A lead-free SMT solder paste for soldering electronic components

Abstract

This invention discloses a lead-free SMT solder paste for soldering electronic components, composed of 88%–92% Sn-Bi based solder alloy powder and 8%–12% flux. The solder alloy powder contains 35%–60% Bi, 0.1%–1.5% Ag, and trace elements selected from Ge and P. The flux contains fully hydrogenated rosin, liquid styrene, a thixotropic agent, a compound solvent, an organic acid activator, and an active and reinforcing complex. The complex consists of nano-nickel powder coated with a styrene-maleic anhydride copolymer after surface modification with a coupling agent. The coating layer isolates the nickel powder during storage and softens and releases it during soldering, generating Ni₃Sn₄ in situ to refine and disperse the Bi-rich phase. The boiling point of the compound solvent matches the soldering temperature of 180℃, and the liquid styrene and fully hydrogenated rosin synergistically resist oxidation. The solder paste exhibits a shear strength of 38.3 MPa after soldering at 180℃, a Bi-rich phase size of 1.9 μm, and a slag ratio of 0.43%.

Description

Technical Field

[0001] This invention belongs to the field of electronic soldering materials technology, specifically relating to a lead-free SMT solder paste for soldering electronic components. Background Technology

[0002] Sn-Bi lead-free solder paste has irreplaceable application value in the field of electronic packaging that requires low-temperature soldering because the melting point of Sn-58Bi eutectic alloy is only 138℃, and the peak soldering temperature can be controlled at around 180℃. It is especially suitable for flexible circuit boards with poor heat resistance, thermistors, and multilayer packaging structures that require stepped soldering. However, Sn-Bi solder has an inherent defect: the Bi phase is prone to segregation during the solidification process of the solder joint, forming a coarse Bi-rich phase. The Bi-rich phase is brittle and becomes a crack source under thermal cycling and mechanical stress. The reliability and mechanical properties of the solder joint are far lower than those of Sn-Ag-Cu solder.

[0003] To address the brittleness of the Bi phase, existing technologies mainly focus on two directions for improvement. One direction is the addition of micro / nano reinforcing particles. CN107999994A mixes micro / nano particle-reinforced tin-based alloy solder powder with SnBi-based low-melting-point alloy solder powder. The β-Sn phase precipitated in the tin-based alloy preferentially bonds with the Cu pad, reducing Sn diffusion in the SnBi eutectic structure and thus suppressing the formation of a Bi-rich layer. This method refines the solder joint microstructure to some extent, but the uniformity of the reinforcing phase particles during welding is difficult to guarantee, and Sn in the SnBi eutectic structure continues to diffuse into the Cu substrate during aging, making it impossible to fundamentally eliminate the Bi-rich layer. CN121423904A uses carbon nanotubes loaded with silver nanoparticles as a reinforcing component. The silver nanoparticles and Sn form the Ag3Sn reinforcing phase, and the carboxyl groups on the surface of the carbon nanotubes reduce the surface tension of the solder. Both work synergistically to promote solder wetting and spreading, and exert a dispersion strengthening effect. This method can improve the mechanical properties of the weld joint to a certain extent, but the interfacial bonding force between carbon nanotubes and the metal matrix is ​​weak, and the strengthening effect gradually diminishes during long-term service.

[0004] Another approach is microalloying with trace elements. CN109082559A adds trace elements such as Ni, Ge, and Ce to SnAgCu-based solders to improve wettability by refining the grain size. CN118808986A adds red phosphorus and germanium to solder alloy powder, utilizing the oxygen-loving skin effect to form a dense surface protective film and reduce the amount of oxide slag. However, relying solely on trace elements has limited effect on improving the brittleness of the Bi phase; its main role is in oxidation resistance and wettability, and the improvement in the mechanical properties of the solder joint cannot meet the requirements of high-reliability packaging.

[0005] Adding nano-nickel powder is a potentially effective way to improve the brittleness of the Bi phase. Nano-nickel powder can react in situ with Sn during welding to form Ni3Sn4 intermetallic compounds. These compounds have high melting points and high hardness, and can act as heterogeneous nucleation sites to refine the solder joint microstructure. Simultaneously, the dispersed Ni3Sn4 particles produce a dispersion strengthening effect. However, this technical approach faces a key obstacle: the flux must contain organic acid activators to remove the oxide film on the pads and solder surfaces. CN118808986A uses dimer fatty acids as activators, while CN111360446B uses succinic acid or glutaric acid in conjunction with activators. The high surface activity of nano-nickel powder makes it highly susceptible to slow reactions with these organic acids during storage. This consumes the activator, reducing flux activity, and the nickel corrosion products cause the solder paste viscosity to continuously increase until failure. If the amount of organic acid is reduced to protect the nano-nickel powder, the flux's ability to remove the oxide film on the pads and solder powder surfaces is insufficient, leading to poor wetting. The contradiction between storage stability and soldering activity is the core technical obstacle to the application of nano-nickel powder in Sn-Bi solder paste systems.

[0006] Furthermore, the low-temperature soldering characteristics of Sn-Bi solder paste impose additional constraints on the solvent system. CN109483089A discloses a flux for high-temperature solder paste using high-boiling-point solvents such as n-decyl ether, with soldering temperatures above 280°C. These solvents have insufficient evaporation rates at the 180°C soldering temperature of Sn-Bi solder paste, leaving a large amount of solvent residue inside and on the surface of the solder joint, forming a weakening layer, reducing the solder joint bond strength, and hindering the spread of molten solder on the substrate surface. CN104308395A discloses a halogen-free flux for SnBi series solder paste using a mixture of ethers with boiling points of 120–138°C and ethers with boiling points of 190–210°C as solvents to meet the low melting point characteristics of SnBi series alloy powders. This approach addresses the matching of solvent boiling points with solder melting points, but no nano-reinforcing particles are added to the active system; the flux only performs the functions of removing oxide films and preventing oxidation, without contributing to the improvement of solder joint mechanical properties and Bi phase microstructure.

