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

Abstract

Some embodiments of the present disclosure relate to lead-free solder pastes with mixed solder powders, which are particularly suitable for high temperature soldering applications involving multi-board reflow operations. In one embodiment, the solder paste is composed of: 10 wt% to 90 wt% of a first solder alloy powder, the first solder alloy powder consisting of a SnSbCuAg solder alloy having a Sn:Sb wt% ratio of 0.75 to 1.1; 10 wt% to 90 wt% of a second solder alloy powder, the second solder alloy powder consisting of a Sn solder alloy including at least 80 wt% Sn; and a balance of flux.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 017,469, filed on April 29, 2020, and entitled “LEAD-FREE SOLDER PASTE WITH MIXED SOLDER POWDER FOR HIGH TEMPERATURE APPLICATIONS,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure relates to lead-free solder paste. Background Art

[0004] Lead (Pb) generated by the disposal of electronic components is considered to be harmful to the environment and human health. Regulations increasingly prohibit the use of lead-based solders in the electronic interconnection and electronic packaging industries. The Restriction of Hazardous Substances Directed (RoHS) directive, which was promulgated on July 1, 2006, has successfully led to the replacement of SnPb solder alloys with lead-free solder alloys. Solders based on SnAg, SnCu and SnAgCu (SAC) have become commonly used solders for forming interconnections in the semiconductor and electronics industries. However, the development of high melting temperature lead-free solders to replace conventional high-lead solders (e.g., Pb-5Sn and Pb-5Sn-2.5Ag) is still in its early stages.

[0005] A common use of high melting temperature solder is the die attachment of semiconductor surface mounted devices / components, where the chip will be bonded between leads or on a lead frame. These surface mounted components / devices are then used for one or more subsequent board level soldering processes. For example, a silicon chip is soldered to a lead frame using a high melting temperature solder to form an assembly. Subsequently, the device (packaged or unpackaged) is attached to a printed wiring board (PWB) by board level reflow. The same board may be exposed to multiple reflows. During the entire process, the internal connection between the silicon chip and the lead frame should remain good. This requires that the high melting temperature solder be able to withstand multiple reflow soldering processes without causing any functional failures.

[0006] Transient liquid phase bonding (TLPB) technology aims to achieve higher remelting temperatures of solder joints by forming intermetallic compounds (IMCs) between low melting temperature alloys and high melting temperature alloys. In the TLPB design, the low melting phase is mostly or even completely consumed during reflow to achieve the high melting temperature target. During reflow, interfacial IMC formation and continued IMC growth on the surface of the high melting alloy occur at the expense of both the low melting alloy and the high melting alloy. Summary of the invention

[0007] Some implementations of the present disclosure are directed to a lead-free solder paste containing a mixed solder powder that is particularly suitable for high temperature soldering applications involving multi-board level reflow operations.

[0008] In one embodiment, the solder paste is composed of: 10 wt % to 90 wt % of a first solder alloy powder consisting of a SnSbCuAg solder alloy having a Sn:Sb wt % ratio of 0.75 to 1.1; 10 wt % to 90 wt % of a second solder alloy powder consisting of a Sn solder alloy containing at least 80 wt % of Sn; and a balance of flux.

[0009] In some implementations, the solder paste is composed of 50wt% to 90wt% of the first solder alloy powder, 10wt% to 50wt% of the second solder alloy powder, and the balance flux. In some implementations, the solder paste is composed of 75wt% to 90wt% of the first solder alloy powder, 10wt% to 25wt% of the second solder alloy powder, and the balance flux.

[0010] In some implementations, the solder paste is composed of less than 50 wt % of the first solder alloy powder, more than 50 wt % of the second solder alloy powder, and the balance of flux.

[0011] In some implementations, the solidus temperature of the first solder alloy powder is 300°C to 360°C, and wherein the solidus temperature of the second solder alloy powder is 200°C to 250°C. In some implementations, the solidus temperature of the second solder alloy powder is 215°C to 245°C. In some implementations, the liquidus temperature of the first solder alloy powder is no greater than 360°C. In some implementations, the liquidus temperature of the second solder alloy powder is no greater than 250°C. In some implementations, the liquidus temperature of the second solder alloy powder is no greater than 245°C.

