Sn-Bi-In-Ag lead-free solder for low-temperature welding and preparation method thereof

Abstract

The application provides a low-temperature Sn-Bi-In-Ag lead-free solder suitable for 180 DEG C and a preparation method thereof, and belongs to the field of low-temperature welding. On the basis of Sn-43Bi binary eutectic composition, the application utilizes a cluster plus connecting atom model to guide the design of multi-component composition, reduces the content of Bi element, and adds a small amount of In element and trace Ag element. The obtained Sn-Bi-In-Ag lead-free solder has a room temperature shear strength of more than 30 MPa. Compared with Sn-43Bi eutectic solder alloy, the wettability is improved by more than 64%, and the melting point is reduced by 9.3 DEG C. After 180 DEG C reflow soldering, the average thickness of IMC of the Sn-Bi-In-Ag solder alloy on the Cu substrate is 1.10 mu m, which is reduced by more than 45% compared with the IMC thickness (about 2 mu m) of the Sn-43Bi eutectic solder, and the Sn-Bi-In-Ag solder alloy is a new type of low-temperature lead-free solder suitable for 180 DEG C.

Description

Technical Field

[0001] This invention belongs to the field of new materials technology, and is particularly applicable to the field of lead-free low-temperature soldering. Specifically, it relates to a Sn-Bi-In-Ag lead-free solder suitable for low-temperature soldering and its preparation method. Background Technology

[0002] For a long time, SnPb solder has been irreplaceable in the field of electronic packaging due to its excellent wetting properties, good conductivity, and mechanical properties. However, the toxicity of Pb to humans and its environmental pollution have forced countries around the world to ban its use, making the search for new high-quality lead-free solders a consensus among researchers. SnBi solder is considered one of the ideal lead-free solders to replace SnPb solder due to its excellent creep resistance, low melting point (139℃), and low cost. However, the brittleness of Bi, the phase coarsening that occurs in Sn-Bi solder during service, and the segregation of Bi elements at the interface in Sn-Bi / Cu joints limit the further application of Sn-Bi solder.

[0003] Studies have found that reducing the Bi content in SnBi eutectic solder and using methods such as multi-element alloying can effectively improve Bi segregation during the soldering process, increase its ductility, and inhibit the formation of intermetallic compounds Cu6Sn5 at the copper substrate solder joint. For example, Wu Renyong et al. from Dalian University of Technology found that as the Bi content in Sn-xBi solder decreases, the melting point of Sn-Bi solder increases, the melting range increases, and the wetting area of ​​the solder gradually increases with decreasing Bi content. Xu Chen et al. from Southeast University added 1~5 wt.% In to SnBi eutectic solder and found that the addition of In lowered the melting point of the eutectic solder, and the elongation of the solder reached its maximum at 2.5 wt.% without affecting the tensile strength. Zhao Xiuchen et al. from Beijing Institute of Technology added different amounts of Ag to SnBi eutectic solder and found that when the Ag content was increased to 1 wt.%, the tensile strength increased by 20%, reaching a maximum of 72 MPa.

[0004] However, while adding a single element to an alloy can improve certain properties of the solder, it can also degrade other properties. For example, adding In can lower the melting point of the solder and significantly reduce its shear strength; adding Ag can increase shear strength but also raise the melting point. Therefore, adding multiple alloying elements often improves the solder's performance better, which is also the development trend of Sn-Bi-based low-temperature solders. For instance, Shalaby found that the Bi-38Sn-2In-2Ag quaternary solder has better creep resistance than solders with In or Ag added alone. Zhang Qingke et al. from the Chinese Academy of Sciences compared adding 1 wt.% Ag and 1.5 wt.% In to Sn-58Bi separately with adding them simultaneously, and found that the combined addition of Ag and In can improve the microhardness and impact toughness of SnBi eutectic solders. A research team led by Huang Mingliang at Dalian University of Technology discovered that the brazing filler metal with the alloy composition Sn-46Bi-3.3In-0.5Ag exhibits higher shear strength and shear displacement compared to brazing filler metals with other elements such as Sn-46Bi-3.3In-0.5X (Cu, Zn).

