Lead-free solder paste for high-temperature applications containing mixed solder powder

Abstract

Some embodiments of the present invention relate to a lead-free solder paste having a mixed solder powder particularly suited for high-temperature soldering applications involving multiple board-level reflow operations. In one embodiment, the solder paste is comprised of 10 to 90 wt. % first solder alloy powder comprised of a SnSbCuAg solder alloy having a Sn:Sb weight ratio of 0.75:1.1, 10 to 90 wt. % second solder alloy powder comprised of a Sn solder alloy containing at least 80 wt. % Sn, and the balance being flux.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 017,469, filed Apr. 29, 2020, entitled "Lead - free Solder Paste with Mixed Solder Powders For High Temperature Applications", which is hereby incorporated by reference in its entirety.

[0002] The present invention relates to a lead - free solder paste.

Background Art

[0003] Lead (Pb) generated by the disposal of electronic assemblies is considered harmful to the environment and human health. In the electronics interconnection and electronics packaging industries, the use of Pb - based solders is being rapidly prohibited by regulations. Due to the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS), which came into force on July 1, 2006, the transition from SnPb solder alloys to Pb - free solder alloys has been successfully progressing. In the semiconductor and electronics industries, SnAg, SnCu, and SnAgCu (SAC) - based solders have become highly evaluated solders for forming interconnections. However, the development of high - melting - point Pb - free solders to replace conventional high - lead - content solders (e.g., Pb - 5Sn and Pb - 5Sn - 2.5Ag) is still in its initial stage.

[0004] The general use of high melting point solder is in die attachment of semiconductor surface mounting devices / components where the chip is joined between leads or on a lead frame. This surface mounting component / device is then used in one or more successive board level soldering processes. For example, a silicon die is soldered to a lead frame using high melting point solder to form an assembly. Subsequently, the device (capsule type or otherwise) is attached onto a printed wiring board (PWB) by board level reflow. The same board can be exposed to multiple reflows. During all processes, the interconnection between the silicon die and the lead frame will be maintained well. High melting point solder is required to withstand multiple reflow soldering processes without causing any malfunction.

[0005] The transient liquid phase bonding (TLPB) technique aims to achieve a higher remelting temperature for solder joints through the formation of intermetallic compounds (IMCs) between a low melting point alloy and a high melting point alloy. In a TLPB structure, in order to reach the high melting point target, most or all of the low melting point phase is consumed during reflow. During reflow, the formation of interfacial IMCs on the surface of the high melting point alloy and the continuous growth of IMCs will sacrifice both the low melting point alloy and the high melting point alloy.

Summary of the Invention

Means for Solving the Problems

[0006] Some embodiments of the present invention relate to a lead-free solder paste having a mixed solder powder particularly suitable for high temperature soldering applications including multiple board level reflow operations.

[0007] In one embodiment, the solder paste is It consists of solder powder and flux, and the solder powder a 10 wt% to 90 wt% first solder alloy powder composed of a SnSbCuAg solder alloy with a weight ratio of Sn:Sb of 0.75 ~ 1.1, and a 10 wt% to 90 wt% second solder alloy powder composed of a Sn solder alloy containing at least 80 wt% Snand is composed of.

[0008] In some embodiments, the solder powder is composed of 50 wt% to 90 wt% of a first solder alloy powder and 10 wt% to 50 wt% of a second solder alloy powder by is composed of. In some embodiments, the solder powder is composed of 75 wt% to 90 wt% of a first solder alloy powder and 10 wt% to 25 wt% of a second solder alloy powder and is composed of.

[0009] In some embodiments, the solder powder is composed of less than 50 wt% of a first solder alloy powder and more than 50 wt% of a second solder alloy powder and is composed of.

[0010] In some embodiments, the solidus temperature of the first solder alloy powder is 300 °C to 360 °C, and the solidus temperature of the second solder alloy powder is 200 °C to 250 °C. In some embodiments, the solidus temperature of the second solder alloy powder is 215 °C to 245 °C. In some embodiments, the liquidus temperature of the first solder alloy powder is 360 °C or less. In some embodiments, the liquidus temperature of the second solder alloy powder is 245 °C or less.

