High-reliability alloy solder capable of being welded at medium temperature and preparation method of high-reliability alloy solder

Abstract

The invention provides a high-reliability alloy solder capable of being welded at medium temperature and a preparation method of the high-reliability alloy solder, and relates to the technical field of welding materials. The soldering flux comprises alloy tin powder and soldering flux, the alloy tin powder comprises first tin-based alloy powder and second tin-based alloy powder, the melting point of the first tin-based alloy powder is higher than that of the second tin-based alloy powder, the first tin-based alloy powder is tin-based alloy powder rich in Sn, and the second tin-based alloy powder is tin-based alloy powder doped with Bi element and with SnIn as the main component. And the doping amount of the Bi element in the second tin-based alloy powder is 0.8 wt%-1.4 wt%. The preparation method comprises the following steps: uniformly premixing the alloy tin powder; raw materials of the soldering flux are pre-stirred, alloy tin powder is put in, and stirring is conducted. Through the proportion of the medium-temperature alloy powder as the main material and the high-low-temperature tin powder as the auxiliary material, the performance is guaranteed, and meanwhile the raw material cost is reduced; and a lead-free formula can be achieved, the requirements of European Union RoHS and other environmental protection laws and regulations are completely met, and the compliance risk of lead-containing solder is avoided.

Description

Technical Field

[0001] This application relates to the field of welding materials technology, and in particular to a highly reliable alloy solder that can be welded at medium temperatures and its preparation method. Background Technology

[0002] As the electronics manufacturing industry moves towards miniaturization and high density, higher demands are being placed on the thermal compatibility and reliability of soldering materials. Existing lead-free solders have significant technical defects: high-temperature lead-free solder paste has a melting point ≥217℃, which can easily damage temperature-sensitive components such as sensors during soldering, and also consumes a lot of energy; low-temperature lead-free solder paste has a melting point ≤173℃, which has a low risk of thermal damage, but its solder joints are brittle and have poor resistance to thermal fatigue, failing to meet the long-term reliability requirements of solder joints.

[0003] To address these issues, the industry has proposed medium-temperature solder paste solutions. However, existing medium-temperature solder pastes mostly employ a single alloy system, resulting in problems such as poor solder joint stability and insufficient wettability. For example, Chinese patent document CN108746523A discloses a Sn-Bi-Ag-Cu medium-temperature solder paste. Although its melting point is controlled at 175-185℃, its bismuth content is as high as 35%-45%, and the solder joint elongation is only 8%-10%, making it prone to cracking after long-term use.

[0004] Therefore, there is an urgent need to develop a medium-temperature mixed solder paste with a stable melting point of 183-217℃, which balances mechanical strength and thermal reliability and is suitable for refined SMT processes. Summary of the Invention

[0005] To address the issue that existing solders cannot simultaneously achieve both mechanical strength and thermal reliability, this application provides a highly reliable alloy solder that can be soldered at medium temperatures and its preparation method.

[0006] Firstly, this application provides a highly reliable alloy solder that can be welded at medium temperatures, employing the following technical solution: A highly reliable alloy solder that can be soldered at medium temperatures includes alloy tin powder and flux. The alloy tin powder includes a first tin-based alloy powder and a second tin-based alloy powder. The melting point of the first tin-based alloy powder is higher than that of the second tin-based alloy powder. The first tin-based alloy powder is a tin-based alloy powder rich in Sn, and the second tin-based alloy powder is a tin-based alloy powder doped with Bi and mainly composed of SnIn. The doping amount of Bi in the second tin-based alloy powder is 0.8wt% to 1.4wt%.

[0007] This application combines two tin-based alloy powders with different melting points to significantly reduce the melting point temperature of the solder. This effectively protects heat-sensitive components during soldering and also greatly improves the mechanical strength and long-term reliability of the solder joint.

[0008] During the soldering process, the second tin-based alloy powder first melts to form a liquid phase, wetting and spreading on the pads and component leads. At this time, the first tin-based alloy powder remains in solid particle form. These solid particles act as a supporting framework dispersed in the liquid phase, effectively suppressing excessive flow and collapse of the liquid solder, and becoming crystallization nuclei during subsequent solidification, refining the grains. After the solder solidifies, the high-melting-point phase exists as a reinforcing phase in the joint, improving the mechanical strength of the solder joint. Simultaneously, the Bi doping in the second tin-based alloy powder effectively lowers the melting point of the SnIn alloy while avoiding excessive Bi leading to coarsening of the brittle phase, thus ensuring the toughness and reliability of the joint while lowering the process temperature.