[0005] Although it is understood that the simultaneous addition of In and Ag can improve solder performance, the investigation into the optimal addition amount is mostly based on experimental research, lacking effective theoretical guidance. This has led to the low efficiency of Sn-Bi-In-Ag solder development at present. The fundamental reason is the lack of atomic structure information of the alloy, making it impossible to establish the correlation between alloy composition and performance.

[0006] In summary, existing SnBi low-temperature eutectic solders offer simple preparation processes, low melting points, low costs, and environmental friendliness. However, they suffer from high brittleness, insufficient wettability, and a relatively thick intermetallic compound (IMC) layer at the copper substrate contact point, making it difficult to improve overall performance. Therefore, the two core issues hindering the development of current SnBi-based multi-element low-temperature solders are: firstly, the need to introduce effective theoretical methods to guide the composition design of multi-element alloys and improve the R&D efficiency of multi-element solders; and secondly, ensuring sufficient strength while improving wettability, reducing IMC thickness, and enhancing the overall performance of the solder. Summary of the Invention

[0007] This invention provides a Sn-Bi-In-Ag lead-free solder suitable for low-temperature soldering and its preparation method. Based on the Sn-Bi binary eutectic bicluster model, superclusters are constructed to obtain a near-programmed structure for the binary Sn-Bi solder. Then, the number of Bi clusters in the superclusters is changed to reduce the Bi content in the alloy. In and Ag are then used to replace Sn atoms in the superclusters, thereby achieving the purpose of composition design. The wettability and IMC thickness of the designed solder are improved compared to simply reducing the Bi content or adding In and Ag. Compared with the Sn-43Bi eutectic solder alloy, the wettability is improved by >64%, and the melting point is reduced by 9.3℃. After reflow soldering at 180℃, the average IMC thickness of the Sn-Bi-In-Ag solder alloy on the Cu substrate is 1.10 μm, which is >45% lower than the IMC thickness (≈2 μm) of the Sn-43Bi eutectic solder. The purpose of this invention is to develop a lead-free Sn-Bi-In-Ag solder alloy suitable for electronic packaging with a room temperature shear strength >30MPa and low solder IMC through precise alloy design.

[0008] The technical solution of this invention is: A Sn-Bi-In-Ag lead-free solder alloy, wherein the alloy comprises, by mass percentage: Sn: 51.50~52.00%; In: 3.35~3.40%; Ag: 0.60~0.65%; Bi: balance; or, Sn: 45.5~46.50%; In: 2.90~3.10%; Ag: 0.28~0.33%; Bi: balance.

[0009] A specific Sn-Bi-In-Ag lead-free solder alloy was designed, wherein the alloy comprises, by mass percentage: Sn: 57.66%; In: 3.79%; Ag: 0.97%; Bi: balance.

[0010] A specific Sn-Bi-In-Ag lead-free solder alloy was designed, wherein the alloy comprises, by mass percentage: Sn: 51.71%; In: 3.38%; Ag: 0.63%; Bi: balance.

[0011] A specific Sn-Bi-In-Ag lead-free solder alloy was designed, wherein the alloy comprises, by mass percentage: Sn: 45.97%; In: 2.99%; Ag: 0.31%; Bi: balance.

[0012] The Sn-Bi-In-Ag lead-free solder alloy described above has a specific microstructure: it consists of a β-Sn phase and a Bi phase. In is dissolved in the β-Sn phase, and as the In content increases, a small amount of petal-shaped InSn4 is formed, while Ag is dispersed in Sn. A Cu6(In,Sn)5 phase is formed on the copper substrate, exhibiting a prismatic structure at the IMC boundary.

