Solder paste and method for producing joined body

Abstract

A solder paste according to the present invention contains a first solder alloy powder, a second solder alloy powder, and flux, and is characterized in that the first solder alloy powder contains Sn, Ag, and In, and has an Ag content of 3.9 mass% or more, and the second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu. According to the solder paste, the meltability of the paste is enhanced, and high speed shear strength and destruction mode of a solder joint part can be optimized.

Description

Solder paste and method for manufacturing bonded body

[0001] This application claims priority to Japanese Patent Application No. 2024-085077, filed May 24, 2024, the contents of which are incorporated herein by reference.

[0002] In the electronics industry, the miniaturization of components is progressing with the increase in system-in-package (SiP) designs for electronic devices. SiP is a type of integrated circuit, a high-density packaging technology that achieves large memory capacity and multi-functionality by stacking multiple ICs or packages. SiP is used in a variety of fields, including smartphones, tablet PCs, digital cameras, in-vehicle electronic devices, medical equipment, and IoT (Internet of Things) devices.

[0003] In SiP designs, soldering involving multiple heating steps may be required to interconnect multiple components into a single assembly. Multi-stage soldering, involving multiple heating steps, requires solder with different liquidus temperatures to prevent remelting of the solder melted in a previous heating step during a subsequent heating step. Typically, SnAgCu solder is used for the first soldering interconnection. SnAgCu solder, which is widely used as a lead-free solder, has a liquidus temperature higher than that of SnPb eutectic solder. Therefore, in soldering performed in the second or subsequent stages, the liquidus temperature of the solder must be set lower than that of SnAgCu solder. Sn—Bi-based lead-free solder is known as a low-temperature solder with a low liquidus temperature. However, the typical alloy composition of Sn—Bi-based lead-free solder is brittle due to its high Bi content. Furthermore, since the melting temperature of the alloy is low, the reliability against thermal fatigue is poor.

[0004] In response to this, conventionally, there have been used solder pastes that combine two or more types of solder alloy powders with different compositions, in which the liquidus temperature of one solder alloy powder is lower than the solidus temperature of the other solder alloy powder, methods for forming assemblies using the solder pastes, and methods for forming solder joints using the solder pastes (see, for example, Patent Document 1).

[0005] International Publication No. 2020 / 227638

[0006] As electronic components become increasingly smaller and more functional, even conventional solder pastes are required to have higher performance. However, in solder pastes that use solder alloy powders with different liquidus temperatures, as described above, there is a risk that the solder alloy powders may not melt depending on the heating conditions. Furthermore, in such solder pastes, the properties of the solder alloy powders differ, making it difficult to control the properties of the solder joints formed (fracture mode, high-speed shear strength, etc.), and it is also difficult to predict the differences in properties that will be achieved depending on the combination of solder alloy powders.

[0007] In particular, simply improving the mechanical strength of a solder alloy can result in stress concentration at the joint interface, making the solder joint more susceptible to fracture, which can be a fracture mode that should be avoided most in solder joints. Therefore, the inventors focused on evaluating solder alloys under more severe conditions than in conventional tests for measuring mechanical strength, and conducted shear strength tests (high-speed shear tests) at speeds much higher than conventional tests. This evaluation is significantly different from conventional shear strength test conditions, and allows for the confirmation of the fracture mode when a solder joint is instantaneously fractured.

[0008] The present invention has been made in view of the above circumstances, and provides a solder paste that improves the melting properties of the paste and optimizes the fracture mode and high-speed shear strength, as well as a method for manufacturing a bonded body using the same.

[0009] In order to solve the above problems, the present invention employs the following configuration.

[0010] [1] A solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is a powder of an alloy containing Sn, Ag, and In, with the Ag content being 3.9 mass% or more, and the second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu.

[0011] [2] The solder paste according to [1], wherein the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less of the first solder alloy powder, and 35 parts by mass or more and 55 parts by mass or less of the second solder alloy powder. [3] The solder paste according to [2], wherein the mixing ratio of the first solder alloy powder to the second solder alloy powder is more than 50 parts by mass and 65 parts by mass or less of the first solder alloy powder, and 35 parts by mass or more and less than 50 parts by mass of the second solder alloy powder.

