Tin-based alloy welding optimization method

Abstract

The invention relates to the technical field of metal materials, in particular to a tin-based alloy welding optimization method. The method comprises the steps that the welding performance improvement requirement is determined, and the performance improvement requirement comprises electrical performance optimization; based on the electrical property optimization, a first tin-based alloy is selected, and the cooling rate of a molten pool in the welding process is regulated and controlled to be 1-2 DEG C / s, so that the supercooling degree of the molten pool reaches 5-10 DEG C, a blocky eutectic Ag3Sn phase or a blocky primary Ag3Sn phase is formed, and the electrical resistivity of a welding spot is reduced; the first tin-based alloy comprises, by weight, 8%-60% of Bi, 0.5%-1.5% of Ag and the balance Sn. According to the scheme provided by the invention, the distribution and the size of the heterogeneous nucleation substrate can be optimized by regulating and controlling the cooling rate and the supercooling degree of the molten pool, and the resistivity of a welding spot can be remarkably reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal materials, in particular to a tin-based alloy welding optimization method. BACKGROUND

[0002] Tin-based alloys are widely used in electronic industries and welding material fields such as electronic packaging, integrated circuit interconnection, and power battery tab welding due to their good weldability, electrical conductivity, and cost advantage, and are key materials for microelectronic manufacturing and power electronic device assembly. Traditional tin-based alloys are mostly single-component or binary alloys (such as Sn-Pb and Sn-Ag), and their microstructures are mainly single-phase or simple eutectic phase. Although they can meet the basic welding requirements, with the development of electronic devices towards high power, miniaturization, and high reliability, more stringent requirements are put forward for the mechanical properties (such as tensile strength and ductility) of the welding points, the electrical properties (such as low resistivity), and the long-term service stability. For example, in high-density chip packaging, the welding points need to withstand both thermal cycle stress and current load. Due to the problems of poor microstructure uniformity and coarse strengthening phase, the traditional alloys are prone to interface cracking or electromigration failure, and have been difficult to meet the performance requirements under complex working conditions. In particular, in scenarios where electrical properties, tensile strength, and ductility need to be optimized simultaneously, it is difficult for traditional methods to achieve multi-objective synergistic optimization through single alloy component or simple process regulation, resulting in that the performance of the welding points cannot meet the high integration and multi-functionalization requirements of modern electronic devices. In addition, the existing technology lacks systematic research on the precise control of the cooling rate and supercooling degree of the molten pool, and it is difficult to achieve directional optimization of performance through microstructure regulation. SUMMARY

[0003] To overcome the problems in the related art, the present application provides a tin-based alloy welding optimization method, which can optimize the distribution and size of heterogeneous nucleation substrates by regulating the cooling rate and supercooling degree of the molten pool, and significantly reduce the resistivity of the welding points.

[0004] The present application provides a tin-based alloy welding optimization method, which comprises the following steps:

[0005] Determine the performance improvement requirements of welding, which include electrical property optimization;

[0006] Based on the electrical property optimization, a first tin-based alloy is selected, the cooling rate of the molten pool during welding is regulated to 1-2 ℃ / s, the supercooling degree of the molten pool is regulated to 5-10 ℃, to form blocky eutectic Ag3Sn phase or blocky primary Ag3Sn phase, and reduce the resistivity of the welding points; the first tin-based alloy comprises 8-60% Bi, 0.5-1.5% Ag, and the balance Sn in terms of weight percentage.

[0007] In some embodiments, the performance requirements also include tensile strength optimization;

[0008] Based on the tensile strength optimization, a second tin-based alloy is selected to control the cooling rate of the molten pool during welding to be 5-10 °C / s, so that the undercooling degree of the molten pool reaches 15-25 °C, to form fine and dispersed eutectic Ag3Sn phases; the second tin-based alloy includes 8-15% Bi, 0.5-1.5% Ag, and the balance Sn in terms of percentage by weight.

[0009] In some embodiments, the performance requirement further includes ductility optimization.

[0010] Based on the ductility optimization, a third tin-based alloy is selected to control the cooling rate of the molten pool during welding to be 1-5 °C / s, so that the undercooling degree of the molten pool reaches 5-15 °C, to form blocky and branched primary Ag3Sn phases; the third tin-based alloy includes 50-60% Bi, 0.5-1.5% Ag, and the balance Sn in terms of percentage by weight.

[0011] In some embodiments, the heterogeneous nucleation substrate of the eutectic Ag3Sn phase includes solid Bi particles and β-Sn grains; and the heterogeneous nucleation substrate of the primary Ag3Sn phase includes liquid Bi particles and β-Sn grains.

[0012] In some embodiments, before welding, the optimization method further includes preparing a first tin-based alloy, and the preparation of the first tin-based alloy specifically includes:

[0013] Sn, Bi, and Ag are added to a crucible in terms of percentage by mass, heated to melt under an argon atmosphere, and uniformly stirred and then cooled and solidified.

[0014] In some embodiments, after the preparation of the first tin-based alloy and before welding, the optimization method further includes subjecting the first tin-based alloy to a solid solution treatment, the temperature of the solid solution treatment being 115-155 °C, and the solid solution time being 4-6 hours, to promote the uniform distribution of Bi particles in Sn and provide a uniform nucleation substrate for heterogeneous nucleation.

[0015] In some embodiments, if the first tin-based alloy is the second tin-based alloy, then the solid Bi particles have an average particle size of 1-3 μm and a distribution density ≥ 105 / mm2 under the condition of solid solution at 145-155 °C for 4-5 hours. 3 ;

[0016] If the first tin-based alloy is the third tin-based alloy, then the liquid Bi particles have a dispersion uniformity ≥ 90% in the molten pool under the condition of solid solution at 115-125 °C for 5-6 hours.

