Cu balls and Sn solder balls are dispersed via a flux has been proposed (see Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, solder paste containing flux as described in Patent Document 1 has a problem that the flux gasified during melting of the solder powder remains in the molten solder, and a large amount of voids are likely to occur during solidification. In particular, in the TLP paste in which a high melting point compound is formed during solidification, the fluidity decreases, and it becomes difficult for the gasified flux to be released to the outside.

[0006] The present invention has been made in view of the above circumstances, and an object thereof is to provide a solder paste that is excellent in heat resistance and can further suppress the generation of voids during solder bonding, and a solder joint formed using the solder paste.

Means for Solving the Problems

[0007] The present invention includes the following aspects. [1] A solder paste containing 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 portion made of an alloy containing Ni and Fe, and a surface layer made of a metal containing Ni that covers the core portion, 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 metal forming the core portion of the second metal powder is 80% by mass or more and 99% by mass or less with respect to the total mass of the metal forming the core portion of the second metal powder, the content of Fe in the metal forming the core portion of the second metal powder is 1% by mass or more and 20% by mass or less with respect to the total mass of the metal forming the core portion of the second metal powder, the content of Ni in the metal forming the surface layer of the second metal powder is 50% by mass or more with respect 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 less than 1.20 μm. [2] The content of the first metal powder is 30 to 99% by mass with respect 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 with respect to the total mass of the first metal powder and the second metal powder. The solder paste according to [1]. [3] A solder joint formed using the solder paste according to [1] or [2].

Effects of the Invention

[0008] According to the present invention, it is possible to provide a solder paste that is excellent in heat resistance and can further suppress the generation of voids during solder bonding.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0010] In this specification, "include" and "contain" each encompass the concepts of "comprise", "consist essentially of", and "consist of".

[0011] In this specification, "the first metal", "the second metal", and "the third metal" may respectively mean "particles formed of the first metal", "particles formed of the second metal", and "particles formed of the third metal". In this specification, "the first metal powder", "the second metal powder", and "the third metal powder" may respectively mean "a group of particles formed of the first metal", "a group of particles formed of the second metal", and "a group of particles formed of the third metal".

[0012] (Solder Paste First Embodiment) The solder paste according to the first embodiment 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 portion made of an alloy containing Ni and Fe, and a surface layer containing Ni that coats the core portion.

[0013] <The First Metal Powder> The metal constituting the first metal powder (hereinafter referred to as the first metal) is a metal containing Sn. The first metal may contain metals 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, Zr, and As. These metals other than Sn may include one kind or two or more kinds. The group of metals other than Sn can be arbitrarily selected from these metals.

[0014] The metal that the first metal may contain may be each simple substance of Sn and metals other than Sn, or may be an alloy obtained by alloying the simple substance of Sn and the simple substance of metals other than Sn.

[0015] The first metal may be, for example, pure Sn, a mixture of a metal other than Sn and Sn, an alloy of a metal other than Sn and Sn, or a mixture of an alloy containing Sn and another metal.

[0016] The first metal may contain inevitable impurities in addition to the aforementioned metals. Even when inevitable impurities are contained, it does not affect the effects of the present invention. The first metal may be of one type or two or more types.

[0017] As used herein, the "melting point of the metal or the melting point of the metal powder" refers to the melting point measured by differential scanning calorimetry (DSC). The melting point of the first metal can be measured, for example, using DSC7020 manufactured by Hitachi High-Tech Science Corporation. The melting point of the second metal described later can be measured, for example, using DSC404-F3 Pegasus manufactured by NETZSCH.

[0018] The melting point of the first metal is preferably 300 °C or lower, may be 250 °C or lower, or may be 80 to 200 °C. When the melting point of the first metal is equal to or lower than the upper limit value of the above preferred range, it becomes easier to ensure the wettability of the solder.

[0019] The content of Sn in the first metal is 20% by mass or more and 100% by mass or less based on the total mass of the first metal. In order for the characteristics of Sn to be fully exhibited, the content of Sn in the first metal is preferably 90% by mass or more, more preferably 95% by mass or more, and still more preferably 100% by mass based on the total mass of the first metal.

[0020] As used herein, the "particle size of the metal or the particle size of the metal powder" refers to the average particle size measured on a volume basis using a laser diffraction / scattering particle size distribution measuring device. The average particle size can be measured, for example, using a laser diffraction / scattering particle size distribution analyzer (MT3300EXII) manufactured by Microtrac Bell Corporation.

[0021] The particle size of the first metal powder is 0.1 to 1000 μm, preferably 1 to 100 μm. When the particle size of the first metal powder is equal to or greater than the lower limit value of the above preferred range, wettability is easily ensured. When it is equal to or less than the upper limit value of the above preferred range, it is easier to form an intermetallic compound.

[0022] <The second metal powder> As illustrated in FIG. 1, the second metal powder 20A has a core portion 201 and a surface layer 202 that covers the entire core portion 201. Rc means the core diameter of the core portion 201. Rs means the thickness of the surface layer 202. The metal forming the core portion 201 is made of an alloy containing Ni and Fe. The metal forming the surface layer 202 is made of a metal containing Ni.

[0023] ≪Surface layer≫ The metal forming the surface layer of the second metal powder may consist only of Ni, or may be a metal containing Ni and a metal other than Ni. The metal containing Ni and a metal other than Ni may be an alloy of the metal other than Ni and Ni. The metal forming the surface layer is preferably Ni alone.

[0024] Examples of the metal other than Ni that the metal forming the surface layer may contain include Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. These metals other than Ni may contain one kind or two or more kinds. The group of metals other than Ni can be arbitrarily selected from these metals.