[0007] The aforementioned issues are interconnected and mutually restrictive. While nano-nickel powder can strengthen solder joints and suppress Bi phase embrittlement, it needs protection against corrosion in acidic flux. Coupling agent modification alone cannot prevent long-term penetration by acidic media, and the solvent boiling point must match the 180°C soldering temperature. Currently, there is no technical solution that unifies these three aspects into a single solder paste system. Summary of the Invention

[0008] The purpose of this invention is to provide a lead-free SMT solder paste for soldering electronic components, so as to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a lead-free SMT solder paste for soldering electronic components, comprising, by mass percentage, 88%–92% solder alloy powder and 8%–12% flux, wherein the solder alloy powder is a Sn-Bi alloy containing 35%–60% Bi, 0.1%–1.5% Ag, and at least one trace element selected from Ge and P, wherein the content of the trace element is 0.001%–0.1%, and the balance is Sn and unavoidable impurities; The flux, by weight, comprises 100 parts in total, including: 24-30 parts of fully hydrogenated rosin, 3-4 parts of liquid styrax, 3-8 parts of thixotropic agent, 35-50 parts of compound solvent, 2-8 parts of organic acid activator, and an active and reinforcing complex. The mass ratio of the fully hydrogenated rosin to the liquid styrax is 4-8:1; The active and reinforced composite is composed of surface-modified nano-nickel powder and a thermoplastic protective resin coating it, wherein the surface-modified nano-nickel powder is 0.5 to 3 parts and the thermoplastic protective resin is 1.5 to 4 parts. The compound solvent is composed of a high-boiling-point solvent and a low-boiling-point solvent mixed in a mass ratio of 1:0.8 to 1.2, wherein the boiling point of the high-boiling-point solvent is 190 to 230°C and the boiling point of the low-boiling-point solvent is 160 to 180°C. The surface-modified nickel nanopowder has an average particle size of 20-70 nm. The surface modification method is as follows: under inert gas protection, the nickel nanopowder is immersed in an organic solvent containing 0.5-5 wt% coupling agent, ultrasonically dispersed for 10-30 minutes, filtered and dried, thereby forming an organic modification layer on the surface of the nickel nanopowder. The thermoplastic protective resin is a styrene-maleic anhydride copolymer with an acid value of 100-200 mg KOH / g.

[0010] Preferably, the trace element is Ge, with a content of 0.005% to 0.015%; or the trace element is P, with a content of 0.005% to 0.02%.

[0011] Preferably, the styrene-maleic anhydride copolymer has an acid value of 140-180 mg KOH / g and is added in an amount of 2-4 parts.

[0012] Preferably, the organic acid activator is at least one of glutaric acid, succinic acid, adipic acid, or itaconic acid, and the amount added is 2 to 6 parts.

[0013] Preferably, the coupling agent is a combination of a silane coupling agent and a titanate coupling agent, with a total concentration of 1-3 wt%; the organic solvent is at least one of ethanol, isopropanol, or acetone.

[0014] Preferably, the high-boiling-point solvent is at least one of diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, or diethylene glycol monobutyl ether, and the low-boiling-point solvent is ethylene glycol butyl ether.

[0015] Preferably, the thixotropic agent is at least one of hydrogenated castor oil, polyamide wax, or ethylene bis-stearamide.

[0016] Preferably, the solder alloy powder contains 0.3%–1.0% Ag, and the trace element is Ge, with a content of 0.007%–0.01%; the fully hydrogenated rosin is 24–28 parts, and the liquid styrene is 3–4 parts; the surface-modified nano-nickel powder has an average particle size of 30–50 nm and is added in an amount of 1–2 parts; the styrene-maleic anhydride copolymer has an acid value of 140–180 mg KOH / g and is added in an amount of 2–4 parts; the organic acid activator is glutaric acid and is added in an amount of 3–5 parts; the compound solvent is a mixture of diethylene glycol monomethyl ether and ethylene glycol butyl ether in a mass ratio of 1:0.8–1.2; the coupling agent is a combination of silane coupling agent KH-560 and chelating titanate coupling agent, with a total concentration of 1.5–3.0 wt%; and the organic solvent is ethanol.

[0017] Preferably, the trace element is Ge, with a content of 0.007% to 0.01%; the fully hydrogenated rosin is 24 parts, and the liquid styrene is 3 parts; the styrene-maleic anhydride copolymer is added in an amount of 3 parts, with an acid value of 150 to 170 mg KOH / g; the surface-modified nano-nickel powder has an average particle size of 40 nm and is added in an amount of 1.5 parts; the glutaric acid is added in an amount of 4 parts; the thixotropic agent is hydrogenated castor oil, and is added in an amount of 5 parts; the compound solvent contains 25 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether.

[0018] Preferably, the preparation method of the active and reinforced composite is as follows: under inert gas protection, surface-modified nano-nickel powder and styrene-maleic anhydride copolymer are added to part of the compound solvent, heated and stirred until the styrene-maleic anhydride copolymer is completely dissolved, and then rapidly cooled to allow the styrene-maleic anhydride copolymer to precipitate on the surface of the nano-nickel powder to form a coating layer, thus obtaining a coated predispersed slurry; the coated predispersed slurry is added to a flux premix composed of fully hydrogenated rosin, the remaining compound solvent, liquid styrene, thixotropic agent and organic acid activator, and mixed evenly.

[0019] Compared with the prior art, the beneficial effects of the present invention are: The solder alloy powder of this invention contains 35%–60% Bi, with the peak welding temperature controlled at around 180°C. The Sn-Bi eutectic composition has a melting point around 138°C, ensuring complete melting and wetting of the solder on the copper substrate surface during welding. The Ag content is 0.1%–1.5%, forming Ag3Sn intermetallic compounds with Sn, which are distributed in the solder matrix and provide reinforcement. Trace elements Ge or P are enriched on the surface of the molten solder, preferentially oxidizing before Sn to form a dense oxide layer that hinders oxygen diffusion into the solder, reducing the amount of oxide slag generated during welding.

[0020] The flux contains 24-30 parts of fully hydrogenated rosin and 3-4 parts of liquid styrax, forming a film-forming system with a mass ratio of 4-8:1. The fully hydrogenated rosin is solid at room temperature, providing viscosity to the solder paste and shape retention during printing. At soldering temperatures, it melts and coats the solder and pad surfaces, preventing re-oxidation of the removed oxide film. The liquid styrax releases volatile terpenes at high soldering temperatures, forming an inert atmosphere layer on the molten solder surface, synergistically inhibiting solder oxidation with the fully hydrogenated rosin.

[0021] Two to eight parts of organic acid activator react chemically with oxides on the surface of the solder pads and solder powder during the soldering preheating stage, removing the oxide film and allowing the molten solder to directly contact the clean metal surface, thus achieving metallurgical bonding.

[0022] Thixotropic agent 3 to 8 parts form a three-dimensional network structure in solder paste, giving the solder paste thixotropic properties. During printing, the viscosity decreases under shear force, and the solder paste is smoothly transferred to the pad through the stencil opening. After the shear force is removed, the viscosity recovers, and the solder paste maintains its shape on the pad without collapsing.