[0011] In some embodiments, the first solder alloy powder is composed of 4 wt% to 10 wt% of Cu, 4 wt% to 20 wt% of Ag, optionally, 0.5 wt% or less of Bi, Co, In, Ge, Ni, P, or Zn, and 0.75 ~ to 1.1 proportion of the balance of Sn and Sb.

[0012] In some embodiments, the second solder alloy powder is SnAg, SnCu, SnAgCu, SnSb, SnAgCuSb, SnAgY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnAgCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), or SnAgCuSbY (Y = Bi, Co, Ge, In, Ni, P 、 or Zn).

[0013] In some embodiments, the second solder alloy powder comprises 0.1 wt% to 4 wt% Ag, 0.1 wt% to 1 wt% Cu, or 0.1 wt% to 11 wt% Sb. In some embodiments, the second solder alloy powder comprises 0.1 wt% to 4 wt% Ag, 0.1 wt% to 1 wt% Cu, and 0.1 wt% to 11 wt% Sb.

[0014] In some embodiments, the second solder alloy powder is doped with Bi, Co, Ge, In, Ni, P, Sb, or Zn.

[0015] In some embodiments, the ratio of the heat absorption from the first melting peak of the solder paste to the heat absorption from the second melting peak of the solder paste is 0.15 or less.

[0016] In one embodiment, the method is a step of applying a solder paste between a substrate and a device to form an assembly, the solder paste , it consists of solder powder and flux, and the solder powder Sn:Sb weight ratio of 0.75 ~ 10 wt% to 90 wt% of a first solder alloy powder composed of a SnSbCuAg solder alloy with a ratio of 1.1, and 10 wt% to 90 wt% of a second solder alloy powder composed of a Sn solder alloy containing at least 80 wt% Sn and Steps of forming, at a peak temperature higher than 320°C, a first reflow soldering process on the assembly to form a solder joint from the solder paste, and after forming the solder joint, at a peak temperature of 230°C to 270°C, performing a second reflow soldering process on the assembly. In some embodiments, the device is a silicon die, the substrate includes a Cu lead frame, and the solder paste is applied between the Cu lead frame and the silicon die. In some embodiments, during the second reflow soldering process, the solder joint maintains a shear adhesion strength of 10 MPa or more.

[0017] In one embodiment, the solder joint is formed in a process including applying a solder paste between the substrate and the device to form an assembly, and reflow soldering the assembly to form the solder joint. The solder paste , it consists of solder powder and flux, and the solder powder has a weight percentage ratio of Sn:Sb of 0.75 ~ is composed of 10% to 90% by weight of a first solder alloy powder composed of a SnSbCuAg solder alloy with a weight percentage ratio of Sn:Sb of 1.1, and 10% to 90% by weight of a second solder alloy powder composed of a Sn solder alloy containing at least 80% by weight of Sn and is composed.

[0018] Other features and aspects of the present invention will become apparent from the following detailed description in conjunction with the accompanying drawings that exemplarily show features according to various embodiments. This summary is not intended to limit the scope of the present invention, which is defined only by the claims appended hereto.

Brief Description of the Drawings

[0019] The technology disclosed herein according to one or more various embodiments will be described in detail with reference to the figures included herein. The figures are provided for illustrative purposes only and merely represent exemplary implementations.

[0020]

Figure 1

[0021]

Figure 2

[0022]

Figure 3

[0023]

Figure 4

[0024] The figures are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It is to be understood that the invention can be practiced with modifications and changes, and that the disclosed technology is limited only by the claims and their equivalents.

Embodiments for Carrying Out the Invention

[0025] To conform to the profile of solder reflow used in the electronics industry, some important characteristics of high - temperature solder can include (i) a solidus temperature of about 260°C or higher (in accordance with a typical SMT solder reflow profile), (ii) good thermal fatigue resistance, (iii) high thermal / electrical conductivity, and / or (iv) low cost. Heretofore, in the power discrete market, high - lead - content solder has been mainstream for die - attach applications.