[0025] The metal forming the surface layer may contain inevitable impurities in addition to the aforementioned metals. Even if inevitable impurities are contained, it will not affect the effects of the present invention.

[0026] When the metal forming the surface layer is a metal containing Ni and a metal other than Ni, the content of Ni in the metal forming the surface layer 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, still more preferably 90% by mass or more, particularly preferably 95% by mass or more, and most preferably 98% by mass or more, based on the total mass of the metal forming the surface layer. When the metal forming the surface layer contains Fe, the content of Fe in the metal forming the surface layer is preferably 0% by mass or more and less than 5% by mass, based on the total mass of the metal forming the surface layer.

[0027] The thickness Rs of the surface layer is 0.05 μm or more and less than 1.20 μm. The thickness Rs of the surface layer is preferably 0.05 μm, 0.10 μm, 0.15 μm, 0.20 μm, 0.25 μm, 0.30 μm, 0.35 μm, 0.40 μm, 0.45 μm, 0.50 μm, 0.55 μm, 0.60 μm, 0.70 μm, 0.75 μm, 0.80 μm, 0.90 μm, 1.00 μm, 1.10 μm, 1.15 μm, and the upper and lower limit values can be appropriately selected from these values. The thickness Rs of the surface layer may be 0.10 μm or more and less than 1.20 μm, or 0.15 μm or more and less than 1.20 μm, or 0.20 μm or more and less than 1.20 μm, or 0.25 μm or more and less than 1.20 μm, or 0.30 μm or more and less than 1.20 μm, or 0.35 μm or more and less than 1.20 μm, or 0.40 μm or more and less than 1.20 μm, or 0.45 μm or more and less than 1.20 μm, or 0.50 μm or more and less than 1.20 μm, or 0.55 μm or more and less than 1.20 μm, or 0.60 μm or more and less than 1.20 μm, or 0.70 μm or more and less than 1.20 μm, or 0.75 μm or more and less than 1.20 μm, or 0.80 μm or more and less than 1.20 μm, or 0.90 μm or more and less than 1.20 μm, or 1.00 μm or more and less than 1.20 μm, or 1.10 μm or more and less than 1.20 μm, or 1.15 μm or more and less than 1.20 μm. The thickness Rs of the surface layer may be 0.05 μm or more and 1.15 μm or less, or 0.05 μm or more and 1.10 μm or less, or 0.05 μm or more and 1.00 μm or less, or 0.05 μm or more and 0.90 μm or less, or 0.05 μm or more and 0.80 μm or less, or 0.05 μm or more and 0.75 μm or less, or 0.05 μm or more and 0.70 μm or less, or 0.05 μm or more and 0.60 μm or less, or 0.05 μm or more and 0.55 μm or less, or 0.05 μm or more and 0.50 μm or less, or 0.05 μm or more and 0.45 μm or less, or 0.05 μm or more and 0.40 μm or less, or 0.05 μm or more and 0.35 μm or less, or 0.05 μm or more and 0.30 μm or less, or 0.05 μm or more and 0.25 μm or less, or 0.05 μm or more and 0.20 μm or less, or 0.05 μm or more and 0.15 μm or less, or 0.05 μm or more and 0.10 μm or less.

[0028] From the perspective of ensuring sufficient heat resistance and further enhancing the void suppression ability, the thickness Rs of the surface layer is preferably not less than 0.10 μm and less than 1.20 μm. From the perspective of ensuring sufficient void suppression ability and further enhancing the heat resistance, the surface layer thickness Rs is preferably not less than 0.05 μm and not more than 1.0 μm, more preferably not less than 0.05 μm and not more than 0.75 μm, still more preferably not less than 0.05 μm and not more than 0.30 μm, particularly preferably not less than 0.05 μm and not more than 0.25 μm, most preferably not less than 0.05 μm and not more than 0.20 μm, and may be not less than 0.05 μm and not more than 0.15 μm, or not less than 0.05 μm and not more than 0.10 μm. When the thickness Rs of the surface layer is not less than the lower limit value, the void suppression ability can be further enhanced. When the thickness Rs of the surface layer is not more than the upper limit value, the heat resistance can be further enhanced.

[0029] In this specification, the thickness Rs of the surface layer of the second metal powder, the core diameter Rc of the core part, and the thickness Ri of the intermediate layer of the second metal powder and the thickness Rm of the metal layer of the second metal powder in the second embodiment described later can be measured from the cross-sectional structure of the second metal powder using an optical microscope, SEM, transmission electron microscope (TEM), etc. Alternatively, the thickness Rs of the surface layer of the second metal can be measured using an Auger electron spectroscopy analyzer based on Auger electron spectroscopy.

[0030] Alternatively, the core diameter Rc of the core part of the second metal powder can be measured as follows. When manufacturing the second metal powder, the particle size of the metal powder prepared for use as the core part can be used as the core diameter Rc.

[0031] The melting point of the metal forming the surface layer is preferably above 300 °C, more preferably 500 °C or higher, and still more preferably 600 - 1600 °C.

[0032] The surface layer may be a plating layer formed by plating treatment. Examples of the plating process include known electroplating, electroless plating, and the like.

[0033] At least a part of the surface of the core portion is covered by a surface layer. In FIG. 1, the entire surface of the core portion is covered by the surface layer. The ratio of the area of the surface of the core portion covered by the surface layer is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, still more preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100% with respect to the total surface area (100%) of the core portion.