[0023] The compound solvent consists of 35-50 parts of a high-boiling-point solvent and a low-boiling-point solvent mixed at a mass ratio of 1:0.8-1.2. The low-boiling-point solvent has a boiling point of 160-180℃ and begins to evaporate during the soldering preheating stage, while the high-boiling-point solvent has a boiling point of 190-230℃ and fully evaporates during the soldering heat preservation stage. The two solvents work together to maintain the appropriate viscosity and open time of the solder paste during the printing and placement stages, and completely evaporate without leaving any residue during the soldering stage.

[0024] The active and reinforced composite consists of surface-modified nickel nanoparticles and a styrene-maleic anhydride copolymer coating them. The average particle size of the nickel nanoparticles is 20–70 nm. The surface is ultrasonically impregnated with an organic solution containing a coupling agent. One end of the coupling agent condenses with the hydroxyl groups on the surface of the nickel nanoparticles to form a chemical bond, while the organic chain segment at the other end extends outward, forming an organic modification layer on the surface of the nickel nanoparticles. This organic modification layer improves the interfacial compatibility between the nickel nanoparticles and the styrene-maleic anhydride copolymer. The styrene-maleic anhydride copolymer is dissolved in a compound solvent by heating, and after rapid cooling, it precipitates on the surface of the nickel nanoparticles to form a dense coating layer. During room temperature storage of the solder paste, the styrene-maleic anhydride copolymer coating layer remains in a glassy state, physically isolating the nickel nanoparticles from the organic acid activator in the flux, preventing chemical reactions between them, maintaining the chemical inertness of the nickel nanoparticles, and keeping the viscosity change rate of the solder paste at a low level during storage. During the welding process, as the temperature rises above the solder's melting point, the styrene-maleic anhydride copolymer coating gradually softens, releasing the coated nano-nickel powder into the molten solder. The nano-nickel powder reacts with Sn in the molten solder, forming Ni3Sn4 intermetallic compound particles in situ. These Ni3Sn4 particles act as heterogeneous nucleation sites during solder solidification, reducing the nucleation work of the Bi and Sn phases and increasing the nucleation rate. This transforms the coarse dendritic morphology of the Bi-rich phase in the solder joint into a fine, uniform equiaxed crystal morphology, refining the average size of the Bi-rich phase to the micrometer level. The Ni3Sn4 particles dispersed throughout the solder joint matrix also act as dislocation pinning points and hinder grain boundary slip, thereby increasing the shear strength of the solder joint.

[0025] The acid value of the styrene-maleic anhydride copolymer is 100-200 mg KOH / g. The anhydride group is partially hydrolyzed into a carboxyl group at the high temperature of welding. The carboxyl group forms a complex bond with the copper atoms on the surface of the solder pad, which enhances the adhesion between the styrene-maleic anhydride copolymer and the solder pad. The styrene-maleic anhydride copolymer remaining after welding forms a protective film on the surface of the solder joint, which plays a role in corrosion prevention and oxidation prevention.

[0026] This invention achieves a balance between the storage stability and welding activity of nano-nickel powder by using styrene-maleic anhydride copolymer-coated nano-nickel powder to maintain chemical inertness during solder paste storage and release and generate Ni3Sn4 reinforcing phase in situ during soldering. Combined with a compound solvent to match the low-temperature soldering volatility characteristics and liquid rosin and fully hydrogenated rosin to synergistically inhibit solder oxidation, the invention achieves a solder joint shear strength of 38.3 MPa at a soldering temperature of 180℃ and refines the average size of the Bi-rich phase to 1.9 μm. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation process of the SMT lead-free solder paste of the present invention.

[0028] Figure 2This is a schematic diagram of the preparation process of the coated pre-dispersed slurry of the present invention.

[0029] Figure 3 This is a schematic diagram of the preparation process of the flux premix of the present invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0031] 100g of nickel nanoparticles with an average particle size of 40nm were immersed in 1L of an ethanol solution containing 2.0wt% silane coupling agent KH-560 and 1.0wt% chelating titanate coupling agent TC-201. Under nitrogen protection, the mixture was dispersed by ultrasonication at 30kHz for 15 minutes, filtered, and dried at 90℃ for 30 minutes to obtain nickel nanoparticles with an organic modified surface layer. Under nitrogen protection, 1.5 parts of the surface-modified nickel nanoparticles and 3 parts of a styrene-maleic anhydride copolymer were added to 15 parts of diethylene glycol monomethyl ether. The acid value of the styrene-maleic anhydride copolymer was 160mgKOH / g. The mixture was heated to 85℃ and stirred at 8000rpm until the styrene-maleic anhydride copolymer was completely dissolved. Then, it was rapidly cooled to room temperature in an ice-water bath. The styrene-maleic anhydride copolymer precipitated on the surface of the nickel nanoparticles to form a coating layer, thus obtaining a coated pre-dispersed slurry. Nitrogen gas was introduced into a sealed reactor for protection, and a reflux condenser was installed. 10 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 105°C. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. 5 parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and 4 parts of glutaric acid were added and stirred until completely dissolved. 3 parts of liquid styrax were added and stirred evenly to obtain a flux premix. At 75°C, a coated pre-dispersed slurry was slowly added to the flux premix, and the mixture was stirred at 1500 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, used as SMT solder powder for soldering electronic components, is a type of lead-free solder powder. Its composition is 57% Bi, 0.9% Ag, and 0.01% Ge, with the balance being Sn and unavoidable impurities. After being heated to 280℃ in a vacuum melting furnace and melted uniformly, it is powdered by gas atomization and sieved to obtain solder alloy powder with a particle size distribution of 15–25 μm. Under vacuum conditions, 89 parts of the solder alloy powder are mixed uniformly with 11 parts of flux to obtain SMT lead-free solder paste. Example

[0032] 100g of nickel nanoparticles with an average particle size of 70nm were immersed in an ethanol solution containing 2.0wt% KH-560 and 1.0wt% TC-201, under nitrogen purging, sonicated at 30kHz for 15 minutes, filtered, and dried at 90℃ for 30 minutes. Under nitrogen protection, 2 parts of modified nickel nanoparticles and 2 parts of styrene-maleic anhydride copolymer were added to 13 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 140mgKOH / g. The mixture was heated to 90℃ and stirred at 10000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate a coating layer, thus obtaining a pre-dispersed slurry. 12 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and placed in a sealed reactor under nitrogen purging and equipped with a reflux condenser. The mixture was heated to 100°C, and 28 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 6 parts of polyamide wax were added and stirred at 100°C until completely melted and evenly dispersed. The mixture was then cooled to 80°C, and 5 parts of succinic acid were added and stirred until completely dissolved. 3.5 parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 80°C and stirred at 1800 rpm for 15 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 45% Bi, 1.0% Ag, 0.008% Ge, and the balance Sn. It was vacuum melted at 270°C, atomized into powder, and sieved to a particle size of 15–25 μm. 90 parts of solder alloy powder and 10 parts of flux were vacuum mixed evenly. Example

