Low-temperature composite tin paste and preparation method thereof

Abstract

This invention provides a low-temperature composite solder paste and its preparation method, belonging to the field of soldering material technology. The method includes the following steps: S1, preparing a hybrid corrosion inhibitor; S2, heating and stirring hydrogenated rosin, acrylic-modified rosin, terpineol, and diethylene glycol monohexyl ether, then adding epoxy silicone oil and KH560, heating and stirring, cooling, adding organic acid and the hybrid corrosion inhibitor, continuing stirring, then adding a thixotropic agent for dispersion, and obtaining a flux system after vacuum degassing and cooling; S3, mixing and stirring Sn42Bi58 tin-bismuth powder, flake SAC305 solder powder, and the flux system, then adding rosin acid-coated Sn-Bi composite nanoparticles, stirring, and vacuum degassing to obtain the low-temperature composite solder paste. This invention can improve the brittle cracking problem of Sn-Bi solder joints and enhance its corrosion resistance and surface insulation stability in immersion liquid cooling environments.

Description

Technical Field

[0001] This invention relates to the field of welding materials technology, specifically to a low-temperature composite solder paste and its preparation method. Background Technology

[0002] Traditional SAC-based lead-free solder paste has good solder joint strength and thermal cycling reliability, but its high melting point usually requires a high peak temperature for reflow soldering, which makes it difficult to meet the low heat load assembly requirements of thermal components, low heat-resistant substrates and high-density electronic components.

[0003] Sn-Bi-based low-temperature solder paste has advantages such as low melting point, low reflow temperature, and low process heat load, enabling soldering at relatively low temperatures. However, the Bi phase in conventional SnBi58 or Sn42Bi58 tin-bismuth solder is hard and brittle, resulting in poor toughness. CN111015021A discloses a low-temperature lead-free solder paste comprising a low-temperature solder and a flux. The low-temperature solder includes both tin-bismuth alloys and carbon nanotubes, while the flux includes rosin, solvent, activator, thickener, and additives. By adding multi-walled carbon nanotubes with a specific aspect ratio to the low-temperature solder and supplementing the flux with specific activators and thickeners, the dispersibility of carbon nanotubes in the solder paste is improved, enhancing their compatibility with other components and increasing the strength and toughness of the solder joints during soldering. However, because Bi is prone to oxidation, it can lead to problems such as oxide inclusions and uneven intermetallic compound layers, which can easily become crack initiation sites, resulting in decreased crack resistance.

[0004] Furthermore, due to the ever-increasing heat density of data center servers, AI computing equipment, and high-power electronic modules, traditional air cooling methods are gradually becoming insufficient to meet the demands for efficient heat dissipation. Unlike traditional air-cooled environments, solder joints, pads, intermetallic compound layers, and flux residues in immersion liquid-cooled equipment are constantly enveloped in coolant and may experience equipment start-up and shutdown, thermal cycling, localized temperature fluctuations, and prolonged immersion. Although coolant is typically an insulating medium, prolonged contact can still affect the wetting, swelling, extraction, or migration of post-soldering residues, reducing the adhesion, barrier properties, and surface insulation stability of the residual film, and further inducing problems such as copper pad corrosion, interface peeling, and electrical connection failure.

[0005] Therefore, there is a need to provide a low-temperature composite solder paste and its preparation method to solve the above-mentioned technical problems. Summary of the Invention

[0006] In view of this, the present invention provides a low-temperature composite solder paste and its preparation method, which can improve the brittle cracking problem of Sn-Bi solder joints and enhance their corrosion resistance and surface insulation stability in immersion liquid cooling environment.

[0007] To achieve the above objectives, the present invention provides a method for preparing low-temperature composite solder paste, comprising the following steps:

[0008] S1. L-histidine is dispersed in an aqueous ethanol solution, and a phenyl isothiocyanate solution is added dropwise. The reaction is carried out under nitrogen protection and heated. After the reaction, the mixture is concentrated, precipitated, filtered, and washed to obtain an L-histidine-phenylthiourea intermediate. The intermediate is further dispersed in an aqueous ethanol solution, and an organosilicon pre-hydrolyzed solution is added dropwise. Then, fumed silica is added to disperse and composite the mixture. After drying, grinding, and sieving, a hybrid corrosion inhibitor is obtained. S2. Hydrogenated rosin, acrylic modified rosin, terpineol and diethylene glycol monohexyl ether are heated and stirred, then epoxy silicone oil and KH560 are added and heated and stirred. After cooling, organic acid and hybrid corrosion inhibitor are added and stirred. Then, thixotropic agent is added and dispersed. After vacuum degassing and cooling, the flux system is obtained. S3. Mix and stir Sn42Bi58 tin-bismuth powder, flake SAC305 solder powder and flux system, then add rosin acid-coated Sn-Bi composite nanoparticles and stir, vacuum degassing to obtain low-temperature composite solder paste.

[0009] This invention uses Sn42Bi58 tin-bismuth powder as the low-temperature melting matrix, and combines it with rosin acid-coated Sn-Bi composite nanoparticles and flake-shaped SAC305 solder powder to form a composite solder system. The rosin acid coating layer reduces the oxidation level of the Sn-Bi composite nanoparticles during storage, paste mixing, and the early stages of reflow. During reflow, it synergistically removes the oxide film and promotes solder wetting and spreading with the organic acid activator, thereby reducing solder joint defects caused by oxide inclusions and insufficient wetting. Flaky SAC305 solder powder, as a small-volume component for microstructure regulation and interface reinforcement, possesses a large interfacial contact area and a short surface diffusion path. After the Sn42Bi58 low-melting-point solder melts, it can be wetted and coated by the Sn-Bi liquid phase, undergoing local dissolution, element diffusion, and metallurgical bonding at the interface. This allows surface Ag and Cu elements to participate in solder joint microstructure regulation in a trace, slow-release manner, which is beneficial for the formation of dispersed Ag-Sn and Cu-Sn intermetallic compound phases or interface strengthening structures, thereby suppressing Bi phase coarsening, continuous segregation, and local stress concentration. Therefore, this invention can improve the cracking problems of Sn-Bi solder joints caused by Bi phase segregation, brittle microstructure, and interfacial stress concentration, improve the crack resistance and interfacial bonding stability of the solder joint, and reduce the risk of interfacial delamination and crack propagation during long-term immersion liquid cooling and temperature fluctuation cycling.