[0034] ≪Core Portion≫ The core portion of the second metal powder is made of an alloy containing Ni and Fe. The alloy in the core portion contains Ni and Fe and has a melting point higher than that of the first metal powder. The melting point of the alloy in the core portion is preferably above 300 °C, more preferably 500 °C or higher, and still more preferably 600 to 1600 °C. When the melting point of the alloy in the core portion exceeds the lower limit value of the above preferred range, the heat resistance can be enhanced, and it becomes easier to increase the shear strength of the solder joint.

[0035] The alloy in the core portion may contain metals other than Ni and Fe. That is, the second metal may be an alloy of Ni and Fe, an alloy of Ni and Fe and other metals, and among these, an alloy of Ni and Fe is preferable.

[0036] Examples of the metals other than Ni and Fe that the alloy in the core portion may contain include Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Mn, Zr, Sn, and As. These metals other than Ni and Fe may contain one kind or two or more kinds. The group of metals other than Ni and Fe can be arbitrarily selected from these metals.

[0037] The metal forming the core part may contain inevitable impurities in addition to the aforementioned metals. Even when inevitable impurities are contained, it does not affect the effects of the present invention. The second metal may be one type or two or more types.

[0038] The content of Ni in the metal forming the core part is 80% by mass or more and 99% by mass or less, preferably 85% by mass or more and 95% by mass or less, based on the total mass of the metal forming the core part. The content of Fe in the metal forming the core part is 1% by mass or more and 20% by mass or less, preferably 5% by mass or more and 15% by mass or less, based on the total mass of the metal forming the core part. When the contents of Ni and Fe in the metal forming the core part are within the above-preferred ranges, intermetallic compounds are formed at an earlier stage, and the heat resistance can be improved.

[0039] The core diameter Rc of the core part 201 of the second metal powder 20A is preferably 0.1 to 1000 μm, more preferably 3 to 300 μm, still more preferably 5 to 100 μm, and particularly preferably 10 to 50 μm.

[0040] Regarding the second metal powder 20A, the ratio of the core diameter Rc of the core part 201 to the thickness Rs of the surface layer 202, as the ratio represented by Rc / Rs, is preferably 0.2 to 20000, more preferably 2 to 2000, still more preferably 10 to 1000, particularly preferably 20 to 500, most preferably 30 to 500, and may also be 30 to 452. The ratio represented by Rc / Rs is preferably 452, 400, 350, 300, 250, 226, 65, 45, 30, and the upper and lower limit values can be appropriately selected from these values. When Rc / Rs is within the above range, it is easy to enhance the heat resistance and suppress voids.

[0041] The second metal powder has a particle size of 0.2 to 1000 μm, preferably 1 to 100 μm, and more preferably 5 to 50 μm. When the particle size of the second metal is equal to or greater than the lower limit value of the preferred range, wettability is easily ensured. When it is equal to or less than the upper limit value of the preferred range, intermetallic compounds are more easily formed.

[0042] The second metal powder 20A may be of one type or two or more types.

[0043] <Relationship between the contents of the first metal powder and the second metal powder> In the solder paste according to the first embodiment, the mixing ratio of the first metal powder and the second metal powder is preferably such that the content of the first metal powder is 30 to 99% by mass, more preferably 60 to 95% by mass, still more preferably 70 to 95% by mass, and particularly preferably 80 to 95% by mass with respect to the total content (100% by mass) of the first metal powder and the second metal powder. The mixing ratio of the first metal powder and the second metal powder is preferably such that the content of the second metal powder is 1 to 70% by mass, more preferably 5 to 40% by mass, still more preferably 5 to 30% by mass, and particularly preferably 5 to 20% by mass with respect to the total content (100% by mass) of the first metal powder and the second metal powder.

[0044] In the solder paste according to the first embodiment, the ratio of the content of the first metal powder to the content of the second metal powder is preferably 4, 8, or 9 as the mass ratio represented by the content of the first metal powder / the content of the second metal powder, and the upper and lower limit values can be appropriately selected from these values. The mass ratio represented by the content of the first metal powder / the content of the second metal powder may be, for example, 1 to 20, 3 to 15, 6 to 10, or 4 to 9.

[0045] <Flux> The solder paste according to the first embodiment contains a flux. The components contained in the flux are not particularly limited as long as the effects of the present invention are achieved. Examples thereof include resin components such as rosin, activators, thixotropic agents, solvents, metal deactivators, surfactants, silane coupling agents, antioxidants, colorants, and the like.

[0046] Examples of the rosin include natural resins containing abietic acid as the main component, i.e., mixtures of abietic acid and its isomers, and chemically modified natural resins.

[0047] Examples of the activator include organic acids, amines, halogen compounds, and the like. Examples of the thixotropic agent include ester-based thixotropic agents, amide-based thixotropic agents, sorbitol-based thixotropic agents, and the like.

[0048] In the solder paste, the content of the flux is preferably 5 to 30% by mass, more preferably 8 to 15% by mass, based on the total mass of the solder paste.

[0049] The solder paste according to 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 portion made of an alloy containing Ni and Fe, and a surface layer containing Ni that coats the core portion. In the solder paste according to the first embodiment, during soldering, since Sn and the alloy containing Ni and Fe form a high-melting-point compound, re-melting can be suppressed even when the solder joint is reheated.

[0050] In the solder paste according to the first embodiment, the second metal powder has a core portion and a surface layer containing Ni that coats the core portion. The content of Ni in the metal forming the surface layer is 50% by mass or more based on 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. The solder paste according to the first embodiment has a surface layer as described above for the second metal powder, so that it has excellent heat resistance and can further suppress the generation of voids during soldering. The reason for obtaining such an effect is not clear, but it is presumed as follows.