[0033] 100g of nickel nanoparticles with an average particle size of 50nm were immersed in an ethanol solution containing 2.5wt% KH-560 and 0.5wt% TC-201, under nitrogen purging, sonicated at 20kHz for 30 minutes, filtered, and dried at 85℃ for 35 minutes. Under nitrogen protection, 1 part of modified nickel nanoparticles and 4 parts of styrene-maleic anhydride copolymer were added to 10 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 180mgKOH / g. The mixture was heated at 80℃ and stirred at 6000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. 15 parts of diethylene glycol monomethyl ether and 20 parts of ethylene glycol butyl ether were mixed and heated to 110°C in a sealed reactor under nitrogen purging. 24 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 3 parts of ethylene bis-stearamide were added, and the mixture was stirred at 110°C until completely melted and evenly dispersed. The temperature was lowered to 70°C, and 2 parts of glutaric acid and 2 parts of itaconic acid were added with stirring until completely dissolved. 4 parts of liquid styrax were added and stirred until homogeneous, yielding a premix. The pre-dispersed slurry was slowly added to the premix at 70°C, and the mixture was stirred at 1200 rpm for 25 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 58% Bi, 0.5% Ag, 0.015% Ge, and the balance Sn. It was vacuum-melted at 260°C and atomized to form powder with a particle size of 15–25 μm. 88 parts of solder alloy powder and 12 parts of flux were vacuum-mixed evenly. Example

[0034] 100g of nickel nanoparticles with an average particle size of 25nm were immersed in an ethanol solution containing 1.0wt% KH-560 and 2.0wt% TC-201, under nitrogen purging, sonicated at 28kHz for 25 minutes, filtered, and dried at 88℃ for 30 minutes. Under nitrogen protection, 0.5 parts of modified nickel nanoparticles and 4 parts of styrene-maleic anhydride copolymer were added to 17 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 170mgKOH / g. The mixture was heated at 88℃ and stirred at 7000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. Eight parts of diethylene glycol monomethyl ether and 23 parts of ethylene glycol butyl ether were mixed and heated to 108°C in a sealed reactor under nitrogen purging. While stirring, 30 parts of fully hydrogenated rosin were added until completely dissolved. Eight parts of hydrogenated castor oil were added, and the mixture was stirred at 108°C until completely melted and evenly dispersed. The temperature was lowered to 78°C, and six parts of adipic acid were added and stirred until completely dissolved. Four parts of liquid styrax were added and stirred until homogeneous, yielding a premix. The pre-dispersed slurry was slowly added to the premix at 78°C, and the mixture was stirred at 1000 rpm for 30 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 42% Bi, 1.5% Ag, 0.005% Ge, and the balance Sn. It was vacuum-melted at 300°C and atomized to form powder with a particle size of 15–25 μm. 91 parts of solder alloy powder and 9 parts of flux were vacuum-mixed evenly. Example

[0035] 100g of nickel nanoparticles with an average particle size of 60nm were immersed in an ethanol solution containing 3.0wt% KH-560 and 0.8wt% TC-201, under nitrogen purging, sonicated at 22kHz for 18 minutes, filtered, and dried at 92℃ for 28 minutes. Under nitrogen protection, 3 parts of modified nickel nanoparticles and 1.5 parts of styrene-maleic anhydride copolymer were added to 10 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 150mgKOH / g. The mixture was heated at 82℃ and stirred at 9000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. 10 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 102°C in a sealed reactor under nitrogen purging. 24 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 4 parts of hydrogenated castor oil were added and stirred at 102°C until completely melted and evenly dispersed. The mixture was then cooled to 72°C, and 3 parts of glutaric acid were added and stirred until completely dissolved. 3.5 parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 72°C and stirred at 1600 rpm for 22 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 50% Bi, 0.8% Ag, 0.012% Ge, and the balance Sn. It was vacuum melted at 290°C and atomized into powder with a particle size of 15–25 μm. 89 parts of solder alloy powder and 11 parts of flux were vacuum mixed evenly. Example

[0036] 100g of nickel nanoparticles with an average particle size of 35nm were immersed in an isopropanol solution containing 1.8wt% KH-560 and 1.2wt% TC-201, under nitrogen purging, sonicated at 26kHz for 22 minutes, filtered, and dried at 90℃ for 30 minutes. Under nitrogen protection, 1.5 parts of modified nickel nanoparticles and 3 parts of styrene-maleic anhydride copolymer were added to 15 parts of diethylene glycol monoethyl ether. The copolymer had an acid value of 165mgKOH / g. The mixture was heated to 86℃ and stirred at 8500rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. Ten parts of diethylene glycol monoethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Five parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and 4 parts of glutaric acid were added and stirred until completely dissolved. Three parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 75°C, and the mixture was stirred at 1500 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous. Example

[0037] 100g of nickel nanoparticles with an average particle size of 45nm were immersed in an ethanol solution containing 2.2wt% KH-560 and 1.5wt% aluminate coupling agent, under nitrogen purging, sonicated at 24kHz for 20 minutes, filtered, and dried at 90℃ for 30 minutes. Under nitrogen protection, 2 parts of modified nickel nanoparticles and 2.5 parts of styrene-maleic anhydride copolymer were added to 14 parts of diethylene glycol monobutyl ether. The copolymer had an acid value of 155mgKOH / g. The mixture was heated to 87℃ and stirred at 7500rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. 11 parts of diethylene glycol monobutyl ether and 24 parts of ethylene glycol butyl ether were mixed and heated to 106°C in a sealed reactor under nitrogen purging. 26 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 5 parts of polyamide wax were added and stirred at 106°C until completely melted and evenly dispersed. The mixture was then cooled to 76°C, and 5 parts of glutaric acid were added and stirred until completely dissolved. 3.3 parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 76°C and stirred at 1400 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 55% Bi, 0.7% Ag, 0.009% Ge, and the balance Sn. It was vacuum melted at 285°C and atomized to form powder with a particle size of 15–25 μm. 90 parts of solder alloy powder and 10 parts of flux were vacuum mixed evenly. Example