[0012] In some implementations, the first solder alloy powder consists of: 4 wt % to 10 wt % Cu; 4 wt % to 20 wt % Ag; optionally, 0.5 wt % or less Bi, Co, In, Ge, Ni, P or Zn; and the balance Sn and Sb in a ratio of 0.75 to 1.1.

[0013] In some implementations, 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, Sb or Zn).

[0014] In some implementations, the second solder alloy powder includes 0.1 to 4 wt % Ag, 0.1 to 1 wt % Cu, or 0.1 to 11 wt % Sb. In some implementations, the second solder alloy powder includes 0.1 to 4 wt % Ag, 0.1 to 1 wt % Cu, and 0.1 to 11 wt % Sb.

[0015] In some implementations, the second solder alloy powder is doped with Bi, Co, Ge, In, Ni, P, Sb, or Zn.

[0016] In some implementations, a ratio of the heat absorption of the first melting peak of the solder paste to the heat absorption of the second melting peak of the solder paste is 0.15 or less.

[0017] In one embodiment, the method includes: applying a solder paste between a substrate and a device to form an assembly, wherein the solder paste is composed of: 10 wt% to 90 wt% of a first solder alloy powder, the first solder alloy powder being composed of a SnSbCuAg solder alloy having a wt% ratio of Sn:Sb of 0.75 to 1.1; 10 wt% to 90 wt% of a second solder alloy powder, the second solder alloy powder being composed of a Sn solder alloy including at least 80 wt% Sn; and a balance of flux; subjecting the assembly to a first reflow soldering process at a peak temperature above 320°C to form a solder joint from the solder paste; and after forming the solder joint, subjecting the assembly to a second reflow soldering process at a peak temperature of 230°C to 270°C. In some implementations, the device is a silicon chip, the substrate includes a Cu lead frame, and the solder paste is applied between the Cu lead frame and the silicon chip. In some implementations, the solder joint maintains a bonding shear strength greater than or equal to 10 MPa during the second reflow soldering process.

[0018] In one embodiment, a solder joint is formed by the following process: applying a solder paste between a substrate and a device to form an assembly; and reflow soldering the assembly to form a solder joint; wherein the solder paste is composed of: 10wt% to 90wt% of a first solder alloy powder, the first solder alloy powder being composed of a SnSbCuAg solder alloy having a Sn:Sb wt% ratio of 0.75 to 1.1; 10wt% to 90wt% of a second solder alloy powder, the second solder alloy being composed of a Sn solder alloy including at least 80% Sn, and a balance of flux.

[0019] Other features and aspects of the present disclosure will become apparent from the following detailed description in conjunction with the accompanying drawings, which illustrate features according to various embodiments by way of example.This summary is not intended to limit the scope of the invention, which is defined solely by the claims appended hereto. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] According to one or more different implementations, the technology disclosed herein will be described in detail with reference to the included drawings. The drawings are provided for illustration purposes only and merely depict example implementations.

[0021] Figure 1 is a graph showing the bond shear strength (MPa) as a function of temperature for several lead-free bonding materials and high-lead solder.

[0022] Figure 2 The phase diagram of SnSb is depicted.

[0023] Figure 3 The figure is a graph of the bonding shear strength (MPa) of five different solder pastes at different test temperatures (°C).

[0024] Figure 4 Shows the Figure 3 Depicted are differential scanning calorimeter (DSC) heating curves of the five solder pastes performed using a TA Q2000 DSC.

[0025] These drawings are not intended to be exhaustive or to limit the invention to the precise forms disclosed. It should be understood that the invention can be practiced with modification and alteration and that the disclosed technology is limited only by the claims and their equivalents. DETAILED DESCRIPTION

[0026] To be compatible with solder reflow profiles used in the electronics industry, some important properties of high temperature solders may include: (i) solidus temperatures of about 260°C and above (based on a typical SMT solder reflow profile), (ii) good thermal fatigue resistance, (iii) high thermal / electrical conductivity, and / or (iv) low cost. To date, high lead content solders still dominate die attach applications in the power discrete market.