[0026] Embodiments of the present invention are directed to lead-free solder pastes having a mixed solder powder particularly suitable for high-temperature soldering applications including multiple board-level reflow operations. The solder paste includes two solder powders and a flux. One solder powder has a melting temperature significantly higher than the other solder powder. For example, one solder powder can have a solidus temperature of a first solder alloy from 300° C. to 360° C., and the other solder powder can have a solidus temperature of a second solder alloy from 200° C. to 250° C. The higher solidus temperature (high-temperature) solder powder is a SnSb-based solder alloy having a Sn:Sb weight ratio between about 0.75 and 1.1. The lower solidus temperature (low-temperature) solder powder is a high-Sn solder, i.e., a solder alloy having a Sn content greater than 80 wt %.

[0027] The high-temperature solder powder comprises 50 wt % to 90 wt % of the solder paste, the low-temperature solder powder comprises 10 wt % to 50 wt % of the solder paste, and the flux comprises 9 wt % to 20 wt % of the solder paste. Due to this composition, as further explained below, the high-temperature solder powder can be superior in the high-temperature performance of the final solder joint including the melting behavior and the thermo-mechanical behavior. The ductile high-Sn solder powder can improve the wetting during reflow and strengthen the ductility of the final joint. Further, the presence of the high-Sn solder in the paste can allow for a relatively low reflow temperature.

[0028] As further described below, in accordance with some embodiments of the present invention, by selecting a low-temperature solder paste and a high-temperature solder paste for the solder paste, one or more of the following characteristics can be satisfied. (1) During the reflow process, the solder powders dissolve completely or partially with respect to each other. (2) After reflow soldering, the remaining amount of the low-melting-phase in the final joint is optimized quantitatively, separated by the high-melting-point SnSb matrix, and the mechanical performance of the desired joint can be maintained at a temperature higher than 270°C. (3) The solder joint can withstand the temperature cycle test for typical die attachment applications. In fact, any solder alloy powder can dissolve completely or partially with respect to each other during reflow. This enables continuous dissolution from the high-temperature solder alloy powder into the solution and does not form interfacial IMCs on the powder surface that would arrest dissolution. In the design, the ratio of the low-temperature alloy powder to the high-temperature alloy powder can be optimized such that the low-melting-phase is separated within the high-melting-point matrix while the high-temperature alloy powder can excel in the high-temperature performance of the final joint.

[0029] The solder paste described in this document can be used for various high-temperature soldering applications, including die attachment and surface mounting of component devices.

[0030] Figure 1 is a chart showing shear adhesion strength in megapascals (MPa) as a function of temperature for joints formed with several lead - free bonding materials and high - lead solder. As shown, many of the lead - free solders have higher shear adhesion strength than high - lead solder. High - Sn solder significantly decreases in shear adhesion strength as it approaches the melting point of the solder (about 217 °C for SnAgCu and 243 °C for Sn10Sb). After reaching the melting point, the high - Sn solder melts, so its shear adhesion strength drops to a value close to zero. Thus, due to the lower melting point of high - Sn solder, it may not be able to maintain shear adhesion strength at high melting temperatures. In comparison, high - lead solder has a shear adhesion strength of about 10 Mpa at 250 °C, and at higher temperatures, its shear adhesion strength further decreases. Surface - mount devices containing high - lead solder as a die - attach material have been proven to withstand subsequent SMT reflow at a peak temperature of up to 260 °C. Therefore, the novel solder paste described in this document is designed to maintain a shear adhesion strength exceeding 10 MPa at 270 °C or higher temperatures, being superior to high - lead counterparts to withstand subsequent SMT processes.