[0038] 100g of nickel nanoparticles with an average particle size of 20nm were immersed in an ethanol solution containing 3.0wt% KH-560 and 1.5wt% TC-201, under nitrogen purging, sonicated at 18kHz for 28 minutes, filtered, and dried at 95℃ for 20 minutes. Under nitrogen protection, 2.5 parts of modified nickel nanoparticles and 1.5 parts of styrene-maleic anhydride copolymer were added to 17 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 145mgKOH / g. The mixture was heated to 83℃ and stirred at 9500rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. Eight parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 104°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Seven parts of ethylene bis-stearamide were added, and the mixture was stirred at 104°C until completely melted and evenly dispersed. The temperature was lowered to 73°C, and two parts of succinic acid and two parts of itaconic acid were added and stirred until completely dissolved. Four parts of liquid styrax were added and stirred until homogeneous, yielding a premix. The pre-dispersed slurry was slowly added to the premix at 73°C, and stirred at 1700 rpm for 18 minutes. After cooling, the flux was obtained. Solder alloy powder, a type of SMT solder powder for electronic component soldering, was composed of 60% Bi, 0.3% Ag, and the balance Sn. It was vacuum-melted at 250°C, atomized, and powdered, with a particle size of 15–25 μm. 92 parts of solder alloy powder and 8 parts of flux were vacuum-mixed evenly. Example

[0039] 100g of nickel nanoparticles with an average particle size of 40nm were immersed in an ethanol solution containing 2.0wt% KH-560 and 1.0wt% TC-201, under nitrogen purging, sonicated at 30kHz for 15 minutes, filtered, and dried at 90℃ for 30 minutes. Under nitrogen protection, 1.5 parts of modified nickel nanoparticles and 3 parts of styrene-maleic anhydride copolymer were added to 15 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 195mgKOH / g. The mixture was heated to 85℃ and stirred at 8000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. Ten parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Five parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and four parts of glutaric acid were added and stirred until completely dissolved. Three parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 75°C, and stirred at 1500 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous.

[0040] Example 10: 100g of nickel nanoparticles with an average particle size of 40nm were immersed in an ethanol solution containing 1.5wt% KH-560 and 0.8wt% TC-201, under nitrogen purging, sonicated at 30kHz for 15 minutes, filtered, and dried at 90℃ for 30 minutes, with a total coupling agent concentration of 2.3wt%. Under nitrogen protection, 1.5 parts of modified nickel nanoparticles and 3 parts of styrene-maleic anhydride copolymer were added to 15 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 160mgKOH / g. The mixture was heated to 85℃ and stirred at 8000rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate the coating layer. Ten parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Five parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and four parts of glutaric acid were added and stirred until completely dissolved. Three parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 75°C, and stirred at 1500 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous.

[0041] Example 11: The nano-nickel powder modification and SMA coating process were the same as in Example 1, with a total coupling agent concentration of 2.3 wt%. The flux premix preparation was the same as in Example 1. The solder alloy powder, used as SMT solder powder for electronic component soldering, is a lead-free solder powder. Its composition is 57% Bi, 0.9% Ag, 0.005% P, with the balance being Sn. P is added in the form of a Sn-0.1P intermediate alloy. It was vacuum melted at 280℃ and atomized to form powder with a particle size of 15–25 μm. 89 parts of solder alloy powder and 11 parts of flux were vacuum mixed uniformly.

[0042] Example 12: The nano-nickel powder modification and SMA coating process are the same as in Example 1. The flux premix preparation is the same as in Example 1. The solder alloy powder, namely an SMT solder powder for soldering electronic components, has the following composition: 57% Bi, 0.9% Ag, 0.02% P, with the balance Sn. It is vacuum melted at 280℃ to produce powder with a particle size of 15-25 μm. 89 parts of solder alloy powder and 11 parts of flux are vacuum mixed evenly.

[0043] Example 13: The nano-nickel powder modification and SMA coating process are the same as in Example 1. The flux premix preparation is the same as in Example 1. The solder alloy powder, namely an SMT solder powder for soldering electronic components, is composed of 35% Bi, 0.1% Ag, 0.01% Ge, and the balance Sn. It is vacuum melted at 260℃, atomized into powder, and has a particle size of 15-25 μm. 89 parts of solder alloy powder and 11 parts of flux are vacuum mixed evenly.

[0044] Example 14: 100g of nickel nanoparticles with an average particle size of 20nm were immersed in an ethanol solution containing 3.0wt% KH-560 and 1.5wt% TC-201, under nitrogen purging, sonicated at 18kHz for 28 minutes, filtered, and dried at 95℃ for 20 minutes. Under nitrogen protection, 2.5 parts of modified nickel nanoparticles and 1.5 parts of styrene-maleic anhydride copolymer were added to 17 parts of diethylene glycol monomethyl ether. The copolymer had an acid value of 145mgKOH / g. The mixture was heated to 83℃ and stirred at 9500rpm until completely dissolved. The mixture was then rapidly cooled in an ice-water bath to precipitate a coating layer, thus obtaining a pre-dispersed slurry. Eight parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 104°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Seven parts of ethylene bis-stearamide were added, and the mixture was stirred at 104°C until completely melted and evenly dispersed. The temperature was lowered to 73°C, and two parts of succinic acid and two parts of itaconic acid were added and stirred until completely dissolved. Four parts of liquid styrax were added and stirred until homogeneous, yielding a premix. The pre-dispersed slurry was slowly added to the premix at 73°C, and stirred at 1700 rpm for 18 minutes. After cooling, the flux was obtained. The solder alloy powder, used as SMT solder powder for electronic component soldering, is a lead-free solder powder. Its composition is 60% Bi, 0.5% Ag, 0.008% Ge, and the balance Sn. It was vacuum melted at 250°C and atomized to form powder with a particle size of 15–25 μm. 92 parts of solder alloy powder and 8 parts of flux were vacuum mixed evenly.

[0045] Example 15: 100g of nickel nanoparticles with an average particle size of 40nm were immersed in an ethanol solution containing 1.0wt% KH-560 and 0.5wt% TC-201, under nitrogen purging, sonicated at 30kHz for 15 minutes, filtered, and dried at 90℃ for 30 minutes, with a total coupling agent concentration of 1.5wt%. Under nitrogen protection, 1.5 parts of modified nickel nanoparticles and 3 parts of styrene-maleic anhydride copolymer were added to 15 parts of diethylene glycol monomethyl ether, with a copolymer acid value of 160mgKOH / g. The mixture was heated to 85℃ and stirred at 8000rpm until completely dissolved, then rapidly cooled in an ice-water bath to precipitate the coating layer. Ten parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin were added until completely dissolved. Five parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and four parts of glutaric acid were added and stirred until completely dissolved. Three parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 75°C, and stirred at 1500 rpm for 20 minutes. After cooling, the flux was obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous.