[0013] The aforementioned Sn-Bi-In-Ag lead-free solder alloy suitable for low-temperature soldering at 180℃ has the following typical properties: room temperature shear strength >30MPa. Compared with Sn-43Bi eutectic solder alloy, the wettability of this alloy is improved by >64%; the melting point is reduced by 9.3℃. After reflow soldering at 180℃, the average IMC thickness of the Sn-Bi-In-Ag solder alloy on the Cu substrate is 1.10μm, which is >45% lower than the IMC thickness (≈2μm) of the Sn-43Bi eutectic solder alloy.

[0014] The aforementioned process for preparing a Sn-Bi-In-Ag lead-free solder alloy suitable for low-temperature (180℃) solder includes the following steps: High-purity alloy particles are weighed according to a mass percentage, and the mixed metal particles are vacuum-sealed in a quartz glass tube. The sealed quartz tube is then placed in a high-temperature muffle furnace at 800℃ for melting for 6 hours, followed by water cooling. During melting, the mixture is shaken every 30 minutes to ensure uniform alloy composition. After water cooling, the metal ingot is cut and polished to produce the final alloy product.

[0015] The design process of the above technical solution involves using the applicant's research group's bi-cluster composition design method to design the composition of a novel Sn-Bi-In-Ag lead-free solder alloy suitable for low-temperature operation at 180℃. Cluster-type structural units are formed based on inter-element interactions, defined as [clusters] (connecting atoms). x That is, consisting of a cluster and x The alloy is composed of connecting atoms, where a cluster is a coordination polyhedron formed by any solute atom at its center and surrounded by matrix atoms in the nearest-neighbor shell. The connecting atoms in the next nearest-neighbor shell are used to match the average density of the alloy. This formula contains information about the structure and composition of the alloy, describing it as a chemical structural unit. Based on this concept, a bi-cluster model is proposed for eutectic materials, which consist of two stable liquid phase structures. That is, each stable liquid phase structure can be represented by a chemical structural unit, such as the Sn-43Bi eutectic composition, which can be represented by the β-Sn chemical structural unit [Sn-Sn]. 10 Sn5 and the structural unit [Bi-Bi6]Bi5, representing Bi, are mixed in a 1:1 ratio to form a double cluster: [Sn-Sn 10 Sn5+[Bi-Bi6]Bi5=Sn 16 Bi 12 =Sn 57.1 Bi42.9 The bicluster structure is scaled up proportionally to obtain a supercluster with sixteen clusters, which are stacked in space in an FCC (Flat Cluster Classification) manner, specifically: [{[Bi-Bi6]Bi5}-{[Sn-Sn... 10 The cluster-based composition characterization method has been successfully applied to the design of various solid solution alloys, such as nickel-based superalloys, magnesium alloys, and high-entropy alloys, providing new ideas and methods for the composition design of high-performance alloys.

[0016] In the Sn-Bi-In-Ag low-temperature solder alloy system, different control measures can be taken to address the impact of alloy composition variations and the amount of trace addition of a third element on solder performance. For example, reducing the Bi content can effectively reduce Bi segregation during soldering, improve solder wettability, and reduce solder brittleness. Therefore, by reducing one Bi cluster and adding one Sn cluster based on the binary Sn-Bi eutectic supercluster, a low-Bi binary Sn-Bi solder can be obtained, denoted as [{[Bi-Bi6]Bi5}-{[Sn-Sn 10 Adding In can increase the ductility of SnBi alloys and lower the melting point temperature. However, excessive In content can generate brittle phases such as BiIn and BiIn2, promoting the formation of Cu6Sn5 phase on the copper substrate and reducing the shear strength of the SnBi solder alloy. Therefore, it needs to be added in small amounts. Since In is often dissolved in Sn in SnBi solder, the upper limit of In atom addition is: replacing [Sn-Sn] in the supercluster formula with 1 In atom. 10 Adding one Sn atom to the connecting atoms of the Sn5 cluster yields a ternary Sn-Bi-In supercluster with low Bi content, represented as: [{[Bi-Bi6]Bi5}-{[Sn-Sn 10The trace addition of Ag can generate the Ag3Sn phase, which plays a role in solid solution strengthening, which is beneficial to improving the shear strength of the solder and can effectively suppress the increase in IMC thickness at the Cu substrate caused by the addition of In. To ensure the wetting performance of the solder, the ratio of Sn-Bi and Sn-Ag in the quaternary solder alloy should meet the eutectic composition ratio, that is, in the low Bi content Sn-Bi-In alloy, the atomic ratio of Sn-Bi should first meet the eutectic ratio of 57:43. At this time, there is a surplus of Sn element, and the surplus Sn element and the added Ag element meet the eutectic ratio of 96.2:3.8. Thus, the number of Ag elements added is 1. Therefore, the upper limit of Ag atom addition is: replacing the supercluster [{[Bi-Bi6]Bi5}-{[Sn-Sn] 10 ]In1Sn4}9{[Bi-Bi6]Bi5}3]-{[Bi-Bi6]Bi5}3, 1 [Sn-Sn 10 The Sn atom at the connection position in the In1Sn4 cluster ultimately yields the composition of the Sn-Bi-In-Ag lead-free solder suitable for 180℃.

