Solder-coated metal particles and method for producing solder-coated metal particles

Abstract

The present invention provides: solder-coated metal particles in which additional stabilization of the solder-coated metal particles is achieved, a sharper particle size distribution (cumulative volume distribution curve) is exhibited, and the solder layer exhibits a melting point of not more than 170°C; and a method for producing solder-coated metal particles.　Provided are solder-coated metal particles each including a ball-shaped metal particle that comprises Ni, 1-15 mass% P, and unavoidable impurities, and a solder layer that covers the metal particle and comprises Sn, 40-67 mass% Bi, and unavoidable impurities, wherein in the cumulative volume distribution curve obtained using a laser diffraction / scattering method for a powder composed of the solder-coated metal particles, the particle diameter ratio (d90-d50) / d50 determined from the particle diameter d50 at 50% of cumulative volume and the particle diameter d90 at 90% of cumulative volume is not greater than 1.5.

Description

Solder-coated metal particles and method for producing solder-coated metal particles

[0001] The present invention relates to solder-coated metal particles and a method for manufacturing the same, and more particularly to solder-coated metal particles coated with a solder layer made of a Sn—Bi alloy and a method for manufacturing the same.

[0002] Solder-coated balls (solder-coated metal particles) have been known. For example, the solder-coated metal particles disclosed in Japanese Patent No. 5367924 (Patent Document 1) are formed by coating ball-shaped metal particles with a solder layer containing Sn and 45% by mass to 65% by mass of Bi. The solder layer is capable of reflow at a temperature of 160°C or less. It is also possible to further include a Ni-plated layer between the ball-shaped metal particles and the solder layer.

[0003] The solder-coated metal particles described above are mainly used to connect components of electrical and electronic devices, and have conventionally been used for input / output terminals of semiconductor packages such as QFP (Quard Flat Package), BGA (Ball Grid Array), and CSP (Chip Size Package). Recently, with the further miniaturization and higher definition of electric and electronic devices, they are being used in a wide variety of applications, such as ACF (Anisotropic Conducting Film), ACP (Anisotropic Conducting Paste), ACAs (Anisotropic Conductive Adhesives), FOB (Flex On Board), FOC (Flex On Chip), FPC (Flexible Printed Circuit), COB (Chip On Board), and COF (Chip On Flex).

[0004] Japanese Patent No. 5367924 Japanese Patent Application Laid-Open No. 2009-197317 Japanese Patent No. 5327582

[0005] As described above, solder-coated metal particles are used in a wide variety of applications. In particular, the solder-coated metal particles disclosed in the aforementioned Japanese Patent No. 5,367,924 are advantageous and useful because the solder layer made of a Sn—Bi alloy has a low melting point, allowing the reflow temperature during mounting to be set low. However, solder-coated metal particles tend to agglomerate and become coarse when forming a solder layer on ball-shaped metal particles. If a powder (particle group) made of solder-coated metal particles contains many particles (coarse particles) that have agglomerated and become coarse, this can cause major problems in products made by arranging individual solder-coated metal particles, such as a significant decrease in production efficiency.

[0006] The object of the present invention is to provide solder-coated metal particles which further stabilize a powder (particle group) composed of solder-coated metal particles, exhibit a sharper particle size distribution (cumulative volume distribution curve), and have a solder layer with a melting point of 170°C or less.

[0007] The inventors conducted a multifaceted investigation into the phenomenon in which a powder (particle group) composed of solder-coated metal particles contains many particles (coarse particles) that have become coarse due to aggregation, and discovered a configuration that can solve the above problem, leading to the invention of this invention.

[0008] The solder-coated metal particles according to the present invention comprise ball-shaped metal particles each consisting of Ni, 1% by mass or more and 15% by mass or less of P, and unavoidable impurities, and a solder layer coating the metal particles and consisting of Sn, 40% by mass or more and 67% by mass or less of Bi, and unavoidable impurities, wherein in an accumulated volume distribution curve of a powder composed of the solder-coated metal particles measured by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50, calculated from the particle size d50 at which the accumulated volume is 50% and the particle size d90 at which the accumulated volume is 90%, is 1.5 or less.

[0009] In the solder-coated metal particles according to the present invention, the particle size ratio (d90-d50) / d50 is preferably 1.0 or less.

[0010] In the solder-coated metal particles according to the present invention, the solder layer preferably contains 46% by mass or more and 64% by mass or less of Bi.

[0011] In the solder-coated metal particles according to the present invention, the solder layer preferably contains 52 mass % or more and 61 mass % or less of Bi.

[0012] The solder-coated metal particles according to the present invention may further have a Cu-plated layer or an Ag-plated layer between the metal particles and the solder layer.

[0013] The solder-coated metal particles according to the present invention may further include a Ni-plated layer between the solder layer and the Cu-plated layer or the Ag-plated layer.

[0014] The above-described solder-coated metal particles can be manufactured by the following manufacturing method: That is, the manufacturing method of solder-coated metal particles according to the present invention includes the steps of: manufacturing ball-shaped metal particles composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities by electroless reduction; and forming a solder layer composed of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities so as to coat the metal particles, thereby forming solder-coated metal particles, wherein the solder layer is formed so that the particle size ratio (d90-d50) / d50, calculated from the particle size d50 at 50% cumulative volume and the particle size d90 at 90% cumulative volume, is 1.5 or less in an integrated volume distribution curve obtained by laser diffraction scattering method for a powder composed of the solder-coated metal particles.

[0015] In the method for producing solder-coated metal particles according to the present invention, the solder layer is preferably formed so that the particle size ratio (d90-d50) / d50 is 1.0 or less.

[0016] In the method for producing solder-coated metal particles according to the present invention, the solder layer is preferably formed so as to contain 46% by mass or more and 64% by mass or less of Bi.

[0017] In the method for producing solder-coated metal particles according to the present invention, the solder layer is preferably formed so as to contain 52 mass % or more and 61 mass % or less of Bi.

[0018] In the method for producing solder-coated metal particles according to the present invention, a Cu-plated layer or an Ag-plated layer can be further formed between the metal particles and the solder layer.

[0019] In the method for producing solder-coated metal particles according to the present invention, a Ni plating layer can be further formed between the solder layer and the Cu plating layer or the Ag plating layer.

[0020] According to this invention, it is possible to provide solder-coated metal particles in which the powder (particle group) composed of solder-coated metal particles exhibits a sharper particle size distribution (cumulative volume distribution curve) and the solder layer exhibits a melting point of 170° C. or less. This allows the reflow temperature during mounting to be set lower, and is expected to significantly improve production efficiency in products that are constructed by arranging individual solder-coated metal particles, as well as contribute to further finer pitches in the arrangement.