[0046] Comparative Example 1: 25 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed and placed in a sealed reactor under nitrogen purging and equipped with a reflux condenser. The mixture was heated to 105°C, and 27 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 5 parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and 4 parts of glutaric acid were added with stirring until completely dissolved. 3 parts of liquid styrax were added and stirred until homogeneous. The mixture was then cooled to obtain the flux. The solder alloy powder, used as SMT solder powder for electronic component soldering (i.e., lead-free solder powder), had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous.

[0047] Comparative Example 2: The modification of nano-nickel powder was the same as in Example 1. Under nitrogen protection, 1.5 parts of modified nano-nickel powder were added to 15 parts of diethylene glycol monomethyl ether, stirred and dispersed at 85°C and 8000 rpm, and rapidly cooled in an ice-water bath to obtain a pre-dispersed slurry without SMA coating. 10 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether were mixed, and nitrogen was purged in a sealed reactor. The mixture was heated to 105°C, and 24 parts of fully hydrogenated rosin were added with stirring until completely dissolved. 5 parts of hydrogenated castor oil were added, and the mixture was stirred at 105°C until completely melted and evenly dispersed. The temperature was lowered to 75°C, and 4 parts of glutaric acid were added with stirring until completely dissolved. 3 parts of liquid styrax were added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry was slowly added to the premix at 75°C, stirred at 1500 rpm for 20 minutes, and cooled to obtain flux. Solder alloy powder, used as SMT solder powder for soldering electronic components, has the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux are vacuum mixed evenly.

[0048] Comparative Example 3: The modification of nano-nickel powder was the same as in Example 1. Under nitrogen protection, 1.5 parts of modified nano-nickel powder and 3 parts of epoxy resin E-51 were added to 15 parts of diethylene glycol monomethyl ether, heated to 85°C, and stirred at 8000 rpm until the epoxy resin was completely dissolved. The mixture was then rapidly cooled in an ice-water bath to obtain a pre-dispersed slurry. The flux premix was prepared in the same manner as in Comparative Example 2. The pre-dispersed slurry was slowly added to the premix at 75°C, stirred at 1500 rpm for 20 minutes, and cooled to obtain the flux. The solder alloy powder, used as SMT solder powder for electronic component soldering, had the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed until homogeneous.

[0049] Comparative Example 4: The nano-nickel powder modification and SMA coating process are the same as in Example 1. 10 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether are mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 27 parts of fully hydrogenated rosin are added until completely dissolved. 5 parts of hydrogenated castor oil are added, and the mixture is stirred at 105°C until completely melted and evenly dispersed. The temperature is lowered to 75°C, and 4 parts of glutaric acid are added and stirred until completely dissolved. Liquid styrax is not added. The mixture is stirred until homogeneous to obtain a premix. The pre-dispersed slurry is slowly added to the premix at 75°C, and stirred at 1500 rpm for 20 minutes. After cooling, the flux is obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, has the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux are vacuum-mixed until homogeneous.

[0050] Comparative Example 5: The nano-nickel powder modification and SMA coating process are the same as in Example 1. 40 parts of diethylene glycol butyl ether and 10 parts of ethylene glycol butyl ether are mixed and heated to 105°C in a sealed reactor under nitrogen purging. While stirring, 24 parts of fully hydrogenated rosin are added until completely dissolved. 5 parts of hydrogenated castor oil are added, and the mixture is stirred at 105°C until completely melted and evenly dispersed. The temperature is lowered to 75°C, and 4 parts of glutaric acid are added and stirred until completely dissolved. 3 parts of liquid styrax are added and stirred until homogeneous to obtain a premix. The pre-dispersed slurry is slowly added to the premix at 75°C, and stirred at 1500 rpm for 20 minutes. After cooling, the flux is obtained. Solder alloy powder, used as SMT solder powder for electronic component soldering, has the same composition as in Example 1. 89 parts of solder alloy powder and 11 parts of flux are vacuum-mixed evenly.

[0051] Comparative Example 6: Flux preparation was the same as in Comparative Example 1. The solder alloy powder, used as SMT soldering powder for electronic component soldering, is a lead-free solder powder composed of 96.5% Sn, 3.0% Ag, and 0.5% Cu. It was vacuum-melted at 350℃ and atomized into powder with a particle size of 15–25 μm. 89 parts of solder alloy powder and 11 parts of flux were vacuum-mixed uniformly.

[0052] Performance testing: Spread rate was measured according to IPC-TM-6502.4.45 standard. Solder paste was printed on copper-clad laminate. Spread rate was measured after reflow soldering of Examples 1-12, 14, 15 and Comparative Examples 1-5 at 180℃. Spread rate was measured after reflow soldering of Example 13 at 210℃. Spread rate was measured after reflow soldering of Comparative Example 6 at 240℃. The average value of 5 samples in each group was taken. Shear strength was measured using lap shear test specimens with copper sheet dimensions of 40mm×10mm×1mm and lap area of ​​5mm×5mm. After reflow soldering at the corresponding peak temperature, the specimens were stretched at a rate of 1mm / min. The average value of 5 samples in each group was taken. Slag rate was measured according to SJ / T11319-2005 standard. Slag rate was calculated after holding Examples 1-12, 14, 15 and Comparative Examples 1-5 at 180℃ for 8 hours, Example 13 at 210℃ for 8 hours, and Comparative Example 6 at 240℃ for 8 hours. Oxide slag was scraped off, weighed, and the slag rate was calculated. The viscosity change rate test involved storing the solder paste in a 60℃ constant temperature chamber for 7 days, then removing it and measuring the viscosity using a Malcom PCU-285 viscometer at 25℃ and 10 r / min. The rate of change from the initial viscosity was calculated. The size of the Bi-rich phase was determined by cold mounting, grinding, and polishing the solder joint cross-section, followed by measurements at five randomly selected fields of view in backscatter mode using a scanning electron microscope, with the average value taken.

[0053]

[0054] Data from Examples 1-15 show that, within the component range defined in the claims, the spreading rate is 85.0%-87.1%, the shear strength is 35.2 MPa-38.3 MPa, the slag rate is 0.43%-0.60%, the viscosity change rate at 60℃ / 7 days is 5.1%-7.0%, and the average size of the Bi-rich phase is 1.9 μm-2.6 μm.