[0017] The preparation method of the alloy designed in this invention is as follows: High-purity (above 99.99 wt.%) alloy particles are weighed according to the mass percentage, and the mixed metal particles are vacuum-sealed in a quartz glass tube. The sealed quartz tube is placed in a high-temperature muffle furnace at 800°C for 6 hours for melting, followed by water cooling. During the first 1.5 hours of melting, the furnace temperature is fully heated to 800°C at a rate of 10°C / min. Then, it is held at 800°C for two hours to ensure that the alloy is fully heated. After that, the quartz tube is shaken in the furnace every 30 minutes, shaking 7-10 times each time, for a total of 5 times to ensure uniform composition of the alloy ingot. The water-cooled metal ingot is then cut and polished to produce the final alloy product.

[0018] The Sn-Bi-In-Ag lead-free solder alloy described above has a reflow temperature below 180℃, making it suitable as a substitute for SnBi solder in electronic packaging products. Currently, the design parameters of soldering equipment are based on the melting point of tin-lead eutectic solders; excessively high soldering temperatures can damage electronic components and printed circuit boards. The good wettability of the solder enables the formation of reliable solder joints on the base material. Based on this, the Sn-Bi-In-Ag lead-free solder alloy has a low melting temperature (below 130℃) and good wettability, with a 10mg solder ball spreading area greater than 5mm². 2 It is suitable for low-temperature soldering. Specifically, the Sn-Bi-In-Ag lead-free solder alloy is used for low-temperature soldering at 180℃.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention is based on the cluster composition method independently developed by the applicant's team, which designs and develops a low-temperature lead-free Sn-Bi-In-Ag solder alloy suitable for 180℃. Compared with commonly used empirical methods, the composition design method based on the cluster model proposed in this invention can more effectively carry out diversified composition design and improve the efficiency of solder composition research and development.

[0020] (2) The series of designed alloys exhibited good wettability and improved mechanical properties under reflow soldering at 180℃. Compared with Sn-43Bi eutectic solder alloy, the Sn-Bi-In-Ag solder alloy prepared by reducing Bi content and adding appropriate amount of In and trace amount of Ag showed improved wettability by more than 64% and a lower melting point by 9.3℃.

[0021] (3) After reflow soldering at 180℃, the average IMC thickness of the Sn-Bi-In-Ag solder alloy on the Cu substrate is 1.10μm, which is more than 45% lower than the IMC thickness (≈2μm) of the Sn-43Bi eutectic solder alloy, and has good soldering performance. Attached Figure Description

[0022] Figure 1 The image shows the EPMA microstructure of the alloy prepared in Example 1.

[0023] Figure 2 The image shows the microstructure of the EPMA of the alloy prepared in Example 1 after welding onto a copper substrate.

[0024] Figure 3 The image shows the EPMA microstructure of the alloy prepared in Example 2.