[0009] Optionally, the first tin-based alloy powder is SnAgCu alloy powder.

[0010] By using SnAgCu alloy powder as a high-melting-point framework phase, the intrinsic strength of the joint can be enhanced, thus laying the foundation for a highly reliable microstructure.

[0011] Optionally, the mass ratio of the first tin-based alloy powder to the second tin-based alloy powder is 5-16:34-45.

[0012] This application limits the amount of two tin-based alloy powders to ensure that at the target welding temperature, there is sufficient low-melting-point liquid phase for good wetting and filling, while there is also sufficient high-melting-point solid phase to provide structural support and reinforcement. This achieves an optimal solid-liquid coexistence state, resulting in a dense and tough solder joint. If there is too little low-melting-point phase, the fluidity is poor, and it is easy to produce a cold solder joint; if there is too much low-melting-point phase, the solid phase skeleton effect is weakened, the solder joint is prone to collapse, and the strength is reduced.

[0013] Optionally, the second tin-based alloy powder is also doped with Ag and Cu elements, and the sum of the doping amounts of Bi, Ag and Cu elements is 4.9wt% to 5.3wt%.

[0014] This application achieves significant dispersion strengthening and grain refinement by doping a small amount of Ag and Cu elements into the second tin-based alloy powder. This results in the formation of fine Ag3Sn and Cu6Sn5 intermetallic compounds, while controlling the total amount of the three elements within the aforementioned narrow range. This allows the melting point to be lowered to a reasonable mid-temperature range by Bi and In, and the formation of optimal strengthening phases by sufficient but not excessive amounts of Ag and Cu. This maximizes strength without sacrificing toughness and avoids problems such as brittleness or increased melting point caused by excessive amounts of a single element.

[0015] Optionally, the second tin-based alloy powder is one or more of Sn75In20Ag2.8Cu0.8Bi1.4, Sn70In25Ag3.0Cu1.0Bi1.0, Sn85.1In10Ag2.5Cu1.2Bi1.2, and Sn72.7In22Ag3.0Cu1.5Bi0.8.

[0016] Optionally, the alloy tin powder further includes a third tin-based alloy powder, the melting point of which is lower than that of the second tin-based alloy powder.

[0017] This application introduces a third tin-based alloy powder with a lower melting point into the alloy tin powder. The three alloy powders with different melting points exhibit gradient melting when heated, which can achieve a smoother liquid phase formation process, facilitate gas discharge and reduce splashing, and form a superior microstructure through a more complex solidification sequence.

[0018] Optionally, the third tin-based alloy powder is SnBi alloy powder.

[0019] This application uses SnBi alloy powder as the third tin-based alloy powder with a lower melting point, which can significantly reduce the melting point and thus melt first at a lower temperature. A small amount of liquid phase can accelerate the heating of the entire solder system, react with the flux in advance, activate the welding interface, and create favorable conditions for the subsequent melting and spreading of the second tin-based alloy powder.

[0020] Optionally, the mass ratio of the first tin-based alloy powder, the second tin-based alloy powder, and the third tin-based alloy powder is 4-6:13:1-3.

[0021] This application limits the amount of the three tin-based alloy powders as described above, controlling the total Bi content of the entire solder joint within a reasonable range. This avoids overall solder joint embrittlement caused by excessive Bi. The trace amount of the third tin-based alloy powder melts first and rapidly wets the solder pad, forming a heat-conducting zone. This greatly promotes the uniform and rapid melting of the subsequent large amount of intermediate-temperature phases, reducing the activation energy for soldering initiation. After soldering, this trace amount of low-melting-point phase is uniformly dispersed in a continuous matrix composed of strong intermediate- and high-melting-point phases. Its potential brittleness is effectively encapsulated and diluted, preventing it from becoming a structural weakness.

[0022] Optionally, the first tin-based alloy powder is a tin-based alloy powder doped with Bi and mainly composed of SnIn, and the mass ratio of the first tin-based alloy powder to the second tin-based alloy powder is 1:1.