[0031] The weight % ratio of Sn:Sb is 0.75 ~ The SnSbCuAgX (X = Bi, Co, Ge, In, Ni, P, Zn, or several other dopants) alloys with a Sn:Sb weight % ratio of 1.1 are a group of high - temperature solder alloys with a solidus / liquidus temperature range between about 300 °C and 360 °C. As shown in Figure 1, the SnSbCuAgX alloys can sufficiently maintain high - temperature shear adhesion strength at temperatures up to about 280 °C. However, the SnSbCuAgX alloys are hard and brittle. In tests of Si on Cu die - attachment, the SnSbCuAgX solder causes the Si die to crack even during temperature - cycle testing or immediately after reflow. This is because the SnSbCuAgX solder is too hard to accommodate the strain due to the mismatch in the thermal expansion coefficients of Si and Cu. Therefore, the SnSbCuAgX solder alloys are themselves unsuitable for die - attachment in die - attachment applications.

[0032] Although the SnSbCuAgX solder alloy and the high-Sn solder alloy may not be individually suitable for high-temperature soldering applications, combining their benefits while minimizing or eliminating each other's weaknesses can provide a viable option for a lead-free solder alloy to replace lead-based solder alloys. According to the present invention, the benefits of both solder alloys are obtained by forming a solder paste that is a combination of high-Sn alloy powder, SnSbCuAgX solder alloy powder, and flux.

[0033] The solder paste described in this document can be reflowed to form a solder joint. The reflowed solder paste demonstrates the benefits of both solder alloys, namely, the ductility of the high-Sn alloy and the high-temperature properties of the SnSbCuAgX solder alloy. To obtain these benefits, the ratio of high-Sn powder to SnSbCuAgX powder may be selected such that the final joint exhibits (1) good mechanical strength (>10 MPa) at a temperature of 270°C or higher, and (2) good ductility to accommodate strain due to the mismatch between the Si die and Cu. To obtain these benefits, a solder joint can be formed in a form where the high-Sn phase of the low-temperature solder alloy, similar to a composite material, is embedded in the SnSb matrix of the high-temperature solder alloy. The separated high-Sn phase within the SnSb matrix will not significantly affect the high-temperature strength of the solder joint because most of the solder joint consists of the SnSb matrix and strengthened Cu-containing and Ag-containing particles. At the same time, the ductile high-Sn phase can be easily deformed by creep to at least partially accommodate the strain due to the CTE mismatch between the Si die and the Cu lead frame.

[0034] As described above, the high-temperature solder alloy that makes up the majority (>50 wt%) in the solder paste can have superior high-melting-point performance at about 270°C or higher. To obtain this benefit, the ratio of Sn to Sb in the high-temperature solder alloy is about 0.75 ~It can be maintained at 1.1, forming the intermetallic compound Sn3Sb2 and the SnSb solid solution (β), and having a melting temperature between about 323 °C and 360 °C. The above is shown by the SnSb phase diagram in Figure 2. To increase ductility, Cu and Ag are alloyed with the SnSb alloy, whereby Ag3Sn particles, Cu6Sn5 particles, and Sn3Sb2 particles precipitate in the SnSb matrix, which can slow down the movement of dislocations and subsequent crack nucleation and growth. However, depending on the amount of Cu and Ag added, the solidus temperature of the solder alloy can be maintained at around 300 °C depending on the content. Therefore, in some embodiments, the high melting point solder alloy preferably has from about 4 wt% to 10 wt% of Cu and from about 4 wt% to 20 wt% of Ag.

[0035] Table 1 below shows examples of SnSbCuAg alloys that can be used for high-temperature solder alloys according to embodiments of the present invention. In some embodiments, dopants of less than 0.5%, such as Bi, Co, In, Ge, Ni, P, or Zn, may be added to the SnSbCuAg alloy to stabilize the deposits and interfacial IMCs.

