A low-temperature Sn-Bi based lead-free solder alloy

Abstract

The present invention discloses a low-temperature Sn-Bi based lead-free solder alloy. The alloy is based on a hypoeutectic double cluster model containing a β-Sn (Bi) solid solution phase. The alloy composition, by mass percentage, comprises 38.2% Bi, 3.7-4.0% In, 0-1.0% Ag, 0-0.8% Cu, and the balance is Sn. The hypoeutectic double cluster model of the alloy contains a structural unit of [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn‑Sn 10 ]Bi3In1+2[Sn‑Sn 10 ]Bi3Sn1+1[Bi‑Bi6]Bi5=Sn 112 Bi 42 In8 = Sn-38.2Bi-3.9In. The low-temperature solder of the present invention has the advantages of a low melting point, high plasticity and toughness, high shear strength, and compatibility with current industrial low-temperature reflow processes. It is suitable as a new low-temperature solder for low-temperature brazing applications such as consumer electronics and new solar cells.

Description

Technical Field

[0001] The present invention relates to the technical field of lead-free solders, in particular to a low-temperature Sn-Bi based lead-free solder alloy. Background Art

[0002] As consumer electronics evolve toward low power consumption, multifunctionality, high integration, miniaturization, and green manufacturing, new requirements are being placed on electronic manufacturing packaging technologies and processes to address the significant reliability challenges facing these products. Currently, the most mature and widely used lead-free solder system is SAC305 (Sn-3.0Ag-0.5Cu). This lead-free solder offers advantages such as excellent wettability, low dross, and high solder joint strength. However, this solder suffers from a high melting point of 217°C, 34°C higher than that of Sn-Pb eutectic solder. Furthermore, SAC305's peak reflow temperature requires reaching 250°C or higher, posing significant challenges to the processes and materials previously used with Sn-Pb eutectic solders. Not only must the impact of high temperatures on device performance be considered, but residual thermal stresses that can cause printed circuit board (PCB) warpage must also be addressed. As electronic packaging continues to miniaturize and become more highly integrated, the thermal management challenges associated with high reflow temperatures are exponentially magnified, becoming even more severe. Therefore, the development of low-temperature solder and low-temperature process is also a development direction of the current electronic packaging industry after lead-free.

[0003] Furthermore, most solar cells in the photovoltaic industry utilize solder-coated copper ribbons and other connector components connected using solder paste. To continuously reduce the cost of solar energy, cell thickness is becoming increasingly thinner. In the coming years, the industry will need to transition from tin-lead solder to lead-free solder. Because the CTE of the copper ribbons and other components used to connect cells together can be 4-5 times greater than that of silicon, stress is applied to the fragile cells when the solder solidifies after processing. As cells become thinner, they become increasingly susceptible to fracture. Using low-melting-point solder alloys and solder pastes can mitigate these issues and reduce the risk of cell fracture.

[0004] In electronic packaging, solder alloys with melting points lower than SAC305 are generally referred to as low-temperature solders. Sn-Bi solders offer advantages such as a low melting point, low cost, good wettability, and high strength, offering greater potential for development compared to other low-temperature lead-free solder systems. However, Sn-Bi eutectic solders also suffer from issues such as high brittleness of the Bi phase, poor ductility, and the tendency for Bi segregation at the Cu3Sn / Cu interface during aging. Poor ductility and low plasticity are particularly critical issues for Sn-Bi solders. Current solutions to this problem primarily encompass two approaches: 1) reducing the Bi content to minimize segregation of the brittle Bi phase; and 2) adding alloying elements such as In and Ag that enhance toughness.