[0012] [4] The solder paste according to any one of [1] to [3], wherein the first solder alloy powder has an Ag content of 3.9 mass% or more and 4.5 mass% or less, an In content of 18 mass% or more and 22 mass% or less, and the remainder is Sn.

[0013] [5] The solder paste according to any one of [1] to [4], wherein the second solder alloy powder has an Ag content of 2.9% by mass or more and 4% by mass or less, a Cu content of 0.3% by mass or more and 1% by mass or less, and the remainder being Sn.

[0014] [6] The solder paste according to any one of [1] to [5], wherein the mass ratio expressed by In / Ag in the first solder alloy powder is 4 or more and 5.5 or less.

[0015] [7] A method for producing a bonded body, comprising a step of obtaining a bonded body by soldering, wherein heating is performed during the soldering using the solder paste according to any one of [1] to [6].

[0016] In this specification, "%" indicating the alloy composition means "% by mass" unless otherwise specified. The solidus temperature and liquidus temperature of the alloy are temperatures measured by differential thermal analysis. The average particle size of the first solder alloy powder and the average particle size of the second solder alloy powder are the average particle size on a volume basis for each solder alloy particle group.

[0017] The failure modes of solder joints include failure of the solder itself (solder bulk) at the joint, delamination failure at the interface between the solder bulk and an intermetallic compound (IMC), and delamination failure at the IMC interface. Because the IMC interface (joint interface) is joined to the electrode, it is not easy to relieve stress at the joint interface. However, physical and electrical loads are primarily applied to the solder joint interface. For this reason, it is believed that stress relief in the bulk, which is relatively easily deformed, can suppress failure. The failure modes of the solder joints are evaluated by performing a high-speed shear test.

[0018] According to the present invention, it is possible to provide a solder paste that can improve the melting properties of the paste and optimize the fracture mode and high-speed shear strength, as well as a method for manufacturing a bonded body using the same.

[0019] 1 is a diagram showing a reflow profile in the evaluation of the melting property of a paste, and FIG. 2 is a diagram showing the evaluation criteria in the evaluation of the melting property of a paste.

[0020] (Solder Paste) One embodiment of the solder paste includes a first solder alloy powder, a second solder alloy powder, and a flux. In the solder paste of this embodiment, the first solder alloy powder is a powder of an alloy containing Sn, Ag, and In, with the Ag content being 3.9 mass% or more. The second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu.

[0021] The solder paste of this embodiment is suitable for soldering applications at low to medium temperatures (150 to 210°C), and is suitable for soldering at temperatures, for example, where the peak soldering temperature is lower than the peak temperature during heating of Sn-Ag-Cu solder. The solder paste of this embodiment uses a first solder alloy powder and a second solder alloy powder having different liquidus temperatures. The first solder alloy powder containing In has a relatively lower liquidus temperature than the second solder alloy powder containing Sn, Ag, and Cu. When using the solder paste of this embodiment, it can be designed to be heated at a peak temperature lower than the solidus temperature of the second solder alloy powder but higher than the liquidus temperature of the first solder alloy powder.

[0022] <First Solder Alloy Powder> In the solder paste of this embodiment, the first solder alloy powder is a powder of an alloy containing Sn, Ag, and In. The Ag content, relative to the total mass of the first solder alloy powder, is 3.9 mass% or more, and may be 3.9 mass% to 5 mass% or less, or 3.9 mass% to 4.5 mass% or less. For example, the Ag content may be 3.9 mass%, 4 mass%, or 4.5 mass%. The In content, relative to the total mass of the first solder alloy powder, may be, for example, 18 mass% or more, 18 mass% to 25 mass% or less, or 18 mass% to 22 mass% or less. For example, the In content may be 19 mass%, 20 mass%, or 21 mass%. The Sn content, which constitutes the remainder, may be 90 mass% or more relative to the total mass of the first solder alloy powder.