[0025] Figure 4 The image shows the microstructure of the EPMA of the alloy prepared in Example 2 after welding onto a copper substrate. Detailed Implementation

[0026] The specific embodiments of the present invention will be described in detail below with reference to the technical solution. Example 1

[0027] Sn 45.97 Bi 50.73 In 2.99 Ag 0.31 wt.% Sn-Bi-In-Ag low-temperature solder alloy Alloy preparation Using high-purity metal raw materials, 40g of metal particles with a mass percentage of 45.97% Sn, 50.73% Bi, 2.99% In, and 0.31% Ag were mixed and vacuum-sealed in a quartz glass tube. The sealed quartz tube was then placed in a high-temperature muffle furnace at 800℃ for 6 hours for melting, followed by water cooling. During melting, the mixture was shaken every 30 minutes to ensure uniform composition of the alloy ingot. After water cooling, the metal ingot was cut and polished to produce the final brazing alloy product. The alloy ingot underwent 6 passes of unidirectional rolling, with a deformation of 20% per pass and a total reduction of 90%, yielding a sheet sample with a thickness of approximately 0.2mm. The sample was then immersed in flux (LR721H2 Alpha flux) and melted in a pure titanium crucible using a 180℃ heating plate to form solder balls. These balls were then cooled in anhydrous ethanol and air-dried for use in welding.

[0028] welding Three copper substrates with dimensions of 40mm × 10mm × 1mm were prepared and their surfaces were ground and polished. Three pre-made 40mg alloy solder balls were taken, and an appropriate amount of flux was applied to their surfaces. They were placed tightly in the center of a 10mm × 10mm area at one end of one of the copper substrates. A section of another copper substrate of the same area was overlapped in a Z-shape. To ensure uniform welding stress, two 0.5mm diameter copper wires were used for support during the overlap, and another copper substrate of the same specifications was used for auxiliary support on one side below. The overlapped joint was then placed in a T200N+ nitrogen-free lead-free reflow oven and reflowed for 90 seconds according to the set curve. After reflow, the residual flux on the joint surface was cleaned with anhydrous ethanol using ultrasonic vibration.

[0029] Organizational structure and melting point performance testing The synthesized metal ingot was cut into 4mm high metal cylinders. One end was successively ground with 500#, 1000#, and 2000# sandpaper, and then polished with W0.5 and W1.5 water-soluble diamond polishing paste until the phase region was clear and there were no obvious scratches under a 40x optical microscope. The morphology of the solder alloy was then observed, and EPMA analysis was performed using a field emission electron probe (JXA-8530F Plus, Japan) to observe the alloy microstructure and structure after solution treatment and aging. Figure 1 As shown, the alloy in this embodiment consists of a gray β-Sn phase, a white Bi phase, and a dark gray irregular Ag3Sn phase. The melting characteristics of the solder alloy were evaluated using a differential scanning calorimeter (DSC822 / TGA / SDTA851, Switzerland) in an argon atmosphere, with a temperature range of 50~300℃ and a heating and cooling rate of 10℃ / min. The melting temperature of the alloy in this embodiment was 129.4℃, and the melting range was 8.8℃.

[0030] Microstructure and mechanical property testing after reflow soldering The microstructure and structure of the alloy in this embodiment after solution treatment and aging, and after welding onto a copper substrate, were examined using TEM and EPMA. Figure 2 As shown, the alloy welded to the copper substrate consists of a gray β-Sn phase, a white Bi phase, and a prismatic Cu6(Sn,In)5 phase at the copper substrate interface. The welded joint was clamped at both ends onto the fixture of an MTS universal tensile testing machine, and the sample was stretched at a rate of 0.3 mm / min. The sample fractured under shear force. Tensile properties at room temperature were measured: the average shear strength of the room temperature joint was 33.2 MPa, and the maximum strength was 33.6 MPa. Example 2