[0023] The first and second tin-based alloy powders are compounded using SnIn-based alloy powders with different melting points. The first tin-based alloy powder, with its higher melting point, can form a high-strength skeleton after solidification, while the second tin-based alloy powder, with its lower melting point, can melt at a lower temperature to form a liquid phase. This liquid phase is pre-wetted and spread on the pads and component leads during the soldering process, and also encapsulates the reinforcing phase particles. Furthermore, both the first and second tin-based alloy powders are based on the Sn-In-Bi system, and they are highly similar in chemical composition, crystal structure, and coefficient of thermal expansion. Therefore, during molten soldering, the interdiffusion resistance between the two phases is small, and the interfacial energy is low. This results in a very clean, continuous, and strong bonding interface between the two phases, making it less likely to form harmful brittle intermetallic compound layers or micro-defects. Thus, the integrity and fracture resistance of the solder joint are fundamentally improved.

[0024] Secondly, this application provides a method for preparing a highly reliable alloy solder that can be soldered at medium temperatures, using the following technical solution: A method for preparing a highly reliable alloy solder that can be soldered at medium temperatures includes the following steps: The alloy tin powder is pre-mixed evenly in an inert environment; The flux raw materials are pre-stirred, then scraped against the wall, and vacuumed to -0.08MPa. Stirring is then performed again. Then, the uniformly mixed alloy tin powder is added, and the mixture is stirred a second time. After this process, the wall is scraped again, and vacuumed to -0.08MPa. Stirring is then performed a third time to obtain the final product.

[0025] In summary, this application includes at least one of the following beneficial effects: 1. Compared with high-temperature solder, the solder provided in this application has a melting point that is 20-40°C lower. Therefore, it can reduce thermal damage to temperature-sensitive components in consumer electronics and automotive electronics during the soldering process, while avoiding the problems of insufficient mechanical strength and poor long-term reliability of low-temperature solder, thus achieving the optimal balance between soldering strength and thermal protection.

[0026] 2. The solder provided in this application does not rely too much on the high-cost precious metals such as bismuth and indium in low-temperature solders. By using medium-temperature alloy powder as the main component and high- and low-temperature tin powder as an auxiliary component, the raw material cost is reduced while ensuring performance. Moreover, a lead-free formula can be achieved, which fully complies with the requirements of environmental regulations such as EU RoHS and avoids the compliance risks of lead-containing solders.

[0027] 3. The solder provided in this application is compatible with most SMT reflow soldering equipment, without the need for large-scale production line modifications. It is compatible with different substrate materials and component types, and the heating and cooling cycles are more reasonable, which helps to improve production efficiency and reduce energy consumption. Attached Figure Description

[0028] Figure 1This is a DSC curve of the alloy solder of Embodiment 1 of the present invention; Figure 2 This is a DSC curve of the alloy solder of Embodiment 2 of the present invention; Figure 3 This is the DSC curve of the alloy solder of Embodiment 3 of the present invention; Figure 4 This is the DSC curve of the alloy solder of Embodiment 4 of the present invention; Figure 5 This is the DSC curve of the alloy solder of Embodiment 5 of the present invention; Figure 6 This is the DSC curve of the alloy solder of Embodiment 6 of the present invention; Figure 7 This is the DSC curve of the alloy solder of Embodiment 7 of the present invention; Figure 8 This is the DSC curve of the alloy solder of Comparative Example 1 of this invention; Figure 9 This is the DSC curve of the alloy solder of Comparative Example 2 of this invention; Figure 10 This is the DSC curve of alloy solder of Comparative Example 3 of the present invention; Figure 11 This is the DSC curve of the alloy solder of Comparative Example 4 of this invention; Figure 12 These are cross-sectional microstructure images of alloy solder and BGA ball solder joints in Examples 1-4 of this invention; Figure 13 These are cross-sectional microstructure images of alloy solder and BGA ball solder joints from Examples 5-7 of this invention; Figure 14 These are cross-sectional micrographs of alloy solder and BGA ball solder joints from Comparative Examples 1-4 of this invention. Detailed Implementation

[0029] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments, further clarifies the invention. Those skilled in the art should understand that the specific descriptions below are illustrative rather than restrictive, and should not be construed as limiting the scope of protection of the present invention.

[0030] To address the problems of poor thermal reliability of high-temperature solders, low mechanical strength of low-temperature solders, and poor solder joint stability of medium-temperature solders in existing technologies, the applicant has proposed the inventive concept of this application. This involves compounding tin powders with different melting points to form a binary or ternary composite alloy system. The binary composite alloy system can be a combination of high-temperature tin powder + medium-temperature tin powder, high-temperature tin powder + low-temperature tin powder, or medium-temperature tin powder + medium-temperature tin powder. The ternary composite alloy system can be a combination of high-temperature tin powder + medium-temperature tin powder + low-temperature tin powder, or high-temperature tin powder + medium-temperature tin powder + medium-temperature tin powder. The core principle is to utilize tin powder with a higher melting point to provide basic strength and reliability, while tin powder with a lower melting point provides good wetting and filling, thereby achieving an optimal solid-liquid coexistence state and forming a dense, strong solder joint. Simultaneously, this compounding lowers the overall melting point temperature of the composite system, allowing for molten soldering at lower welding temperatures.