[0021]

[0023] Figure 1 is a diagram schematically showing the cross-sectional structure of a solder-coated metal particle according to an embodiment of the present invention, which has a solder layer on the surface of the metal particle.

[0024] Figure 2 is a diagram schematically showing the cross-sectional structure of a solder-coated metal particle according to an embodiment of the present invention, which has a Cu or Ag plating layer and a solder layer, in this order, on the surface of the metal particle.

[0025] Figure 3 is a diagram schematically showing the cross-sectional structure of a solder-coated metal particle according to an embodiment of the present invention, which has a Ni plating layer, a Cu or Ag plating layer, and a solder layer, in this order, on the surface of the metal particle.

[0026] Figure 4 is a diagram schematically showing the cross-sectional structure of a solder-coated metal particle according to an embodiment of the present invention, which has a Cu or Ag plating layer, a Ni plating layer, and a solder layer, in this order, on the surface of the metal particle. 1 is a diagram showing a cross-sectional configuration of a solder-coated metal particle according to an embodiment of the present invention, which has a Ni plating layer, a Cu plating layer or an Ag plating layer, a Ni plating layer, and a solder layer on the surface of the metal particle in this order.

[0022] Hereinafter, specific examples of embodiments of the solder-coated metal particles and the manufacturing method of solder-coated metal particles according to the present invention will be described. Note that the term "particle" will be used unless a distinction is intended between a single particle and a particle group (a collection of particles). Furthermore, the term "single particle" will be used when a single particle is specifically intended, and the term "powder" will be used when a particle group (a collection of particles) is specifically intended. Furthermore, the content (mass %) of elements constituting a particle is expressed as a ratio to a single particle or a predetermined amount of powder. Furthermore, the content (mass %) of elements constituting a solder layer is expressed as a ratio to a predetermined amount of solder layer.

[0023] 1 is an embodiment of the solder-coated metal particle according to the present invention, and is a diagram schematically illustrating the cross-sectional structure of a solder-coated metal particle 1A having a metal particle 10 and a solder layer 11. Hereinafter, a single solder-coated metal particle 1A will also be referred to as a single particle 1A, and a collection of single particles 1A will be referred to as a powder 1A.

[0024] The solder-coated metal particle 1A (single particle 1A) shown in FIG. 1 comprises a ball-shaped metal particle 10 composed of Ni (nickel), 1% by mass to 15% by mass of P (phosphorus), and unavoidable impurities, and a solder layer 11 coating the metal particle 10 and composed of Sn (tin), 40% by mass to 67% by mass of Bi (bismuth), and unavoidable impurities. The metal particle 10 constituting the solder-coated metal particle 1A preferably has a particle size of 0.5 μm to 100 μm, depending on its intended use. The particle size of the metal particle 10 refers to the d50 value described below. The metal particle 10 can be stably formed, for example, by a manufacturing method including an electroless plating process using a reduction precipitation method. For example, the reduction-precipitation type spherical NiP microparticles described in JP 2009-197317 A (Patent Document 2) or Japanese Patent No. 5327582 A (Patent Document 3) are suitable for the metal particles 10. For reference, the entire contents of the above-mentioned JP 2009-197317 A are incorporated herein by reference.

[0025] The metal particles 10 are primarily composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities. An appropriate amount of P promotes particle formation during the formation of the metal particles 10, such as by increasing the particle formation rate and facilitating particle size control. The P content not only promotes particle formation but also affects various properties of the metal particles 10, such as the volume resistivity (conductivity). Therefore, the P content is preferably set to a value (content) deemed necessary to achieve the desired properties, such as the volume resistivity (conductivity). Note that if the P content exceeds 15% by mass, problems such as an increase in the volume resistivity of the metal particles 10 occur. Furthermore, if the P content is less than 1% by mass, the aforementioned effect of promoting particle formation of the metal particles 10 cannot be expected.

[0026] Furthermore, the metal particles 10 may further contain Cu (copper) in addition to Ni and P due to their manufacturing method. In this case, the Cu content is preferably 0.01% by mass or more and 18% by mass or less. Furthermore, the metal particles 10 may contain Sn (tin) in addition to the Cu due to their manufacturing method. In this case, the Sn content is preferably 0.05% by mass or more and 5% by mass or less. As with the case of P, Cu and Sn affect the hardness and volume resistivity (conductivity) of the metal particles 10, but within the above ranges, the monodispersity of the metal particles 10 can be expected to be improved. The concept of volume resistivity of metal particles (single particles) in this invention and the method for measuring the volume resistivity of metal particles (single particles) will be described later.

[0027] Furthermore, due to the manufacturing method, the metal particles 10 inevitably contain impurities in addition to Ni and P. In this case, the main impurities (unavoidable impurities) contained in the metal particles 10 are C (carbon) and O (oxygen). Metal particles containing excessive impurities increase their volume resistivity, causing inconveniences such as reduced adhesion to the solder layer. Therefore, it is desirable to limit the impurities contained in the metal particles 10, for example, to 0.1% by mass or less of C, 0.8% by mass or less of O, and the total of C, O, and other impurities to 1.0% by mass or less.

[0028] As described above, the solder-coated metal particle 1A shown in FIG. 1 has a solder layer 11 that coats a ball-shaped metal particle 10. The solder layer 11 is composed of Sn, 40% by mass or more and 67% by mass or less of Bi, and unavoidable impurities. A binary Sn—Bi alloy composed of Sn and Bi is known to be an alloy that forms a eutectic structure. The Sn—Bi alloy reaches its eutectic point when the Bi content is 58% by mass, and the temperature (melting point) at this point is 139°C. Therefore, by using a Sn—Bi alloy with a Bi content set within an appropriate range that includes the eutectic point, a solder layer with a relatively low melting point can be formed.

[0029] From the above perspective, the solder layer 11 coating the ball-shaped metal particles 10 is composed of a solder alloy consisting of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities, i.e., a Sn—Bi alloy containing 40% by mass to 67% by mass of Bi. The solder layer 11 composed of a Sn—Bi alloy containing 40% by mass to 67% by mass of Bi has a melting point of 170°C or lower. By having the melting point of the solder layer 11 of the solder-coated metal particles 1A be 170°C or lower, the thermal effects upon reflowing the solder layer 11 are reduced, thereby preventing defects in the product due to the thermal effects after reflow. The thickness of the solder layer 11 coating the metal particles 10 depends on the application, but is preferably 0.5 μm to 100 μm. The particle size of the solder-coated metal particles 1A can be controlled by controlling the thickness of the solder layer 11. The particle size of the solder-coated metal particles 1A refers to the d50 described below.