[0017] In some embodiments, after the solution treatment and before the soldering, the optimization method further comprises obtaining an actual liquidus temperature of the first Sn-based alloy by a DSC cycle method, specifically comprising the following steps:

[0018] (1) heating the first Sn-based alloy to the solidus temperature and holding at the solidus temperature for 30 min; then heating the first Sn-based alloy to 30-60℃ above the solidus temperature at a heating rate of 10℃ / min;

[0019] (2) cooling the Sn-based alloy to 30-60℃ below the solidus temperature at the same cooling rate as the heating rate;

[0020] (3) cyclically performing steps (1) and (2) with an isothermal temperature increasing by 0.5℃ as a gradient until no endothermic peak appears in the heating process;

[0021] (4) recording the highest temperature T1 at which an endothermic peak exists and the lowest temperature T2 at which no endothermic peak appears in the heating process, and the actual liquidus temperature is the arithmetic mean of T1 and T2, which is used to calculate the solder pool supercooling degree.

[0022] In some embodiments, the regulating the cooling rate of the solder pool during the soldering process specifically comprises the following steps:

[0023] forced cooling is performed by blowing compressed cold air to the solder pool area, the flow rate of the cold air is controlled by a flow regulating valve, and the inlet temperature of the cold air is set to a specific range; the temperature of the solder pool is monitored in real time by an infrared thermometer during the forced cooling, and the heating power is adjusted by a PID heating control to compensate for the temperature fluctuation during the cooling.

[0024] Compared with the prior art, the technical scheme provided by the application has the following beneficial effects:

[0025] By targeting the design of heterogeneous nucleation substrates, the application regulates the solidification behavior of Sn-based alloys under a low supercooling degree of 5-10℃, induces the growth of Ag3Sn phase in a blocky morphology using nucleation driving force, reduces the number of grain boundaries of traditional fine dispersed phase, plays a role in reducing the electron scattering interface, and thus reduces the electron transport resistance. At the same time, the blocky Ag3Sn phase forms a continuous conductive framework in the Sn matrix, eliminates the electronic transport bottleneck caused by isolated fine phase under high supercooling, improves the uniformity of current distribution, and plays a role in inhibiting the electromigration failure caused by local current concentration. In addition, through the synergistic control of a cooling rate of 1-2℃ / s and a supercooling degree of 5-10℃, the solder joint resistivity can be significantly reduced compared with the existing conventional soldering process, which can provide key technical support for high-end electronic packaging scenarios such as power devices, high-density interconnection substrates, etc. BRIEF DESCRIPTION OF DRAWINGS

[0026] The above and other objects, features and advantages of this application will become more apparent from the more detailed description of exemplary embodiments thereof in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments thereof.

[0027] Figure 1 These are the DSC cyclic test curves for Sn-10Bi-1Ag and Sn-57Bi-1Ag; where (a, c) are the DSC cyclic test program settings; (b, d) are the heating curves during the cyclic test. In b and d, the numbers next to the curves represent the number of cycles.

[0028] Figure 2 This is a flowchart of the statistical analysis of different Bi particle sizes in Sn-10Bi-1Ag of Example 3;

[0029] Figure 3 Here are the thermal expansion coefficient curves for Sn-10Bi-1Ag in Example 3 and Sn-57Bi-1Ag in Example 4:

[0030] Figure 4 The images are SEM images of the Sn-10Bi-1Ag alloy of Example 1 and the Sn-57Bi-1Ag alloy of Example 4 at different strain levels and after fracture, with a strain rate of 5×10-4 s-1; where (ae) is the Sn-10Bi-1Ag alloy and (fj) is the Sn-57Bi-1Ag alloy. Detailed Implementation

[0031] Preferred embodiments of the present application will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the present application are shown in the drawings, it should be understood that the present application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0032] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0033] It should be understood that although the terms "first," "second," "third," etc., may be used in this application to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this application, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0034] Tin-based alloys are key materials in electronic packaging and welding, and their microstructure and performance control directly affect the reliability and service life of electronic devices. Traditional tin-based alloys mostly rely on single-component or simple binary systems, which can meet basic welding requirements. However, with the development of electronic devices towards high power and high density, more stringent requirements are placed on the mechanical strength, ductility, and electrical performance of solder joints. Sn-Bi-Ag ternary alloys offer potential for performance optimization by adjusting the element ratio and microstructure. However, traditional tin-based alloys are dominated by a single phase or a simple eutectic phase, resulting in insufficient microstructure uniformity. This leads to problems such as increased resistivity and interface failure in high-power scenarios. For example, in the welding of battery tabs, the solder joints need to withstand large current loads, and traditional alloys, due to the uneven distribution of reinforcing phases, cannot meet the low resistivity requirements.

[0035] To address the aforementioned issues, this application provides a method for optimizing the welding of tin-based alloys. By adjusting the alloy composition and solidification process, the nucleation and growth of the Ag3Sn phase can be controlled. A specific Bi content can alter the undercooling behavior of the molten pool. Combined with cooling rate regulation, bulk eutectic or primary Ag3Sn phases can be directionally formed, thereby achieving optimized electrical performance.

[0036] To better illustrate this technical solution, this article defines some technical terms:

[0037] Heterogeneous nucleation in this paper refers to the process in which, during the solidification of a metal alloy, a new phase does not spontaneously and uniformly form in the liquid phase, but preferentially attaches to an existing substrate (such as solid particles or grain surfaces) to form nuclei.

[0038] In this paper, supercooling refers to the difference between the actual liquid phase temperature and the equilibrium solidification temperature.

[0039] The actual liquid phase temperature in this article refers to the true temperature of the liquid phase alloy during the actual welding or solidification process.

[0040] In this paper, the equilibrium solidification temperature refers to the theoretical temperature at which a metallic alloy begins to solidify (precipitate solid phase) from the liquid phase under thermodynamic equilibrium, i.e., the liquidus temperature on the phase diagram. At this point, the Gibbs free energies of the liquid phase and the solid phase are equal, and the solidification process is in dynamic equilibrium.

[0041] In this paper, tensile strength refers to the maximum load-bearing capacity of a material to resist fracture, which directly affects the material's resistance to damage under tensile, bending and other loads.

[0042] In this paper, ductility refers to the ability of a material to undergo plastic deformation before fracture, reflecting the material's ability to buffer deformation under stress and avoid brittle fracture.

[0043] In this paper, resistivity refers to the core parameter of a material's conductivity, which directly affects the energy consumption and signal transmission stability of conductive components.

[0044] The equilibrium solidification temperature of the first tin-based alloy of this application can be determined by thermodynamic calculations (e.g., JMatPro software).