[0031] It should be noted that the high-temperature tin powder refers to tin-based metal powder with a melting point higher than 200°C, the medium-temperature tin powder refers to tin-based metal powder with a melting point of 170~200°C, and the low-temperature tin powder refers to tin-based metal powder with a melting point lower than 170°C.

[0032] The solution of this application will be further illustrated below through specific embodiments and comparative examples. Example

[0033] Example 1 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder consists of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5, and the second tin-based alloy powder is medium-temperature tin powder B, specifically Sn75In20Ag2.8Cu0.8Bi1.4. Based on the mass of the alloy tin powder, the high-temperature tin powder accounts for 25% of the mass, and the medium-temperature tin powder accounts for 75% of the mass.

[0034] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, as detailed below: (1) Powder mixing: Weigh one or more types of tin powder according to the set mass fractions, and after weighing, put all the tin powder into the three-dimensional powder mixing equipment. Fill the container with nitrogen and rotate slowly and uniformly for 10 minutes to mix the powder evenly. (2) Solder paste mixing: Add 12 parts by weight of flux to the mixing pot for pre-mixing at a speed of 50 rpm for 10 min; after completion, scrape the wall, apply vacuum at -0.08 MPa, and stir at 50 rpm for 5 min. After the flux is mixed, add the mixed solder powder to the mixing pot for one stirring at a speed of 50 rpm for 20 min; after completion, scrape the wall, apply vacuum at -0.08 MPa, and stir at 50 rpm for 40 min to obtain medium-temperature mixed solder paste. Example

[0035] Example 2 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder consists of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5, and the second tin-based alloy powder is medium-temperature tin powder B, specifically Sn85.1In10Ag2.5Cu1.2Bi1.2. Based on the mass of the alloy tin powder, the high-temperature tin powder accounts for 30% of the mass, and the medium-temperature tin powder accounts for 70% of the mass.

[0036] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here. Example

[0037] Example 3 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder consists of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5, and the second tin-based alloy powder is medium-temperature tin powder B, specifically Sn72.7In22Ag3.0Cu1.5Bi0.8. Based on the mass of the alloy tin powder, the high-temperature tin powder accounts for 32% of the mass, and the medium-temperature tin powder accounts for 68% of the mass.

[0038] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here. Example

[0039] Example 4 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder consists of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5. The second tin-based alloy powder is a mixture of two medium-temperature tin powders B: Sn75In20Ag2.8Cu0.8Bi1.4 (medium-temperature tin powder B1) and Sn85.1In10Ag2.5Cu1.2Bi1.2 (medium-temperature tin powder B2). By mass, the high-temperature tin powder accounts for 10%, medium-temperature tin powder B1 accounts for 30%, and medium-temperature tin powder B2 accounts for 60%.

[0040] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here. Example

[0041] Example 5 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder is composed of a first tin-based alloy powder, a second tin-based alloy powder, and a third tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5; the second tin-based alloy powder is medium-temperature tin powder B, specifically Sn75In20Ag2.8Cu0.8Bi1.4; and the third tin-based alloy powder is medium-temperature tin powder C, specifically Sn42Bi57Ag1. By mass, high-temperature tin powder A accounts for 30%, medium-temperature tin powder B accounts for 65%, and low-temperature tin powder C accounts for 5%.

[0042] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here. Example

[0043] Example 6 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder is composed of a first tin-based alloy powder, a second tin-based alloy powder, and a third tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5; the second tin-based alloy powder is medium-temperature tin powder B, specifically Sn70In25Ag3.0Cu1.0Bi1.0; and the third tin-based alloy powder is medium-temperature tin powder C, specifically Sn42Bi57Ag1. By mass, high-temperature tin powder A accounts for 20%, medium-temperature tin powder B accounts for 65%, and low-temperature tin powder C accounts for 15%.

[0044] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here. Example

[0045] Example 7 provides a highly reliable alloy solder suitable for medium-temperature soldering, comprising alloy tin powder and flux. The alloy tin powder consists of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is medium-temperature tin powder B1, specifically Sn85.1In10Ag2.5Cu1.2Bi1.2, and the second tin-based alloy powder is medium-temperature tin powder B2, specifically Sn75In20Ag2.8Cu0.8Bi1.4. Based on the mass of the alloy tin powder, the mass percentage of medium-temperature tin powder B1 is 50%, and the mass percentage of medium-temperature tin powder B2 is 50%.