[0030] The solder layer 11 preferably contains 46% by mass or more and 64% by mass or less of Bi. That is, the solder layer 11 is preferably made of an Sn—Bi alloy containing Sn, 46% by mass or more and 64% by mass or less of Bi, and unavoidable impurities. The solder layer 11 made of an Sn—Bi alloy containing 46% by mass or more and 64% by mass or less of Bi has a melting point of 160° C. or less, and therefore the thermal effects when the solder layer 11 is reflowed are further reduced.

[0031] Furthermore, the solder layer 11 more preferably contains 52% by mass or more and 61% by mass or less of Bi. That is, the solder layer 11 is more preferably made of an Sn—Bi alloy containing Sn, 52% by mass or more and 61% by mass or less of Bi, and unavoidable impurities. The solder layer 11 made of an Sn—Bi alloy containing 52% by mass or more and 61% by mass or less of Bi has a melting point of 150° C. or less, and therefore the thermal effects when the solder layer 11 is reflowed are sufficiently reduced.

[0032] In the production of the above-described solder-coated metal particles 1A, the solder layer 11 can be formed by electroplating (electroplating) so as to coat the metal particles 10. Depending on the production method, when a solder layer is formed on the metal particles (single particles), the metal particles (single particles) on whose surfaces a solder plating structure is being formed may aggregate close to each other, forming particles with irregular shapes that are not ball-shaped. Most of the irregularly shaped particles aggregate to form distorted, coarse particles (hereinafter referred to as "coarse particles"). Providing a powder with a high content of coarse particles can cause major problems, such as a significant decrease in production efficiency, in products constructed by arranging individual solder-coated metal particles.

[0033] The solder-coated metal particles 1A described above have a particle size ratio (d90-d50) / d50, calculated by subtracting the particle size d50 at 50% of the cumulative volume from the particle size d90 at 90% of the cumulative volume, in a cumulative volume distribution curve measured by a laser diffraction scattering method for the powder 1A composed of the solder-coated metal particles 1A (single particles 1A), of 1.5 or less (preferably, 1.0 or less). Powder 1A having a particle size ratio of 1.5 or less contains a small proportion of particles coarsened by aggregation, while powder 1A having a particle size ratio of 1.0 or less contains an even smaller proportion of particles coarsened by aggregation. Therefore, providing powder 1A having a particle size ratio of 1.5 or less (preferably, 1.0 or less) to the above-described products can significantly improve the production efficiency of the products. The particle size ratio in the cumulative volume distribution curve measured by a laser diffraction scattering method has been confirmed experimentally and will be described later.

[0034] 2 is a diagram schematically illustrating the cross-sectional structure of a solder-coated metal particle 1B according to an embodiment of the present invention, which has a Cu plating layer 12a or an Ag plating layer 12b and a solder layer 11, in this order, on the surface of a metal particle 10. Hereinafter, a single solder-coated metal particle 1B will also be referred to as a single particle 1B, and a collection of single particles 1B will be referred to as a powder 1B.

[0035] 2 includes a ball-shaped metal particle 10 composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities, and a solder layer 11 composed of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities, coating the metal particle 10. Unlike the solder-coated metal particle 1A described above, the solder-coated metal particle 1B further includes a Cu plating layer 12a or an Ag plating layer 12b between the metal particle 10 and the solder layer 11.

[0036] The metal particles 10 and solder layer 11 constituting the solder-coated metal particles 1B have the same configuration as the metal particles 10 and solder layer 11 constituting the above-described solder-coated metal particles 1A. Therefore, for the specific configurations such as the composition and thickness of the metal particles 10 and solder layer 11 shown in Figure 2, please refer to the explanation of the metal particles 10 and solder layer 11 constituting the above-described solder-coated metal particles 1A.

[0037] As described above, the solder-coated metal particles 1B have a Cu plating layer 12a or an Ag plating layer 12b. The Cu plating layer 12a or the Ag plating layer 12b can be formed by electroless plating or electrolytic plating so as to coat the metal particles 10. The Cu plating layer 12a or the Ag plating layer 12 has good wettability with the solder layer 11, which is primarily composed of Sn and Bi, and can improve the adhesion between the metal particles 10 and the solder layer 11 compared to when the solder layer 11 is formed directly on the metal particles 10, which is primarily composed of Ni and P. The thickness of the Cu plating layer 12a or the Ag plating layer 12b depends on the application, but is preferably 0.1 μm or more and 50 μm or less.

[0038] Due to the properties of the main components Ni and P, the metal particles 10 have a higher volume resistivity than Cu particles primarily composed of Cu. Therefore, solder-coated metal particles 1A having metal particles 10 as their cores have a higher volume resistivity than solder-coated Cu particles having Cu particles as their cores. Therefore, when the solder-coated metal particles 1A are used in place of solder-coated Cu particles, for example, in conductive regions such as conductive materials or electrical circuits, the high volume resistivity of the solder-coated metal particles 1A can cause problems such as heat generation due to current flow or insufficient current at constant voltage. For such applications, it is preferable to employ solder-coated metal particles 1B having a Cu plating layer 12a or Ag plating layer 12b between the metal particles 10 and the solder layer 11. By providing a Cu plating layer 12a or an Ag plating layer 12b to coat the metal particle 10 and then providing the solder layer 11, the volume resistivity of the solder-coated metal particle 1B (single particle 1B) can be made smaller than that of the solder-coated metal particle 1A (single particle 1A) due to the characteristics of Cu or Ag, which have a sufficiently small volume resistivity.

[0039] In the solder-coated metal particles 1B described above, in an integrated volume distribution curve of the powder 1B composed of the solder-coated metal particles 1B (single particles 1B) measured by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50 described above is 1.5 or less (preferably 1.0 or less). Powder 1B having a particle size ratio of 1.5 or less (preferably 1.0 or less) contains a small proportion of particles that have become coarse due to aggregation. Therefore, like the powder 1A described above, powder 1B is suitable for use in products composed of an array of solder-coated metal particles.

[0040] 3 is a diagram schematically illustrating the cross-sectional structure of a solder-coated metal particle 1C according to an embodiment of the present invention, which has, in this order, a Ni plating layer 13, a Cu plating layer 12a or an Ag plating layer 12b, and a solder layer 11 on the surface of a metal particle 10. Hereinafter, a single solder-coated metal particle 1C will also be referred to as a single particle 1C, and a collection of single particles 1C will be referred to as a powder 1C.