[0010] In addition, in this invention, phenyl isothiocyanate reacts with L-histidine to generate a histidine-phenylthiourea derivative, the structural formula of which is as follows:

[0011] The obtained histidine-phenylthiourea derivative molecule contains an imidazole ring, a carboxyl group, a thiourea group, and a benzene ring structure. The nitrogen, oxygen, and sulfur atoms in the imidazole ring, carboxyl group, and thiourea group can adsorb, complex, or coordinate with copper pads, Sn-Bi solder joint surfaces, and Cu-Sn interfaces, forming a corrosion-inhibiting adsorption layer on the metal surface. The benzene ring structure helps improve the hydrophobic shielding effect at the interface. Furthermore, after the organosilicon pre-hydrolyzed solution is combined with the histidine-phenylthiourea derivative, an organosilicon network can be formed through hydrolysis and condensation, fixing the corrosion-inhibiting active structure in the hybrid system and reducing its migration in post-soldering residues and the risk of wetting, swelling, or extraction by the insulating coolant. Fumed silica, as an inorganic dispersed phase and barrier component, can improve the dispersion stability of the hybrid corrosion-inhibiting component in the flux system and, together with the organosilicon network, form an organic-inorganic barrier structure. Therefore, the hybrid corrosion inhibitor can form a corrosion-inhibiting protective layer in the post-soldering residual film that has the functions of coordination adsorption, hydrophobic shielding and barrier, thereby improving the corrosion resistance and surface insulation stability of the solder joint and post-soldering residual system in the immersion liquid cooling environment, and reducing the risk of copper pad corrosion, Cu-Sn interface corrosion, ion migration and interface peeling.

[0012] Therefore, this invention can improve the problems of brittle cracking and interface peeling of Sn-Bi solder joints while maintaining the advantages of low-temperature reflow soldering of Sn-Bi system, and improve its corrosion resistance and surface insulation stability in immersion liquid cooling environment.

[0013] Optionally, the phenyl isothiocyanate solution is obtained by dissolving 1.3-1.5 parts by weight of phenyl isothiocyanate in 20-25 parts by weight of tetrahydrofuran; the organosilicon pre-hydrolyzed solution is obtained by adding 0.4-0.6 parts by weight of KH560, 1.3-1.8 parts by weight of tetraethyl orthosilicate and 0.4-0.6 parts by weight of methyltriethoxysilane to 25-35 parts by weight of 80 vol% ethanol aqueous solution, adjusting the pH to 4.5-5.0 with 5 wt% acetic acid aqueous solution, and stirring and hydrolyzing for 30-50 min.

[0014] Optionally, in step S1, 1.5-1.6 parts by weight of L-histidine are added to 60-70 parts by weight of 75 vol% ethanol aqueous solution and stirred to disperse. Triethylamine is added to adjust the pH to 8.0-8.5. 21.3-26.5 parts by weight of phenyl isothiocyanate solution is added dropwise. The mixture is heated to 45-50°C under nitrogen protection and reacted for 10-12 hours. After cooling to room temperature, the mixture is concentrated under reduced pressure, ethyl acetate is added to precipitate the product, and the mixture is filtered. The product is washed successively with ethanol and ethyl acetate to obtain the L-histidine-phenylthiourea intermediate. Further add 40-50 parts by weight of 80 vol% ethanol aqueous solution, add triethylamine to adjust pH to 7.0-7.5, stir and disperse for 30-40 min, then add 28-38 parts by weight of organosilicon pre-hydrolyzed solution, heat to 45-50℃ and stir to react for 2-4 h, then add 2-3 parts by weight of fumed silica and ultrasonically disperse for 30-50 min, continue stirring for 60-80 min, vacuum dry at 50-60℃ for 6-8 h, grind and pass through a 200 mesh sieve to obtain the hybrid corrosion inhibitor component.

[0015] Optionally, in step S2, hydrogenated rosin, acrylic-modified rosin, terpineol, and diethylene glycol monohexyl ether are added to a reaction vessel, heated to 90-100°C under nitrogen protection, and stirred for 40-60 minutes. Then, epoxy silicone oil and KH560 are added, the temperature is raised to 100-110°C, and stirring is continued for 1.5-3 hours. The temperature is then lowered to 70-75°C, organic acid and hybrid corrosion inhibitor are added, and stirring is continued for 40-60 minutes. Next, a thixotropic agent is added, and the mixture is stirred and dispersed at 75-80°C for 40-60 minutes. Then, the mixture is vacuum degassed under -0.08 to -0.09 MPa for 20-30 minutes and cooled to room temperature to obtain the flux system. The organic acid consists of 1.5-2.5 parts by mass of sebacic acid and 0.8-1.2 parts by mass of adipic acid. The thixotropic agent consists of 4-5 parts by mass of hydrogenated castor oil and 1.5-2.5 parts by mass of polyamide wax.

[0016] This invention uses sebacic acid and adipic acid as organic acid components, which can gently remove oxides from solder and pad surfaces during reflow, improve wetting and spreading properties, and reduce the risk of strong corrosion residues. Hydrogenated castor oil and polyamide wax are used as thixotropic agents to form a reversible thixotropic network, preventing solder sedimentation and improving the storage stability and printability of solder paste.

[0017] Optionally, the flake-shaped SAC305 solder powder is prepared by adding 40-50 parts by weight of SAC305 solder powder into a planetary ball mill jar, adding 500 parts by weight of zirconia grinding balls and 500 parts by weight of ethylene glycol, purging the air in the ball mill jar with nitrogen for 10-15 minutes, sealing it, ball milling at 400 r / min for 70-90 minutes, vacuum filtering the resulting slurry under nitrogen protection, washing it with anhydrous ethanol 3-5 times, and then vacuum drying it at 40-45℃ for 2-3 hours.

[0018] This invention produces sheet-like SAC305 solder powder by planetary ball milling of SAC305 solder powder in a wet ethylene glycol medium. The shearing, extrusion, and impact effects generated by the zirconia grinding balls cause the SAC305 solder particles with a certain degree of ductility to undergo plastic deformation and gradually flatten, thereby producing sheet-like SAC305 solder powder. At the same time, ethylene glycol can reduce the risk of cold soldering and agglomeration of the powder, and nitrogen protection and low-temperature vacuum drying can reduce surface oxidation during the ball milling and drying process.

[0019] Optionally, the rosin acid-coated Sn-Bi composite nanoparticles are prepared by adding 0.3-0.5 parts by weight of rosin acid to 100-120 mL of ethanol, heating to 40-45°C and stirring for 20-30 min, cooling to room temperature, adding 3.5-4.5 parts by weight of sodium borohydride and stirring in an ice-water bath for 10-15 min, then adding a Sn / Bi metal salt mixed solution dropwise under nitrogen protection and an ice-water bath, continuing to stir for 60-80 min, then heating to room temperature and stirring for another 80-120 min, collecting the solid by centrifugation, washing it 3-5 times with deionized water and anhydrous ethanol, and then vacuum drying at 50-55°C for 4-5 h.