[0051] Until reflow is started and a certain period of time has elapsed, the surface layer of the second metal powder serves to protect the core part of the second metal powder. Since Ni contained in the surface layer of the second metal powder has a lower reactivity with Sn than an alloy containing Ni and Fe, it is possible to suppress the rapid formation of a compound between the alloy containing Ni and Fe and Sn. As a result, the vaporized flux is more easily discharged from the solder paste, and the generation of voids during soldering can be further suppressed. When the thickness of the surface layer is 0.05 μm or more, the rapid formation of a compound between the alloy containing Ni and Fe and Sn can be suppressed, and the generation of voids can be further suppressed.

[0052] While a certain period of time elapses after reflow is started, gradually, the Ni in the surface layer diffuses into the molten Sn. As the Ni in the surface layer disappears, the alloy containing Ni and Fe, which is the core part, reacts with Sn to form a high-melting-point compound. When the thickness of the surface layer is less than 1.20 μm, the Ni in the surface layer easily diffuses into the molten Sn, so that a high-melting-point compound can be formed and the heat resistance of the solder joint is improved.

[0053] The solder paste according to the first embodiment has its heat resistance enhanced by forming a high-melting-point compound between the core part of the second metal powder made of an alloy containing Ni and Fe and the Sn of the first metal powder during reflow.

[0054] The reaction for forming the high-melting-point compound is an exothermic reaction. The heat generation amounts for the first metal powder and the second metal powder contained in the solder paste according to the present embodiment are preferably within the following ranges.

[0055] First, a mixed powder composed of the first metal powder and the second metal powder contained in the solder paste according to the present embodiment to be targeted is prepared (hereinafter, this mixed powder is referred to as the "target sample"). The mixing ratio (mass ratio) of the first metal powder and the second metal powder in the target 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 according to the target embodiment.

[0056] In the target sample, a mixed powder is prepared which is the same except that the second metal powder is replaced with the metal powder of the core part of the second metal powder (hereinafter, this mixed powder is referred to as the "reference sample"). In the reference sample, the mixing ratio (mass ratio) of the first metal powder and the metal powder of the core part of 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 according to the target embodiment.

[0057] First, for the reference sample, the heat generation amount is measured by differential scanning calorimetry (DSC) according to the following procedure. In the measurement, EXSTAR DSC7020 (manufactured by Hitachi High-Tech Science Corporation) is used as the measurement device, and the sample can be measured by putting it in an aluminum pan. The measurement program is set to 100 to 350 °C with a heating rate of 5 °C / min.

[0058] The result of the differential scanning calorimetry is obtained as a DSC curve with the horizontal axis being time t and the vertical axis being heat flow (unit: [W] or [J][s] -1 ). Or it is obtained as a DSC curve with the horizontal axis being temperature T and the vertical axis being heat flow. The obtained DSC curve becomes a large negative value (i.e., largely endothermic) around 235 °C where the first metal powder containing Sn melts.

[0059] Based on the DSC curve, by integrating the heat flow per unit mass (mg) of the reference sample with respect to time, the integrated value Q0' per unit mass (mg) of the reference sample from time t0 to time t (unit: [J]) is calculated. Here, time t0 is the time when the temperature of the reference sample reaches 200 °C. In the range from 200 °C to 350 °C, the time t when the integrated value Q0' of the heat flow of the reference sample is minimized is tin0 Let it be. At time t in0 is the time when the heat generation of the reference sample starts. At time t in0 From time t to time t, the heat generation amount is taken as the heat generation amount of the reference sample (hereinafter, this is referred to as "reference heat generation amount Q0"). The reference heat generation amount Q0 is the heat generation amount per unit mass (mg) of the reference sample.

[0060] The reference heat generation amount Q0 is the heat generation amount from temperature T in0 to temperature T. It can also be said so. Here, at time t in0 the temperature of the reference sample is T in0 and at time t, the temperature of the reference sample is T. T in0 is the temperature of the reference sample when the integrated value Q0' of the heat flow of the reference sample is minimized in the range from 200°C to 350°C.

[0061] Next, for the target sample, in the same manner as the reference sample, a DSC curve is obtained by differential scanning calorimetry. The T of the reference sample in0 is calculated in the same manner as the T of the target sample in For the target sample, the heat generation amount from temperature T in to temperature T is calculated (hereinafter, this is referred to as "target heat generation amount Q"). The target heat generation amount Q is the heat generation amount per unit mass (mg) of the target sample. The target heat generation amount Q is the heat generation amount from temperature T in to temperature T. It can also be said so. Here, at time t in the temperature of the target sample is T in and at time t, the temperature of the target sample is T. T in is the temperature of the target sample when the integrated value of the heat flow of the target sample is minimized in the range from 200°C to 350°C.

[0062] As will be described later in the examples, the target heat generation amount Q from temperature T in to temperature T is calculated as shown in FIG. 6, for example. In FIG. 6, the reference heat generation amount Q0 from temperature T in0 to temperature T is the heat generation amount of Comparative Example 1.

[0063] temperature T in The ratio of the target calorific value Q of the target sample in the temperature range from 100 to 300°C is in0 It is preferable that the calorific value is 10% or more and less than 100% of the reference calorific value Q0 (100%) of the reference sample in the temperature range of 0 to 300°C. The lower limit of the proportion of the target calorific value is preferably 10% or more, more preferably 15% or more, even more preferably 20% or more, particularly preferably 25% or more, most preferably 30% or more, and may be 40% or more, 50% or more, or 60% or more. The upper limit of the proportion of the target calorific value is not particularly limited, but may be less than 100% or may be 80% or less.

[0064] As will be described later in the examples, the temperature T in0 The temperature T of the target sample for the reference calorific value Q0 (100%) in the range of in The proportion of the target calorific value Q in the range of 0 to 300°C is calculated, for example, as shown in FIG.