[0041] 3 includes a ball-shaped metal particle 10 composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities, and a solder layer 11 composed of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities, coating the metal particle 10. Furthermore, unlike the solder-coated metal particles 1A and 1B described above, the solder-coated metal particle 1C includes a Cu plating layer 12a or an Ag plating layer 12b between the metal particle 10 and the solder layer 11, and further includes a Ni plating layer 13 between the metal particle 10 and the Cu plating layer 12a or the Ag plating layer 12b.

[0042] The metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting solder-coated metal particles 1C have the same configurations as the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting the above-described solder-coated metal particles 1A to 1 B. Therefore, for specific configurations such as the composition and thickness of the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b shown in Figure 3, please refer to the explanations for the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting the above-described solder-coated metal particles 1A to 1B.

[0043] As described above, the solder-coated metal particles 1C have a Ni plating layer 13. The Ni plating layer 13 is composed of Ni and may further contain an appropriate amount of P. The Ni plating layer 13 can be formed to coat the metal particles 10 by electrolytic plating or electroless plating containing P. When a Cu plating layer 12a or an Ag plating layer 12b is directly formed on the surface of the metal particles 10, poor adhesion tends to occur, although various factors are thought to be involved. Therefore, in applications where the Cu plating layer 12a or the Ag plating layer 12b is required and adhesion between the metal particles 10 and the Cu plating layer 12a or the Ag plating layer 12b is also required, the Ni plating layer 13 is preferably provided between the metal particles 10 and the Cu plating layer 12a or the Ag plating layer 12b. The Ni plating layer 13 between the metal particles 10 and the Cu plating layer 12 a or Ag plating layer 12 b is effective in improving the adhesion between the metal particles 10 and the Cu plating layer 12 a or Ag plating layer 12 b. In this case, the thickness of the Ni plating layer 13 depends on the application, but is preferably 0.1 μm or more and 50 μm or less.

[0044] In the solder-coated metal particles 1C described above, in an integrated volume distribution curve of powder 1C composed of solder-coated metal particles 1C (single particles 1C) measured by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50 described above is 1.5 or less (preferably 1.0 or less). Powder 1C having a particle size ratio of 1.5 or less (preferably 1.0 or less) contains a small proportion of particles that have become coarse due to aggregation. Therefore, like powders 1A and 1B described above, powder 1C is suitable for use in products composed of an array of solder-coated metal particles.

[0045] 4 is a diagram schematically illustrating the cross-sectional structure of a solder-coated metal particle 1D according to an embodiment of the present invention, which has, in this order, a Cu plating layer 12a or an Ag plating layer 12b, a Ni plating layer 14, and a solder layer 11 on the surface of a metal particle 10. Hereinafter, a single solder-coated metal particle 1D will also be referred to as a single particle 1D, and a collection of single particles 1D will be referred to as a powder 1D.

[0046] 4 includes a ball-shaped metal particle 10 composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities, and a solder layer 11 composed of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities, coating the metal particle 10. Furthermore, unlike the solder-coated metal particles 1A, 1B, and 1C described above, the solder-coated metal particle 1D includes a Cu plating layer 12a or an Ag plating layer 12b between the metal particle 10 and the solder layer 11, and further includes a Ni plating layer 14 between the solder layer 11 and the Cu plating layer 12a or the Ag plating layer 12b.

[0047] The metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting solder-coated metal particle 1D have the same configurations as the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting the above-described solder-coated metal particles 1A to 1C. Therefore, for specific configurations such as the composition and thickness of the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b shown in Figure 4, please refer to the explanation of the metal particles 10, solder layer 11, Cu plating layer 12a, and Ag plating layer 12b constituting the above-described solder-coated metal particles 1A to 1C.

[0048] As described above, the solder-coated metal particle 1D has a Ni plating layer 14. The Ni plating layer 14 is made of Ni and may further contain an appropriate amount of P. The Ni plating layer 14 can be formed so as to cover the Cu plating layer 12a or the Ag plating layer 12b by electrolytic plating or electroless plating containing P. If the solder layer 11 is formed directly on the surface of the Cu plating layer 12a or the Ag plating layer 12b, the Sn contained in the solder layer 11 and the Cu contained in the Cu plating layer 12a or the Ag contained in the Ag plating layer 12b will form a brittle intermetallic compound, which may result in poor adhesion or reduced connection reliability. Therefore, in applications where a Cu plating layer 12a or an Ag plating layer 12b is required and adhesion or connection reliability between the metal particles 10 and the Cu plating layer 12a or the Ag plating layer 12b is also required, a Ni plating layer 14 is preferably provided between the solder layer 11 and the Cu plating layer 12a or the Ag plating layer 12b. The Ni plating layer 14 between the solder layer 11 and the Cu plating layer 12a or the Ag plating layer 12b is expected to improve adhesion or connection reliability between the solder layer 11 and the Cu plating layer 12a or the Ag plating layer 12b. In this case, the thickness of the Ni plating layer 14 depends on the application, but is preferably 0.1 μm or more and 50 μm or less.

[0049] In the solder-coated metal particles 1D described above, in an integrated volume distribution curve of powder 1D composed of solder-coated metal particles 1D (single particles 1D) measured by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50 described above is 1.5 or less (preferably 1.0 or less). Powder 1D having a particle size ratio of 1.5 or less (preferably 1.0 or less) contains a small proportion of particles that have become coarse due to aggregation. Therefore, like powders 1A to 1C described above, powder 1D is suitable for use in products composed of an array of solder-coated metal particles.

[0050] 5 is a diagram schematically illustrating the cross-sectional structure of a solder-coated metal particle 1E according to an embodiment of the present invention, which has, in this order, a Ni plating layer 13, a Cu plating layer 12a or an Ag plating layer 12b, a Ni plating layer 14, and a solder layer 11 on the surface of a metal particle 10. Hereinafter, a single solder-coated metal particle 1E will also be referred to as a single particle 1E, and a collection of single particles 1E will be referred to as a powder 1E.

[0051] 5 includes a ball-shaped metal particle 10 composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities, and a solder layer 11 composed of Sn, 40% by mass to 67% by mass of Bi, and unavoidable impurities, coating the metal particle 10. Furthermore, unlike the solder-coated metal particles 1A, 1B, 1C, and 1D described above, the solder-coated metal particle 1E includes a Cu plating layer 12a or an Ag plating layer 12b between the metal particle 10 and the solder layer 11, a Ni plating layer 13 between the metal particle 10 and the Cu plating layer 12a or the Ag plating layer 12b, and a Ni plating layer 14 between the solder layer 11 and the Cu plating layer 12a or the Ag plating layer 12b.