[0027] Implementations of the present disclosure relate to lead-free solder pastes having mixed solder powders that are particularly suitable for high temperature soldering applications involving multi-board level reflow operations. The solder paste includes two solder powders and a flux. One of the solder powders has a significantly higher melting temperature than the other solder powder. For example, one of the solder powders may have a solidus temperature of a first solder alloy of 300 to 360°C, while the other of the solder powders may have a solidus temperature of a second solder alloy of 200 to 250°C. The higher solidus temperature (higher 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 (lower temperature) solder powder is a Sn-rich solder or solder alloy in which the Sn content is greater than 80 weight percent.

[0028] The higher temperature solder powder accounts for 50wt% to 90wt% of the solder paste, the lower temperature solder powder accounts for 10wt% to 50wt% of the solder paste, and the flux accounts for 9wt% to 20wt% of the solder paste. As further described below, due to the composition, the higher temperature solder powder can dominate the high temperature properties of the final bonded joint, including melting and thermomechanical behavior. The ductile Sn-rich solder powder can improve wettability during reflow and enhance the ductility of the final joint. In addition, the presence of Sn-rich solder in the solder paste can allow for relatively low reflow temperatures.

[0029] According to some implementations of the present disclosure, as further described below, the selection of the lower temperature solder powder and the higher melting temperature solder powder in the solder paste can be performed to meet one or more of the following properties: (1) the solder powders fully or partially dissolve into each other during the reflow process; (2) the remaining low melting phase in the final joint after reflow soldering is optimized in quantity and isolated by the high melting SnSb matrix to maintain the desired joint mechanical properties at temperatures above 270°C; and (3) the solder joint is able to withstand temperature cycling tests for typical chip attach applications. In fact, during reflow, the two solder alloy powders can fully or partially dissolve into each other. This allows the high temperature solder alloy powder to continuously dissolve into the liquid solution without interfacial IMC formation on the powder surface to slow down the dissolution. In the design, the ratio of the lower temperature alloy powder to the higher temperature alloy powder can be optimized to allow the higher temperature alloy powder to dominate the high temperature properties of the final joint while isolating the low melting phase in the high melting phase matrix.

[0030] The solder pastes described herein can be used in a variety of high temperature soldering applications including chip attach and surface mounting of component devices.

[0031] Figure 1is a graph showing the bond shear strength (megapascals (MPa)) of joints formed by several lead-free bonding materials and high-lead solders as a function of temperature. As depicted, most lead-free solders have higher bond shear strengths than high-lead solders. The bond shear strength of Sn-rich solders drops significantly when approaching the melting temperature of the solder (about 217°C for SnAgCu and about 243°C for Sn10Sb). After reaching the melting temperature, the bond shear strength of Sn-rich solders drops to near zero because they become molten. Therefore, due to the lower melting temperature of Sn-rich solders, they may not be able to maintain any bond shear strength at high melting temperatures. In contrast, high-lead solders have a bond shear strength of about 10Mpa at 250°C, and at higher temperatures, the bond shear strength drops even lower. Surface mount devices using high-lead solders as chip attach materials have been shown to be able to withstand subsequent SMT reflow at peak temperatures up to 260°C. Therefore, the novel solder paste described in this paper is designed to maintain a bond shear strength of more than 10 MPa, superior to the high-lead solder paste control, even at a temperature of 270°C or higher, to withstand subsequent SMT processes.