[0005] Existing public literature discloses that reducing the Bi content within a certain range can effectively improve the toughness of Sn-Bi-based solder alloys. Chinese patent application publication number CN115383349A discloses a method for microalloying and microstructuring a high-toughness lead-free tin-bismuth solder. The solder contains 40% Bi by weight, a major component. Its elongation is 26.6% higher than that of Sn-58Bi, and it avoids excessive IMC growth during aging. Existing public literature also discloses that adding a small amount of alloying elements to Sn-Bi can improve the impact resistance of solder joints. Chinese patent application publication number CN111683785A discloses a solder alloy containing 35-68% Bi by weight, 0.5-3% In by weight, and other trace elements such as Pd, Co, and Ge. This lead-free solder effectively refines the brittle Bi phase and exhibits excellent impact resistance, but the alloy's toughness still fails to meet the requirements of practical applications. Adding the element In can improve the toughness of the alloy to a certain extent, but its low content makes it difficult to significantly improve the performance of the alloy. In addition, the excessive number of trace alloy types added to this solder is not conducive to the uniformity of the structure and the stability of the performance. Chinese patent application publication number CN117900692A discloses a Sn-Bi-In-Ag lead-free solder based on the traditional {cluster} (connected atom) model, in which the weight percentage of the main component Sn is 45.5-46.5% or 51.5-52.0%, and the weight percentage of In is 2.9-3.1% or 3.35-3.4%. This lead-free solder alloy can slightly improve the shear strength, but due to the composition deviation caused by the double cluster model used without taking into account the influence of the solid solution phase, its shear strength is only 28.6MPa. Summary of the Invention

[0006] The purpose of the present invention is to enhance the performance of existing low-temperature Sn-Bi based lead-free solder, further improve the plasticity and ductility of the low-temperature Sn-Bi based lead-free solder, and lower the melting point, so as to better apply it to the low-temperature welding process of high-reliability low-temperature Sn-Bi based lead-free solder alloy.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy, the alloy is based on a hypoeutectic double cluster model containing a β-Sn(Bi) solid solution phase, the alloy is based on a hypoeutectic double cluster model containing a β-Sn(Bi) solid solution phase, In, Ag, and Cu atoms are added to replace Sn atoms to obtain a high-reliability lead-free solder alloy, the alloy composition, by mass percentage, includes 38.2% Bi, 3.7-4.0% In, 0-1.0% Ag, 0-0.8% Cu, and the balance is Sn; the hypoeutectic double cluster model of the alloy, the structural unit containing the β-Sn(Bi) solid solution phase is [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn-Sn 10 ]Bi3In1+2[Sn-Sn 10 ]Bi3Sn1+1[Bi-Bi6]Bi5=Sn 112 Bi 42 In8=Sn-38.2Bi-3.9In.

[0009] Preferably, in the alloy, the mass content of In is 3.9%, and the mass content of Ag is 0.94%; the hypoeutectic double cluster model of the alloy, 8[Sn-Sn 10 ]Bi3In1+2[Sn-Sn 10 ]Bi3Ag1+1[Bi-Bi6]Bi5=Sn 110 Bi 42 In8Ag2=Sn-38.2Bi-3.9In-0.94Ag.

[0010] In the traditional {cluster} (connected atoms) model, Sn 57 Bi 43 The double cluster model of the eutectic alloy consists only of the structural units representing Sn [Sn-Sn 10 ]Sn5 and the structural unit representing Bi [Bi-Bi6]Bi5 are simply mixed in a ratio of 1:1, and the cluster analytical formula is: 1[Sn-Sn 10 ]Sn5+1[Bi-Bi6]Bi5=Sn 16 Bi 12 =Sn 57.1 Bi 42.9 This model does not take into account the influence of the β-Sn(Bi) solid solution phase. However, the low-temperature Sn-Bi based lead-free solder of the present invention, in its dual cluster model, contains the structural unit of the β-Sn(Bi) solid solution phase as [Sn-Sn 10 ]Bi3Sn1, after optimization, Sn 57 Bi 43 The analytical formula of the double cluster of eutectic alloy is: 2[Sn-Sn 10]Bi3Sn1+1[Bi-Bi6]Bi5=Sn 24 Bi 18 =Sn 57.1 Bi 42.9 .

[0011] Based on the double cluster model including the solid solution phase, the present invention further analyzes the Sn-Bi binary hypoeutectic double cluster analytical formula and the multi-cluster analytical formula of the Sn-Bi-In-X alloy composition. For example: 10[Sn-Sn 10 ]Bi3Sn1+1[Bi-Bi6]Bi5=Sn 120 Bi 42 =Sn-38.2Bi and 8[Sn-Sn 10 ]Bi3In1+2[Sn-Sn 10 ]Bi3Ag1+1[Bi-Bi6]Bi5=Sn 110 Bi 42 In8Ag2=Sn-38.2Bi-3.9In-0.94Ag. The present invention designs the multi-element alloy composition more accurately, thereby obtaining an alloy with good comprehensive properties.