[0052] The metal particles 10, solder layer 11, Cu plating layer 12a, Ag plating layer 12b, Ni plating layer 13, and Ni plating layer 14 constituting solder-coated metal particle 1E have the same configurations as the metal particles 10, solder layer 11, Cu plating layer 12a, Ag plating layer 12b, Ni plating layer 13, and Ni plating layer 14 constituting the above-described solder-coated metal particles 1A to 1D. Therefore, for specific configurations such as the composition and thickness of the metal particles 10, solder layer 11, Cu plating layer 12a, Ag plating layer 12b, Ni plating layer 13, and Ni plating layer 14 shown in Figure 5, please refer to the explanation for the metal particles 10, solder layer 11, Cu plating layer 12a, Ag plating layer 12b, Ni plating layer 13, and Ni plating layer 14 constituting the above-described solder-coated metal particles 1A to 1D.

[0053] In the solder-coated metal particles 1E described above, in an integrated volume distribution curve of powder 1E composed of solder-coated metal particles 1E (single particles 1E) measured by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50 described above is 1.5 or less (preferably 1.0 or less). Powder 1E having a particle size ratio of 1.5 or less (preferably 1.0 or less) contains a small proportion of particles that have become coarse due to aggregation. Therefore, like powders 1A to 1D described above, powder 1E is suitable for use in products composed of an array of solder-coated metal particles.

[0054] Here, the concept of the volume resistivity of metal particles (metal particles 10) in this invention and a method for measuring the volume resistivity of metal particles (metal particles 10) will be described. The ball-shaped metal particles 10 are conductive particles composed of Ni, 1% by mass to 15% by mass of P, and unavoidable impurities. Although single-particle metal particles (metal particles 10) exhibit conductivity, it is currently technically difficult to directly and accurately determine the volume resistivity of a single particle. Therefore, in this invention, the volume resistivity of a powder, which is an assembly of conductive metal particles (single particles), is approximately measured, and this measured value is defined as the volume resistivity of the metal particles. Therefore, the volume resistivity of a metal particle (single particle) is synonymous with the volume resistivity of a powder, which is an assembly of metal particles, and the volume resistivity of a metal particle (single particle) is the same as the volume resistivity of a powder.

[0055] The volume resistivity of a powder, which is a collection of conductive metal particles (single particles), is determined using the powder as a sample using an apparatus such as that shown in FIG. 6 . FIG. 6 is a diagram showing the essential components of an apparatus for determining the volume resistivity of a powder. The apparatus shown in FIG. 6 includes a cylinder 21 with an inner diameter d that accommodates sample powder 20, a copper jig 22 attached to the bottom of the cylinder 21, a copper piston 23 that slides within the cylinder 21, a power source, and a resistance meter. The power source and resistance meter are connected between the copper jig 22 and the copper piston 23. The resistance meter is, for example, a Hioki 3541 Resistance Meter. The copper jig 22 and the copper piston 23 have approximately the same resistance value. The distance between the copper jig 22 and the copper piston 23 is detectable. The pressing force exerted by the copper piston 23 on the sample powder 20 is detectable.

[0056] In the present invention, the Rv value obtained by the following measurement and calculation procedures is defined as the volume resistivity Rv (Ωm) of the powder.

[0057] The specific measurement procedure is as follows. First, the sample powder 20 (weighing 1.15 g) is placed in the cylinder 21. Next, the copper piston 23 is moved in the direction indicated by the arrow 24 to press the sample powder 20, thereby applying a pressing force (approximately 22 MPa) to the sample powder 20. The position of the copper piston 23 is then maintained, and the pressing force on the sample powder 20 and the distance between the copper jig 22 and the copper piston 23 are maintained constant. In this state, electricity is passed between the copper jig 22 and the copper piston 23, and the resistance value is measured with a resistance meter. The resistance value obtained by this measurement procedure is the resistance value of the entire electrical circuit including the sample powder 20.

[0058] The specific calculation procedure is as follows. The resistance value of the entire electric circuit including the sample powder 20 obtained by the above-mentioned measurement procedure is defined as R1 (Ω). Furthermore, the resistance values ​​of the copper jig 22 and the copper piston 23 are defined as R2 (Ω), the distance between the copper jig 22 and the copper piston 23 is defined as L (m), and the inner diameter of the cylinder 21 is defined as d (m). Then, the value of (R1-R2) x π x (d / 2)2 / L is calculated. The value calculated by this calculation procedure is the volume resistivity Rv (Ωm) of the powder.

[0059] The effects of the solder-coated metal particles and the method for manufacturing the solder-coated metal particles according to the present invention were confirmed by experiments as follows.

[0060] <Metal Particles> Ball-shaped metal particles composed of Ni, P, and unavoidable impurities were produced by electroless plating. The production method for reducing and depositing spherical NiP microparticles described in JP 2009-197317 A (Patent No. 5327582) was employed in the production of the metal particles. As a result, ball-shaped metal particles (powder) designated as particle number 10 in Table 1 were obtained. The values ​​shown in Table 1 were obtained for the particle diameters d10, d50, d90, and particle diameter ratio (d90-d50) / d50 at 10%, 50%, and 90% cumulative volumes from the cumulative volume distribution curve of the metal particles (powder) designated as particle number 10 obtained by laser diffraction scattering. The volume resistivity Rv of the metal particles (powder) designated as particle number 10 was also determined using the measurement and calculation procedures described above, yielding the values ​​shown in Table 1. The chemical composition of the metal particles was determined by ICP analysis (Inductively Coupled Plasma analysis).

[0061]

[0062] <Solder-coated metal particles 1A> Next, solder-coated metal particles were produced by electrolytic plating using metal particles (powder) of particle number 10 shown in Table 1, forming a solder layer composed of Sn, Bi, and unavoidable impurities so as to coat the metal particles. This resulted in solder-coated metal particles 1A (powder 1A) as shown in Figure 1, which have metal particles and a solder layer and are shown in Experiments 1 to 10 in Table 2. Then, from the cumulative volume distribution curve of solder-coated metal particles 1A (powder 1A) obtained by laser diffraction scattering, the values ​​shown in Table 2 were obtained for particle diameters d10, d50, d90, and the particle diameter ratio (d90-d50) / d50. The chemical composition of the solder layer was determined by cross-sectional analysis of the metal particles using SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray spectroscopy). The thickness of the solder layer was determined by assuming that half the difference between the d50 of the solder-coated metal particles (powder) and the d50 of the metal particles (powder) corresponds to the median thickness of the solder layer, and half of this difference was used.