[0031] Sn 51.71 Bi 44.28 In 3.38 Ag 0.63 wt.% Sn-Bi-In-Ag low-temperature solder alloy Alloy preparation Using high-purity raw materials, 40g of metal particles with a mass percentage of 45.97% Sn, 50.73% Bi, 2.99% In, and 0.31% Ag were mixed and vacuum-sealed in a quartz glass tube. The sealed quartz tube was then placed in a high-temperature muffle furnace at 800℃ for 6 hours for melting, followed by water cooling. During melting, the mixture was shaken every 30 minutes to ensure uniform composition of the alloy ingot. After water cooling, the metal ingot was cut and polished to produce the final brazing alloy product. Next, the alloy ingot for welding was subjected to 6 passes of unidirectional rolling. The deformation per pass was 20%, and the total reduction was 90%, resulting in a sheet sample with a thickness of approximately 0.5mm. This sample was then immersed in flux (LR721H2 Alpha flux) and melted in a pure titanium crucible using a 180℃ heating plate to form solder balls. The balls were then cooled in anhydrous ethanol and air-dried for welding use.

[0032] welding Three copper substrates with dimensions of 40mm × 10mm × 1mm were prepared and their surfaces were ground and polished. Three pre-made 40mg alloy solder balls were taken, and an appropriate amount of flux was applied to their surfaces. They were placed tightly in the center of a 10mm × 10mm area at one end of one of the copper substrates. A section of another copper substrate of the same area was overlapped in a Z-shape. To ensure uniform welding stress, two 0.5mm diameter copper wires were used for support during the overlap, and another copper substrate of the same specifications was used for auxiliary support on one side below. The overlapped joint was then placed in a T200N+ nitrogen-free lead-free reflow oven and reflowed for 90 seconds according to the set curve. After reflow, the residual flux on the joint surface was cleaned with anhydrous ethanol using ultrasonic vibration.

[0033] Organizational structure and melting point performance testing The microstructure and structure of the alloy after solution aging were detected using TEM and EPMA, such as Figure 3 As shown, the alloy prepared in this embodiment consists of a gray β-Sn phase, a white Bi phase, and a dark gray irregular Ag3Sn phase. The melting characteristics of the solder alloy were evaluated using differential scanning calorimetry (DSC) in an argon atmosphere, with a temperature range of 50–300 °C and a heating and cooling rate of 10 °C / min. The melting temperature of the alloy in this embodiment was 129.1 °C, and the melting range was 7.2 °C.

[0034] Microstructure and mechanical property testing after reflow soldering The microstructure and structure of the alloy material after solution treatment and aging following reflow soldering were examined using TEM and EPMA. Figure 4 As shown, the welded alloy consists of a gray β-Sn phase, a white Bi phase, and a prismatic Cu6(Sn,In)5 phase at the copper substrate interface. Tensile properties at room temperature were measured using an MTS universal tensile testing machine: the average shear strength of the joint at room temperature was 32.7 MPa, and the maximum strength was 37.3 MPa. Example 3

[0035] Sn 57.66 Bi 37.59 In 3.79 Ag 0.97 wt.% Sn-Bi-In-Ag low-temperature solder alloy A Sn-Bi-In-Ag low-temperature solder alloy with a mass percentage content of 57.66% Sn, 37.59% Bi, 3.79% In, and 0.97% Ag was prepared according to the method of Example 2. The melting temperature and tensile properties at room temperature of the alloy were also tested according to the method of Example 2. The melting temperature of the alloy in this example was 127.0℃, the melting range was 40.8℃, and the average shear strength of the joint at room temperature was 28.6 MPa and the maximum strength was 30.4 MPa.

[0036] The performance data of the three Sn-Bi-In-Ag low-temperature solder alloys are shown in Table 1.