Table 1

[0036] As described above, the lower melting point solder alloy can be a high-Sn solder alloy (Sn > 80 wt%). For example, the alloy can be SnAg, SnCu, SnAgCu, SnSb, SnAgY (Y = Bi, Co, Ge, In, Ni, P, Sb or other dopants), SnCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or other dopants), or SnAgCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, Zn, or other dopants). The solidus temperature of the low-temperature solder alloy can range from about 200°C to 250°C. Table 2 below shows examples of high-Sn alloys that can be used for low-temperature solder alloys according to embodiments of the present invention. In some embodiments, additives of Bi, In, and / or Ni can be included in the solder alloy to enhance ductility or improve wetting performance. [Table 2]

[0037] In some embodiments, the high-temperature solder powder contains 10 wt% to 90 wt% of the solder paste, and the low-temperature solder powder contains 10 wt% to 90 wt% of the solder paste. To maintain the high-temperature performance and ductility of the final solder joint, the ratio of the first solder alloy to the second solder alloy can be adjusted. If the ratio (by weight) of the first solder alloy to the second solder alloy is insufficient, the high-temperature performance of the final solder joint may not be maintained. On the other hand, if the ratio (by weight) of the first solder alloy to the second solder alloy is more than necessary, the solder joint may become too hard and may lead to the breakage of the Si die. Therefore, the relative ratio of the first solder alloy to the second solder alloy in the paste may be selected to satisfy both high-temperature performance and sufficient ductility. For this purpose, in some embodiments, the high-temperature solder powder may preferably contain 50 wt% to 90 wt% of the solder paste, and the low-temperature solder powder may contain 10 wt% to 50 wt% of the solder paste. In some embodiments, the high-temperature solder powder may preferably contain 75 wt% to 90 wt% of the solder paste, and the low-temperature solder powder may contain 10 wt% to 25 wt% of the solder paste. In some embodiments, the low-temperature solder powder may contain more than 50 wt% of the solder paste. Such embodiments of the solder paste may be particularly suitable for power module applications that do not necessarily require high-temperature strength.

[0038] Table 3 shows the solder alloy element composition, (wt%) of the solder paste according to the present invention and the solder element composition (wt%) of the reflow solder joint. This solder paste can be used for HTLF (high-temperature lead-free) die attach applications or other suitable high-temperature soldering applications. As shown in these examples, the ratio of the first solder alloy to the second solder alloy may be in the range of 9:1 to 5:5.

Table 3

[0039] In some embodiments including the use of solder paste in a power discrete device / application, the ratio of Sn:Sb in the solder joint formed from the solder paste can be approximately from 1:1 to 1.3:1. In some embodiments including the use of solder paste in a power module device / application, the ratio of Sn:Sb in the solder joint formed from the solder paste can be approximately from 1:1 to 1.8:1.

[0040] Figure 3 is a chart showing the shear adhesion strength (MPa) at different test temperatures (°C) of solder joints formed from five different solder pastes. Specifically, Figure 3 shows the shear adhesion strength of the first five solder pastes in Table 3, where 874-33-1 represents the first solder paste in Table 1 and 847-33-2 represents the second solder paste in Table 3. As shown here, three of the five solder pastes (874-33-1, 874-33-2, and 874-33-3) maintain a shear adhesion strength of at least 10 MPa at 270 °C or higher.

[0041] Figure 4 shows the differential scanning calorimetry (DSC) heating curves measured by TA Q2000 DSC for the five solder pastes shown in Figure 3. As illustrated, for the three solder pastes (874-33-1, 874-33-2, and 874-33-3) that maintain good high-temperature mechanical strength, the ratio of H1 / H2 (H1: heat absorption from the first peak of the DSC, and H2: heat absorption from the second peak of the DSC) is less than 0.09, while the ratio of H1 / H2 for the other two solder pastes (874-33-4 and 874-33-5) is higher than 0.10. Table 4 below shows the characteristics of the DSC curves.

Table 4

[0042] Based on DSC tests conducted on solder pastes according to the present invention, it has been found that a more preferred ratio of the heat absorption from the first melting peak of the solder paste to the heat absorption from the second melting peak of the solder paste is about 0.15 or lower. At this ratio, joints formed from the solder paste can maintain sufficient high-temperature shear strength (even at 280 °C, > 15 MPa), and good ductility (e.g., by having a sufficient amount of high-melting-point solder powder). In an embodiment, the selection of the solder powder and the relative weight % of the solder powder can be based on the ratio of H1 / H2.