[0063] The melting points of the solder layers shown in Table 2 are estimated values ​​obtained from a well-known Sn—Bi alloy (binary) phase diagram. Specifically, a Sn—Bi alloy (binary) phase diagram was used in which the melting point at 58% by mass Bi is 139°C (eutectic point), the melting point at 100% by mass Sn is 232°C, and the melting point at 100% by mass Bi is 272°C. In this phase diagram, the melting point is assumed to decrease linearly (slope a) from 100% by mass Sn to the eutectic point and to increase linearly (slope b) from the eutectic point to 100% by mass Bi. The value of the line with the slope a or b at a specific Bi content (mass%) was determined. This value was used as the center value of the range in which the melting point exists, and the amplitude of the approximation by the line was set to ±5°C to determine the melting points shown in Table 2.

[0064]

[0065] Experiment No. 1 is a comparative example. The solder-coated metal particles (powder) of Experiment No. 1 were ball-shaped metal particles (powder) coated with metal particles (powder) designated by particle symbol 10 in Table 1, i.e., ball-shaped metal particles composed of Ni, approximately 6.9% by mass of P, approximately 3.8% by mass of Cu, approximately 0.3% by mass of Sn, and unavoidable impurities, and had a solder layer of approximately 0.9 μm in thickness composed of approximately 84.2% by mass of Sn, approximately 15.8% by mass of Bi, and unavoidable impurities. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.37, which is below the preferred value of 1.0. Therefore, for example, a sufficient improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. However, this solder layer is outside the recommended range of 40% by mass or more and 67% by mass or less of Bi relative to Sn, and its melting point is estimated to be approximately 207±5°C, so it is highly likely that the recommended temperature will not be 170°C or less, and the reflow temperature during mounting will have to be set quite high.

[0066] Experiment No. 2 is a comparative example. The solder-coated metal particles (powder) of Experiment No. 2 coated metal particles designated as particle number 10 in Table 1, and contained approximately 70.7% Sn by mass, approximately 29.3% Bi by mass, and unavoidable impurities, forming a solder layer approximately 0.6 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.36, which is below the preferred value of 1.0. Therefore, for example, a sufficient improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. However, this solder layer had a Bi to Sn ratio outside the recommended range of 40% to 67% by mass. Its melting point was estimated to be approximately 185±5°C, making it unlikely to reach the recommended 170°C or lower, necessitating a relatively high reflow temperature during mounting.

[0067] Experiment No. 3 is a comparative example. The solder-coated metal particles (powder) of Experiment No. 3 coated metal particles designated as particle number 10 in Table 1, had the same composition as Experiment No. 2, and had a solder layer approximately 0.7 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.33, which is below the preferred value of 1.0. Therefore, for example, a sufficient improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. However, this solder layer has a Bi to Sn ratio outside the recommended range of 40% by mass to 67% by mass. Since its melting point is estimated to be approximately 185±5°C, it is unlikely to reach the recommended 170°C or lower, which necessitates a relatively high reflow temperature during mounting.

[0068] Experiment No. 4 is an example of the present invention. The solder-coated metal particles (powder) of Experiment No. 4 coated metal particles designated as particle number 10 in Table 1, and contained approximately 54.6% Sn by mass, approximately 45.4% Bi by mass, and unavoidable impurities, forming a solder layer approximately 0.6 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.36, which is below the recommended 1.5 (preferably 1.0 or less), and therefore, sufficient improvements in production efficiency can be expected, for example, in products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the recommended range of 40% to 67% by mass, and its melting point was estimated to be approximately 159±5°C, which is likely to be below the recommended 170°C, allowing the reflow temperature during mounting to be set relatively low.

[0069] Experiment No. 5 is an example of the present invention. The solder-coated metal particles (powder) of Experiment No. 5 coated metal particles designated as particle number 10 in Table 1, and contained approximately 50.0% Sn by mass, approximately 50.0% Bi by mass, and unavoidable impurities, forming a solder layer approximately 0.7 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.37, which is below the recommended 1.5 or less (preferably 1.0 or less). Therefore, for example, a sufficient improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the more preferred range of 52% to 61% by mass, and its melting point was estimated to be approximately 152±5°C, making it highly likely to be below the preferred 160°C, allowing for a lower reflow temperature during mounting.

[0070] The volume resistivity Rv of the solder-coated metal particles (powder) of Experiment No. 5 was determined by the above-described measurement and calculation procedures, similar to the case of the metal particles of particle symbol 10. The volume resistivity of these solder-coated metal particles (powder) was found to be approximately 0.50×10 －５ Ωm, and the volume resistivity of the metal particles (powder) of particle number 10 having no solder layer (approximately 21.1 × 10 shown in Table 1) －５ The resistance was about 1 / 42 of the original resistance (Ωm).

[0071] Experiment No. 6 is an example of the present invention. The solder-coated metal particles (powder) of Experiment No. 6 coated metal particles designated as particle number 10 in Table 1, and contained approximately 43.9% Sn by mass, approximately 56.1% Bi by mass, and unavoidable impurities, forming a solder layer approximately 0.7 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.36, which is below the recommended 1.5 (preferably 1.0 or less), and therefore, sufficient improvements in production efficiency can be expected, for example, in products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the more preferred range of 52% to 61% by mass, and its melting point was estimated to be approximately 142±5°C, which is likely to be below the more preferred 150°C, allowing the reflow temperature during mounting to be set sufficiently low.

[0072] Experiment No. 7 is an example of the present invention. The solder-coated metal particles (powder) of Experiment No. 7 coated metal particles designated as particle number 10 in Table 1, and contained approximately 43.2 mass% Sn, approximately 56.8 mass% Bi, and unavoidable impurities, forming a solder layer approximately 0.4 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.40, which is below the recommended 1.5 or less (preferably 1.0 or less). Therefore, for example, sufficient improvements in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the more preferred range of 52 mass% to 61 mass%. Since its melting point was estimated to be approximately 141±5°C, it was likely to be below the more preferred 150°C, allowing the reflow temperature during mounting to be set sufficiently low.

[0073] Experiment No. 8 is an example of the present invention. The solder-coated metal particles (powder) of Experiment No. 8 coated metal particles designated as particle number 10 in Table 1, and contained approximately 39.2 mass% Sn, approximately 60.8 mass% Bi, and unavoidable impurities, forming a solder layer approximately 0.7 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.39, which is below the recommended 1.5 (preferably 1.0 or less), and therefore, sufficient improvements in production efficiency can be expected, for example, in products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the more preferred range of 52 mass% to 61 mass%, and its melting point was estimated to be approximately 148±5°C, which may be more preferably 150°C or less, allowing the reflow temperature during mounting to be set sufficiently low.