[0065] In summary, the calorific values of the first metal powder and the second metal powder are preferably as follows: Temperature T of the target sample per unit mass (mg) measured by differential scanning calorimetry in The ratio of the target calorific value Q in the range of 0 to 300°C is the temperature T of the reference sample per unit mass (mg). in0 It is preferable that the calorific value is 10% or more and less than 100% of the reference calorific value Q0 (100%) in the range of 0°C to 300°C. The target sample is a mixed powder consisting of a first metal powder and a second metal powder, and the mixing ratio (mass ratio) of the first metal powder to the second metal powder in the target sample is the same as the mixing ratio (mass ratio) of the first metal powder to the second metal powder in the solder paste. The reference sample is a mixed powder composed of the first metal powder and the metal powder of the core part of the second metal powder. The mixing ratio (mass ratio) of the first metal powder and the metal powder of the core part 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, respectively. Temperature T in is the temperature of the target sample when the integrated value of the heat flow of the target sample is minimized in the range from 200 °C to temperature T. The temperature T of the target sample can take a temperature from 200 °C to 350 °C. Temperature T in0 is the temperature of the target sample when the integrated value of the heat flow of the reference sample is minimized in the range from 200 °C to temperature T. The temperature T of the reference sample can take a temperature from 200 °C to 350 °C.

[0066] 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 are more likely to diffuse into the molten Sn than Ni. Therefore, the surface layer mainly composed of Cu, Au, Ag, or Pd cannot sufficiently suppress the rapid formation of a compound between the alloy containing Ni and Fe and Sn. Even when the main component of the surface layer of the second metal powder is not Ni but Bi, since Bi dissolves in Sn, the rapid formation of a compound between the alloy containing Ni and Fe and Sn cannot be sufficiently suppressed. 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 will melt in a short time after the reflow starts, so the rapid formation of a compound between the alloy containing Ni and Fe and Sn cannot be sufficiently suppressed.

[0067] In addition, Cu, Au, Ag, and Pd are more likely to react with Sn to form a compound than Ni. Therefore, the surface layer containing Cu, Au, Ag, or Pd will form a compound with Sn, inhibiting the formation of a high-melting-point compound between Sn and the alloy containing Ni and Fe. As a result, the heat resistance of the solder joint cannot be sufficiently enhanced.

[0068] Embodiments of the present invention are not limited to the first embodiment and may be other embodiments other than the first embodiment. As other embodiments, the second to fourth embodiments will be described.

[0069] (Second Embodiment) The solder paste according to the second embodiment is obtained by changing the second metal powder 20A to the second metal powder 20B in the solder paste according to the first embodiment. The solder paste according to the second embodiment is the same as that of the first, third, or fourth embodiment except that it contains the second metal powder 20B. Examples of the core portion 201 and the surface layer 202 in the second metal powder 20B are the same as those described above for the second metal powder 20A.

[0070] As illustrated in FIG. 2, the second metal powder 20B has an intermediate layer 203 that covers the core portion 201 between the core portion 201 and the surface layer 202 that covers the core portion 201.

[0071] The intermediate layer may cover a part of the surface of the core portion or may cover the entire surface of the core portion, and it is preferable that the entire surface of the core portion is covered. In FIG. 2, the intermediate layer 203 covers the entire surface of the core portion 201. The ratio of the surface area of the core portion covered by the intermediate layer is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, still more preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100% with respect to the total surface area (100%) of the core portion.

[0072] Ri means the thickness of the intermediate layer 203. The intermediate layer may be a single layer or may be two or more layers. In the second metal powder 20B illustrated in FIG. 2, the intermediate layer 203 is a single layer.

[0073] The composition of the metal forming the intermediate layer 203 is different from the metal forming the core part 201 and the metal forming the surface layer 202. The metal forming the intermediate layer may be a single metal or an alloy formed of two or more metal elements.

[0074] Examples of metals that the intermediate layer may contain include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. These metals may be included singly or in combination of two or more. The group of metals that the intermediate layer may contain can be arbitrarily selected from these metals.

[0075] The intermediate layer may contain inevitable impurities in addition to the aforementioned metals. Even when inevitable impurities are contained, it does not affect the effects of the present invention. The metal forming the intermediate layer may be one type or two or more types.

[0076] The thickness Ri of the intermediate layer may be, for example, 0.01 μm or more and 100 μm or less, 0.05 μm or more and 50 μm or less, or 0.1 μm or more and 10 μm or less.

[0077] The intermediate layer may be a plating formed by a plating process. Examples of the plating method include known electroplating, electroless plating, etc.

[0078] The second metal powder 20B may be one type or two or more types.

[0079] Since the second metal powder 20B has the intermediate layer 203, it becomes easy to provide the surface layer 202 of the second metal powder 20B.

[0080] (Third Embodiment) In the solder paste according to the third embodiment, the surface layer of the second metal powder is covered with a metal layer. The solder paste according to the third embodiment is the same as that of the first, second, or fourth embodiment, except that the second metal powder has a metal layer.

[0081] As illustrated in FIG. 3, the second metal powder 20C has a core portion 201, a surface layer 202 covering the core portion 201, and a metal layer 204 covering the surface layer 202.

[0082] At least a part of the surface of the surface layer is covered with the metal layer. In FIG. 3, the entire surface of the surface layer 202 is covered with the metal layer 204. The ratio of the area of the surface of the surface layer covered by the metal layer is preferably 50% or more and 100% or less, more preferably 70% or more and 100% or less, still more preferably 90% or more and 100% or less, particularly preferably 95% or more and 100% or less, and most preferably 100% with respect to the total surface area (100%) of the surface layer.