[0020] This invention utilizes Sn 2+ and Bi 3+ Sn-Bi nanoparticles are formed by synchronous reduction under the action of sodium borohydride. The carboxyl or carboxylic acid groups in the rosin acid molecule can coordinate and adsorb with the surface of the newly formed metal nanoparticles, and form a thin organic protective structure on the particle surface by utilizing its hydrophobic resin acid skeleton, thereby reducing the risk of particle agglomeration and oxidation.

[0021] Optionally, the Sn / Bi metal salt mixed solution is obtained by adding SnCl2·2H2O and BiCl3 to a hydrochloric acid / ethanol mixed solution and stirring for 30-40 minutes under nitrogen protection.

[0022] Preparing a Sn / Bi metal salt mixed solution under nitrogen protection can reduce Sn 2+ Oxidized by air to Sn 4+ This reduces the risk of metal salt hydrolysis and oxidation precipitation, making Sn 2+ and Bi 3+ Maintaining good dissolution stability in the system is beneficial for subsequent simultaneous reduction and uniform nucleation of Sn-Bi nanoparticles.

[0023] Optionally, the hydrochloric acid / ethanol mixed solution is prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1; the Sn / Bi metal salt mixed solution comprises the following raw materials in parts by mass: 3.8~4.2 parts SnCl2·2H2O, 4.2~4.7 parts BiCl3 and 80~100 parts hydrochloric acid / ethanol mixed solution.

[0024] Optionally, in step S3, Sn42Bi58 tin-bismuth powder, flake SAC305 solder powder and flux system are mixed and stirred for 8-15 minutes at 25-30°C and a vacuum degree of -0.09MPa. Then, rosin acid-coated Sn-Bi composite nanoparticles are added and stirred for 20-30 minutes, followed by vacuum degassing for 15-30 minutes to obtain low-temperature composite solder paste.

[0025] The present invention also provides a low-temperature composite solder paste, comprising the following raw materials in parts by weight: 68-76 parts Sn42Bi58 tin-bismuth powder, 8-12 parts flake SAC305 solder powder, 10-15 parts flux system and 4-6 parts rosin acid-coated Sn-Bi composite nanoparticles; wherein the flux system comprises the following raw materials in parts by weight: 22-26 parts hydrogenated rosin, 10-15 parts acrylic acid-modified rosin, 15-20 parts terpineol, 12-16 parts diethylene glycol monohexyl ether, 3-4 parts epoxy silicone oil, 0.5-0.7 parts KH560, 2.3-3.7 parts organic acid, 0.5-0.6 parts hybrid corrosion inhibitor and 5.5-7.5 parts thixotropic agent.

[0026] The low-temperature composite solder paste prepared by the above component ratio can take into account low-temperature melting and wetting, solder joint structure strengthening and interface bonding stability.

[0027] The above-described technical solution of the present invention has at least the following beneficial effects: 1. This invention uses Sn42Bi58 tin-bismuth powder as the low-temperature melting matrix, and combines it with rosin acid-coated Sn-Bi composite nanoparticles and flake-shaped SAC305 solder powder to form a composite solder system. The rosin acid coating layer can reduce the oxidation of nanoparticles and synergistically remove oxide films, promote wetting and spreading, and reduce solder joint defects with organic acid activators. The flake-shaped SAC305, as a small amount of microstructure regulation and interface enhancement component, can undergo wetting, local dissolution, and interface diffusion in the Sn-Bi liquid phase, allowing trace amounts of Ag and Cu elements on the surface to participate in microstructure regulation. This is beneficial for the formation of Ag-Sn and Cu-Sn intermetallic compound phases or interface strengthening structures, inhibiting Bi phase coarsening, segregation, and stress concentration, thereby improving the crack resistance and interfacial bonding stability of the solder joint.

[0028] 2. This invention involves the reaction of phenyl isothiocyanate with L-histidine to generate a histidine-phenylthiourea derivative. The nitrogen, oxygen, and sulfur atoms in the imidazole ring, carboxyl group, and thiourea group can adsorb, complex, or coordinate with copper pads, Sn-Bi solder joint surfaces, and Cu-Sn interfaces. The benzene ring structure helps improve the hydrophobic shielding of the interface. Furthermore, through the hydrolysis and condensation of the organosilicon pre-hydrolysis solution and the inorganic barrier effect of fumed silica, the corrosion-inhibiting active structure is fixed in the organic-inorganic hybrid system, reducing its migration in post-soldering residues and the risk of being wetted, swollen, or extracted by insulating coolant. This improves the corrosion resistance and surface insulation stability of solder joints and post-soldering residues in an immersion liquid-cooled environment, and reduces the risk of interfacial corrosion, ion migration, and interfacial peeling. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. The described embodiments are some embodiments of the present invention, and all other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0030] Example 1 4.0 g SnCl2·2H2O and 4.5 g BiCl3 were added to 90 g hydrochloric acid / ethanol mixed solution (the hydrochloric acid / ethanol mixed solution was prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1). The mixture was stirred for 35 min under nitrogen protection to obtain a Sn / Bi metal salt mixed solution. 0.4 g rosin acid was added to 110 mL ethanol, the temperature was raised to 43 °C and stirred for 25 min, cooled to room temperature, 4.0 g sodium borohydride was added and stirred in an ice-water bath for 12 min, and then the Sn / Bi metal salt mixed solution was added dropwise under nitrogen protection and an ice-water bath. The mixture was stirred for another 70 min, then raised to room temperature and stirred for another 100 min. The solid was collected by centrifugation, washed four times with deionized water and anhydrous ethanol, and dried under vacuum at 52 °C for 4.5 h to obtain rosin acid-coated Sn-Bi composite nanoparticles.

[0031] 45g of SAC305 solder powder (particle size 25~45μm) was added to a planetary ball mill jar, along with 500g of zirconia grinding balls and 500g of ethylene glycol. Nitrogen gas was introduced to replace the air in the ball mill jar for 12 minutes, and then the jar was sealed. The jar was ball-milled at 400r / min for 80 minutes. The resulting slurry was vacuum filtered under nitrogen protection and washed four times with anhydrous ethanol. The slurry was then vacuum-dried at 42℃ for 2.5 hours to obtain flake-like SAC305 solder powder.