[0046] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here.

[0047] Comparative Example 1 Comparative Example 1 provides an alloy solder, comprising alloy tin powder and flux, wherein the alloy tin powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5.

[0048] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here.

[0049] Comparative Example 2 Comparative Example 2 provides an alloy solder comprising alloy tin powder and flux. The alloy tin powder is composed of a first tin-based alloy powder and a second tin-based alloy powder. The first tin-based alloy powder is high-temperature tin powder A, specifically Sn96.5Ag3.0Cu0.5, and the second tin-based alloy powder is low-temperature tin powder C, specifically Sn42Bi57Ag1.0. Based on the mass of the alloy tin powder, the high-temperature tin powder A accounts for 50% of the total mass, and the low-temperature tin powder C accounts for 50% of the total mass.

[0050] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here.

[0051] Comparative Example 3 Comparative Example 3 provides an alloy solder, comprising alloy tin powder and flux, wherein the alloy tin powder is medium-temperature tin powder B, specifically Sn75In20Ag2.8Cu0.8Bi1.4.

[0052] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here.

[0053] Comparative Example 4 Comparative Example 4 provides an alloy solder, comprising alloy tin powder and flux, wherein the alloy tin powder is medium-temperature tin powder B, specifically Sn85.1In10Ag2.5Cu1.2Bi1.2.

[0054] The flux consists of 30 wt% rosin, 10 wt% activator (glutaric acid), 5 wt% thixotropic agent (hydrogenated castor oil), and 55 wt% solvent (diethylene glycol hexyl ether). The high-reliability alloy solder is prepared by mixing alloy tin powder and flux at a mass ratio of 9:1, using the same method as in Example 1, which will not be repeated here.

[0055] Table 1. Formulations of alloy tin powder in Examples 1-7 and Comparative Examples 1-4

[0056] The high-reliability alloy solders obtained in the above embodiments and comparative examples were subjected to performance testing, and the test results are shown in Table 2. The test items included: 1. Differential Scanning Calorimetry (DSC) Analysis: The solders of Examples 1-7 and Comparative Examples 1-4 were thermally analyzed using a differential scanning calorimeter. Details can be found in [link to details]. Figures 1-11Furthermore, it should be noted that the melting points listed in Table 2 are characterized by the peak temperatures in the DSC curves.

[0057] 2. Tensile strength and elongation after fracture tests: Refer to GB / T 228.1-2021 Metallic Materials Tensile Tests.

[0058] 3. Solder Joint Push Force Test: The test board used was Suzhou Younuo's self-developed PCB board, model TB04 (Cu+OSP); the test equipment was a DAGE4000 push machine; the chip was a 1210 ceramic capacitor; Solder Joint Reflow: Different embodiments were reflowed at 205℃ for 180s; the test method was as follows: solder paste was printed on the PCB board, reflowed at 205℃ with nitrogen, the reflowed board was cut and placed on the push machine. The push head of the push machine was pushed perpendicularly to the chip until the solder joint and chip separated and broke, and the maximum force was measured.

[0059] Table 2 Test results of Examples 1-7 and Comparative Example 1

[0060] As shown in Table 2, the alloy solders provided in Examples 1-7 can control the melting point below 200℃ and achieve reflow at 205℃, meeting the requirements for medium-temperature soldering. In contrast, the melting point of Comparative Example 1 reaches 217℃, significantly higher than that of Examples 1-7. Furthermore, compared to Comparative Example 1, Examples 1-7 exhibit a substantial increase in tensile strength despite a lower melting point. Comparative Example 2, using a blend of high-temperature and low-temperature tin powder, achieves medium-temperature soldering, but its elongation after fracture is too low, posing a risk of high brittleness. Comparative Examples 3 and 4, both using medium-temperature tin powder, meet the melting point requirements for medium-temperature soldering, but their tensile strength and solder joint thrust are significantly lower than those of Examples 1-7.

[0061] The present invention also uses the alloy solder provided in the above embodiments and comparative examples for encapsulation soldering with BGA balls (SAC305). Among them, Comparative Example 1 is a high-temperature tin powder of the same material as the BGA ball, so the soldering temperature is 245°C and the reflow time is 180s. The other embodiments and comparative examples all use the same soldering temperature of 205°C and the reflow time of 180s.