[0023] The alloy constituting the first solder alloy powder may contain metal elements other than Sn, Ag, and In. Examples of metal elements other than Sn, Ag, and In include Cu, Ni, Bi, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and As. The first solder alloy powder may also contain unavoidable impurities. The solder paste of this embodiment may contain two or more types of first solder alloy powders with different alloy compositions and particle sizes.

[0024] A preferred first solder alloy powder is a powder of an alloy containing Sn, Ag, and In, with the Ag content being 3.9% by mass or more and 4.5% by mass or less and the In content being 18% by mass or more and 22% by mass or less; a more preferred first solder alloy powder is a powder of an Sn-Ag-In alloy with the Ag content being 3.9% by mass or more and 4.5% by mass or less, the In content being 18% by mass or more and 22% by mass or less, and the remainder being Sn.

[0025] In the first solder alloy powder, the mass ratio of In / Ag is preferably 4 or more and 5.5 or less, and may be 4.2 or more and 5.4 or less, or 4.4 or more and 5.2 or less. When the mass ratio of In / Ag is equal to or greater than the lower limit of the above-mentioned preferred range, excessive bulk strength is prevented in the solder itself (solder bulk) at the joint, making it easier to control the fracture mode to a more appropriate one. Furthermore, when the mass ratio of In / Ag is equal to or greater than the lower limit of the above-mentioned preferred range, excessive shear strength is prevented in the bulk of the solder itself (solder bulk) at the joint, making it easier to obtain appropriate shear strength. On the other hand, when the mass ratio is equal to or less than the upper limit of the above-mentioned preferred range, a significant decrease in joint strength is prevented, making it easier to obtain the desired high-speed shear strength.

[0026] The solidus temperature of the first solder alloy powder is, for example, 135°C or higher and 150°C or lower, and may be 140°C or higher and 145°C or lower. The liquidus temperature of the first solder alloy powder is, for example, 180°C or higher and 200°C or lower, and may be 190°C or higher and 200°C or lower. The particle size of the first solder alloy powder is not particularly limited and may be a particle size similar to that used in ordinary solder pastes. If the first solder alloy powder has a size (particle size distribution) corresponding to symbols 1 to 8 in the powder size classification (Table 2) in JIS Z 3284-1:2014, soldering to fine components becomes possible. For example, the first solder alloy powder has an average particle size of 1 μm or higher and 1000 μm or lower. The average particle size of the first solder alloy powder here refers to the particle size at 50% of the integrated value in the particle size distribution measured by a laser diffraction / scattering particle size distribution analyzer.

[0027] <Second Solder Alloy Powder> In the solder paste of this embodiment, the second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu. The Ag content, relative to the total mass of the second solder alloy powder, may be, for example, 2.9% by mass or more and 4% by mass or less, or 3% by mass or more and 4% by mass or less. For example, the Ag content may be 2.9%, 3%, 3.2%, 3.5%, or 4% by mass. The Cu content, relative to the total mass of the second solder alloy powder, may be, for example, 0.3% by mass or more and 1.5% by mass or less, 0.3% by mass or more and 1% by mass or less, or 0.3% by mass or more and 0.7% by mass or less. For example, the Cu content may be 0.3%, 0.5%, 0.7%, 0.75%, 0.8%, or 0.9% by mass. The content of Sn relative to the total mass of the second solder alloy powder is the remainder, and may be 90 mass % or more.

[0028] The alloy constituting the second solder alloy powder may contain metal elements other than Sn, Ag, and Cu. Examples of metal elements other than Sn, Ag, and Cu include Ni, Bi, In, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and As. The second solder alloy powder may also contain unavoidable impurities. The solder paste of this embodiment may contain two or more types of second solder alloy powders with different alloy compositions and particle sizes.

[0029] A preferred second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu, with the Ag content being 2.9% by mass or more and 4% by mass or less and the Cu content being 0.3% by mass or more and 1% by mass or less; a more preferred second solder alloy powder is a powder of an Sn-Ag-Cu alloy with the Ag content being 3% by mass or more and 4% by mass or less, the Cu content being 0.3% by mass or more and 0.7% by mass or less, and the remainder being Sn.