[0032] SnSbCuAgX (X = Bi, Co, Ge, In, Ni, P, Zn or some other dopant) alloys with a Sn:Sb wt% ratio of about 0.75 to 1.1 are a group of high temperature solder alloys with a solidus / liquidus temperature range of about 300°C to 360°C. Figure 1 As depicted, SnSbCuAgX alloys are able to maintain high temperature bond shear strength well at temperatures up to about 280°C. However, SnSbCuAgX alloys are rigid and brittle. For Si die attach testing on Cu, SnSbCuAgX solders will cause Si chips to break during temperature cycling testing, or even just after reflow, because they are too hard to withstand the strain caused by the mismatch in the coefficient of thermal expansion (CTE) between Si and Cu. Therefore, SnSbCuAgX solder alloys themselves are not suitable for die attach in die attach applications.

[0033] Although SnSbScuAgX solder alloy and Sn-rich solder alloy may not be suitable for high temperature soldering applications alone, if the advantages of the two solder alloys can be combined while minimizing or eliminating their respective disadvantages, the combination can be a suitable lead-free replacement for lead-based solder alloys. According to the present disclosure, the advantages of the two solder alloys can be achieved by forming a solder paste that is a combination of Sn-rich solder alloy powder, SnSbCuAgX solder alloy powder and flux.

[0034] The solder paste described herein can be reflowed to form a solder joint. The reflowed solder joint can demonstrate the advantages of both solder alloys: the ductility of the Sn-rich alloy and the high temperature performance of the SnSbCuAgX solder alloy. To achieve these advantages, the ratio of Sn-rich powder to SnSbCuAgX powder can be selected so that the final joint exhibits: (1) good mechanical strength (>10MPa) at 270°C or even higher; (2) and good ductility to accommodate the strain caused by the CTE mismatch between the Si chip and Cu. To achieve these advantages, the solder joint can be formed into a morphology in which the Sn-rich phase of the low-temperature solder alloy is embedded in the SnSb matrix of the high-temperature solder alloy, similar to a composite material. The Sn-rich phase isolated in the SnSb matrix may not significantly affect the high temperature strength of the solder joint because the majority of the solder joint is the SnSb matrix and the reinforced Cu-containing and Ag-containing particles. At the same time, the ductile Sn-rich phase can easily deform by creep to accommodate the strain caused at least in part by the CTE mismatch between the Si chip and the Cu lead frame.

[0035] As described above, the majority (>50wt%) of the higher temperature solder alloy in the solder paste can dominate the high melting temperature performance at temperatures of about 270°C and above. To achieve this advantage, the ratio between Sn and Sb in the higher temperature solder alloy can be maintained at about 0.75 to 1.1, wherein Sn3Sb2 intermetallic compounds and SnSb solid solutions (β) are formed, and have a melting temperature between about 323°C and 360°C. This is due to Figure 2 As depicted in the SnSb phase diagram. In order to improve ductility, Cu and Ag are alloyed into the SnSb alloy so that Ag3Sn and Cu6Sn5 particles are precipitated inside the SnSb matrix together with Sn3Sb2 particles to slow down dislocation movement 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 approximately 300°C depending on the content. Therefore, in some implementations, it is preferred that the high melting temperature solder alloy has about 4wt% to 10wt% Cu and about 4wt% to 20wt% Ag.

[0036] Table 1 below shows examples of SnSbCuAg alloys that can be used for higher temperature solder alloys according to implementations of the present disclosure. In some implementations, less than 0.5% dopants such as Bi, Co, In, Ge, Ni, P, or Zn can be added to the SnSbCuAg solder alloy to stabilize precipitation and interfacial IMC.

[0037] Table 1

[0038] # Sn(wt%) Sb(wt%) Cu(wt%) Ag (wt%) 1 38 38 7 17 2 38 38 10 14 3 38 42 5 15 4 38 42 8 12 5 39 41 5 15 6 39 41 8 12 7 39 41 10 10 8 40 40 5 15 9 40 40 8 12 10 40 41 5 14 11 40 41 8 11 12 41 39 8 12 13 41 39 10 10 14 41 41 5 13 15 41 41 8 10 16 42 42 6 12 17 42 42 5 13 18 42 43 5 10 19 42 43 8 7 20 43 42 5 10 21 43 43 5 9 22 43 43 8 6 23 44 42 5 9 24 44 42 8 6 25 44 43 5 8 26 44 44 5 7 27 44 44 8 4