[0045] This application proposes an optimized method for welding tin-based alloys, comprising the following steps:

[0046] Determine the performance improvement requirements for welding, including electrical performance optimization;

[0047] Based on the aforementioned electrical performance optimization, a first tin-based alloy is selected, and the cooling rate of the molten pool during the welding process is controlled to be 1℃~2℃ / s, so that the undercooling of the molten pool reaches 5℃~10℃, in order to form a blocky eutectic Ag3Sn phase or a blocky primary Ag3Sn phase, thereby reducing the resistivity of the solder joint; the first tin-based alloy comprises 8%~60% Bi, 0.5%~1.5% Ag and the balance Sn by weight percentage.

[0048] Specifically, during the welding process, the addition of Bi increases the viscosity of the melt and slows down atomic diffusion. The molten pool cooling rate refers to the rate at which the temperature of the molten pool decreases per unit time. Molten pool cooling can be achieved using compressed air forced cooling combined with PID heating compensation. By precisely controlling the molten pool cooling rate within the range of 1℃ to 2℃ / s, the undercooling of the molten pool can be stably controlled within 5℃ to 10℃. Under this undercooling condition, the heterogeneous nucleation of the alloy solidification has a moderate driving force, and the Ag3Sn phase grows in a blocky morphology, thereby reducing the number of grain boundaries caused by traditional fine dispersed phases. Since grain boundaries are the main interfaces for electron scattering, the blocky phase structure reduces electron transport resistance, thus achieving the technical objective of reducing the resistivity of the weld joint. Furthermore, the blocky Ag3Sn phase forms a continuously distributed conductive framework in the Sn matrix, which can avoid the electron transport bottleneck caused by the isolated dispersion of fine Ag3Sn phase under high supercooling, improve the uniformity of current distribution, effectively suppress the risk of electromigration failure caused by local current concentration, meet the welding scenarios of high-power electronic devices, improve conductivity while maintaining welding reliability, and solve the problem of electrical performance bottleneck caused by unreasonable phase structure of traditional alloys.

[0049] Correspondingly, the temperature monitoring involved in this application can be carried out in real time using an infrared thermal imager.

[0050] In some embodiments, the performance requirements also include tensile strength optimization;

[0051] Based on the aforementioned tensile strength optimization, a second tin-based alloy is selected, and the cooling rate of the molten pool during the welding process is controlled to be 5℃~10℃ / s, so that the undercooling of the molten pool reaches 15℃~25℃, in order to form a fine and dispersed eutectic Ag3Sn phase; the second tin-based alloy comprises 8%~15%Bi, 0.5%~1.5%Ag and the balance Sn by weight percentage.

[0052] Specifically, when welding requires increased tensile strength, a tin-based alloy system with a Bi content of 8%–15% and an Ag content of 0.5%–1.5% can be selected. This composition design ensures sufficient eutectic reaction and avoids embrittlement caused by excessive Bi. During welding, the molten pool is forcibly cooled by compressed air, with the cooling rate precisely controlled within the range of 5°C–10°C / s, resulting in a supercooling of 15°C–25°C. At this supercooling, heterogeneous nucleation during alloy solidification has a moderately high driving force, causing the Ag3Sn phase to heterogeneously nucleate at the β-Sn grain boundaries in a eutectic form. The high cooling rate inhibits grain growth, ultimately forming a fine-sized and uniformly distributed eutectic Ag3Sn phase. These dispersed hard phases effectively hinder dislocation movement, while the fine microstructure reduces stress concentration, thereby improving the tensile strength of the weld joint.

[0053] Through the above technical solution, this application can achieve the following in electronic packaging welding: by selecting a tin-based alloy with a specific Bi content and adjusting the supercooling by controlling the cooling rate, the fine and dispersed eutectic Ag3Sn phase formed can maintain good interfacial bonding while improving strength. This significantly reduces the risk of interfacial peeling or breakage of solder joints under high stress conditions, and extends the service life of electronic devices. It is particularly suitable for solder joints subjected to cyclic thermal stress in high-density chip packaging, as well as scenarios requiring high-reliability connections such as power battery tab welding.

[0054] In some embodiments, the performance requirements also include scalability optimization;

[0055] Based on the aforementioned ductility optimization, a third tin-based alloy is selected, and the cooling rate of the molten pool during the welding process is controlled to be 1℃~5℃ / s, so that the undercooling of the molten pool reaches 5℃~15℃, in order to form blocky and branched primary Ag3Sn phases; the third tin-based alloy comprises 50%~60% Bi, 0.5%~1.5% Ag and the balance Sn by weight percentage.

[0056] Specifically, when welding requires increased tensile strength, a tin-based alloy system with a Bi content of 50%–60% and an Ag content of 0.5%–1.5% can be selected. After the third tin-based alloy melts, the cooling rate is controlled within the range of 1℃–5℃ / s, maintaining the supercooling of the molten pool within the range of 5℃–15℃. Under these conditions, liquid Bi particles act as a heterogeneous nucleation substrate, promoting the formation of blocky nuclei in the Ag3Sn phase during the early stages of solidification. As the temperature gradient changes, the Ag3Sn phase extends and grows in multiple directions into a branched structure. The dispersed liquid Bi particles in the high-Bi content alloy not only provide nucleation sites but also inhibit the continuous growth of the Sn matrix, forming an interwoven Ag3Sn phase network. This composite structure, through the stress dispersion effect of the branched Ag3Sn phase and the interfacial slip effect of Bi particles, jointly enhances the plastic deformation capacity of the weld joint.

[0057] Through the above technical solution, this application effectively solves the problem of decreased ductility of high Bi content weld joints due to excessive brittle phase. By controlling the morphological distribution of Ag3Sn phase and the dispersion state of Bi particles, the weld joint can disperse local stress through the mechanical interlocking of the branch structure and the plastic flow of Bi particles when subjected to bending or tensile loads, thereby avoiding brittle fracture. It is suitable for welding scenarios of power battery tabs that require both high strength and deformation capacity.