[0083] The metal layer may be a single layer or two or more layers. In the second metal powder 20C illustrated in FIG. 3, the metal layer is a single layer.

[0084] The composition of the metal forming the metal layer is different from that of the metal forming the surface layer. The metal forming the metal layer may be a single type of simple metal or an alloy formed of two or more metal elements.

[0085] Examples of the metal that the metal layer may contain include Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, and As. These metals may be included singly or in combination of two or more. The group of metals that the core portion may contain can be arbitrarily selected from these metals.

[0086] The metal layer may contain inevitable impurities in addition to the aforementioned metals. Even when inevitable impurities are contained, it does not affect the effects of the present invention. The metal for forming the metal layer may be one type or two or more types.

[0087] When the metal for forming the metal layer contains Fe, the content of Fe in the metal for forming the metal layer is preferably 0% by mass or more and less than 5% by mass based on the total mass of the metal for forming the metal layer.

[0088] The thickness Rm of the metal layer may be, for example, 0.01 μm or more and 100 μm or less, or 1 μm or more and 50 μm or less, or 2 μm or more and 10 μm or less.

[0089] The metal layer may be a plating formed by a plating process. Examples of the plating process include known electroplating, electroless plating, etc.

[0090] Since the second metal powder 20C has the metal layer 204 and the metal layer contains Sn, the wettability is improved when the solder paste is melted, and further reduction of voids can be expected. The metal layer melts after the start of heating when melting the solder paste, and the surface layer of the third metal is exposed, so the effects of the present application are not inhibited.

[0091] (Fourth Embodiment) The solder paste according to the fourth embodiment further contains a metal powder other than the first metal powder and the second metal powder (hereinafter also referred to as "third metal powder"). The solder paste according to the fourth embodiment is the same as the first, second, or fourth embodiment except that it further contains the third metal powder.

[0092] The third metal powder is not particularly limited in composition as long as it is different from the first metal powder and the second metal powder. For example, powders composed of single metals such as Ni, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, As, or alloys formed of two or more of these single metals are preferable.

[0093] As the third metal powder, for example, it may have a core part and a surface layer made of a metal containing Ni that covers the core part. The core part is not particularly limited in composition as long as it is different from the core parts of the first and second embodiments. For example, powders made of each single metal of Ni, Ag, Cu, In, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, Sn, As, or alloys formed of two or more of these single metals are preferable.

[0094] The third 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. The metal constituting the third metal powder may contain one kind or two or more kinds. There may be one kind of the third metal powder, or two or more kinds.

[0095] By containing the third metal powder, it is possible to increase the thermal conductivity of the solder paste.

[0096] (Solder joint) The solder joint according to the present invention is formed using the solder paste according to the above embodiment. The object to be joined by the solder joint is not particularly limited. For example, a semiconductor element and a substrate can be joined by the solder joint. Examples of the semiconductor element include a silicon carbide (SiC) chip and an Si chip. Examples of the substrate include a circuit board, a ceramic substrate, a metal substrate, and a direct copper bonding (DCB) substrate. The electrode on the substrate may be, for example, a Cu electrode or a Cu electrode treated with any one of Sn plating, Ni plating, Ni - Au plating, Ni - Pd plating, or Ni - Pd - Au plating.

[0097] As a method for joining an object to be joined using the solder paste according to the above-described embodiment, for example, a reflow method can be mentioned. The reflow conditions can be appropriately set with reference to known methods. During reflow, the inside of the reflow furnace may be depressurized. By depressurizing, the gasified flux is easily discharged from the solder paste, and the generation of voids is easily suppressed. The temperature at the time of joining the semiconductor element and the substrate is preferably, for example, 120°C or higher and 400°C or lower, may be 200°C or higher and 400°C or lower, or may be 250°C or higher and 400°C or lower. The atmosphere at the time of joining the object may be an air atmosphere, an inert atmosphere such as a nitrogen atmosphere, or a reducing atmosphere.

[0098] As described above, since the solder joint according to the present invention uses the solder paste according to the above-described embodiment containing the first metal powder, the second metal powder, and the flux, it has excellent heat resistance and shear strength. The solder joint according to the present invention is particularly useful for applications that require a high-temperature solder that does not melt during operation under high-temperature conditions such as a power semiconductor element.

Examples

[0099] Hereinafter, the present invention will be described by way of examples, but the present invention is not limited to the following examples.

[0100] <Preparation of Paste> (Examples 1 to 7, Comparative Examples 1 to 2) The metal powder having the composition shown in Table 1 and the flux were mixed to prepare the pastes of the examples and the comparative examples. In the pastes of Examples 1 to 5 and 7, the flux is 12% by mass, and the total mass of the first metal powder and the second metal powder is 88% by mass. 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. The paste of Comparative Example 1 has a flux of 12% by mass and the total mass of the first metal powder and Metal Powder B is 88% by mass. The paste of Comparative Example 2 has a flux of 12% by mass and the total mass of the first metal powder and Metal Powder A is 88% by mass. The numerical values in the table represent the content of each metal powder with respect to the total mass (100% by mass) of the metal powders in the paste. The blank spaces in the table mean "0".

[0101] The particle size of the metal powder and the particle size Rc of the core part were measured based on volume using a laser diffraction / scattering particle size distribution measuring device (MT3300EXII) manufactured by Microtrac Bell Corporation. The melting point of the metal powder was determined by differential scanning calorimetry (DSC). For the first metal powder, it was measured using DSC7020 manufactured by Hitachi High-Tech Science Corporation, and for the second metal powder and Metal Powders A to C, it was measured using DSC404-F3 Pegasus manufactured by NETZSCH. The thickness Rs of the surface layer in the second metal powders (1) to (5), Metal Powder A, and Metal Powder C was measured using an Auger electron spectroscopy analyzer based on Auger electron spectroscopy.