[0037] Table 1 Performance data of three alloy materials

[0038] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A Sn-Bi-In-Ag lead-free solder alloy, characterized in that, The alloy comprises, by mass percentage: Sn: 51.50~52.00%; In: 3.35~3.40%; Ag: 0.60~0.65%; Bi: Balance; Based on the Sn-Bi binary eutectic bicluster model, superclusters were constructed to obtain the near-programmed structure of the binary Sn-Bi solder. Then, the number of Bi clusters in the superclusters was changed to reduce the Bi content in the alloy. Next, In and Ag were used to replace Sn atoms in the superclusters. The Sn-43Bi eutectic composition was represented by the chemical structural unit [Sn-Sn]. 10 Sn5 and the structural unit [Bi-Bi6]Bi5, representing Bi, are mixed in a 1:1 ratio to form a double cluster: [Sn-Sn 10 Sn5+[Bi-Bi6]Bi5=Sn 16 Bi 12 =Sn 57.1 Bi 42.9 ; Replace [Sn-Sn] in the supercluster with one In atom. 10 One Sn atom in the linking atoms of the Sn5 cluster is replaced by an Ag tuple in the supercluster formula [{[Bi-Bi6]Bi5}-{[Sn-Sn]]. 10 ]In1Sn4}9{[Bi-Bi6]Bi5}3]-{[Bi-Bi6]Bi5}3, 1 [Sn-Sn 10 The composition of Sn-Bi-In-Ag lead-free solder is obtained by connecting one Sn atom at the linking position in the In1Sn4 cluster.

2. The Sn-Bi-In-Ag lead-free solder alloy according to claim 1, characterized in that, The alloy comprises, by mass percentage: Sn: 51.71%; In: 3.38%; Ag: 0.63%; Bi: Balance.

3. A Sn-Bi-In-Ag lead-free solder alloy, characterized in that, The alloy comprises, by mass percentage: Sn: 45.5~46.50%; In: 2.90~3.10%; Ag: 0.28~0.33%; Bi: Balance; Based on the Sn-Bi binary eutectic bicluster model, superclusters were constructed to obtain the near-programmed structure of the binary Sn-Bi solder. Then, the number of Bi clusters in the superclusters was changed to reduce the Bi content in the alloy. Next, In and Ag were used to replace Sn atoms in the superclusters. The Sn-43Bi eutectic composition was represented by the chemical structural unit [Sn-Sn]. 10 Sn5 and the structural unit [Bi-Bi6]Bi5, representing Bi, are mixed in a 1:1 ratio to form a double cluster: [Sn-Sn 10 Sn5+[Bi-Bi6]Bi5=Sn 16 Bi 12 =Sn 57.1 Bi 42.9 ; Replace [Sn-Sn] in the supercluster with one In atom. 10 One Sn atom in the linking atoms of the Sn5 cluster is replaced by an Ag tuple in the supercluster formula [{[Bi-Bi6]Bi5}-{[Sn-Sn]]. 10 ]In1Sn4}9{[Bi-Bi6]Bi5}3]-{[Bi-Bi6]Bi5}3, 1 [Sn-Sn 10 The composition of Sn-Bi-In-Ag lead-free solder is obtained by connecting one Sn atom at the linking position in the In1Sn4 cluster.

4. The Sn-Bi-In-Ag lead-free solder alloy according to claim 3, characterized in that, The alloy comprises, by mass percentage: Sn: 45.97%; In: 2.99%; Ag: 0.31%; Bi: Balance.

5. The method for preparing Sn-Bi-In-Ag lead-free solder alloy according to any one of claims 1-4, characterized in that, Includes the following steps: High-purity Sn, Bi, In, and Ag metal particles are mixed and sealed in a vacuum-sealed quartz glass tube, then melted in a high-temperature muffle furnace at 750-800 ℃ for 6-6.25 hours, followed by water cooling to obtain the Sn-Bi-In-Ag lead-free solder alloy.

6. The preparation method according to claim 5, characterized in that, During the smelting process, the metal ingot is shaken every 30 minutes. After water cooling, the metal ingot is cut and polished to produce the Sn-Bi-In-Ag lead-free solder alloy.

7. The application of the Sn-Bi-In-Ag lead-free solder alloy according to any one of claims 1-4, characterized in that, The Sn-Bi-In-Ag lead-free solder alloy is used for low-temperature soldering.

8. The application according to claim 7, characterized in that, The Sn-Bi-In-Ag lead-free solder alloy is used for low-temperature soldering at 180℃.

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