[0074] Experiment No. 9 is a comparative example. The solder-coated metal particles (powder) of Experiment No. 9 coated metal particles designated as particle number 10 in Table 1, and contained approximately 33.3% Sn by mass, approximately 66.7% Bi by mass, and unavoidable impurities, forming a solder layer approximately 2.4 μm thick. However, the particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 3.02, exceeding the recommended 1.5. Therefore, it is highly likely that production efficiency will not be improved, for example, in products constructed by arranging individual solder-coated metal particles. Furthermore, the Bi to Sn ratio of this solder layer was within the recommended range of 40% to 67% by mass, and its melting point was estimated to be approximately 167±5°C, which may be below the recommended 170°C, allowing the reflow temperature during mounting to be set relatively low.

[0075] Experiment No. 10 is a comparative example. The solder-coated metal particles (powder) of Experiment No. 10 coated metal particles designated as particle No. 10 in Table 1, and contained approximately 20.0% Sn by mass, approximately 80.0% Bi by mass, and unavoidable impurities, forming a solder layer approximately 0.7 μm thick. The particle size ratio (d90-d50) / d50 of these solder-coated metal particles (powder) was approximately 0.36, which is below the preferred value of 1.0. Therefore, for example, a sufficient improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. However, this solder layer fell outside the recommended range of 40% to 67% Bi by mass relative to Sn, and its melting point was estimated to be approximately 209±5°C. Therefore, it was highly likely that the solder layer would not reach the recommended 170°C or lower, necessitating a significantly higher reflow temperature during mounting.

[0076] <Cu Plating Layer> Next, Cu-coated metal particles were produced by electroplating metal particles (powder) having particle number 10 shown in Table 1 to form a Cu plating layer covering the metal particles. As a result, Cu-coated metal particles (powders) having metal particles and a Cu plating layer but no solder layer, as shown in Experiments 11 to 13 in Table 3, were obtained. The cumulative volume distribution curves of these Cu-coated metal particles (powders) obtained by laser diffraction scattering were used to obtain the particle sizes d10, d50, d90, and particle size ratio (d90-d50) / d50 shown in Table 3. The volume resistivity Rv of these Cu-coated metal particles (powders) was also determined using the measurement and calculation procedures described above, similar to the metal particles, to obtain the values ​​shown in Table 3. The thickness of the Cu plating layer was determined by assuming that half the difference between the d50 of the Cu-coated metal particles (powder) and the d50 of the metal particles (powder) corresponds to the median value of the thickness of the Cu plating layer, and half of this difference was used.

[0077]

[0078] Experiments Nos. 11 to 13 are reference examples and relate to Cu-coated metal particles (powder) before solder coating. The Cu-coated metal particles (powder) before solder coating of Experiments Nos. 11 to 13 have metal particles of particle No. 10 shown in Table 1 and a Cu plating layer coating the metal particles. The Cu-coated metal particles (powder) of Experiment No. 11 have a Cu plating layer with a thickness of approximately 0.7 μm. The Cu-coated metal particles (powder) have a volume resistivity of approximately 0.12×10 －５ Ωm, and the volume resistivity of the metal particles (powder) of particle number 10 having no Cu plating layer (approximately 21.1 × 10 shown in Table 1) －５ The Cu-coated metal particles (powders) of Experiments 12 and 13 both had a Cu plating layer with a thickness of about 2.5 μm. The Cu-coated metal particles (powders) of Experiments 12 and 13 both had a volume resistivity of about 0.07×10 －５ Ωm, and the volume resistivity of the metal particle of particle number 10 having no Cu plating layer (approximately 21.1 × 10 shown in Table 1) －５ The resistance was about 1 / 300 of the original resistance (Ωm).

[0079] When compared with the volume resistivity of the metal particles (powder) designated particle number 10 in Table 1, the volume resistivity of the solder-coated metal particles (powder) designated experiment number 5, which did not have a Cu plating layer, was approximately 1 / 42, while the volume resistivity of the Cu-coated metal particles (powder) designated experiments numbers 11 to 13, which had a Cu plating layer, was approximately 1 / 180 to 1 / 300. This demonstrates that solder-coated metal particles (powder) with a Cu plating layer are suitable for applications requiring solder-coated metal particles (powder) with a low volume resistivity. In this case, an Ag plating layer composed of Ag, which has a volume resistivity slightly lower than that of Cu, is also effective in place of the Cu plating layer. Furthermore, a Cu strike plating layer can be further provided to improve the adhesion of the Ag plating layer.

[0080] <Ag Plating Layer> Next, Ag-coated metal particles were produced by electroless plating using metal particles (powder) having particle number 10 shown in Table 1, forming an Ag plating layer so as to coat the metal particles. As a result, Ag-coated metal particles (powders) having metal particles and an Ag plating layer but no solder layer, as shown in Experiments 14 and 15 in Table 5, were obtained. The cumulative volume distribution curves of these Ag-coated metal particles (powders) measured by a laser diffraction scattering method gave the values ​​shown in Table 4 for particle diameters d10, d50, d90, and particle diameter ratio (d90-d50) / d50. The volume resistivity Rv of these Ag-coated metal particles (powders) was determined using the measurement and calculation procedures described above, similar to the metal particles, to obtain the values ​​shown in Table 4. The thickness of the Ag plating layer was determined by assuming that half the difference between the d50 of the Ag-coated metal particles (powder) and the d50 of the metal particles (powder) corresponds to the median value of the thickness of the Ag plating layer, and half of this difference was used.

[0081]

[0082] Experiments Nos. 14 and 15 are reference examples and relate to Ag-coated metal particles (powder) before solder coating. The Ag-coated metal particles (powder) before solder coating of Experiments Nos. 14 and 15 have metal particles of particle No. 10 shown in Table 1 and an Ag plating layer covering the metal particles. The Ag-coated metal particles (powder) of Experiment No. 14 have an Ag plating layer with a thickness of about 0.6 μm. The Ag-coated metal particles (powder) of Experiment No. 14 have a volume resistivity of about 0.10×10 －５ Ωm, and the volume resistivity of the metal particle (powder) of particle number 10 having no Ag plating layer (approximately 21.1 × 10 shown in Table 1) －５ The Ag-coated metal particles (powder) of Experiment No. 15 had an Ag plating layer with a thickness of about 0.4 μm. The Ag-coated metal particles (powder) of Experiment No. 15 had a volume resistivity of about 0.25×10 －５ Ωm, and the volume resistivity of the metal particle of particle number 10 having no Ag plating layer (approximately 21.1 × 10 shown in Table 1) －５ The resistance was about 1 / 100 of the original resistance (Ωm).