[0102] First metal powder: Metal powder of Sn 100% by mass (Sn 100% by mass powder) The first metal powder satisfied Symbol 4 in the powder size classification in Table 2 of JIS Z 3284-1:2004 (particle size distribution). That is, the content of particles with a particle size of 38 μm or less in the first metal powder was 99% by mass or more with respect to the total mass of the first metal powder. The average particle size of the first metal powder was 34.54 μm. The melting point of the first metal powder was 232°C.

[0103] Second metal powders (1) to (5): In the second metal powders (1) to (5), the core part was a metal powder (Ni-10% by mass Fe powder) made of an alloy of Ni 90% by mass and Fe 10% by mass. The Ni-10 mass% Fe powder that is the core part had a size (particle size distribution) that satisfied symbol 5 in the powder size classification in Table 2 of JIS Z 3284-1:2004. That is, in the Ni-10 mass% Fe powder, the content of particles with a particle diameter of 25 μm or less was 99 mass% or more with respect to the total mass of the Ni-10 mass% Fe powder. The particle diameter of the Ni-10 mass% Fe powder that is the core part (that is, the core diameter Rc) was 22.59 μm. The melting point of the core part was 1444 °C.

[0104] In the second metal powders (1) to (5), metal powder A, and metal powder C, the metal forming the surface layer covering the entire surface of the core part was a metal of 100 mass% Ni. This surface layer was obtained by coating the entire core part with Ni plating. All of these Ni platings were formed by electroplating. The thickness Rs of the surface layer (that is, the thickness of the Ni plating) was (1) 0.05 μm, (2) 0.10 μm, (3) 0.35 μm, (4) 0.50 μm, and (5) 0.75 μm, respectively. The melting point of Ni forming the surface layer was 1455 °C.

[0105] Metal powders A to C: As metal powder A, one having a core part and a surface layer covering the core part and made of Ni was used. Metal powder A was the same metal powder as the second metal powder (1) except that the thickness Rs of the surface layer was 1.20 μm. The particle diameter of the core part of metal powder A (that is, the core diameter Rc) was 22.59 μm. The melting point of the core part was 1444 °C. The melting point of Ni forming the surface layer was 1455 °C.

[0106] As metal powder B, the same metal powder as the metal powder (Ni-10 mass% Fe powder) used as the core part of the second metal powders (1) to (5) was used. The third metal powder was a metal powder without a surface layer. The particle diameter of metal powder B was 22.59 μm. The melting point of metal powder B was 1444 °C.

[0107] As the metal powder C, one having a core portion and a surface layer covering the core portion and made of Ni was used. The thickness of the surface layer of the metal powder was 1 to 3 μm. The core portion was a metal powder made of Cu (Cu 100 mass% powder). The particle size of the core portion of the metal powder C (i.e., the core diameter Rc) was 55.3 μm. The melting point of the core portion was 1085 °C. The melting point of Ni forming the surface layer was 1455 °C.

[0108] As the flux, SDC5 (manufactured by Senju Metal Industry Co., Ltd.) was used.

[0109] In accordance with the evaluation methods described in the following <Evaluation>, evaluation of <<Void suppression ability evaluation>>, <<Compound formation evaluation>>, and <<Heat resistance evaluation>> was carried out. These evaluation results are shown in Table 1 and Figure 8.

[0110] <Evaluation> <<Void suppression ability evaluation>> (1) Evaluation method Using a mask (opening: size 3 mm × 3 mm, thickness 0.15 mm) and a squeegee, the paste of each example was printed on a Cu substrate (size 50 mm × 50 mm, thickness 0.3 mm), a Si chip (size 3 mm × 3 mm) was mounted, and then reflowed. The reflow profile was such that the temperature was raised from 30 °C to the peak temperature (250 °C) at a rate of 3 °C / sec. In a nitrogen atmosphere, the peak time was set to 2.5 minutes, and the pressure was reduced during the first 2 minutes of the peak. Thereafter, voids were observed using an X-ray observation device.

[0111] (2) Judgment criteria A A small amount of voids similar to the case of using the paste of Comparative Example 2 were generated. B More voids were generated than in the case of using the paste of Comparative Example 2 and less than in the case of using the paste of Comparative Example 1. C A large amount of voids similar to the case of using the paste of Comparative Example 1 were generated.

[0112] X-ray observation images of Examples 1 to 3, 5, 6, and Comparative Examples 1 to 2 are shown in Fig. 4. In Examples 1 to 3, 5, 6, and Comparative Example 2, it was confirmed that voids were suppressed as compared with Comparative Example 1. Although not shown, in Example 4, voids were suppressed to the same extent as in Example 3 and Example 5. Although not shown, in Example 7, it was confirmed that voids were suppressed as compared with Comparative Example 1.

[0113]

Table 1

[0114] ≪Evaluation of Heat Resistance≫ (1) Evaluation Method The heat resistance of the solder joint is enhanced by the formation of a high-melting-point compound between Sn contained in the solder paste and an alloy containing Ni and Fe. The reaction for forming this compound is an exothermic reaction. Therefore, the heat resistance was evaluated by measuring the heat generation amount during compound formation.