[0058] In some embodiments, the heterogeneous nucleation substrate of the eutectic Ag3Sn phase comprises solid Bi particles and β-Sn grains; the heterogeneous nucleation substrate of the primary Ag3Sn phase comprises liquid Bi particles and β-Sn grains.

[0059] Specifically, when the Bi content is 8%–15%, Bi preferentially precipitates as solid particles during alloy solidification, forming a heterogeneous nucleation substrate together with β-Sn grains. This is because the solid Bi particle substrate has the advantages of low interfacial energy and high nucleation efficiency, and under the action of higher undercooling, it has a higher nucleation driving force, thereby promoting the nucleation of fine eutectic Ag3Sn phase. When the Bi content is 50%–60%, Bi exists in the molten pool in the form of liquid particles, forming a substrate with β-Sn grains. The liquid Bi particle substrate has high interfacial energy and low nucleation efficiency, and achieves a lower nucleation driving force through lower undercooling, thereby helping to form coarse bulky primary Ag3Sn phase.

[0060] When applying the technical solution of this application, once the welding performance requirements are determined, the tensile strength of the solder joint can be improved by selecting a tin-based alloy with a Bi content of 8% to 15% and high undercooling to form a fine, dispersed eutectic Ag3Sn phase, according to the desired improvement in welding performance. The ductility of the solder joint can be improved by selecting a tin-based alloy with a Bi content of 50% to 60% and low undercooling to form a bulky primary Ag3Sn phase. When the requirement to improve welding performance includes reducing the resistivity of the solder joint, a wider range of Bi content can be selected, requiring only a lower undercooling to achieve a lower nucleation driving force. Specifically, when adapting to low-temperature welding scenarios, a tin-based alloy with a higher Bi content can be selected, using a liquid substrate as a heterogeneous nucleation site.

[0061] In some embodiments, to improve the uniformity of the microstructure, the optimization method further includes preparing a first tin-based alloy before welding, wherein preparing the first tin-based alloy specifically includes:

[0062] Sn, Bi, and Ag were added to the crucible according to their mass percentages, heated to melt under an argon atmosphere, stirred evenly, and then cooled and solidified.

[0063] Specifically, after adding metal raw materials in a preset ratio into a sealed melting furnace, the air inside the furnace is continuously replaced by argon gas. When the temperature rises to the point where the metal is completely liquefied, a stirring device is activated to create a vortex motion in the liquid alloy. During this process, Bi and Sn matrix achieve full miscibility, and Ag is uniformly dispersed in the melt in atomic form. The melt is slowly cooled to its solidification point under continuous stirring, forming a uniformly composed alloy ingot. This solves the problem of uneven microstructure caused by oxidation and segregation in traditional alloy preparation processes, providing a high-quality raw material guarantee for controlling the undercooling of the weld pool and strengthening phase nucleation in welding processes.

[0064] To improve the uniformity of liquid phase heat dissipation, this application also controls the distribution of the nucleation substrate by holding time and temperature. Specifically, after the preparation of the first tin-based alloy and before welding, the optimization method further includes a solution treatment of the first tin-based alloy. The solution treatment temperature is 115℃~155℃ and the solution treatment time is 4~6 hours to promote the uniform distribution of Bi particles in Sn and provide a uniform nucleation substrate for heterogeneous nucleation.

[0065] Specifically, before welding, the alloy undergoes solution treatment, resulting in a uniform, dispersed distribution of Bi particles within the Sn matrix. As the molten pool cools, the uniformly distributed Bi particles act as a heterogeneous nucleation substrate, promoting the preferential attachment of the Ag3Sn phase to the Bi particle surface during solidification, thereby inhibiting the coarsening or segregation of the Ag3Sn phase. For example, after solution treatment at 145°C for 5 hours, the average spacing between Bi particles can be controlled within the micrometer range, allowing for more precise control of the molten pool undercooling.

[0066] Through the above technical solution, this application solves the problem of insufficient heterogeneous nucleation substrate caused by uneven distribution of Bi particles, enabling the Ag3Sn phase to obtain uniform and dense nucleation sites during solidification, thereby improving the compactness and uniformity of the internal structure of the weld joint, and laying the microstructure foundation for subsequent control of the undercooling degree and cooling rate of the molten pool.

[0067] In some embodiments, if the first tin-based alloy is a second tin-based alloy, then the solid Bi particles are dissolved at 145°C to 155°C for 4 to 5 hours, the average particle size of the solid Bi particles is 1 to 3 μm, and the distribution density in the Sn matrix is ​​≥10. 5 pcs / mm 3 ;

[0068] If the first tin-based alloy is a third tin-based alloy, it is dissolved at 115–125°C for 5–6 hours to ensure that the uniformity of the dispersion of liquid Bi particles in the molten pool is ≥90%.

[0069] Solution treatment refers to holding the alloy at a specific temperature, which can be achieved using a constant temperature chamber or heat treatment furnace. The uniform diffusion of alloying elements is achieved through coordinated control of temperature and time. The average particle size of solid Bi particles refers to the median particle diameter measured by a metallographic microscope or scanning electron microscope. This can be achieved through mechanical crushing combined with annealing. Particle size control directly affects the spatial distribution density of the heterogeneous nucleation substrate. The dispersion uniformity of liquid Bi particles refers to the spatial consistency of the liquid Bi phase in the molten pool. This can be achieved by matching and controlling the stirring rate and cooling rate. See [link to relevant documentation]. Figure 2 , Figure 2This is a flowchart illustrating the analysis process for different types of Bi particle sizes in the Sn-10Bi-1Ag alloy. The statistical analysis first involves dividing the regions into areas for each type of Bi particle, and the uniformity values ​​are obtained through statistical calculations of the microstructure images using image analysis software.

[0070] When processing the second tin-based alloy, the higher temperature and shorter solution time promote the formation of fine solid particles of Bi in the Sn matrix, and provide a uniform nucleation substrate for the eutectic Ag3Sn phase through dual control of particle size and distribution density; when processing the third tin-based alloy, the lower temperature and longer solution time maintain the liquid properties of Bi particles, and the constraint of dispersion uniformity ensures that the primary Ag3Sn phase can grow uniformly along the surface of liquid Bi to form a branched structure when the molten pool solidifies.