[0032] 0.5 g KH560 (CAS No.: 2530-83-8), 1.5 g tetraethyl orthosilicate, and 0.5 g methyltriethoxysilane were added to 30 g of 80 vol% aqueous ethanol solution. The pH was adjusted to 4.8 with 5 wt% aqueous acetic acid solution, and the mixture was stirred and hydrolyzed for 40 min to obtain an organosilicon pre-hydrolyzed solution. 1.4 g phenyl isothiocyanate was dissolved in 23 g tetrahydrofuran to obtain a phenyl isothiocyanate solution. 1.55 g L-histidine was added to 65 g of 75 vol% aqueous ethanol solution and stirred to disperse. Triethylamine was added to adjust the pH to 8.2, and 24.4 g of phenyl isothiocyanate solution was added dropwise. The mixture was heated to 48 °C under nitrogen protection and reacted for 11 h. After cooling to room temperature, the mixture was concentrated under reduced pressure, precipitated with ethyl acetate, filtered, and washed successively with ethanol and ethyl acetate to obtain an L-histidine-phenylthiourea intermediate. Further, 45 g of... Triethylamine was added to an 80 vol% aqueous ethanol solution to adjust the pH to 7.2, and the mixture was stirred and dispersed for 35 min. Then, 33 g of organosilicon pre-hydrolyzed solution was added dropwise, and the mixture was heated to 48 °C and stirred for 3 h. Subsequently, 2.5 g of fumed silica was added and ultrasonically dispersed for 40 min. The mixture was stirred for another 70 min, and then vacuum dried at 55 °C for 7 h. The mixture was then ground and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

[0033] 24g of hydrogenated rosin (CAS No.: 65997-06-0), 12g of acrylic-modified rosin (CAS No.: 82692-93-1), 18g of terpineol (CAS No.: 8000-41-7), and 14g of diethylene glycol monohexyl ether were added to a reaction vessel. The mixture was heated to 95°C and stirred for 50 minutes under nitrogen protection. Subsequently, 3.5g of epoxy silicone oil and 0.6g of... KH560 was heated to 105℃ and stirred for 2 hours. The temperature was then lowered to 72℃, and 2.0 g sebacic acid, 1.0 g adipic acid, and 0.55 g of hybrid corrosion inhibitor were added. The mixture was stirred for 50 minutes. Then, 4.5 g hydrogenated castor oil (CAS No.: 61788-85-0) and 2.0 g polyamide wax (particle size 5~7 μm) were added. The mixture was stirred and dispersed at 78℃ for 50 minutes. Subsequently, the mixture was degassed under vacuum at -0.085 MPa for 25 minutes and cooled to room temperature to obtain the flux system.

[0034] 72.5g of Sn42Bi58 tin-bismuth powder (particle size 25~45μm), 10g of flake SAC305 solder powder and 12.5g of flux system were mixed and stirred for 12min at 28℃ and vacuum degree -0.09MPa. Then, 5.0g of rosin acid-coated Sn-Bi composite nanoparticles were added and stirred for 25min. Vacuum degassing was carried out for 20min to obtain low-temperature composite solder paste.

[0035] Example 2 3.8 g SnCl2·2H2O and 4.2 g BiCl3 were added to 80 g hydrochloric acid / ethanol mixed solution (the hydrochloric acid / ethanol mixed solution was prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1). The mixture was stirred for 30 min under nitrogen protection to obtain a Sn / Bi metal salt mixed solution. 0.3 g rosin acid was added to 100 mL ethanol, heated to 40 °C and stirred for 20 min. After cooling to room temperature, 3.5 g sodium borohydride was added and the mixture was stirred in an ice-water bath for 10 min. Then, the Sn / Bi metal salt mixed solution was added dropwise under nitrogen protection and an ice-water bath, and the mixture was stirred for another 60 min. After that, the mixture was heated to room temperature and stirred for another 80 min. The solid was collected by centrifugation, washed three times with deionized water and anhydrous ethanol, and dried under vacuum at 50 °C for 4 h to obtain rosin acid-coated Sn-Bi composite nanoparticles.

[0036] 40g of SAC305 solder powder (particle size 25~45μm) was added to a planetary ball mill jar, along with 500g of zirconia grinding balls and 500g of ethylene glycol. After purging the jar with nitrogen for 10 minutes, the jar was sealed and milled at 400r / min for 70 minutes. The resulting slurry was then vacuum filtered under nitrogen protection and washed three times with anhydrous ethanol. Finally, it was vacuum dried at 40℃ for 2 hours to obtain flake-like SAC305 solder powder.

[0037] 0.4 g KH560 (CAS No.: 2530-83-8), 1.3 g tetraethyl orthosilicate, and 0.4 g methyltriethoxysilane were added to 25 g of 80 vol% aqueous ethanol solution. The pH was adjusted to 4.5 with 5 wt% aqueous acetic acid solution, and the mixture was stirred and hydrolyzed for 30 min to obtain an organosilicon pre-hydrolyzed solution. 1.3 g phenyl isothiocyanate was dissolved in 20 g tetrahydrofuran to obtain a phenyl isothiocyanate solution. 1.5 g L-histidine was added to 60 g of 75 vol% aqueous ethanol solution and stirred to disperse. Triethylamine was added to adjust the pH to 8.0, and 21.3 g of phenyl isothiocyanate solution was added dropwise. The mixture was heated to 45 °C under nitrogen protection and reacted for 10 h. After cooling to room temperature, the mixture was concentrated under reduced pressure, precipitated with ethyl acetate, filtered, and washed successively with ethanol and ethyl acetate to obtain an L-histidine-phenylthiourea intermediate. Further, 40 g of... Triethylamine was added to an 80 vol% aqueous ethanol solution to adjust the pH to 7.0, and the mixture was stirred and dispersed for 30 min. Then, 28 g of organosilicon pre-hydrolyzed solution was added dropwise, and the mixture was heated to 45 °C and stirred for 2 h. Subsequently, 2 g of fumed silica was added and ultrasonically dispersed for 30 min, and the mixture was stirred for another 60 min. The mixture was then vacuum dried at 50 °C for 6 h, ground, and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

[0038] 22g of hydrogenated rosin (CAS No.: 65997-06-0), 10g of acrylic-modified rosin (CAS No.: 82692-93-1), 15g of terpineol (CAS No.: 8000-41-7), and 12g of diethylene glycol monohexyl ether were added to a reaction vessel. The mixture was heated to 90℃ and stirred for 40 minutes under nitrogen protection. Then, 3g of epoxy silicone oil and 0.5g of... KH560 was heated to 100℃ and stirred for 1.5h. The temperature was then lowered to 70℃, and 1.5g sebacic acid, 0.8g adipic acid, and 0.5g hybrid corrosion inhibitor were added and stirred for 40min. Then, 4g hydrogenated castor oil (CAS No.: 61788-85-0) and 1.5g polyamide wax (particle size 5~7μm) were added and stirred and dispersed at 75℃ for 40min. Subsequently, vacuum degassing was performed at -0.08MPa for 20min, and the mixture was cooled to room temperature to obtain the flux system.

[0039] 74g of Sn42Bi58 tin-bismuth powder (particle size 25~45μm), 8g of flake SAC305 solder powder and 10g of flux system were mixed and stirred for 8min at 25℃ and vacuum degree -0.09MPa. Then, 4g of rosin acid-coated Sn-Bi composite nanoparticles were added and stirred for 20min. Vacuum degassing was carried out for 15min to obtain low-temperature composite solder paste.