[0115] First, mixed powders composed of metal powders contained in each of the solder pastes of Examples 1 to 3, 5, and Comparative Examples 1 to 2 were prepared. Hereinafter, these mixed powders are referred to as the mixed powders of Examples 1 to 3, 5, and Comparative Examples 1 to 2, respectively. In each mixed powder, the mixing ratio (mass ratio) of each metal powder was set to be the same as the mixing ratio (mass ratio) in the solder paste.

[0116] Each mixed powder was placed in an aluminum pan, and the exothermic peak was measured by DSC. As the measuring device, EXSTAR DSC7020 (manufactured by Hitachi High-Technologies Corporation) was used. The measurement program was set at 100 to 350 °C with a heating rate of 5 °C / min.

[0117] The measurement results (DSC curves) of the exothermic peak (heat flow) per unit mass (mg) for each of the mixed powders of Examples 1 to 3, 5, and Comparative Examples 1 to 2 are shown in Fig. 5. In Fig. 5, the large peak near 230 °C indicates the endothermic peak when Sn melts.

[0118] Next, based on the DSC curves, the heat flow per unit mass (mg) was integrated over time to calculate the integrated value per unit mass (mg) (unit: [J]) for each of the mixed powders of Examples 1 to 3, 5, and Comparative Examples 1 and 2. In the mixed powder of Comparative Example 1, in the range from 200°C to 350°C, the temperature T at which the heat generation amount reaches the minimum value was designated as T in0 In each of the mixed powders of Examples 1 to 3, 5, and Comparative Example 2, in the range from 200°C to 350°C, the temperature T at which the integrated value reaches the minimum value was designated as T in respectively. Temperature T in The heat generation amount per unit mass (mg) of each of the mixed powders of Examples 1 to 3, 5, and Comparative Example 2 in the range from temperature T in0 to 350°C, and the heat generation amount per unit mass (mg) of the mixed powder of Comparative Example 1 in the range from T

[0119] Next, in the range from temperature T in to 300°C, the ratio of the heat generation amount of each of the mixed powders of Examples 1 to 3, 5, and Comparative Example 2 was calculated as the ratio to the heat generation amount (100%) of the mixed powder of Comparative Example 1 in the range from temperature T in0 to 300°C. The calculation results of the ratio of the heat generation amount are shown in FIG. 7. The higher the ratio of the heat generation amount, the more the compound formation progresses during soldering and the higher the heat resistance.

[0120] From the above, in the examples, it was confirmed that the thicker the Ni plating thickness, the later the start of the exothermic reaction and the more the reaction to form a compound was suppressed.

[0121] (2) Judgment Criteria A The ratio of the heat generation amount of the mixed powder was 60% or more with respect to the heat generation amount of the mixed powder of Comparative Example 1. B The ratio of the heat generation amount of the mixed powder was 20% or more and less than 60% with respect to the heat generation amount of the mixed powder of Comparative Example 1. C The ratio of the heat generation amount of the mixed powder was less than 20% with respect to the heat generation amount of the mixed powder of Comparative Example 1.

[0122] In Examples 1 to 7 and Comparative Example 1, it was confirmed that they had higher heat resistance than Comparative Example 2.

[0123] ≪Evaluation of Compound Formation≫ Similar to the evaluation of void suppression ability, the paste of each example was printed on a substrate and reflowed without mounting an Si chip. The solder after reflow was photographed.

[0124] The photographed images of Examples 1 to 3, 5, 6 and Comparative Examples 1 to 2 are shown in Fig. 8. The white portion in the photographed image is the portion with weak metallic luster, indicating a compound of an alloy composed of Ni and Fe and Sn. The black portion in the photographed image is the portion with strong metallic luster, indicating unreacted Sn where compounding has not progressed.

[0125] In Examples 1 to 3, 5, 6 and Comparative Example 1, it was confirmed that more compounds were formed compared to Comparative Example 2. Although not shown in the photographed image, in Example 4, compounds were formed to the same extent as in Example 3 and Example 5. Although not shown in the photographed image, in Example 7, more compounds were formed compared to Comparative Example 2. In Comparative Example 2, since the Ni plating was too thick, the core portion was not completely exposed during bonding, and it is presumed that a sufficient amount of compounds were not formed to exhibit heat resistance between the core portion and the first metal. From these results, it was confirmed that Examples 1 to 7 and Comparative Example 1 had higher heat resistance than Comparative Example 2.

Explanation of Signs

[0126] 20A, 20B, 20C Second metal powder 201 Core portion 202 Surface layer 203 Intermediate layer 204 Metal layer Rc Core diameter of the core portion Rs Thickness of the surface layer Ri Thickness of the intermediate layer Rm Thickness of the metal layer

Claims

1. A solder paste containing 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 portion made of an alloy containing Ni and Fe, and a surface layer made of a metal containing Ni and covering the core portion, the content of Sn in the first metal powder is 20% by mass or more and 100% by mass or less based on the total mass of the first metal powder, the content of Ni in the metal forming the core portion of the second metal powder is 80% by mass or more and 99% by mass or less based on the total mass of the metal forming the core portion of the second metal powder, the content of Fe in the metal forming the core portion of the second metal powder is 1% by mass or more and 20% by mass or less based on the total mass of the metal forming the core portion of the second metal powder, the content of Ni in the metal forming the surface layer of the second metal powder is 50% by mass or more based on 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, the thickness of the surface layer of the second metal powder is 0.05 μm or more and less than 1.20 μm, a solder paste.

2. The content of the first metal powder is 30 to 99% by mass based on the total mass of the first metal powder and the second metal powder, the content of the second metal powder is 1 to 70% by mass based on the total mass of the first metal powder and the second metal powder, The solder paste according to Claim 1.

3. A solder joint formed using the solder paste according to Claim 1 or 2.

Images