[0012] The solder alloy of the present invention reduces the Bi content on the basis of the Sn-58Bi eutectic alloy. Under the condition of ensuring a low reflow process temperature, the Bi content is reduced to the composition range of hypoeutectic Sn-Bi, thereby reducing the risk of Bi atoms being segregated at the interface due to the brittle Bi phase during the reflow process, and reducing the tendency of Bi to segregate in Cu3Sn / Cu during the aging process. In addition, the hypoeutectic Sn-Bi component is transformed from the Sn-Bi eutectic structure to a mixed structure of Sn-Bi eutectic and β-Sn, and the toughness of the β-Sn phase is improved compared to the eutectic structure. A certain amount of In element is added on the basis of reducing the Bi content. The crystal structures of In and Sn are both tetragonal structures, so In is more likely to replace Bi as a solute atom in β-Sn, which can reduce the lattice distortion of β-Sn and thus improve toughness. In addition, adding In can also reduce the liquidus temperature of the solder. In, as a primary additive element, is added to Sn-Bi hypoeutectic solders, potentially improving their toughness and lowering their processing temperature, thereby enhancing their low-temperature solderability. The amount of indium added significantly influences the properties of Sn-Bi-based solders. When the indium content is below 2 wt.%, the toughness of the solder alloy is not significantly improved. When the indium content exceeds 5 wt.%, precipitation of the BiIn phase is observed in the microstructure. The formation of this compound leads to a sharp deterioration in performance and increased brittleness. Furthermore, the maximum solid solubility of indium in Sn is approximately 7 wt.%. In the designed Sn-Bi-based solder, when the Bi content is 35-45 wt.%, the corresponding maximum solid solubility of indium should be within the range of 3.85-4.55 wt.% to avoid the formation of the InSn4 phase. In summary, adjusting the In / Sn ratio to achieve an indium content of 3.7-4.0 wt.% can most effectively optimize the properties of Sn-Bi-based alloys.

[0013] The effect of trace addition of Cu element has little effect on the melting point of solder. Its main function is to form high melting point Cu6Sn5 IMC during smelting, which acts as a site for heterogeneous nucleation and refines the microstructure. The presence of this second phase hinders the movement of dislocations, thereby helping to improve the strength of the solder joint. However, both strengthening effects depend on the size and specific distribution of the second phase. The effect of trace addition of Ag is similar to that of Cu, but the addition of 0.1-1.5% Ag is conducive to the formation of granular Ag3Sn, which has a very significant effect on matrix dispersion strengthening. In addition, the addition of Ag can further improve the toughness of the solder.

[0014] The Sn-Bi based lead-free solder of the present invention has the following beneficial effects:

[0015] 1) The liquidus of the solder alloy included in the present invention is lower than that of the hypoeutectic Sn-Bi solder with the same Bi content, and can be reduced by as low as 10°C. The solder alloy can match the low-temperature reflow process with a peak value of about 185°C and can be applied to low-temperature welding or mixed assembly fields.

[0016] 2) The solder alloy of the present invention reduces the Bi content to the hypoeutectic Sn-Bi composition range while maintaining a low reflow process temperature. This inhibits the segregation of Bi atoms at the interface during reflow to form a brittle phase, thereby increasing the toughness of the β-Sn matrix. This improves the drop resistance of the solder joint without reducing its strength.

[0017] 3) Adding In: In dissolves in the β-Sn matrix, strengthening it. By adjusting the In / Sn mass ratio, the poor plasticity and insufficient toughness of the Sn-Bi-based solder alloy can be improved; at the same time, the formation of InSn4-type intermetallic compound (IMC) phases is avoided. During the interfacial reaction, In reacts with Cu and Sn. The formation of an appropriately thick IMC can simultaneously improve the shear strength and shear displacement of the solder joint. Furthermore, the addition of In can lower the melting point of the solder.

[0018] 4) Adding Ag and / or Cu alloying elements to the Sn-Bi-In alloy forms corresponding intermetallic compounds, which refine the microstructure and address the problem of microstructural coarsening caused by the addition of In. The addition of Ag and Cu improves the wettability of the solder joint, improving the deterioration of wettability caused by the addition of In.