[0071] Compared with existing technologies, traditional solution treatment typically uses fixed temperature and time parameters, which cannot adapt to alloy systems with different Bi contents. This solution sets different treatment conditions for high-Bi and low-Bi alloys, and solves the problem of microstructure coarsening caused by uneven distribution of Bi particles during solidification by controlling the correlation between temperature-time combination and phase state.

[0072] Through the above technical solutions, this application has achieved optimization of the pretreatment of tin-based alloys with different compositions before welding, ensuring that Bi particles have controllable size and spatial distribution characteristics when used as heterogeneous nucleation substrates, thereby promoting the uniform precipitation of Ag3Sn phase during subsequent welding process and effectively improving the mechanical and electrical performance stability of the weld joint.

[0073] In some embodiments, after solution treatment and before welding, the optimization method further includes obtaining the actual liquidus temperature of the first tin-based alloy by DSC cycling, specifically including the following steps:

[0074] (1) Heat the first tin-based alloy to the solidus temperature and hold it at the solidus temperature for 30 min; then heat the first tin-based alloy to 30℃~60℃ above the solidus temperature at a heating rate of 10℃ / min.

[0075] (2) Cool the tin-based alloy to 30°C to 60°C below the solidus temperature using the same cooling rate as the heating rate;

[0076] (3) The isothermal temperature is increased in increments of 0.5℃, and steps (1) and (2) are repeated until no endothermic peaks appear during the heating process;

[0077] (4) Record the highest temperature T1 with an endothermic peak and the lowest temperature T2 without an endothermic peak during the heating process. The actual liquid phase temperature is the arithmetic mean of T1 and T2. The actual liquid phase temperature is used to calculate the undercooling of the molten pool.

[0078] The DSC cyclic method refers to a thermal analysis method that uses a differential scanning calorimeter to perform multiple heating and cooling cycles. Specifically, it can be implemented using standard DSC equipment with a programmed temperature control module. Its purpose is to improve the accuracy of liquid phase temperature measurement by eliminating the influence of the material's thermal history. The solidus temperature refers to the critical temperature at which the alloy begins to melt, which can be determined using alloy phase diagrams or thermal analysis curves. Its function is to provide a reference temperature range for heating and cooling cycles. The endothermic peak refers to the peak signal formed on the DSC curve due to the endothermic reaction during the material's phase transformation. It can be detected by heat flow changes, and its function is to determine whether a melting phase transformation has occurred by observing the presence or absence of the peak. The actual liquid phase temperature refers to the true temperature at which the material completely melts. It can be calculated after eliminating thermal hysteresis errors through multiple cyclic tests, and its function is to provide an accurate temperature reference for calculating the supercooling of the molten pool.

[0079] Specifically, by performing multiple heating and cooling cycles using the DSC (Digital Superheated Circulation) method, the influence of the material's thermal history can be gradually eliminated, making the measured actual liquid phase temperature closer to the true value. In each cycle, the isothermal temperature increases with a fixed gradient until no more endothermic peaks appear during heating, indicating that the material has completely melted. By recording the critical temperatures T1 and T2 and taking the average, deviations caused by equipment errors or thermal hysteresis in a single test can be effectively eliminated, thus providing a reliable basis for controlling the undercooling of the molten pool during subsequent welding processes.

[0080] The specific measurement process of this DSC cyclic method is as follows:

[0081] In the first DSC cycle, the sample was heated to the melting initiation temperature (209℃ for Sn-10Bi-1Ag alloy and 134℃ for Sn-57Bi-1Ag alloy), at which point the alloy exists in a state of coexistence of liquid and solid phases. This temperature was held for 30 min. Subsequently, heating was continued at a rate of 10℃ / min to 250℃, followed by cooling at the same rate to 150℃ (100℃ for Sn-57Bi-1Ag alloy). In subsequent cycles, the isothermal temperature was increased by 0.5℃ each time until no endothermic peak was observed during heating. The absence of a significant endothermic peak indicates that the alloy had completely melted at this isothermal temperature, and no solid phase remained. Figure 1 As shown by the red curve, the liquidus temperature is defined as the average of the highest temperature with an endothermic peak and the lowest temperature without an endothermic peak. The Sn-10Bi-1Ag alloy exhibits an endothermic peak at an isotherm of 213℃, which disappears at an isotherm of 213.5℃; therefore, its liquidus point temperature is 213.3℃. Similarly, the liquidus point temperature of the Sn-57Bi-1Ag alloy is 137.3℃. The error in measuring the alloy liquidus point temperature using this method is within ±0.25℃.

[0082] In some embodiments, regulating the cooling rate of the molten pool during welding specifically includes the following steps:

[0083] Compressed cold air is blown into the molten pool area for forced cooling, and the flow rate of the cold air is controlled by a flow regulating valve, with the cold air inlet temperature set to a specific range. During the forced cooling process, the temperature of the molten pool is monitored in real time by an infrared thermometer, and the heating power is adjusted by PID heating control to compensate for temperature fluctuations during the cooling process.

[0084] Compressed cold air refers to the low-temperature airflow generated by a gas compression device, specifically an air compressor combined with a refrigeration unit. Its function is to accelerate heat dissipation from the molten pool through forced convection heat transfer. A flow control valve is a control element that linearly regulates the gas flow rate, specifically a proportional control valve or an electric control valve. Its function is to precisely control the cooling intensity by changing the cold air velocity. An infrared thermometer is a non-contact temperature sensor based on the principle of infrared radiation, specifically a short-wave infrared detector. Its function is to achieve real-time dynamic monitoring of the molten pool surface temperature. PID heating control is a closed-loop temperature regulation system based on a proportional-integral-derivative algorithm, specifically a programmable logic controller combined with a resistance heating module. Its function is to maintain temperature stability by dynamically adjusting the heating power.