[0043] The embodiments described herein do not rely on an increase in the melting point of the solder joint through the formation of IMCs on the surface of a high-melting-point alloy by consuming a low-melting-point alloy, in contrast to the TLPB technique. In a TLPB design, during reflow, the low-melting-point alloy is consumed and IMCs are formed and grow on the surface of the melting metal / alloy powder. As the thickness of the IMC layer increases, the growth rate becomes significantly slower. In contrast, solder joints formed by the solder pastes described herein do not rely on IMC formation during reflow to achieve high-temperature performance. Rather, high-temperature performance can be obtained by selecting an appropriate percentage of the high-temperature melting phase in the final joint and, after solidification, limiting the low-temperature melting phase to the high-temperature melting phase.

[0044] Although various embodiments of the disclosed technology have been described above, it should be understood that they are presented by way of example only and not by way of limitation. Similarly, although the various figures may depict an exemplary architecture or other configuration for the disclosed technology, this is done to assist in the understanding of the features and functions that may be included in the disclosed technology. The illustrated embodiments and their various alternatives can be implemented without being limited to the illustrated examples. Further, with respect to the flow diagrams, operational descriptions, and method claims, the order in which steps are presented herein is not intended to obligate the various embodiments to be implemented so as to perform the recited functions in the same order, unless the context otherwise indicates.

[0045] The disclosed technology has been described from the perspective of various exemplary embodiments and implementations, but the various features, aspects, and functionalities described in one or more of the individual embodiments are not limited in their applicability to the particular embodiments in which they are described. Instead, such embodiments can be applied singly or in various combinations to one or more of the other embodiments of the disclosed technology, regardless of whether such embodiments are described and regardless of whether such features are shown as being part of the embodiments in which they are described. Accordingly, the breadth and scope of the technology disclosed herein should not be limited by any of the above-described exemplary embodiments.

[0046] The terms and phrases used in this book, and variations thereof, should be construed as open-ended and not limiting, unless specifically stated otherwise. By way of example, the term "including" should be read to mean "including without limitation"; the term "example" is used to provide an illustrative example of the item under discussion and is not an exhaustive or limiting list; the terms "a" or "an" should be read to mean "at least one", "one or more", etc. Also, adjectives such as "conventional", "traditional", "usual", "standard", "known", and terms of similar meaning should not be construed as limiting the item described to items available at a given period or point in time, but should be read to include prior art, conventional techniques, ordinary techniques, or standard techniques that are available or known at any point in the present or future. Similarly, when this book refers to techniques that are obvious or known to those skilled in the art, such techniques include those that are obvious or known to those skilled in the art at any point in the present or future.

[0047] The presence of broad terms such as "one or more", "at least", "but not limited to", or other similar phrases in an example should not be read as a substitution for a narrow case being intended or required in an example where such broad phrases may not be present.

Claims

1. A solder paste comprising solder powder and a flux, wherein the solder powder is 50% to 90% by weight of a first solder alloy powder, and the first solder alloy powder is 4% to 10% by weight of Cu, 4% to 20% by weight of Ag, optionally, 0.5% by weight or less of Bi, Co, In, Ge, Ni, P, Zn or other dopants, and the balance being Sn and Sb in a ratio of 0.75 to 1.1, a first solder alloy powder; and 10% to 50% by weight of a second solder alloy powder composed of a Sn-based solder alloy containing at least 80% by weight of Sn, the second solder alloy powder being SnAg, SnCu, SnAgCu, SnSb, SnAgCuSb, SnAgY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnAgCuY (Y = Bi, Co, Ge, In, Ni, P, Sb or Zn), or SnAgCuSbY (Y = Bi, Co, Ge, In, Ni, P, or Zn), a second solder alloy powder, wherein the solidus temperature of the first solder alloy powder is 300°C to 360°C and the solidus temperature of the second solder alloy powder is 200°C to 250°C. A solder paste.

2. The solder paste according to claim 1, wherein the solder powder comprises 75% to 90% by weight of the first solder alloy powder and 10% to 25% by weight of the second solder alloy powder.

3. The solder paste according to claim 1, wherein the solidus temperature of the second solder alloy powder is 215°C to 245°C.