[0019] 5) The elements contained in the solder alloy of the present invention are non-toxic and harmless, and meet the green and environmental protection requirements in the development of the electronics industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a 100× scanning image of the microstructure of Example 1-6;

[0021] Figure 2 is a 100× scanning image of the microstructure of Comparative Examples 1-9;

[0022] Figure 3 Characterization of the gray phase, phase boundary and white phase regions of Comparative Example 1, Example 1 and Example 5;

[0023] Figure 4 300× scanning images of the microstructures of Example 5 and Comparative Example 8;

[0024] Figure 5 is the shear strength of each solder joint;

[0025] Figure 6is the shear displacement of each solder joint;

[0026] Figure 7 is the wetted area of ​​each solder. DETAILED DESCRIPTION

[0027] In order to more clearly illustrate the content of the present invention, the present invention will be described in detail below with reference to embodiments and drawings.

[0028] Example 1

[0029] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic dual-cluster model containing a β-Sn(Bi) solid solution phase, and In is added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.9% In, and the balance is Sn. The purity of Sn, Bi, and In is 99.99%. The structural unit of the hypoeutectic dual-cluster model of the alloy containing the β-Sn(Bi) solid solution phase is [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn-Sn 10 ]Bi3In1+2[Sn-Sn 10 ]Bi3Sn1+1[Bi-Bi6]Bi5=Sn 112 Bi 42 In8=Sn-38.2Bi-3.9In.

[0030] The preparation method of the solder alloy is as follows:

[0031] 1) Prepare the solder alloy in proportion by mass percentage and place it in a high temperature resistant quartz tube;

[0032] 2) Use a hydrogen flame to melt and seal one end of the quartz tube, melt the other end into a narrow opening, and use a vacuum pump to evacuate the tube. After exhausting all the air in the tube, melt and seal the narrow opening again.

[0033] 3) Place the quartz tube in a high-frequency melting furnace, heat it to 700℃ and keep it at 700℃ for at least 5 hours. During the melting period, shake the quartz tube every half an hour to ensure that the liquid solder alloy in the quartz tube is more uniform.

[0034] 4) After the smelting is completed, the furnace is cooled to 300°C, and then the quartz tube is taken out and quenched in cold water to ensure that the equilibrium structure of the solder alloy can be completely cooled and solidified at high temperature;

[0035] 5) After the temperature of the quartz tube drops to room temperature, break the quartz tube and take out the solder alloy ingot.

[0036] Example 2

[0037] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic twin-cluster model containing a β-Sn(Bi) solid solution phase, and In and Cu are added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 4.0% In, 0.28% Cu, and the remainder is Sn. The purity of Sn, Bi, In, and Cu is 99.99%. The hypoeutectic twin-cluster model of the alloy contains a structural unit of [Sn-Sn] containing a β-Sn(Bi) solid solution phase. 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn-Sn 10 ]Bi3In1+1[Sn-Sn 10 ]Bi3Sn+1[Sn-Sn 10 ]Bi3Cu1+1[Bi-Bi6]Bi5=Sn 111 Bi 42 In8Cu1=Sn-38.2Bi-4.0In-0.28Cu.

[0038] The preparation method of the solder alloy is the same as that in Example 1.

[0039] Example 3

[0040] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic twin-cluster model containing a β-Sn(Bi) solid solution phase, and In and Ag are added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.9% In, 0.47% Ag, and the remainder is Sn. The purity of Sn, Bi, In, and Ag is 99.99%. The hypoeutectic twin-cluster model of the alloy contains a structural unit of [Sn-Sn] containing a β-Sn(Bi) solid solution phase. 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn-Sn 10 ]Bi3In1+1[Sn-Sn 10 ]Bi3Sn1+1[Sn-Sn 10 ]Bi3Ag1+1[Bi-Bi6]Bi5=Sn 111 Bi 42 In8Ag1=Sn-38.2Bi-3.9In-0.47Ag.

[0041] The preparation method of the solder alloy is the same as that of Example 1.

[0042] Example 4

[0043] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic dual-cluster model containing a β-Sn(Bi) solid solution phase, and In and Cu are added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.76% In, 0.70% Cu, and the balance is Sn. The purity of Sn, Bi, In, and Cu is 99.99%. The structural unit of the hypoeutectic dual-cluster model of the alloy containing the β-Sn(Bi) solid solution phase is [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 15[Sn-Sn 10 ]Bi3In1+5[Sn-Sn 10 ]Bi3Cu1+2[Bi-Bi6]Bi5=Sn 220 Bi 84 In 15 Cu5=Sn-38.2Bi-3.76In-0.7Cu.