[0085] Specifically, during welding, compressed cold air is blown onto the surface of the molten pool at a set flow rate, achieving initial cooling by forcibly carrying away heat through convection. A flow control valve adjusts the cold air flow rate according to the target cooling rate; for example, reducing the flow rate when a lower cooling rate is needed and increasing the flow rate when a higher cooling rate is required. An infrared thermometer continuously collects molten pool temperature data and feeds it back to the control system. A PID algorithm dynamically adjusts the heating power based on the deviation between the measured temperature and the set value; for example, increasing the heating power to compensate for heat loss when the temperature drops too quickly and reducing the heating power to enhance cooling when the temperature drops too slowly. Controlling the cold air inlet temperature avoids cooling efficiency deviations caused by ambient temperature fluctuations; for example, a lower inlet temperature can be used in high-temperature environments during summer to maintain the cooling effect.

[0086] In some specific implementations, the cold air velocity can be set to 5–20 m / s. 3 The inlet temperature can be controlled between -5℃ and 10℃ per hour. The sampling frequency of the infrared thermometer can be set to more than 10 times per second to ensure real-time temperature monitoring. The parameters of the PID controller can be tuned according to the heat capacity characteristics of the molten pool, for example, the proportional coefficient can be set to 0.8, the integral time to 120 seconds, and the derivative time to 30 seconds.

[0087] Through the above technical solution, this application can precisely control the heat transfer efficiency during the cooling process of the molten pool, effectively suppressing the microstructure inhomogeneity caused by abrupt temperature gradient changes. This control method can ensure the repeatability of molten pool undercooling in different batches of welding processes, avoid abnormal Ag3Sn phase morphology caused by uncontrolled cooling rate, and thus ensure the resistivity stability and mechanical property consistency of the weld joint.

[0088] This application further proposes specific steps for controlling the cooling rate of the molten pool during welding, including using compressed cold air to blow into the molten pool area for forced cooling, and controlling the flow rate of the cold air to be 5-20 m / s using a flow regulating valve. 3 The cooling rate is 1℃~5℃ / s, and the inlet temperature of the cold air is -5℃~10℃. During the forced cooling process, the temperature of the molten pool is monitored in real time by an infrared thermometer, and the heating power is adjusted from 0 to 500W by PID heating control to compensate for temperature fluctuations during the cooling process. If the cooling rate is 1℃~5℃ / s, the cold air flow rate is set to 5~12m. 3 / h, PID heating control adjusts the heating power to 100~300W; when the cooling rate needs to be adjusted to 5℃~10℃ / s, the cold air flow rate is set to 12~20m. 3 / h, PID heating control adjusts the heating power from 0 to 100W.

[0089] Forced cooling by blowing compressed cold air into the molten pool area refers to actively cooling the molten pool by generating a low-temperature airflow from an external air source. This can be achieved using an air compressor with temperature control and directional nozzles. Controlling the airflow speed and temperature directly affects the heat dissipation efficiency of the molten pool. Controlling the cold air flow rate using a flow regulating valve involves changing the gas flow rate per unit time by adjusting the valve opening. This can be achieved using a proportional-integral (PI) control valve with a flow sensor in closed-loop control to precisely match the heat dissipation intensity required for the target cooling rate. PID heating control adjusts the heating power by dynamically adjusting the welding heat source output based on temperature feedback signals. This can be achieved by collecting real-time temperature data using thermocouples or infrared thermometers and calculating the compensation power value using a proportional-integral-derivative (PID) algorithm to offset temperature fluctuations caused by forced cooling.

[0090] Specifically, during the welding process, when a medium-low cooling rate of 1℃~5℃ / s is required, this is achieved by setting a cooling rate of 5~12m. 3 A cold air flow rate of 100-300W provides basic heat dissipation, while a PID heating power of 100-300W is activated to compensate for the temperature of the molten pool, keeping the cooling curve linear and flat. When switching to high-speed cooling of 5-10℃ / s is required, the cold air flow rate is increased to 12-20m / s. 3To enhance heat dissipation, the PID heating power is reduced to 0-100W to allow for faster cooling. Real-time temperature data monitored by an infrared thermometer is transmitted to the control system. By comparing the preset cooling rate with the actual temperature change rate, the system dynamically adjusts the flow control valve opening and heating power output, forming a closed-loop control.

[0091] Through the above technical solution, this application can flexibly adjust the cooling strategy according to the difference in alloy composition, achieve precise control of the target cooling rate while maintaining the stability of the molten pool temperature, ensure that the morphology and distribution of the Ag3Sn phase meet the preset performance requirements, and effectively improve the uniformity and performance consistency of the weld joint structure.

[0092] The present invention will be further illustrated below with specific embodiments.

[0093] Example 1

[0094] 1) Weigh out 89wt% Sn, 10wt% Bi and 1wt% Ag by mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 350℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-10Bi-1Ag alloy ingot.

[0095] 2) Solution treatment was performed on the Sn-10Bi-1Ag alloy ingot at a temperature of 150℃ for 4 hours.

[0096] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 1 m³ / s. 3 Set the cold air temperature to 10℃ and the heating power to 30W. Control the cooling rate to 1℃ / s to make the solder joint supercooled to 5℃. Then continue to maintain the cooling rate of 1℃ / s until the solder joint solidifies.

[0097] Example 2

[0098] 1) Weigh 89wt% Sn, 10wt% Bi and 1wt% Ag according to mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 350℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-10Bi-1Ag alloy ingot.

[0099] 2) Solution treatment was performed on the Sn-10Bi-1Ag alloy ingot at a temperature of 150℃ for 4 hours.

[0100] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 2m. 3Set the cold air temperature to 5℃ and the heating power to 60W. Control the cooling rate to 2℃ / s to make the solder joint supercooled to 10℃. Then continue to maintain the cooling rate of 2℃ / s until the solder joint solidifies.

[0101] Example 3

[0102] 1) Weigh 89wt% Sn, 10wt% Bi and 1wt% Ag according to mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 350℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-10Bi-1Ag alloy ingot.

[0103] 2) Solution treatment was performed on the Sn-10Bi-1Ag alloy ingot at a temperature of 150℃ for 4 hours.

[0104] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 4m³ / h. 3 Set the cold air temperature to 5℃ and the heating power to 100W. Control the cooling rate to 5℃ / s to make the solder joint supercooled to 20℃. Then continue to maintain the cooling rate of 5℃ / s until the solder joint solidifies.

[0105] Example 4

[0106] 1) Weigh 42wt% Sn, 57wt% Bi and 1wt% Ag according to the mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 200℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-57Bi-1Ag alloy ingot.