[0030] In the second solder alloy powder, the mass ratio expressed by Ag / Cu is preferably 10 or less, and may be 3 to 9, 5 to 8.5, or 6 to 8. When the mass ratio expressed by Ag / Cu is within the above range, the melting property of the paste is easily improved, and both the fracture mode and high-speed shear strength are easily optimized.

[0031] The solidus temperature of the second solder alloy powder is, for example, 205°C or higher and 225°C or lower, and may be 210°C or higher and lower than 220°C. The liquidus temperature of the second solder alloy powder is, for example, 215°C or higher and 235°C or lower, and may be 215°C or higher and lower than 230°C. The particle size of the second solder alloy powder is not particularly limited and may be a particle size similar to that used in ordinary solder pastes. If the second solder alloy powder has a size (particle size distribution) corresponding to symbols 1 to 8 in the powder size classification (Table 2) of JIS Z 3284-1:2014, soldering to fine components becomes possible. For example, the second solder alloy powder has an average particle size of 1 μm or higher and 1000 μm or lower. The average particle size of the second solder alloy powder here refers to the particle size at 50% of the integrated value in the particle size distribution measured by a laser diffraction / scattering particle size distribution analyzer.

[0032] <Mixing ratio of first solder alloy powder to second solder alloy powder> In the solder paste of this embodiment, the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less of the first solder alloy powder, and 35 parts by mass or more and 55 parts by mass or less of the second solder alloy powder. From the viewpoint of optimizing the fracture mode and high-speed shear strength, the mixing ratio of the two is preferably 50 parts by mass or more and 65 parts by mass or less of the first solder alloy powder and 35 parts by mass or more and 50 parts by mass or less of the second solder alloy powder, more preferably more than 50 parts by mass and 65 parts by mass or less of the first solder alloy powder and 35 parts by mass or more and less than 50 parts by mass of the second solder alloy powder, more preferably 55 parts by mass or more and 65 parts by mass or less of the first solder alloy powder and 35 parts by mass or more and 45 parts by mass or less of the second solder alloy powder, and even more preferably 60 parts by mass or more and 65 parts by mass or less of the first solder alloy powder and 35 parts by mass or more and 40 parts by mass or less of the second solder alloy powder.

[0033] <Flux> In the solder paste of this embodiment, the flux is not particularly limited and may contain, for example, rosin as a resin component, a solvent, an activator, a thixotropic agent, etc. Examples of the activator include organic acids, halogen compounds, and amine compounds. Examples of the thixotropic agent include ester-based thixotropic agents, amide-based thixotropic agents, sorbitol-based thixotropic agents, and cellulose nanofibers.

[0034] In the solder paste of this embodiment, the content of the flux is preferably 5% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 15% by mass or less, relative to the total mass of the solder paste; and the total content of the first solder alloy powder and the second solder alloy powder is preferably 70% by mass or more and 95% by mass or less, and more preferably 85% by mass or more and 95% by mass or less, relative to the total mass of the solder paste.

[0035] The solder paste of this embodiment may optionally contain a metal powder other than the first solder alloy powder and the second solder alloy powder. There are no limitations on the composition of this metal powder, and examples of metals that may be contained include Sn, Ag, Cu, In, Bi, Ni, Ge, P, Co, Ga, Zn, Sb, Pb, Au, Al, Pt, Pd, Fe, Mn, Zr, and As. These metals may be contained alone or in combination with two or more types of metal powder. The metal powder may be two or more types of powder with different compositions or particle sizes. Furthermore, the surface of the metal powder may be plated with a metal different from the metal constituting the metal powder, or it may contain a resin powder or carbon fiber. For example, the resin powder may be a plastic powder, and the carbon fiber may be carbon fiber. The resin powder or carbon fiber may be two or more types of powder with different compositions or particle sizes. Furthermore, the surface of the resin powder or carbon fiber may be plated with a metal.