[0040] Example 3 4.2 g SnCl2·2H2O and 4.7 g BiCl3 were added to 100 g hydrochloric acid / ethanol mixed solution (the hydrochloric acid / ethanol mixed solution was prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1). The mixture was stirred for 40 min under nitrogen protection to obtain a Sn / Bi metal salt mixed solution. 0.5 g rosin acid was added to 120 mL ethanol, heated to 45 °C and stirred for 30 min. After cooling to room temperature, 4.5 g sodium borohydride was added and the mixture was stirred in an ice-water bath for 15 min. Then, the Sn / Bi metal salt mixed solution was added dropwise under nitrogen protection and an ice-water bath, and the mixture was stirred for another 80 min. After that, the mixture was heated to room temperature and stirred for another 120 min. The solid was collected by centrifugation, washed five times with deionized water and anhydrous ethanol, and dried under vacuum at 55 °C for 5 h to obtain rosin acid-coated Sn-Bi composite nanoparticles.

[0041] 50g of SAC305 solder powder (particle size 25~45μm) was added to a planetary ball mill jar, along with 500g of zirconia grinding balls and 500g of ethylene glycol. After purging the jar with nitrogen for 15min, the jar was sealed and milled at 400r / min for 90min. The resulting slurry was then vacuum filtered under nitrogen protection and washed five times with anhydrous ethanol. Finally, it was vacuum dried at 45℃ for 3h to obtain flake-like SAC305 solder powder.

[0042] 0.6 g KH560 (CAS No.: 2530-83-8), 1.8 g tetraethyl orthosilicate, and 0.6 g methyltriethoxysilane were added to 35 g of 80 vol% aqueous ethanol solution. The pH was adjusted to 5.0 with 5 wt% aqueous acetic acid solution, and the mixture was stirred and hydrolyzed for 50 min to obtain an organosilicon pre-hydrolyzed solution. 1.5 g phenyl isothiocyanate was dissolved in 25 g tetrahydrofuran to obtain a phenyl isothiocyanate solution. 1.6 g L-histidine was added to 70 g of 75 vol% aqueous ethanol solution and stirred to disperse. Triethylamine was added to adjust the pH to 8.5, and 26.5 g of phenyl isothiocyanate solution was added dropwise. The mixture was heated to 50 °C under nitrogen protection and reacted for 12 h. After cooling to room temperature, the mixture was concentrated under reduced pressure, precipitated with ethyl acetate, filtered, and washed successively with ethanol and ethyl acetate to obtain an L-histidine-phenylthiourea intermediate. Further, 50 g of... Triethylamine was added to an 80 vol% aqueous ethanol solution to adjust the pH to 7.5, and the mixture was stirred and dispersed for 40 min. Then, 38 g of organosilicon pre-hydrolyzed solution was added dropwise, and the mixture was heated to 50 °C and stirred for 4 h. Subsequently, 3 g of fumed silica was added and ultrasonically dispersed for 50 min, and the mixture was stirred for another 80 min. The mixture was then vacuum dried at 60 °C for 8 h, ground, and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

[0043] 26g of hydrogenated rosin (CAS No.: 65997-06-0), 15g of acrylic-modified rosin (CAS No.: 82692-93-1), 20g of terpineol (CAS No.: 8000-41-7), and 16g of diethylene glycol monohexyl ether were added to a reaction vessel. The mixture was heated to 100℃ and stirred for 60 minutes under nitrogen protection. Subsequently, 4g of epoxy silicone oil and 0.7g of... KH560 was heated to 110℃ and stirred for 3 hours. The temperature was then lowered to 75℃, and 2.5g sebacic acid, 1.2g adipic acid, and 0.6g hybrid corrosion inhibitor were added. The mixture was stirred for 60 minutes. Then, 5g hydrogenated castor oil (CAS No.: 61788-85-0) and 2.5g polyamide wax (particle size 5~7μm) were added. The mixture was stirred and dispersed at 80℃ for 60 minutes. Subsequently, the mixture was degassed under vacuum at -0.09MPa for 30 minutes and cooled to room temperature to obtain the flux system.

[0044] 68g of Sn42Bi58 tin-bismuth powder (particle size 25~45μm), 12g of flake SAC305 solder powder and 15g of flux system were mixed and stirred for 15min at 30℃ and vacuum degree -0.09MPa. Then, 6g of rosin acid-coated Sn-Bi composite nanoparticles were added and stirred for 30min. Vacuum degassing was carried out for 30min to obtain low-temperature composite solder paste.

[0045] Example 4 3.9 g SnCl2·2H2O and 4.3 g BiCl3 were added to 85 g hydrochloric acid / ethanol mixed solution (the hydrochloric acid / ethanol mixed solution was prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1). The mixture was stirred for 32 min under nitrogen protection to obtain a Sn / Bi metal salt mixed solution. 0.35 g rosin acid was added to 105 mL ethanol, heated to 41 °C and stirred for 22 min. After cooling to room temperature, 3.8 g sodium borohydride was added and the mixture was stirred in an ice-water bath for 11 min. Then, under nitrogen protection and an ice-water bath, the Sn / Bi metal salt mixed solution was added dropwise and stirred for another 65 min. The mixture was then heated to room temperature and stirred for another 90 min. The solid was collected by centrifugation, washed three times with deionized water and anhydrous ethanol, and dried under vacuum at 51 °C for 4 h to obtain rosin acid-coated Sn-Bi composite nanoparticles.

[0046] 42g of SAC305 solder powder (particle size 25~45μm) was added to a planetary ball mill jar, along with 500g of zirconia grinding balls and 500g of ethylene glycol. After purging the jar with nitrogen for 11 minutes, the jar was sealed and milled at 400r / min for 75 minutes. The resulting slurry was then vacuum filtered under nitrogen protection and washed three times with anhydrous ethanol. Finally, it was vacuum dried at 41℃ for 2 hours to obtain flake-like SAC305 solder powder.