[0062] Figure 12 These are cross-sectional microstructure images of alloy solder and BGA ball solder joints in Examples 1-4 of the present invention, wherein a corresponds to Example 1, b corresponds to Example 2, c corresponds to Example 3, and d corresponds to Example 4. Figure 13 These are cross-sectional micrographs of alloy solder and BGA ball solder joints in Examples 5-7 of the present invention, wherein e corresponds to Example 5, f corresponds to Example 6, and g corresponds to Example 7. Figure 14These are cross-sectional micrographs of alloy solder and BGA ball solder joints in Comparative Examples 1-4 of this invention, wherein h corresponds to Comparative Example 1, i corresponds to Comparative Example 2, j corresponds to Comparative Example 3, and k corresponds to Comparative Example 4.

[0063] from Figures 12-14 The comparison shows that the solder paste and pads or solder balls in Comparative Examples 2-4 have poor soldering effects. This is because the In content in the solder paste is high, and the In-rich compound exhibits excessive flow during reflow. In contrast, Examples 1-7 each consist of multiple components. In particular, the addition of the high-temperature component maintains a solid particle morphology within the solder joint. These solid particles act as a supporting framework dispersed in the liquid phase, effectively suppressing excessive flow and collapse of the liquid solder, and ensuring that the solder can spread uniformly and coat the PCB pads. Furthermore, the high-temperature component further dilutes the In element, preventing excessive diffusion of In into the BGA solder balls and causing softening of the balls.

[0064] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A highly reliable alloy solder suitable for medium-temperature welding, characterized in that, The material includes alloy tin powder and flux. The alloy tin powder includes a first tin-based alloy powder and a second tin-based alloy powder. The melting point of the first tin-based alloy powder is higher than that of the second tin-based alloy powder. The first tin-based alloy powder is a tin-based alloy powder rich in Sn, and the second tin-based alloy powder is a tin-based alloy powder doped with Bi and mainly composed of SnIn. The doping amount of Bi in the second tin-based alloy powder is 0.8wt% to 1.4wt%.

2. The high-reliability alloy solder capable of medium-temperature welding according to claim 1, characterized in that, The first tin-based alloy powder is SnAgCu alloy powder.

3. The high-reliability alloy solder capable of medium-temperature welding according to claim 2, characterized in that, The mass ratio of the first tin-based alloy powder to the second tin-based alloy powder is 5-16:34-45.

4. The high-reliability alloy solder capable of medium-temperature welding according to claim 2, characterized in that, The second tin-based alloy powder is also doped with Ag and Cu elements, and the sum of the doping amounts of Bi, Ag and Cu elements is 4.9wt%~5.3wt%.

5. The high-reliability alloy solder capable of medium-temperature welding according to claim 4, characterized in that, The second tin-based alloy powder is one or more of Sn75In20Ag2.8Cu0.8Bi1.4, Sn70In25Ag3.0Cu1.0Bi1.0, Sn85.1In10Ag2.5Cu1.2Bi1.2, and Sn72.7In22Ag3.0Cu1.5Bi0.

8.

6. The high-reliability alloy solder capable of medium-temperature welding according to claim 2, characterized in that, The alloy tin powder also includes a third tin-based alloy powder, the melting point of which is lower than that of the second tin-based alloy powder.

7. The high-reliability alloy solder capable of medium-temperature welding according to claim 6, characterized in that, The third tin-based alloy powder is SnBi alloy powder.

8. The high-reliability alloy solder capable of medium-temperature welding according to claim 7, characterized in that, The mass ratio of the first tin-based alloy powder, the second tin-based alloy powder and the third tin-based alloy powder is 4-6:13:1-3.

9. The high-reliability alloy solder capable of medium-temperature welding according to claim 1, characterized in that, The first tin-based alloy powder is a tin-based alloy powder doped with Bi element and with SnIn as the main component, and the mass ratio of the first tin-based alloy powder to the second tin-based alloy powder is 1:

1.

10. A method for preparing a highly reliable alloy solder capable of medium-temperature soldering according to any one of claims 1-9, characterized in that, Includes the following steps: The alloy tin powder is pre-mixed evenly in an inert environment; The flux raw materials are pre-stirred, then scraped against the wall, and vacuumed to -0.08MPa. Stirring is then performed again. Then, the uniformly mixed alloy tin powder is added, and the mixture is stirred a second time. After this process, the wall is scraped again, and vacuumed to -0.08MPa. Stirring is then performed a third time to obtain the final product.

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