[0083] <Solder-Coated Metal Particles 1B> Next, using Cu-coated metal particles designated by Experiment No. 13 in Table 3, a solder layer composed of Sn, Bi, and unavoidable impurities was formed by electrolytic plating so as to coat the Cu-coated metal particles, thereby producing solder-coated metal particles. As a result, solder-coated metal particles 1B (powder 1B) as shown in FIG. 2 , which have metal particles, a Cu plating layer, and a solder layer, were obtained, as designated by Experiment No. 16 in Table 5. The values ​​shown in Table 4 were obtained for particle diameters d10, d50, d90, and the particle diameter ratio (d90-d50) / d50 from the cumulative volume distribution curve of solder-coated metal particles 1B (powder 1B) obtained by laser diffraction scattering. Furthermore, the volume resistivity Rv of these solder-coated metal particles 1B (powder 1B) was determined by the measurement and calculation procedures described above, similar to the case of the metal particles designated by particle symbol 10, to obtain the values ​​shown in Table 5. The chemical composition of the solder layer was determined by SEM-EDX as described above. In addition, the thickness of the solder layer was determined by assuming that half the difference between the d50 of the solder-coated metal particles (powder) and the d50 of the Cu-coated metal particles (powder) corresponds to the median value of the thickness of the solder layer, and half of that difference was used.

[0084]

[0085] Experiment No. 16 is an example of the present invention. The solder-coated metal particles 1B (powder 1B) of Experiment No. 16 are metal particles (powder) with particle symbol 10 shown in Table 1, that is, comprised of Ni, approximately 6.9 mass % of P, approximately 3.8 mass % of Cu, approximately 0.3 mass % of Sn, and unavoidable impurities, and had a volume resistivity of approximately 21.1 × 10 －５The solder-coated metal particles (powder 1B) in Experiment 16 had a particle size ratio (d90-d50) / d50 of approximately 0.46, which is below the preferred range of 1.0, and therefore, a substantial improvement in production efficiency can be expected for products constructed by arranging individual solder-coated metal particles. Furthermore, the solder layer had a Bi to Sn ratio of 52% to 61% by mass, which is more preferable. The solder layer's melting point was estimated to be approximately 142±5°C, which is likely to be below the preferred range of 150°C, allowing for a sufficiently low reflow temperature during mounting.

[0086] The solder-coated metal particles 1B (powder 1B) of experiment number 16 had a volume resistivity of about 0.23×10 －５ The volume resistivity of the solder-coated metal particles 1A (powder 1A) of Experiment No. 5, which did not have a Cu plating layer, was about 0.50×10 －５ Ωm, it was found that the volume resistivity of the solder-coated metal particles (powder) is further reduced by a configuration having a Cu plating layer between the metal particles and the solder layer. Also, from the same viewpoint as the volume resistivity of the solder-coated metal particles (powder) is further reduced by a configuration having a Cu plating layer between the metal particles and the solder layer, it is considered that the volume resistivity of the solder-coated metal particles (powder) is also likely to be further reduced by a configuration having an Ag plating layer between the metal particles and the solder layer.

[0087] 1A to 1E Solder-coated metal particles 10 Metal particles 11 Solder layer 12a Cu plating layer 12b Ag plating layer 13, 14 Ni plating layers 20 Sample powder 21 Cylinder 22 Jig 23 Piston 24 Arrow

Claims

1. Solder-coated metal particles comprising ball-shaped metal particles consisting of Ni, 1% by mass or more and 15% by mass or less of P, and unavoidable impurities, and a solder layer coating the metal particles and consisting of Sn, 40% by mass or more and 67% by mass or less of Bi, and unavoidable impurities, wherein in an accumulated volume distribution curve of a powder consisting of the solder-coated metal particles by a laser diffraction scattering method, the particle size ratio (d90-d50) / d50 calculated from the particle size d50 at an accumulated volume of 50% and the particle size d90 at an accumulated volume of 90% is 1.5 or less.

2. The solder-coated metal particles according to claim 1, wherein the particle size ratio (d90-d50) / d50 is 1.0 or less.

3. The solder-coated metal particle according to claim 1 or 2, wherein the solder layer contains 46 mass % or more and 64 mass % or less of Bi.

4. The solder-coated metal particle according to claim 1 or 2, wherein the solder layer contains 52 mass % or more and 61 mass % or less of Bi.

5. The solder-coated metal particle according to claim 1 or 2, further comprising a Cu-plated layer or an Ag-plated layer between the metal particle and the solder layer.

6. The solder-coated metal particle according to claim 5, further comprising a Ni plating layer between the metal particle and the Cu plating layer or the Ag plating layer.

7. The solder-coated metal particle according to claim 5, further comprising a Ni plating layer between the solder layer and the Cu plating layer or the Ag plating layer.

8. A method for producing solder-coated metal particles, comprising: a step of producing ball-shaped metal particles consisting of Ni, 1% by mass or more and 15% by mass or less of P, and unavoidable impurities by an electroless reduction method; and a step of producing solder-coated metal particles by forming a solder layer consisting of Sn, 40% by mass or more and 66% by mass or less of Bi, and unavoidable impurities so as to cover the metal particles, wherein the solder layer is formed so that in an accumulated volume distribution curve obtained by a laser diffraction scattering method for a powder consisting of the solder-coated metal particles, the particle size ratio (d90-d50) / d50 calculated from the particle size d50 at an accumulated volume of 50% and the particle size d90 at an accumulated volume of 90% is 1.5 or less.

9. The method for producing solder-coated metal particles according to claim 8, wherein the solder layer is formed so that the particle size ratio (d90-d50) / d50 is 1.0 or less.

10. A method for producing solder-coated metal particles as described in claim 8 or 9, wherein the solder layer is formed so as to contain 46 mass % or more and 63 mass % or less of Bi.

11. A method for producing solder-coated metal particles according to claim 8 or 9, wherein the solder layer is formed so as to contain 51 mass % or more and 60 mass % or less of Bi.

12. The method for producing solder-coated metal particles according to claim 8 or 9, further comprising forming a Cu-plated layer or an Ag-plated layer between the metal particles and the solder layer.

13. The method for producing solder-coated metal particles according to claim 12, further comprising forming a Ni plating layer between the metal particles and the Cu plating layer or the Ag plating layer.

14. The method for producing solder-coated metal particles according to claim 12, further comprising forming a Ni plating layer between the solder layer and the Cu plating layer or the Ag plating layer.