[0036] As described above, the solder paste of this embodiment includes first and second solder alloy powders having specific alloy compositions, and a flux. The first solder alloy powder is an alloy powder containing Sn, Ag, and In, with the Ag content being 3.9% by mass or more. The inclusion of In in the first solder alloy powder lowers the peak soldering temperature. Furthermore, unlike brittle Bi, the use of In enhances meltability while preventing excessive solder bulk strength. The Ag content of 3.9% by mass or more enhances the strength of the solder bulk at the joint. Furthermore, by using the first solder alloy powder in combination with a second solder alloy powder containing Sn, Ag, and Cu, which have different liquidus temperatures, the solder alloys melt well during heating. Therefore, the solder paste of this embodiment enhances the meltability of the paste and optimizes the fracture mode and high-speed shear strength.

[0037] A suitable mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less of the first solder alloy powder, and 35 parts by mass or more and 55 parts by mass or less of the second solder alloy powder. By controlling the mixing ratio of the two within this suitable range, the properties of both the first solder alloy powder and the second solder alloy powder are fully exhibited. A more suitable mixing ratio of the first solder alloy powder to the second solder alloy powder is more than 50 parts by mass and 65 parts by mass or less of the first solder alloy powder, and 35 parts by mass or more and less than 50 parts by mass of the second solder alloy powder. By controlling the mixing ratio of the two within this more suitable range, the meltability of the paste is improved, and the fracture mode and high-speed shear strength of the solder joint are more easily optimized.

[0038] When soldering is performed using the solder paste of this embodiment, for example, during heating, the first solder alloy powder with a lower solidus temperature melts first. At the same time, the second solder alloy powder with a relatively higher solidus temperature can be partially or completely melted into the alloy (first solder alloy powder) that has been melted by heating. The solder joint produced in this manner has optimized fracture modes and high-speed shear strength.

[0039] A preferred form of solder paste is a solder paste consisting of a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is an alloy powder containing Sn, Ag, and In, with the Ag content being 3.9 mass% or more, and the second solder alloy powder is an alloy powder containing Sn, Ag, and Cu, and the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 mass parts or more and 65 mass parts or less of the first solder alloy powder and 35 mass parts or more and 55 mass parts or less of the second solder alloy powder.

[0040] Another preferred form of solder paste is a solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is an alloy powder containing Sn, Ag, and In, with an Ag content of 3.9 to 4.5 mass% and an In content of 18 to 22 mass%, with the remainder being Sn; and the second solder alloy powder is an alloy powder containing Sn, Ag, and Cu, with an Ag content of 2.9 to 4 mass%, an Cu content of 0.3 to 1 mass%, and the remainder being Sn; and the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 to 65 parts by mass of the first solder alloy powder and 35 to 55 parts by mass of the second solder alloy powder.

[0041] Another preferred embodiment of the solder paste is a solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder contains Sn, Ag, and In, the Ag content is 3.9 mass % or more and 4.5 mass % or less, the In content is 18 mass % or more and 22 mass % or less, and the remainder is Sn, and the mass ratio expressed by In / Ag in the first solder alloy powder is 4 or more and 5.5 or less. The second solder alloy powder is an alloy powder containing Sn, Ag, and Cu, with an Ag content of 2.9% by mass or more and 4% by mass or less, a Cu content of 0.3% by mass or more and 1% by mass or less, and the remainder being Sn, and the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less, and the second solder alloy powder is 35 parts by mass or more and 55 parts by mass or less, to form a solder paste.

[0042] Another preferred form of solder paste is a solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is a powder of an Sn-Ag-In alloy having an Ag content of 3.9% by mass or more and 4.5% by mass or less, an In content of 18% by mass or more and 22% by mass or less, and the remainder being Sn, and the second solder alloy powder is a powder of an Sn-Ag-Cu alloy having an Ag content of 3% by mass or more and 4% by mass or less, a Cu content of 0.3% by mass or more and 0.7% by mass or less, and the remainder being Sn, and the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less, and 35 parts by mass or more and 55 parts by mass or less of the second solder alloy powder.

[0043] Another preferred form of solder paste is a solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is a powder of an Sn-Ag-In alloy having an Ag content of 3.9% by mass or more and 4.5% by mass or less, an In content of 18% by mass or more and 22% by mass or less, and the remainder being Sn, and wherein the mass ratio of In / Ag in the first solder alloy powder is 4 or more and 5.5 or less, and the second solder alloy powder is a powder of an Sn-Ag-Cu alloy having an Ag content of 3% by mass or more and 4% by mass or less, a Cu content of 0.3% by mass or more and 0.7% by mass or less, and the remainder being Sn, and the mixing ratio of the first solder alloy powder to the second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less of the first solder alloy powder and 35 parts by mass or more and 55 parts by mass or less of the second solder alloy powder.