[0039] As discussed above, the lower melting temperature solder alloy may be a Sn-rich solder alloy (Sn>80wt%). For example, the alloy may be SnAg, SnCu, SnAgCu, SnSb, SnAgY (Y=Bi, Co, Ge, In, Ni, P, Sb or some other dopant), SnCuY (Y=Bi, Co, Ge, In, Ni, P, Sb or some other dopant), or SnAgCuY (Y=Bi, Co, Ge, In, Ni, P, Sb, Zn or some other dopant). The solidus temperature of the lower temperature solder alloy may be in the range of about 200°C to 250°C. Table 2 below shows examples of Sn-rich alloys that may be used for the lower temperature solder alloy according to the present disclosure. In some implementations, additives of Bi, In and / or Ni may be included in the solder alloy to enhance its ductility or improve wetting properties.

[0040] Table 2

[0041]

[0042] In some implementations, the higher temperature solder powder may account for 10wt% to 90wt% of the solder paste, and the lower temperature solder powder may account for 10wt% to 90wt% of the solder paste. In order 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 may be adjusted. If the wt% of the first solder alloy relative to the second solder alloy is insufficient, the final solder joint may not be able to maintain high temperature performance. On the other hand, if the wt% of the first solder alloy relative to the second solder alloy is sufficient, the solder joint may be too hard and cause damage to the Si chip. Therefore, the relative ratio of the first solder alloy and the second solder alloy in the solder paste can be selected so that high temperature performance and sufficient ductility can be met. To this end, in some implementations, the higher temperature solder powder may more preferably account for 50wt% to 90wt% of the solder paste, and the lower temperature solder powder may account for 10wt% to 50wt% of the solder paste. In some implementations, the higher temperature solder powder may more preferably account for 75wt% to 90wt% of the solder paste, and the lower temperature solder powder may account for 10wt% to 25wt% of the solder paste. In some implementations, the lower temperature solder powder may account for more than 50wt% of the solder paste. This implementation of the solder paste may be particularly suitable for power module applications that do not necessarily require high temperature strength.

[0043] Table 3 shows the solder alloy elemental composition (in wt%) of the solder paste according to the present disclosure and the joint solder elemental composition (in wt%) of the reflow soldering. The solder paste can be used for HTLF (high temperature lead-free) chip attach applications or other suitable high temperature soldering applications. As depicted in these embodiments, the ratio of the first solder alloy to the second solder alloy can be in the range of about 9:1 to 5:5.

[0044] Table 3

[0045]

[0046]

[0047] In some implementations involving use of the solder paste in power discrete devices / applications, the ratio of Sn:Sb in the solder joints formed by the solder paste may be approximately 1:1 to 1.3: 1. In some implementations involving use of the solder paste in power module devices / applications, the ratio of Sn:Sb in the solder joints formed by the solder paste may be approximately 1:1 to 1.8:1.

[0048] Figure 3 is a graph showing the bond shear strength (MPa) of solder joints formed by five different solder pastes at different test temperatures (°C). Specifically, Figure 3 The bond shear strengths of the first five solder pastes in Table 3 are shown, where 874-33-1 refers to the first solder paste in Table 1, 874-33-2 refers to the second solder paste in Table 3, etc. As depicted, three out of five solder pastes (874-33-1, 874-33-2, and 874-33-3) maintain a bond shear strength of at least 10 MPa at temperatures of 270°C and above.

[0049] Figure 4 The data measured by TA Q2000 DSC are shown. Figure 3 Differential Scanning Calorimeter (DSC) heating curves of five solder pastes are depicted. As shown, for 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 of the first peak of DSC, and H2: heat absorption of the second peak) is less than 0.09, while the ratio of H1 / H2 of the other two solder pastes (874-33-4 and 874-33-5) is higher than 0.10. The following Table 4 shows the characteristics of the DSC curve.