[0044] The preparation method of the solder alloy is the same as that of Example 1.

[0045] Example 5

[0046] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic dual-cluster model containing a β-Sn(Bi) solid solution phase, and In and Ag are added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.9% In, 0.94% Ag, and the balance is Sn. The purity of Sn, Bi, In, and Ag is 99.99%. The structural unit of the hypoeutectic dual-cluster model of the alloy containing the β-Sn(Bi) solid solution phase is [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 8[Sn-Sn 10 ]Bi3In1+2[Sn-Sn 10 ]Bi3Ag1+1[Bi-Bi6]Bi5=Sn 110 Bi 42 Ag2In8=Sn-38.2Bi-3.9In-0.94Ag.

[0047] The preparation method of the solder alloy is the same as that of Example 1.

[0048] Example 6

[0049] A high-reliability, low-temperature Sn-Bi-based lead-free solder alloy is disclosed. The alloy is based on a hypoeutectic dual-cluster model containing a β-Sn(Bi) solid solution phase, and In, Ag, and Cu are added to obtain a high-reliability lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.76% In, 0.42% Cu, 0.47% Ag, and the balance is Sn. The purity of Sn, Bi, In, Cu, and Ag is 99.99%. The hypoeutectic dual-cluster model of the alloy, containing a β-Sn(Bi) solid solution phase, has a structural unit of [Sn-Sn 10 ]Bi3Sn1, the analytical expression of the hypoeutectic double cluster is: 15[Sn-Sn 10 ]Bi3In1+3[Sn-Sn 10 ]Bi3Cu1+2[Sn-Sn 10 ]Bi3Ag1+2[Bi-Bi6]Bi5=Sn 220 Bi 84 In 15 Cu5Ag2=Sn-38.2Bi-3.76In-0.47Ag-0.42Cu.

[0050] Comparative Example 1

[0051] A low-temperature Sn-Bi based lead-free solder alloy, wherein the components by mass percentage are as follows: Bi: 38.2%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0052] Comparative Example 2

[0053] A low-temperature Sn-Bi based lead-free solder alloy, wherein the components by mass percentage are as follows: Bi: 42.3%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0054] Comparative Example 3

[0055] A low-temperature Sn-Bi based lead-free solder alloy, wherein the components by mass percentage are as follows: Bi: 58%, the balance being Sn. The solder alloy is prepared in the same manner as in Example 1.

[0056] Comparative Example 4

[0057] A low-temperature Sn-Bi-based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0058] Comparative Example 5

[0059] A low-temperature Sn-Bi-based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, Cu: 0.26%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0060] Comparative Example 6

[0061] A low-temperature Sn-Bi based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, Ag: 0.45%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0062] Comparative Example 7

[0063] A low-temperature Sn-Bi-based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, Cu: 0.68%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0064] Comparative Example 8

[0065] A low-temperature Sn-Bi based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, Ag: 0.92%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0066] Comparative Example 9

[0067] A low-temperature Sn-Bi-based lead-free solder alloy, wherein the components by mass percentage are: Bi: 42.3%, In: 3.5%, Cu: 0.40%, Ag: 0.45%, and the balance is Sn. The solder alloy is prepared in the same manner as in Example 1.

[0068] The solders used in the examples and comparative examples were subjected to DSC testing to determine their melting points and determine the appropriate brazing process. The testing temperature range was 50°C to 250°C, with a heating and cooling rate of 10°C / min. The melting points of the various solder alloy components are shown in Table 1.

[0069] Table 1. Melting points of various solder alloys

[0070]

[0071] The microstructure images of Examples 1 to 5 are shown in Figure 1 The microstructure images of comparative examples 1 to 9 are shown in Figure 2 .

[0072] Comparative Example 1 does not add In, while Example 1 does. In is primarily dissolved in the β-Sn phase, providing solid solution strengthening. Examples 2 to 6 add certain amounts of Cu and / or Ag to the corresponding compositions of Example 1, resulting in significantly refined microstructures.

[0073] The areas of the gray phase, phase boundary and white phase of Comparative Example 1, Example 1 and Example 5 were statistically analyzed. Figure 3 As shown in the figure, the gray-white phase interface area of ​​Example 1 is significantly increased due to the addition of In. After adding In and Ag elements at the same time, the white phase area of ​​Example 5 is larger, and the Bi phase is more dispersed and uniform.