[0044] (Method for manufacturing a bonded body) One embodiment of the method for manufacturing a bonded body is a manufacturing method including a step of obtaining a bonded body by soldering. In the method for manufacturing a bonded body of this embodiment, heating is performed during the soldering using the above-mentioned solder paste.

[0045] Hereinafter, a method for manufacturing a bonded body according to this embodiment will be described, in which a solder paste application operation, a component mounting operation, and a reflow operation as an example of a heating method are carried out in this order.

[0046] Solder Paste Application Operation In the solder paste application operation, the above-described solder paste is applied to the surface of a substrate. Examples of the substrate include a printed wiring board and a wafer. Methods for applying the solder paste include, for example, a method of printing and applying the solder paste using a mask with an opening, a method of discharging the solder paste using a dispenser, and a method of transferring the solder paste using a probe pin.

[0047] In the component mounting operation, components such as chips, integrated circuits, transistors, diodes, resistors, capacitors, semiconductor packages, and substrates are mounted in predetermined positions on the substrate on which the solder paste has been printed.

[0048] Reflow Operation The atmosphere for the reflow operation is not particularly limited as long as the effects of the present invention are achieved, but it may be, for example, a nitrogen gas atmosphere or a reducing gas atmosphere. The reducing gas atmosphere may be formed, for example, by volatilizing a reducing compound in a reflow furnace, or by supplying a reducing gas obtained by passing nitrogen through a liquid reducing compound to the reflow furnace. Formic acid is preferred as the reducing compound. When the reflow operation is performed in a reducing gas atmosphere, the amount of activator in the solder paste used may be reduced, or the solder paste may not contain an activator at all. This can further reduce flux residue.

[0049] In the reflow operation, the substrate after component mounting is heated in a reflow furnace at a temperature (i.e., peak temperature) that is lower than the solidus temperature of the second solder alloy powder contained in the solder paste and higher than the liquidus temperature of the first solder alloy powder (this is called main heating). The heating temperature may be, for example, 10 to 20°C higher than the liquidus temperature of the first solder alloy powder. The heating time may be, for example, 10 seconds to 3 minutes. The reflow operation may also include preheating, in which the substrate after component mounting is heated at a temperature lower than the heating temperature of the main heating, before the main heating.

[0050] During the reflow process, when the heating temperature rises above the solidus temperature of the first solder alloy powder, the first solder alloy powder begins to melt and become molten solder. This molten solder spreads and begins to wet the joining surface between the component and the substrate. At the same time, the second solder alloy powder begins to melt into the molten solder. As the heating continues, the molten solder completely melts, and the second solder alloy powder continues to melt into the molten solder, forming a solder joint. The second solder alloy powder continues to melt into the molten solder, forming a homogeneous liquid state. As a result, the mechanical performance of the solder joint is improved.

[0051] According to the manufacturing method of the bonded body of the present embodiment described above, the specific solder paste described above is employed, and therefore the fracture mode and high-speed shear strength of the manufactured bonded body are optimized, thereby improving the bonding strength.

[0052] The specific solder paste described above and the method for manufacturing a bonded body using the same are useful techniques for soldering applications in fields such as smartphones, tablet PCs, digital cameras, in-vehicle electronic devices, medical equipment, and IoT devices.