[0050] Table 4

[0051]

[0052] Based on DSC testing of solder pastes according to the present disclosure, it was found that a more preferred ratio of the heat absorption of the first melting peak of the solder paste to the heat absorption of the second melting peak of the solder paste was about 0.15 or less. At these ratios, it was found that joints formed from the solder pastes can maintain sufficient high temperature shear strength (greater than 15 MPa even at 280°C) (e.g., from having a sufficient amount of higher melting temperature solder powder) and good ductility (e.g., from having a sufficient amount of Sn-rich powder). In implementation, the selection of solder powders and the relative wt.% of the solder powders can be based on the H1 / H2 ratio.

[0053] In contrast to the TLPB technique, the implementation described herein does not rely on increasing the melting temperature of the solder joint by forming IMCs on the surface of the high melting alloy at the expense of the low melting alloy. In the TLPB design, during reflow, IMCs form and grow on the surface of the high melting metal / alloy powder at the expense of the low melting alloy. The growth rate slows significantly as the thickness of the IMC layer increases. In contrast, the solder joints formed by the solder pastes described herein do not rely on the formation of IMCs during reflow to achieve high temperature performance. Instead, high temperature performance is achieved by selecting the appropriate percentage of the high melting phase in the final joint and confining the low melting phase with the high melting phase after solidification.

[0054] Although various embodiments of the disclosed technology have been described above, it should be understood that they are presented in an exemplary and non-restrictive manner. Similarly, various figures may depict example architectures or other configurations of the disclosed technology, which are used to help understand the features and functions that may be included in the disclosed technology. The illustrated embodiments and various alternatives thereof may be implemented without limitation to the illustrated embodiments. In addition, with respect to flow charts, operating instructions, and method claims, unless the context otherwise dictates, the order of the steps presented herein should not mandate the implementation of the various embodiments in the same order to perform the functions described.

[0055] Although the disclosed technology is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functions described in one or more individual embodiments are not limited to their applicability to the specific embodiments described, but may be applied alone or in various combinations to one or more of the other embodiments of the disclosed technology, whether or not such embodiments are described and whether or not such features are presented as part of such embodiments. Therefore, the breadth and scope of the technology disclosed herein should not be limited by any of the exemplary embodiments described above.

[0056] Unless expressly stated otherwise, the terms and phrases used in this document and their variations should be interpreted as open-ended and not restrictive. As examples of the foregoing: "including" should be understood as "including but not limited to" and the like; the term "embodiment" is used to provide an illustrative example of the item in question, rather than an exhaustive or limiting list thereof; the term "a" or "an" should be understood as "at least one", "one or more", and the like; adjectives and terms of similar meaning such as "conventional", "traditional", "normal", "standard", "known", etc. should not be interpreted as limiting the items described to a given time period or limiting the items available at a given time, but should be understood to include conventional, traditional, normal, or standard technologies that are available or known at any time now or in the future. Similarly, if this document refers to technology that is obvious or known to a person of ordinary skill in the art, such technology includes those that are obvious or known to a person of ordinary skill in the art at any time now or in the future.

[0057] In some cases, the presence of expanded words and phrases such as "one or more," "at least," "but not limited to," or other similar phrases should not be understood to mean that a narrower context is intended or required where such expanded phrases may not be present.

Claims

1. A solder paste, comprising: 50 wt % to 90 wt % of a first solder alloy powder; 10 wt % to 50 wt % of a second solder alloy powder; and 9wt% to 20wt% flux, wherein the sum of the percentage contents of each component of the solder paste is equal to 100%, and wherein the first solder alloy powder is composed of a SnSbCuAg solder alloy having a Sn:Sb wt% ratio of 0.75 to 1.1, and The second solder alloy powder consists of a Sn solder alloy including at least 80 wt % of Sn. 2 . The solder paste according to claim 1 , wherein the first solder alloy powder is 75 wt % to 90 wt %, and the second solder alloy powder is 10 wt % to 25 wt %. 3 . The solder paste according to claim 1 , wherein the solidus temperature of the first solder alloy powder is 300° C. to 360° C., and wherein the solidus temperature of the second solder alloy powder is 200° C. to 250° C. 4 . The solder paste according to claim 3 , wherein the solidus temperature of the second solder alloy powder is 215° C. to 245° C. 5 . The solder paste according to claim 3 , wherein the liquidus temperature of the first solder alloy powder is not greater than 360° C. 6 . The solder paste according to claim 3 , wherein the liquidus temperature of the second solder alloy powder is not greater than 250° C.