[0107] 2) Solution treatment was performed on the Sn-57Bi-1Ag alloy ingot at a temperature of 120℃ for 6 hours.

[0108] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 1 m³ / s. 3 Set the cold air temperature to 10℃ and the heating power to 30W. Control the cooling rate to 1℃ / s to make the solder joint supercooled to 5℃. Then continue to maintain the cooling rate of 1℃ / s until the solder joint solidifies.

[0109] Example 5

[0110] 1) Weigh 42wt% Sn, 57wt% Bi and 1wt% Ag according to the mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 200℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-57Bi-1Ag alloy ingot.

[0111] 2) Solution treatment was performed on the Sn-57Bi-1Ag alloy ingot at a temperature of 120℃ for 6 hours.

[0112] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 2m. 3 Set the cold air temperature to 5℃ and the heating power to 60W. Control the cooling rate to 2℃ / s to make the solder joint supercooled to 10℃. Then continue to maintain the cooling rate of 2℃ / s until the solder joint solidifies.

[0113] Example 6

[0114] 1) Weigh 42wt% Sn, 57wt% Bi and 1wt% Ag according to the mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 200℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-57Bi-1Ag alloy ingot.

[0115] 2) Solution treatment was performed on the Sn-57Bi-1Ag alloy ingot at a temperature of 120℃ for 6 hours.

[0116] 3) Welding is performed using the solution-treated alloy ingot. During the welding process, the flow rate of cold air is set to 4m³ / h. 3 Set the cold air temperature to 5℃ and the heating power to 100W. Control the cooling rate to 5℃ / s to make the solder joint supercooled to 20℃. Then continue to maintain the cooling rate of 5℃ / s until the solder joint solidifies.

[0117] Comparative Example 1

[0118] 1) Weigh 89wt% Sn, 10wt% Bi and 1wt% Ag according to mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 350℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-10Bi-1Ag alloy ingot.

[0119] 2) Solution treatment was performed on the Sn-10Bi-1Ag alloy ingot at a temperature of 150℃ for 4 hours.

[0120] 3) Weld the alloy ingot after solution treatment and let it cool naturally until the weld solidifies.

[0121] Comparative Example 2

[0122] 1) Weigh 42wt% Sn, 57wt% Bi and 1wt% Ag according to the mass percentage, then place Sn, Bi and Ag in a high-purity graphite crucible, heat to 200℃ under argon protection, stir thoroughly and then cool and solidify to obtain Sn-57Bi-1Ag alloy ingot.

[0123] 2) Solution treatment was performed on the Sn-57Bi-1Ag alloy ingot at a temperature of 120℃ for 6 hours.

[0124] 3) Weld the alloy ingot after solution treatment and let it cool naturally until the weld solidifies.

[0125] The resistivity of the solder joints obtained in Examples 1-6 was measured using a four-probe tester, and the results are shown in Table 1 below:

[0126] Table 1: Test results of solder joints in Examples 1-6 and Comparative Examples 1-2

[0127]

[0128] Example 7

[0129] The difference between Example 7 and Example 1 is that the supercooling of Example 7 is 15°C and the cooling rate is 3°C / s.

[0130] Example 8

[0131] The difference between Example 8 and Example 1 is that the supercooling of Example 8 is 25°C and the cooling rate is 10°C / s.

[0132] The tensile strength and ductility of the welds obtained in Examples 3, 7, 8 and Comparative Example 1 were tested, and the results are shown in Table 2 below:

[0133] Table 2: Test results of solder joints in Examples 3, 7, 8 and Comparative Example 1

[0134] Example 3 Example 7 Example 8 Comparative Example 1 Tensile strength (MPa) 64 66 67 52 Ductility (%) 39 38 25 15

[0135] It is evident that higher supercooling can promote the formation of fine and dispersed eutectic Ag3Sn phase and improve tensile strength, but it will lose some ductility. The tensile strength of Example 3 is about 15% higher than that of Comparative Example 1, but the ductility is slightly reduced, indicating that fine and dispersed eutectic Ag3Sn phase can enhance strength, but will sacrifice some plasticity.

[0136] Example 9

[0137] The difference between Example 9 and Example 4 is that the supercooling of Example 9 is 15°C and the cooling rate is 3°C / s.

[0138] The tensile strength and ductility of the welds obtained in Examples 4, 5, 9, and Comparative Example 2 were tested, and the results are shown in Table 3 below:

[0139] Table 3: Test results of solder joints in Examples 4, 5, 9 and Comparative Example 2

[0140] Example 4 Example 5 Example 9 Comparative Example 2 Tensile strength (MPa) 61 55 48 45 Ductility (%) 54 48 50 20

[0141] It can be seen that the size of the primary Ag3Sn phase at the weld joint increases, exhibiting a more regular blocky morphology. The ductility of Example 4 is about 20% higher than that of Comparative Example 2, while the tensile strength is slightly lower. This indicates that the blocky branched primary Ag3Sn phase can improve plasticity through stress dispersion, but the strength is slightly reduced.

[0142] In summary, this application achieves precise control of undercooling (5℃~10℃) by adjusting the cooling rate (1℃~2℃ / s), which can directionally induce the formation of bulk eutectic / primary Ag3Sn phase, while optimizing electrical properties (low resistivity) and mechanical properties (strength or ductility). Low-Bi alloys (8%~15%) are suitable for high undercooling to enhance strength, while high-Bi alloys (50%~60%) are suitable for low undercooling to improve ductility, providing customized solutions for different application scenarios (such as high-conductivity solder joints or high-ductility solder joints).

[0143] Performance characterization:

[0144] Figure 1 These are the DSC cyclic test curves for Sn-10Bi-1Ag and Sn-57Bi-1Ag; where (a, c) are the DSC cyclic test program settings; (b, d) are the heating curves during the cyclic test. In b and d, the numbers next to the curves represent the number of cycles.