[0053] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0054] <Preparation of Solder Paste> (Examples 1 to 5, Comparative Examples 1 to 35) Each solder alloy having the alloy composition shown in Tables 1 to 3 was atomized to prepare a first solder alloy powder and a second solder alloy powder, each having an average particle size of 32 μm when measured on a volume basis. The solidus and liquidus temperatures of some of the prepared solder alloy powders are shown below. Sn-4Ag-20In: solidus temperature 143 (°C), liquidus temperature 198 (°C) Sn-3.2Ag-20In: solidus temperature 141 (°C), liquidus temperature 195 (°C)

[0055] Sn-3Ag-0.5Cu Solidus temperature 217 (℃), liquidus temperature 220 (℃) Sn-2.9Ag-0.5Cu Solidus temperature 217 (℃), liquidus temperature 220 (℃) Sn-4Ag-0.5Cu Solidus temperature 217 (℃), liquidus temperature 229 (℃) Sn-3.8Ag-0.5Cu Solidus temperature 217 (℃), liquidus temperature 217 (℃) Sn-3Ag-0.3Cu Solidus temperature 218 (℃), liquidus temperature 221 (℃) Sn-3Ag-1Cu Solidus temperature 216 (℃), liquidus temperature 224 (℃) Sn-3Ag-2Cu Solidus temperature 216 (℃), liquidus temperature 277 (℃)

[0056] Sn-3Ag Solidus temperature 221 (℃), liquidus temperature 222 (℃) Sn-3.5Ag Solidus temperature 220 (℃), liquidus temperature 221 (℃) Sn-5Ag Solidus temperature 220 (℃), liquidus temperature 245 (℃) Sn-5Sb Solidus temperature 240 (℃), liquidus temperature 243 (℃)

[0057] Next, the first solder alloy powder and the second solder alloy powder were mixed in the mixing ratio shown in Tables 1 to 3, and then mixed with a flux having the following composition to prepare the solder paste of each example. The mixing ratio of the solder alloy powder and the flux at this time was 88.5 parts by mass of the mixture of the first solder alloy powder and the second solder alloy powder, and 11.5 parts by mass of the flux.

[0058] Flux composition: 40% by mass of hydrogenated acid-modified rosin, 40% by mass of solvent, 10% by mass of organic acid, 10% by mass of thixotropic agent

[0059] <Evaluation> The solder paste of each example was evaluated for paste meltability, fracture mode, and high-speed shear strength as follows. The evaluation results are shown in Tables 1 to 3.

[0060] [Paste Meltability] The substrate, components, reflow equipment, and observation equipment used are as follows: Substrate: FR-4, substrate size 105 mm x 105 mm x 0.8 mm Components: QFP Reflow equipment: SNR-615 manufactured by Senju Metal Industry Co., Ltd. Observation equipment: Optical microscope, VHX-6000 manufactured by Keyence Corporation

[0061] - Solder paste application operation, component mounting operation: Solder paste was applied to the surface of the substrate, and the components were mounted in the designated positions on the substrate where the solder paste had been applied. - Reflow operation: Next, the substrate with the components mounted was heated in the reflow device. The heating conditions were as follows: in a nitrogen atmosphere, the substrate was heated to 190°C at a temperature increase rate of 1°C / s, and then heated continuously for 150 seconds in the range from 190°C to 210°C (peak temperature). The reflow profile at this time is shown in Figure 1.

[0062] The meltability of the paste was evaluated by slowly cooling the substrate after the reflow operation, observing a predetermined position on the substrate where the solder paste was applied (the portion where the terminal of the component was attached) with the observation device, and judging the melting state of the solder alloy powder (melted or unmelted) based on the following evaluation criteria. These evaluation criteria are shown in Figure 2. Evaluation criteria A: The solder alloy powder was melted, and no surface irregularities were observed (no surface irregularities). B: The solder alloy powder was melted, but surface irregularities were observed (surface irregularities). C: The solder alloy powder was not melted sufficiently (insufficient melting). D: The solder alloy powder was not melted (unmelted).

[0063] [Fracture Mode and High-Speed ​​Shear Strength] A high-speed shear (HSS) test was carried out to evaluate the fracture mode and high-speed shear strength as characteristics of the solder joints.

[0064] - Preparation of substrate for HSS test The solder paste was printed on a substrate using a mask with a mask opening (diameter) of 0.91 mm and a mask thickness of 0.24 mm, and then reflowed to obtain a substrate for HSS test equipped with solder joints. Reflow details Reflow device: SNR-615 manufactured by Senju Metal Industry Co., Ltd. Reflow conditions: Peak temperature 220°C, peak time 35 seconds, nitrogen atmosphere; the peak temperature was set to 220°C to prevent differences in melting properties from affecting this evaluation.