[0074] In Examples 2, 4, 6, Comparative Examples 5, 7 and 9, the solder alloys contain Cu element, and precipitation of dark Cu6(Sn,In)5 phase can be observed.

[0075] In Examples 3, 5, 6, Comparative Examples 6, 8 and 9, the solder alloys contain Ag element, and precipitation of gray Ag3 (Sn, In) phase can be observed.

[0076] The microstructure images of Example 5 and Comparative Example 8 at 300× are as follows: Figure 4 As shown in the figure, it can be observed that the size of the Ag3 (Sn, In) precipitated phase in Example 5 (Sn-38.2Bi-3.9In-0.94Ag) is significantly smaller than that in Comparative Example 8 (Sn-42.3Bi-3.5In-0.92Ag). Large precipitates are not conducive to improving the mechanical properties of the solder joint.

[0077] The shear strength test was carried out on the embodiment and comparative alloys. The solder joints prepared from the embodiment and comparative alloys were loaded on a tensile machine and shear tests were carried out at a rate of 0.3 mm / min. The shear strength results and shear displacement data are shown in Table 1. Figure 5 and Figure 6 .

[0078] Example 5 (Sn-38.2Bi-3.9In-0.94Ag) has both the best shear strength and shear displacement.

[0079] Wetting tests were performed on the example and comparative alloys as follows: 1) Prepare three 10 mg solder balls and place them on a polished copper sheet. 2) Reflow was performed according to the set reflow curve. After reflow, residual flux was cleaned with ethanol. 3) Wetting area was measured using Photoshop. Three groups of tests were performed for each component and the average value was taken. The area data after wetting is shown in Figure 2. Figure 7 .

[0080] Compared with Sn-Bi alloy, the addition of In element improves the plasticity of Sn-Bi-In alloy, but the wettability decreases. Based on the Sn-Bi-In alloy, the addition of Ag element further improves the plasticity of Sn-Bi-In-Ag alloy and also improves its wettability.

[0081] The Examples and Comparative Examples demonstrate that the melting point of the low-temperature Sn-Bi-based lead-free solder of the present invention is significantly lowered, all to below 128.7°C. Each Example (Sn-38.2Bi-X) corresponds to the Comparative Example (Sn-42.3Bi-X) with the same elements added, with only a significant reduction in Bi content and slight changes in In, Ag, and Cu. While maintaining a significant increase in strength, plasticity and ductility are significantly improved, enabling optimal application in low-temperature soldering processes.

[0082] The low-temperature solder of the present invention has the advantages of low melting point, high plasticity and toughness, high shear strength, and can match the current industrial medium and low temperature reflow process. It is suitable as a new low-temperature solder for use in low-temperature brazing application scenarios such as consumer electronic products or new solar cells.

[0083] Unless otherwise specified, all percentages described in the present invention are by mass.

Claims

1. A low-temperature Sn-Bi based lead-free solder alloy, characterized in that: The alloy is based on a hypoeutectic double cluster model containing a β-Sn(Bi) solid solution phase, and In, Ag, and Cu atoms are added to replace Sn atoms to obtain a high-reliability, low-temperature lead-free solder alloy. The alloy composition, by mass percentage, includes 38.2% Bi, 3.7-4.0% In, 0-1.0% Ag, 0-0.8% Cu, and the balance is Sn. The hypoeutectic double cluster model of the alloy contains a structural unit of [Sn-Sn] 10 ] Bi3Sn1, the hypoeutectic double cluster analytical expression is: 8[Sn-Sn 10 ] Bi3In1 + 2[Sn-Sn 10 ] Bi3Sn1 + 1[Bi-Bi6] Bi5 = Sn 112 Bi 42 In8 = Sn-38.2Bi-3.9In.

2. A low-temperature Sn-Bi based lead-free solder alloy according to claim 1, characterized in that: The alloy has a composition of 38.2% Bi, 3.9% In, 0.94% Ag, and the remainder Sn in terms of mass percentage. The hypoeutectic double cluster model of the alloy is 8[Sn-Sn 10 ] Bi3In1 + 2[Sn-Sn 10 ] Bi3Ag1 + 1[Bi-Bi6]Bi5 = Sn 110 Bi 42 In8Ag2 = Sn-38.2Bi-3.9In-0.94Ag.

Images