[0055] Comparative Example 1, without nano-nickel powder and SMA coating, had a spread rate of 82.0%, a shear strength of 28.3 MPa, a slag ratio of 0.69%, and a Bi-rich phase size of 3.9 μm. Compared to Comparative Example 1, Example 1 showed an increase in spread rate from 82.0% to 87.1%, a shear strength increase from 28.3 MPa to 38.3 MPa (an improvement of 35.3%), a decrease in slag ratio from 0.69% to 0.43%, and a refinement of the Bi-rich phase size from 3.9 μm to 1.9 μm. During welding, the SMA-coated nano-nickel powder releases nickel particles, which react with Sn to form the Ni3Sn4 intermetallic compound. This compound acts as heterogeneous nucleation sites during solder solidification, increasing the nucleation rate and refining the Bi and Sn phase grains. The dispersed Ni3Sn4 particles also hinder dislocation movement, resulting in dispersion strengthening.

[0056] Comparative Example 2, using only a coupling agent to modify the nano-nickel powder without SMA coating, exhibited a shear strength of 32.8 MPa, higher than Comparative Example 1's 28.3 MPa but lower than Example 1's 38.3 MPa. The viscosity change rate of Comparative Example 2 at 60℃ for 7 days reached 13.8%, while Example 1 showed only 5.1%. Without SMA coating, the nano-nickel powder was directly exposed to the acidic environment of glutaric acid, resulting in slow corrosion during storage. This corrosion consumed the acid in the flux and generated nickel salts, leading to a continuous increase in system viscosity. In Example 1, the SMA coating layer remained in a glassy state at room temperature, effectively isolating the acidic medium from the nano-nickel powder. At welding temperatures, the SMA layer softened, allowing the nano-nickel powder to be released and participate in the welding reaction, achieving a balance between storage stability and welding activity.

[0057] Comparative Example 3 used epoxy resin E-51 instead of SMA, and its shear strength was 30.1 MPa, which was 1.8 MPa higher than that of Comparative Example 1. The epoxy resin underwent cross-linking and curing at a welding temperature of 180°C, forming a three-dimensional network that trapped the nano-nickel powder within, preventing it from being released to participate in the welding reaction. The gradient between 38.3 MPa in Example 1, 32.8 MPa in Comparative Example 2, and 30.1 MPa in Comparative Example 3 indicates that the characteristic of SMA not undergoing cross-linking at welding temperature and gradually softening to release the nano-nickel powder is a necessary condition for the nano-nickel powder to exert its reinforcing effect.

[0058] Comparative Example 4, which did not contain liquid rosin, had a slag ratio of 0.73%, an increase of 0.30 percentage points compared to 0.43% in Example 1. Liquid rosin releases volatile terpenes at the high welding temperatures, forming an inert atmosphere layer on the surface of the molten solder, hindering oxygen diffusion to the solder surface and synergistically inhibiting solder oxidation with fully hydrogenated rosin. The spreading rate of Comparative Example 4 was 83.5%, 3.6 percentage points lower than that of Example 1. The volatile components of liquid rosin also played an auxiliary role in reducing the surface tension of the molten solder during the welding process.

[0059] Comparative Example 5 used diethylene glycol butyl ether as a high-boiling-point solvent, achieving a spreading rate of 78.4% and a shear strength of 24.1 MPa. Diethylene glycol butyl ether has a boiling point of 230°C, and its volatilization rate is less than 20% at a soldering temperature of 180°C. A large amount of solvent remains inside and on the surface of the solder joint, hindering the spreading of molten solder on the copper substrate surface and forming a weakening layer after the solder joint solidifies. Example 1 used a compound solvent of diethylene glycol monomethyl ether and ethylene glycol butyl ether, with boiling points of 194°C and 171°C respectively, which can fully volatilize at a soldering temperature of 180°C.

[0060] Comparative Example 6 used SAC305 solder alloy powder, with a soldering temperature of 240°C, a shear strength of 42.1 MPa, and a spread of 85.5%. Example 1, with a shear strength of 38.3 MPa at 180°C, is 91.0% of Comparative Example 6. The Sn-Bi solder paste of this application can achieve mechanical properties close to those of conventional high-temperature SAC305 solder paste under low-temperature soldering conditions, making it suitable for substrates and components that cannot withstand soldering temperatures above 240°C.

[0061] Example 10 had the same formulation and process as Example 1, and was prepared and tested independently. The spreading rate of Example 10 was 86.7%, while that of Example 1 was 87.1%, a deviation of 0.4 percentage points; the shear strength of Example 10 was 37.8 MPa, while that of Example 1 was 38.3 MPa, a deviation of 1.3%; the slag rate of Example 10 was 0.44%, while that of Example 1 was 0.43%, a deviation of 0.01 percentage points; the viscosity change rate of Example 10 was 5.4%, while that of Example 1 was 5.1%, a deviation of 0.3 percentage points; the size of the Bi-rich phase in Example 10 was 2.0 μm, while that in Example 1 was 1.9 μm, a deviation of 0.1 μm. The deviations of the two independent experiments were all within the normal experimental fluctuation range, indicating good repeatability of the formulation and process.

[0062] Examples 11 and 12 replaced Ge with 0.005% and 0.02% P, respectively, resulting in spread rates of 86.9% and 86.3%, shear strengths of 37.4 MPa and 36.3 MPa, and slag ratios of 0.45% and 0.47%. P exhibits an oxygen-loving skin effect in Sn-Bi solder, accumulating on the surface of the molten solder and preferentially oxidizing before Sn, forming a dense oxide layer that hinders oxygen diffusion into the solder interior. The slag ratio of 0.47% in Example 12 is lower than that of Comparative Example 1 (0.69%), and the antioxidant effect remains significant even with a P content of 0.02%.

[0063] Example 13 had a Bi content of 35%, an Ag content of 0.1%, and a liquidus temperature of approximately 195–205°C. At 180°C, it was in the solid-liquid two-phase region and could not completely melt. The welding temperature was adjusted to 210°C to ensure complete melting of the alloy. Under these conditions, the spread rate was 85.4%, the shear strength was 35.2 MPa, and the slag rate was 0.60%. Compared to Example 1, the reduced Bi content decreased the volume fraction of the Bi-rich phase in the solder, increased the surface tension and viscosity, and correspondingly reduced the spread rate. Increasing the welding temperature from 180°C to 210°C increased the slag rate to 0.60% because the increased temperature accelerated the oxidation rate of the solder surface. SMA-coated nano-nickel powder effectively refined the microstructure and strengthened the solder joints even under low Bi conditions. The Bi-rich phase size of Example 13 (2.5 μm) was smaller than that of Comparative Example 1 (3.9 μm), and the shear strength of 35.2 MPa was 24.4% higher than that of Comparative Example 1.