[0047] 0.45 g KH560 (CAS No.: 2530-83-8), 1.4 g tetraethyl orthosilicate, and 0.45 g methyltriethoxysilane were added to 28 g of 80 vol% aqueous ethanol solution. The pH was adjusted to 4.6 with 5 wt% aqueous acetic acid solution, and the mixture was stirred and hydrolyzed for 35 min to obtain an organosilicon pre-hydrolyzed solution. 1.35 g phenyl isothiocyanate was dissolved in 21 g tetrahydrofuran to obtain a phenyl isothiocyanate solution. 1.52 g L-histidine was added to 62 g of 75 vol% aqueous ethanol solution and stirred to disperse. Triethylamine was added to adjust the pH to 8.1, and 22.35 g of phenyl isothiocyanate solution was added dropwise. The mixture was heated to 46 °C under nitrogen protection and reacted for 10.5 h. After cooling to room temperature, the mixture was concentrated under reduced pressure, precipitated with ethyl acetate, filtered, and washed successively with ethanol and ethyl acetate to obtain an L-histidine-phenylthiourea intermediate. Further, 42 g of... Triethylamine was added to an 80 vol% aqueous ethanol solution to adjust the pH to 7.1, and the mixture was stirred and dispersed for 32 min. Then, 30 g of organosilicon pre-hydrolyzed solution was added dropwise, and the mixture was heated to 46 °C and stirred for 2.5 h. Subsequently, 2.2 g of fumed silica was added and ultrasonically dispersed for 35 min. The mixture was stirred for another 65 min and then vacuum dried at 52 °C for 6.5 h. The mixture was then ground and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

[0048] 23g of hydrogenated rosin (CAS No.: 65997-06-0), 11g of acrylic-modified rosin (CAS No.: 82692-93-1), 16g of terpineol (CAS No.: 8000-41-7), and 13g of diethylene glycol monohexyl ether were added to a reaction vessel. The mixture was heated to 92℃ and stirred for 45 minutes under nitrogen protection. Then, 3.2g of epoxy silicone oil and 0.55g of... KH560 was heated to 102℃ and stirred for 2 hours. The temperature was then lowered to 71℃, and 1.7g sebacic acid, 0.9g adipic acid, and 0.52g hybrid corrosion inhibitor were added. The mixture was stirred for 45 minutes. Then, 4.2g hydrogenated castor oil (CAS No.: 61788-85-0) and 1.7g polyamide wax (particle size 5~7μm) were added. The mixture was stirred and dispersed at 76℃ for 45 minutes. Subsequently, the mixture was degassed under vacuum at -0.08MPa for 22 minutes and cooled to room temperature to obtain the flux system.

[0049] 73g of Sn42Bi58 tin-bismuth powder (particle size 25~45μm), 9g of flake SAC305 solder powder and 11g of flux system were mixed and stirred for 10min at 26℃ and vacuum degree -0.09MPa. Then, 4.5g of rosin acid-coated Sn-Bi composite nanoparticles were added and stirred for 22min. Vacuum degassing was carried out for 18min to obtain low-temperature composite solder paste.

[0050] Example 5 4.0 g SnCl2·2H2O and 4.4 g BiCl3 were added to 88 g hydrochloric acid / ethanol mixed solution (the hydrochloric acid / ethanol mixed solution was prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1). The mixture was stirred for 36 min under nitrogen protection to obtain a Sn / Bi metal salt mixed solution. 0.38 g rosin acid was added to 108 mL ethanol, the temperature was raised to 42 °C and stirred for 24 min, cooled to room temperature, 3.9 g sodium borohydride was added and stirred in an ice-water bath for 13 min, and then the Sn / Bi metal salt mixed solution was added dropwise under nitrogen protection and an ice-water bath. The mixture was stirred for another 68 min, then raised to room temperature and stirred for another 95 min. The solid was collected by centrifugation, washed four times with deionized water and anhydrous ethanol, and dried under vacuum at 53 °C for 4.5 h to obtain rosin acid-coated Sn-Bi composite nanoparticles.

[0051] 46g of SAC305 solder powder (particle size 25~45μm) was added to a planetary ball mill jar, along with 500g of zirconia grinding balls and 500g of ethylene glycol. After purging the jar with nitrogen for 13min, the jar was sealed and ball-milled at 400r / min for 78min. The resulting slurry was then vacuum filtered under nitrogen protection and washed four times with anhydrous ethanol. Subsequently, it was vacuum-dried at 43℃ for 2.5h to obtain flake-like SAC305 solder powder.

[0052] 0.48 g KH560 (CAS No.: 2530-83-8), 1.6 g tetraethyl orthosilicate, and 0.48 g methyltriethoxysilane were added to 31 g of 80 vol% aqueous ethanol solution. The pH was adjusted to 4.7 with 5 wt% aqueous acetic acid solution, and the mixture was stirred and hydrolyzed for 42 min to obtain an organosilicon pre-hydrolyzed solution. 1.42 g phenyl isothiocyanate was dissolved in 22 g tetrahydrofuran to obtain a phenyl isothiocyanate solution. 1.54 g L-histidine was added to 66 g of 75 vol% aqueous ethanol solution and stirred to disperse. Triethylamine was added to adjust the pH to 8.3, and 23.42 g of phenyl isothiocyanate solution was added dropwise. The mixture was heated to 47 °C under nitrogen protection and reacted for 11 h. After cooling to room temperature, the mixture was concentrated under reduced pressure, precipitated with ethyl acetate, filtered, and washed successively with ethanol and ethyl acetate to obtain an L-histidine-phenylthiourea intermediate. Further, 46 g of... Triethylamine was added to an 80 vol% aqueous ethanol solution to adjust the pH to 7.3, and the mixture was stirred and dispersed for 36 min. Then, 34 g of organosilicon pre-hydrolyzed solution was added dropwise, and the mixture was heated to 47 °C and stirred for 3 h. Subsequently, 2.6 g of fumed silica was added and ultrasonically dispersed for 42 min. The mixture was stirred for another 72 min, and then vacuum dried at 56 °C for 7 h. The mixture was then ground and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

[0053] 24g of hydrogenated rosin (CAS No.: 65997-06-0), 13g of acrylic-modified rosin (CAS No.: 82692-93-1), 17g of terpineol (CAS No.: 8000-41-7), and 14g of diethylene glycol monohexyl ether were added to a reaction vessel. The mixture was heated to 96℃ and stirred for 48 minutes under nitrogen protection. Subsequently, 3.6g of epoxy silicone oil and 0.6g of... KH560 was heated to 106℃ and stirred for 2 hours. The temperature was then lowered to 73℃, and 2.1g sebacic acid, 1.0g adipic acid, and 0.56g hybrid corrosion inhibitor were added. The mixture was stirred for 52 minutes. Then, 4.6g hydrogenated castor oil (CAS No.: 61788-85-0) and 2.1g polyamide wax (particle size 5~7μm) were added. The mixture was stirred and dispersed at 78℃ for 52 minutes. Subsequently, the mixture was degassed under vacuum at -0.085MPa for 26 minutes and cooled to room temperature to obtain the flux system.

[0054] 71g of Sn42Bi58 tin-bismuth powder (particle size 25~45μm), 11g of flake SAC305 solder powder and 13g of flux system were mixed and stirred for 13min at 27℃ and vacuum degree -0.09MPa. Then, 5.2g of rosin acid-coated Sn-Bi composite nanoparticles were added and stirred for 26min. Vacuum degassing was carried out for 22min to obtain low-temperature composite solder paste.

[0055] The present invention also includes comparative examples and related experiments.