7. The solder paste according to claim 4, wherein the liquidus temperature of the second solder alloy powder is not greater than 245°C.

8. The solder paste according to claim 1, wherein the first solder alloy powder consists of: 4 to 10 wt % Cu; 4 wt % to 20 wt % Ag; Optionally, 0.5 wt % or less of Bi, Co, In, Ge, Ni, P or Zn; and The balance ratio is 0.75 to 1.1 of Sn and Sb. 9 . The solder paste according to claim 1 , wherein the second solder alloy powder is SnAg, SnCu, SnAgCu, SnSb, SnAgCuSb.

10. The solder paste according to claim 1, wherein the second solder alloy powder is SnAgY, SnCuY, SnAgCuY or SnAgCuSbY, wherein Y is Bi, Co, Ge, In, Ni, P or Zn. 11 . The solder paste according to claim 9 , wherein the second solder alloy powder comprises 0.1 to 4 wt % of Ag, 0.1 to 1 wt % of Cu, or 0.1 to 11 wt % of Sb. 12 . The solder paste according to claim 11 , wherein the second solder alloy powder comprises 0.1 to 4 wt % of Ag, 0.1 to 1 wt % of Cu, and 0.1 to 11 wt % of Sb.

13. The solder paste according to claim 12, wherein the second solder alloy powder is doped with Bi, Co, Ge, In, Ni, P or Zn. 14 . The solder paste according to claim 1 , wherein a ratio of heat absorption of a first melting peak of the solder paste to heat absorption of a second melting peak of the solder paste is 0.15 or less.

15. A welding method, comprising: A solder paste is applied between the substrate and the device to form an assembly, wherein the solder paste consists of: 50 wt % to 90 wt % of a first solder alloy powder, 10 wt % to 50 wt % of a second solder alloy powder, and 9wt% to 20wt% flux, wherein the sum of the percentage contents of each component of the solder paste is equal to 100%, and wherein the first solder alloy powder is composed of a SnSbCuAg solder alloy having a Sn:Sb wt% ratio of 0.75 to 1.1, and The second solder alloy powder consists of a Sn solder alloy including at least 80 wt % Sn; performing a first reflow soldering process on the assembly at a peak temperature greater than 320° C. to form solder joints from the solder paste; and After forming the solder joints, the assembly is subjected to a second reflow soldering process at a peak temperature of 230°C to 270°C.

16. The method of claim 15, wherein the device is a silicon chip, the substrate comprises a Cu lead frame, and the solder paste is applied between the Cu lead frame and the silicon chip.

17. The method of claim 15, wherein: The solidus temperature of the first solder alloy powder is 300° C. to 360° C.; and The second solder alloy powder has a solidus temperature of 200°C to 250°C. 18 . The method of claim 15 , wherein during the second solder reflow process, the solder joint maintains a bond shear strength greater than or equal to 10 MPa.

19. A weld joint formed by the following method: applying solder paste between the substrate and the device to form an assembly; and reflow soldering the component to form the solder joint; The solder paste is composed of: 50 wt % to 90 wt % of a first solder alloy powder, 10 wt % to 50 wt % of a second solder alloy powder, and 9wt% to 20wt% flux, wherein the sum of the percentage contents of each component of the solder paste is equal to 100%, and wherein the first solder alloy powder is composed of a SnSbCuAg solder alloy having a Sn:Sb wt% ratio of 0.75 to 1.1, and The second solder alloy powder consists of a Sn solder alloy including at least 80 wt % of Sn.

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