[0064] Example 14 had a Bi content of 60%, Ag content of 0.5%, Ge content of 0.008%, and a welding temperature of 180℃. It exhibited a spread rate of 85.2%, a shear strength of 35.5 MPa, a slag ratio of 0.49%, and a Bi-rich phase size of 2.3 μm. Under conditions of 60% Bi and trace elements, all performance characteristics were superior to Comparative Example 1. The synergistic effect of SMA-coated nano-nickel powder and Ge remained effective even under the upper limit of Bi content.

[0065] The coupling agent in Example 15 had a total concentration of 1.5 wt%, a spreading rate of 86.8%, a shear strength of 37.8 MPa, a slag ratio of 0.44%, and a Bi-rich phase size of 2.0 μm. Compared with the performance data of Example 10 (concentration 2.3 wt%) and Example 1 (concentration 3.0 wt%), the spreading rate, shear strength, slag ratio, and Bi-rich phase size of the solder paste remained stable when the coupling agent concentration varied within the range of 1.5 wt% to 3.0 wt%, indicating that the coupling agent concentration within this range could form an effective organic modified layer on the surface of the nano-nickel powder.

[0066] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0067] The above description is only used to illustrate the technical solution of the present invention and is not intended to limit it. Any other modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention, as long as they do not depart from the spirit and scope of the technical solution of the present invention, should be covered within the scope of the claims of the present invention.

Claims

1. A lead-free SMT solder paste for soldering electronic components, comprising, by weight percentage, 88%–92% solder alloy powder and 8%–12% flux, characterized in that, The solder alloy powder is a Sn-Bi alloy containing 35% to 60% Bi, 0.1% to 1.5% Ag, and at least one trace element selected from Ge and P. The content of the trace element is 0.001% to 0.1%, and the balance is Sn and unavoidable impurities. The flux, by weight, comprises 100 parts in total, including: 24-30 parts of fully hydrogenated rosin, 3-4 parts of liquid styrax, 3-8 parts of thixotropic agent, 35-50 parts of compound solvent, 2-8 parts of organic acid activator, and an active and reinforcing complex. The mass ratio of the fully hydrogenated rosin to the liquid styrax is 4-8:1; The active and reinforced composite is composed of surface-modified nano-nickel powder and a thermoplastic protective resin coating it, wherein the surface-modified nano-nickel powder is 0.5 to 3 parts and the thermoplastic protective resin is 1.5 to 4 parts. The compound solvent is composed of a high-boiling-point solvent and a low-boiling-point solvent mixed in a mass ratio of 1:0.8 to 1.2, wherein the boiling point of the high-boiling-point solvent is 190 to 230°C and the boiling point of the low-boiling-point solvent is 160 to 180°C. The surface-modified nickel nanopowder has an average particle size of 20-70 nm. The surface modification method is as follows: under inert gas protection, the nickel nanopowder is immersed in an organic solvent containing 0.5-5 wt% coupling agent, ultrasonically dispersed for 10-30 minutes, filtered and dried, thereby forming an organic modification layer on the surface of the nickel nanopowder. The thermoplastic protective resin is a styrene-maleic anhydride copolymer with an acid value of 100-200 mg KOH / g.

2. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The trace element is Ge, with a content of 0.005% to 0.015%; or the trace element is P, with a content of 0.005% to 0.02%.

3. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The styrene-maleic anhydride copolymer has an acid value of 140-180 mgKOH / g and is added in amounts of 2-4 parts.

4. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The organic acid activator is at least one of glutaric acid, succinic acid, adipic acid, or itaconic acid, and its addition amount is 2 to 6 parts.

5. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The coupling agent is a combination of silane coupling agent and titanate coupling agent, with a total concentration of 1-3 wt%; the organic solvent is at least one of ethanol, isopropanol or acetone.

6. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The high-boiling-point solvent is at least one of diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, or diethylene glycol monobutyl ether, and the low-boiling-point solvent is ethylene glycol butyl ether.

7. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The thixotropic agent is at least one of hydrogenated castor oil, polyamide wax, or ethylene bis-stearamide.

8. The SMT lead-free solder paste for soldering electronic components according to claim 1, characterized in that, The solder alloy powder contains 0.3%–1.0% Ag, and the trace element is Ge, with a content of 0.007%–0.01%. The fully hydrogenated rosin is 24–28 parts, and liquid styrene is 3–4 parts. The surface-modified nano-nickel powder has an average particle size of 30–50 nm and is added in an amount of 1–2 parts. The styrene-maleic anhydride copolymer has an acid value of 140–180 mg KOH / g and is added in an amount of 2–4 parts. The organic acid activator is glutaric acid, added in an amount of 3–5 parts. The compound solvent is a mixture of diethylene glycol monomethyl ether and ethylene glycol butyl ether at a mass ratio of 1:0.8–1.

2. The coupling agent is a combination of silane coupling agent KH-560 and chelated titanate coupling agent, with a total concentration of 1.5–3.0 wt%. The organic solvent is ethanol.

9. The SMT lead-free solder paste for soldering electronic components according to claim 8, characterized in that, The trace element is Ge, with a content of 0.007% to 0.01%; the fully hydrogenated rosin is 24 parts, and the liquid styrene is 3 parts; the styrene-maleic anhydride copolymer is added in an amount of 3 parts, with an acid value of 150 to 170 mg KOH / g; the surface-modified nano-nickel powder has an average particle size of 40 nm and is added in an amount of 1.5 parts; the glutaric acid is added in an amount of 4 parts; the thixotropic agent is hydrogenated castor oil, added in an amount of 5 parts; the compound solvent contains 25 parts of diethylene glycol monomethyl ether and 25 parts of ethylene glycol butyl ether.

10. A lead-free SMT solder paste for soldering electronic components according to any one of claims 1 to 9, characterized in that, The preparation method of the active and reinforced composite is as follows: under inert gas protection, surface-modified nano-nickel powder and styrene-maleic anhydride copolymer are added to part of the compound solvent, heated and stirred until the styrene-maleic anhydride copolymer is completely dissolved, and then rapidly cooled to allow the styrene-maleic anhydride copolymer to precipitate on the surface of the nano-nickel powder to form a coating layer, thus obtaining a coated predispersed slurry; the coated predispersed slurry is added to a flux premix composed of fully hydrogenated rosin, the remaining compound solvent, liquid styrene, thixotropic agent and organic acid activator, and mixed evenly.

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