[0056] Comparative Example 1 Compared with Example 1, the only difference is that no hybrid corrosion inhibitor was prepared. The other preparation methods and components are completely consistent, and the low-temperature composite solder paste is finally obtained.

[0057] Comparative Example 2 Compared with Example 1, the only difference is that no sheet-like SAC305 solder powder was added. The other preparation methods and components are completely consistent, and the low-temperature composite solder paste is finally obtained.

[0058] Comparative Example 3 Compared with Example 1, the only difference is that rosin acid was not added to coat the Sn-Bi composite nanoparticles. The other preparation methods and components are completely the same, and the low-temperature composite solder paste is finally obtained.

[0059] Performance testing The performance of the low-temperature composite solder pastes prepared in Examples 1-5 and Comparative Examples 1-3 was tested: Wetting and spreading performance tests were conducted according to GB / T38265.10-2019 "Test Methods for Solder Flux - Part 10: Wetting and Spreading Tests of Solder Flux", and the solder spreading rate was calculated. The viscosity and thixotropic index of the low-temperature composite solder pastes were tested using a Brookfield viscometer. The low-temperature composite solder pastes prepared in Examples 1-5 and Comparative Examples 1-3 were printed onto copper pad test boards of the same specifications, and after mounting chip components of the same specifications, reflow soldering was performed. The reflow peak temperature was 180℃, and the peak holding time was 70s. After cooling to room temperature, the initial shear strength of the solder joints was tested using a solder joint shear tester. Simultaneously, temperature cycling tests were conducted according to GB / T2423.22-2012 "Environmental Testing - Part 2: Test Methods - Test N: Temperature Change". After 500 cycles, the solder joints were observed by metallographic sectioning, and the proportion of cracks was statistically analyzed. Copper corrosion performance was tested according to GB / T38265.15-2021 "Test Methods for Soldering Flux - Part 15: Copper Corrosion Test". The low-temperature composite solder pastes prepared in Examples 1-5 and Comparative Examples 1-3 were coated onto the pretreated copper sheet surface. After reflow treatment, they were placed in a constant temperature and humidity environment of 40℃ and 90% relative humidity for 72 hours. After the test, the discoloration, corrosion spots, and residual corrosion of the copper sheet surface were observed, and the degree of copper sheet corrosion was rated. Surface insulation resistance was tested according to GB / T38265.17-2022 "Test Methods for Soldering Flux - Part 17: Surface Insulation Resistance Comb Test and Electrochemical Migration Test of Flux Residue" to evaluate the surface insulation stability and electrochemical migration risk of the low-temperature composite solder paste residue after soldering. The specific performance test results are shown in Table 1.

[0060] Table 1

[0061] Table 1 shows that the low-temperature composite solder pastes prepared in Examples 1-5 had a spreading rate of 86.1-89.6%, a viscosity of 168-188 Pa·s, and a thixotropic index of 0.56-0.65, indicating that the solder pastes prepared by this invention have good wetting and spreading properties. The initial shear strength of Examples 1-5 was 40.5-43.8 MPa, and the proportion of cracked solder joints after 500 temperature cycles was only 1.6-3.4%, indicating that this invention, through the combination of rosin acid-coated Sn-Bi nanoparticles and flake-shaped SAC305 solder powder, can improve the bonding strength of solder joints and reduce the risk of brittle cracking of Sn-Bi solder joints during temperature changes. In contrast, Comparative Example 2, without the addition of flake-shaped SAC305 solder powder, had an initial shear strength of only 32.4 MPa, and the proportion of cracked solder joints increased to 20.5% after 500 temperature cycles. In Comparative Example 3, without the addition of rosin acid to coat Sn-Bi nanoparticles, the spreading rate decreased to 80.5%, and the proportion of cracked weld points increased to 13.6%.

[0062] Based on the copper corrosion rating and surface insulation resistance results, the copper corrosion rating of Examples 1-5 is all 0, and the surface insulation resistance is 2.2 × 10⁻⁶. 11 ~3.4×10 11 The Ω indicates that the low-temperature composite solder paste prepared by this invention exhibits good corrosion resistance and surface insulation stability after soldering. Comparative Example 1, without the prepared hybrid corrosion inhibitor, although its spreading rate, viscosity, thixotropic index, and initial shear strength are similar to those of the examples, shows an increase in copper corrosion grade to level 2 and a decrease in surface insulation resistance to 4.2 × 10⁻⁶. 9 Ω.

[0063] To further evaluate the applicability of this invention in an immersion cooling environment, the low-temperature composite solder pastes prepared in Examples 1-5 and Comparative Examples 1-3 were reflowed and soldered onto copper pad test boards. After cooling, they were placed in a simulated immersion coolant and immersed at 60°C for 168 hours. After the test, the samples were removed, and the corrosion of the solder joints and copper pad surfaces was observed. The surface insulation resistance retention rate after immersion was also tested. The specific performance test results are shown in Table 2.

[0064] Table 2

[0065] As shown in Table 2, after being immersed in simulated immersion coolant at 60°C for 168 hours, the solder joint surfaces of Examples 1-5 remained intact with no obvious corrosion spots, and the copper pads showed virtually no discoloration. The surface insulation resistance retention rate was 90.4-94.1%, indicating that the low-temperature composite solder paste prepared by this invention has good corrosion resistance and surface insulation retention in the immersion coolant environment. In contrast, Comparative Example 1, which did not prepare hybrid corrosion inhibitors, showed slight corrosion marks at the edges of the solder joints after immersion in coolant, and discoloration and corrosion spots appeared on the surface of the copper pads. The surface insulation resistance retention rate was only 60.8%, significantly lower than that of Example 1. Comparative Examples 2 and 3, which still contained hybrid corrosion inhibitors, had higher insulation retention rates after immersion, but Comparative Example 3 showed uneven wetting and edge defects in some areas of the solder joints. The above are preferred embodiments of this invention. Those skilled in the art can make several improvements and modifications without departing from the principle of this invention, and these improvements and modifications should also be considered within the scope of protection of this invention.

Claims

1. A method for preparing a low-temperature composite solder paste, characterized in that, Includes the following steps: S1. Disperse L-histidine in an aqueous ethanol solution, add phenyl isothiocyanate solution dropwise, and react under nitrogen protection. After the reaction, concentrate, precipitate, filter and wash to obtain L-histidine-phenylthiourea intermediate. Further dispersed in an ethanol-water solution, reacted with the addition of organosilicon pre-hydrolyzed solution, then dispersed and compounded with fumed silica, and after drying, grinding and sieving, a hybrid corrosion inhibitor component was obtained; S2. Hydrogenated rosin, acrylic modified rosin, terpineol and diethylene glycol monohexyl ether are heated and stirred, then epoxy silicone oil and KH560 are added and heated and stirred. After cooling, organic acid and hybrid corrosion inhibitor are added and stirred. Then, thixotropic agent is added and dispersed. After vacuum degassing and cooling, the flux system is obtained. S3. Mix and stir Sn42Bi58 tin-bismuth powder, flake SAC305 solder powder and flux system, then add rosin acid-coated Sn-Bi composite nanoparticles and stir, vacuum degassing to obtain low-temperature composite solder paste.

2. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, The phenyl isothiocyanate solution is obtained by dissolving 1.3-1.5 parts by weight of phenyl isothiocyanate in 20-25 parts by weight of tetrahydrofuran; the organosilicon pre-hydrolyzed solution is obtained by adding 0.4-0.6 parts by weight of KH560, 1.3-1.8 parts by weight of tetraethyl orthosilicate and 0.4-0.6 parts by weight of methyltriethoxysilane to 25-35 parts by weight of 80 vol% ethanol aqueous solution, adjusting the pH to 4.5-5.0 with 5 wt% acetic acid aqueous solution, and stirring and hydrolyzing for 30-50 min.

3. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, In step S1, 1.5-1.6 parts by weight of L-histidine are added to 60-70 parts by weight of 75 vol% ethanol aqueous solution and stirred to disperse. Triethylamine is added to adjust the pH to 8.0-8.5, and 21.3-26.5 parts by weight of phenyl isothiocyanate solution is added dropwise. The mixture is heated to 45-50°C under nitrogen protection and reacted for 10-12 hours. After cooling to room temperature, the mixture is concentrated under reduced pressure, ethyl acetate is added to precipitate the product, and the mixture is filtered. The product is washed successively with ethanol and ethyl acetate to obtain the L-histidine-phenylthiourea intermediate. The mixture is added to 40-50 parts by weight of 80 vol% ethanol aqueous solution, and triethylamine is added to adjust the pH to 7.0-7.

5. The mixture is stirred and dispersed for 30-40 min, and then 28-38 parts by weight of organosilicon pre-hydrolyzed solution is added dropwise. The mixture is heated to 45-50℃ and stirred for 2-4 h. Then 2-3 parts by weight of fumed silica are added and ultrasonically dispersed for 30-50 min. The mixture is stirred for 60-80 min and then vacuum dried at 50-60℃ for 6-8 h. The mixture is then ground and passed through a 200-mesh sieve to obtain the hybrid corrosion inhibitor component.

4. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, In step S2, hydrogenated rosin, acrylic acid-modified rosin, terpineol, and diethylene glycol monohexyl ether are added to a reaction vessel. Under nitrogen protection, the mixture is heated to 90-100°C and stirred for 40-60 minutes. Then, epoxy silicone oil and KH560 are added, and the mixture is heated to 100-110°C and stirred for 1.5-3 hours. The mixture is then cooled to 70-75°C, and organic acid and hybrid corrosion inhibitors are added and stirred for 40-60 minutes. Next, a thixotropic agent is added, and the mixture is stirred and dispersed at 75-80°C for 40-60 minutes. Then, the mixture is vacuum degassed under -0.08 to -0.09 MPa for 20-30 minutes and cooled to room temperature to obtain the flux system. The organic acid is composed of 1.5 to 2.5 parts by weight of sebacic acid and 0.8 to 1.2 parts by weight of adipic acid; the thixotropic agent is composed of 4 to 5 parts by weight of hydrogenated castor oil and 1.5 to 2.5 parts by weight of polyamide wax.

5. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, The flake-shaped SAC305 solder powder was prepared by adding 40-50 parts by weight of SAC305 solder powder into a planetary ball mill jar, adding 500 parts by weight of zirconia grinding balls and 500 parts by weight of ethylene glycol, purging the air in the ball mill jar with nitrogen for 10-15 minutes, sealing it, and ball milling at 400 r / min for 70-90 minutes. The resulting slurry was then vacuum filtered under nitrogen protection and washed 3-5 times with anhydrous ethanol, followed by vacuum drying at 40-45°C for 2-3 hours.

6. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, The rosin acid-coated Sn-Bi composite nanoparticles were prepared by adding 0.3-0.5 parts by weight of rosin acid to 100-120 mL of ethanol, heating to 40-45°C and stirring for 20-30 min, cooling to room temperature, adding 3.5-4.5 parts by weight of sodium borohydride and stirring in an ice-water bath for 10-15 min, then adding a Sn / Bi metal salt mixed solution dropwise under nitrogen protection and an ice-water bath, continuing to stir for 60-80 min, then heating to room temperature and stirring for another 80-120 min, collecting the solid by centrifugation, washing it 3-5 times with deionized water and anhydrous ethanol, and then vacuum drying at 50-55°C for 4-5 h.

7. The method for preparing a low-temperature composite solder paste according to claim 6, characterized in that, The Sn / Bi metal salt mixed solution was obtained by adding SnCl2·2H2O and BiCl3 to a hydrochloric acid / ethanol mixed solution and stirring for 30-40 minutes under nitrogen protection.

8. The method for preparing a low-temperature composite solder paste according to claim 7, characterized in that, The hydrochloric acid / ethanol mixed solution is prepared by mixing 1.0 mol / L hydrochloric acid aqueous solution and anhydrous ethanol at a volume ratio of 1:1; the Sn / Bi metal salt mixed solution comprises the following raw materials in parts by mass: 3.8~4.2 parts SnCl2·2H2O, 4.2~4.7 parts BiCl3 and 80~100 parts hydrochloric acid / ethanol mixed solution.

9. The method for preparing a low-temperature composite solder paste according to claim 1, characterized in that, In step S3, Sn42Bi58 tin-bismuth powder, flake SAC305 solder powder and flux system are mixed and stirred for 8-15 minutes at 25-30°C and a vacuum degree of -0.09MPa. Then, rosin acid is added to coat Sn-Bi composite nanoparticles and stirred for 20-30 minutes. Vacuum degassing is performed for 15-30 minutes to obtain low-temperature composite solder paste.

10. A low-temperature composite solder paste, characterized in that, The low-temperature composite solder paste is prepared using the preparation method according to any one of claims 1 to 9, comprising the following raw materials in parts by weight: 68-76 parts Sn42Bi58 tin-bismuth powder, 8-12 parts flake SAC305 solder powder, 10-15 parts flux system, and 4-6 parts rosin acid-coated Sn-Bi composite nanoparticles; wherein the flux system comprises the following raw materials in parts by weight: 22-26 parts hydrogenated rosin, 10-15 parts acrylic acid-modified rosin, 15-20 parts terpineol, 12-16 parts diethylene glycol monohexyl ether, 3-4 parts epoxy silicone oil, 0.5-0.7 parts KH560, 2.3-3.7 parts organic acid, 0.5-0.6 parts hybrid corrosion inhibitor, and 5.5-7.5 parts thixotropic agent.

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