[0065] - HSS Test The obtained substrate for HSS test was attached to a jig in an HSS tester (DAGE-4000HS manufactured by Nordson Dage), and five solder joints prepared by the above-mentioned preparation procedure were sheared, and the strength required for shearing (high-speed shear strength) was measured. The conditions for the HSS test were as follows: shear speed 4000 mm / sec, shear height 10 μm

[0066] Evaluation of Fracture Mode After the HSS test, the HSS test substrate was observed with an optical microscope (VHX-6000 manufactured by Keyence Corporation) to evaluate the fracture mode. The fracture mode was evaluated by judging the fracture pattern due to shear based on the following evaluation criteria. Evaluation criteria For five solder joints, A: There was no peeling fracture at the intermetallic compound (IMC) interface across the entire surface of all five bumps, and one or more fractures in the solder itself (solder bulk) were found across an area of ​​90% or more of the bump surface. B: In all five bumps, peeling fracture at the IMC interface and bulk fracture occurred simultaneously, and the fracture area of ​​the solder itself on the bump surface was less than 90%. C: One or more peeling fractures were found across the entire surface of the bump at the IMC interface.

[0067] Evaluation of High-Speed ​​Shear Strength The evaluation of high-speed shear strength was carried out by using the strength measured in the HSS test (high-speed shear strength) as an index and judging based on the following evaluation criteria. The high-speed shear strength here needs to be a certain value or more, but it is not enough to simply have a high value. If the shear strength is too high, the probability of occurrence of an inappropriate fracture mode increases, so the value needs to be within a certain range as per the evaluation criteria below. Evaluation criteria For five solder joints, A: The average high-speed shear strength is greater than 10.5 N and less than 13 N. C: The average high-speed shear strength is 13 N or more, or 10.5 N or less.

[0068]

[0069]

[0070]

[0071] From the results shown in Tables 1 to 3, it can be confirmed that the solder pastes of Examples 1 to 5 had improved paste meltability and optimized fracture mode and high-speed shear strength. On the other hand, some of the solder pastes of Comparative Examples 1 to 35 had insufficient high-speed shear strength. The solder pastes of Comparative Examples 1 to 35 did not show any improvement in paste meltability or optimization of fracture mode and high-speed shear strength.

[0072] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments. Addition, omission, substitution, and other modifications of the configuration are possible within the scope of the spirit of the present invention. The present invention is not limited by the above description, but is limited only by the scope of the appended claims.

Claims

1. A solder paste comprising a first solder alloy powder, a second solder alloy powder, and a flux, wherein the first solder alloy powder is a powder of an alloy containing Sn, Ag, and In, with the Ag content being 3.9 mass% or more, and the second solder alloy powder is a powder of an alloy containing Sn, Ag, and Cu.

2. The solder paste according to claim 1, wherein the mixing ratio of said first solder alloy powder to said second solder alloy powder is 45 parts by mass or more and 65 parts by mass or less, and 35 parts by mass or more and 55 parts by mass or less of said second solder alloy powder.

3. The solder paste according to claim 2, wherein the mixing ratio of said first solder alloy powder to said second solder alloy powder is more than 50 parts by mass and not more than 65 parts by mass, and said second solder alloy powder is not less than 35 parts by mass and not more than 50 parts by mass.

4. The solder paste according to claim 1, wherein the first solder alloy powder has an Ag content of 3.9 mass % or more and 4.5 mass % or less, an In content of 18 mass % or more and 22 mass % or less, and the remainder being Sn.

5. The solder paste according to claim 1, wherein the second solder alloy powder has an Ag content of 2.9 mass % or more and 4 mass % or less, a Cu content of 0.3 mass % or more and 1 mass % or less, and the remainder being Sn.

6. The solder paste according to claim 1, wherein the mass ratio of In / Ag in the first solder alloy powder is 4 or more and 5.5 or less.

7. A method for producing a bonded body, comprising the step of obtaining a bonded body by soldering, wherein heating is carried out during the soldering using the solder paste according to any one of claims 1 to 6.

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