[0145] Figure 2 This is a flowchart illustrating the analysis of different Bi particle sizes in Sn-10Bi-1Ag from Example 3;

[0146] Figure 3 Here are the thermal expansion coefficient curves for Sn-10Bi-1Ag in Example 3 and Sn-57Bi-1Ag in Example 4:

[0147] Figure 4 At a strain rate of 5×10 -4 s -1 SEM images of Sn-10Bi-1Ag alloy of Example 1 and Sn-57Bi-1Ag alloy of Example 4 at different strain levels and after fracture; wherein, (ae) Sn-10Bi-1Ag alloy; (fj) Sn-57Bi-1Ag alloy.

[0148] In summary, this invention systematically studies the heterogeneous nucleation mechanism and undercooling effect of Sn-Bi-Ag ternary alloys, constructing a complete technical path encompassing nucleation substrate design, quantitative control of undercooling, and directional optimization of microstructure and properties. Firstly, it reveals that in low-bismuth-content tin-based alloys, the eutectic Ag3Sn phase nucleates on a substrate of solid Bi particles and β-Sn grains; while in high-bismuth-content tin-based alloys, the primary Ag3Sn phase nucleates on a substrate of liquid Bi particles and β-Sn grains. For different substrate characteristics, undercooling control criteria are established: a high undercooling of approximately 20°C is used for low-bismuth-content tin-based alloys to promote the formation of fine, dispersed eutectic Ag3Sn phases; while a low undercooling of approximately 10°C is used for high-bismuth-content tin-based alloys to induce the growth of blocky, branched primary Ag3Sn phases. This provides a theoretical basis and technical support for the composition design and welding process optimization of tin-based alloys.

[0149] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method of optimizing a soldering of a tin-based alloy, characterized in that, The method comprises the following steps: determining the performance improvement requirement of the solder, the performance improvement requirement comprising electrical property optimization; based on the electrical property optimization, selecting a first tin-based alloy, controlling the cooling rate of the molten pool during soldering to be 1-2℃ / s, and making the undercooling degree of the molten pool reach 5-10℃, so as to form block-shaped eutectic Ag3Sn phases or block-shaped primary Ag3Sn phases, and reduce the resistivity of the solder joint; the first tin-based alloy comprises 8-60% Bi, 0.5-1.5% Ag and the balance Sn in terms of mass percentage.

2. The tin-based alloy solder optimization method of claim 1, wherein, The performance requirement further comprises tensile strength optimization; based on the tensile strength optimization, selecting a second tin-based alloy, controlling the cooling rate of the molten pool during soldering to be 5-10℃ / s, and making the undercooling degree of the molten pool reach 15-25℃, so as to form fine and dispersed eutectic Ag3Sn phases; the second tin-based alloy comprises 8-15% Bi, 0.5-1.5% Ag and the balance Sn in terms of mass percentage.

3. The tin-based alloy solder optimization method of claim 2, wherein, The performance requirement further comprises ductility optimization; based on the ductility optimization, selecting a third tin-based alloy, controlling the cooling rate of the molten pool during soldering to be 1-5℃ / s, and making the undercooling degree of the molten pool reach 5-15℃, so as to form block-shaped and branched primary Ag3Sn phases; the third tin-based alloy comprises 50-60% Bi, 0.5-1.5% Ag and the balance Sn in terms of mass percentage.

4. The tin-based alloy solder optimization method of claim 3, wherein, The heterogeneous nucleation substrate of the eutectic Ag3Sn phases comprises solid Bi particles and β-Sn grains; and the heterogeneous nucleation substrate of the primary Ag3Sn phases comprises liquid Bi particles and β-Sn grains.

5. The tin-based alloy solder optimization method of claim 4, wherein, Before soldering, the optimization method further comprises preparing the first tin-based alloy, and the preparation of the first tin-based alloy specifically comprises: adding Sn, Bi and Ag into a crucible in terms of mass percentage, heating to melting under an argon atmosphere, and cooling and solidifying after uniform stirring.

6. The tin-based alloy solder optimization method of claim 5, wherein, After the preparation of the first tin-based alloy and before soldering, the optimization method further comprises subjecting the first tin-based alloy to solid solution treatment, the temperature of the solid solution treatment being 115-155℃, and the solid solution time being 4-6 hours, so as to promote the uniform distribution of Bi particles in Sn and provide uniform nucleation substrate for heterogeneous nucleation.

7. The tin-based alloy solder optimization method of claim 6, wherein, If the first tin-based alloy is the second tin-based alloy, then the solid Bi particles have an average particle size of 1-3μm under the condition of solid solution at 145-155℃ for 4-5 hours; If the first tin-based alloy is the third tin-based alloy, then the liquid Bi particles have a dispersion uniformity of ≥90% in the molten pool under the condition of solid solution at 115-125℃ for 5-6 hours.

8. The tin-based alloy solder optimization method of claim 6, wherein, After the solid solution treatment and before soldering, the optimization method further comprises obtaining the actual liquidus temperature of the first tin-based alloy by a DSC cycle method, and the obtaining specifically comprises the following steps: (1) heating the first tin-based alloy to the solidus temperature, and keeping the first tin-based alloy at the solidus temperature for 30min; then heating the first tin-based alloy to 30-60℃ above the solidus temperature at a temperature increasing rate of 10℃ / min; (2) cooling the tin-based alloy to 30-60℃ below the solidus temperature at the same cooling rate as the temperature increasing rate; and (3) repeating the heating and cooling steps (1) and (2) for 2-3 times. (3) increasing the isothermal temperature by 0.5℃ as a gradient, and cyclically performing step (1) and step (2) until no endothermic peak appears in the heating process; (4) recording the highest temperature T1 at which an endothermic peak exists and the lowest temperature T2 at which no endothermic peak exists in the heating process, and the actual liquidus temperature is the arithmetic mean of T1 and T2, which is used to calculate the degree of supercooling of the molten pool.

9. The tin-based alloy solder optimization method of claim 3, wherein, The regulation of the cooling rate of the molten pool in the welding process specifically comprises the following steps: compressed cold air is blown to the molten pool area for forced cooling, the flow rate of the cold air is controlled through a flow regulating valve, and the cold air inlet temperature is set to a specific range; the molten pool temperature is monitored in real time through an infrared thermometer during the forced cooling, and the heating power is adjusted through PID heating control to compensate for the temperature fluctuation in the cooling process.

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