4. The solder paste according to claim 1, wherein the liquidus temperature of the first solder alloy powder is 360°C or less.

5. The solder paste according to claim 1, wherein the liquidus temperature of the second solder alloy powder is 250°C or less.

6. The solder paste according to claim 3, wherein the liquidus temperature of the second solder alloy powder is 245°C or less.

7. The solder paste according to claim 1, wherein the second solder alloy powder contains 0.1% to 4% by weight of Ag, 0.1% to 1% by weight of Cu, or 0.1% to 11% by weight of Sb.

8. The second solder alloy powder contains 0.1 wt% to 4 wt% of Ag, 0.1 wt% to 1 wt% of Cu, and 0.1 wt% to 11 wt% of Sb, and the solder paste according to claim 7.

9. The second solder alloy powder of the solder paste according to claim 8 is doped with Bi, Co, Ge, In, Ni, P, or Zn.

10. For the solder paste according to claim 1, the ratio of the heat absorption from the first melting peak of the solder paste to the heat absorption from the second melting peak of the solder paste is 0.15 or less.

11. A step of applying a solder paste between a substrate and a device to form an assembly, wherein the solder paste consists of a solder powder and a flux, The solder powder is 50 wt% to 90 wt% of a first solder alloy powder, wherein the first solder alloy powder consists of 4 wt% to 10 wt% of Cu, 4 wt% to 20 wt% of Ag, optionally 0.5 wt% or less of Bi, Co, In, Ge, Ni, P, Zn or other dopants, and the balance of Sn and Sb in a ratio of 0.75 to 1.1; a first solder alloy powder; 10 wt% to 50 wt% of a second solder alloy powder composed of a Sn solder alloy containing at least 80 wt% of Sn, wherein the second solder alloy powder is SnAg, SnCu, SnAgCu, SnSb, SnAgCuSb, SnAgY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnAgCuY (Y = Bi, Co, Ge, In, Ni, P, Sb or Zn), or SnAgCuSbY (Y = Bi, Co, Ge, In, Ni, P, or Zn); a second solder alloy powder; and a step; Performing a first reflow soldering process on the assembly at a peak temperature higher than 320 °C so as to form a solder joint from the solder paste; After forming the solder joint, performing a second reflow soldering process on the assembly at a peak temperature of 230 °C to 270 °C, and the method. The solidus temperature of the first solder alloy powder is 300 °C to 360 °C, and the solidus temperature of the second solder alloy powder is 200 °C to 250 °C.

12. The method according to claim 11, wherein the device is a silicon die, the substrate includes a Cu lead frame, and the solder paste is applied between the Cu lead frame and the silicon die.

13. The method according to claim 11, wherein during the second reflow soldering process, the solder joint maintains a shear adhesion strength of 10 MPa or more.

14. A soldering method comprising: applying a solder paste between a substrate and a device to form an assembly; reflow soldering the assembly to form a solder joint, wherein the solder paste comprises a solder powder and a flux; the solder powder is 50 wt% to 90 wt% of a first solder alloy powder, the first solder alloy powder comprising 4 wt% to 10 wt% Cu, 4 wt% to 20 wt% Ag, optionally up to 0.5 wt% Bi, Co, In, Ge, Ni, P, Zn or other dopants, and the balance Sn and Sb in a ratio of 0.75 to 1.1; a first solder alloy powder; is 10 wt% to 50 wt% of a second solder alloy powder composed of a Sn solder alloy containing at least 80 wt% Sn, the second solder alloy powder being SnAg, SnCu, SnAgCu, SnSb, SnAgCuSb, SnAgY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnCuY (Y = Bi, Co, Ge, In, Ni, P, Sb, or Zn), SnAgCuY (Y = Bi, Co, Ge, In, Ni, P, Sb or Zn), or SnAgCuSbY (Y = Bi, Co, Ge, In, Ni, P, or Zn); a second solder alloy powder; The soldering method, wherein the solidus temperature of the first solder alloy powder is 300 °C to 360 °C and the solidus temperature of the second solder alloy powder is 200 °C to 250 °C.

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