Low temperature tin paste and method of making same

Abstract

The present application belongs to the solder paste material field of surface assembly technology, and relates to a low-temperature solder paste and a preparation method thereof. The low-temperature solder paste contains a tin-bismuth-based alloy powder and a soldering paste. The soldering paste contains rosin, an active agent, a thixotropic agent, a surfactant, a corrosion inhibitor and an organic solvent. The active agent is a composite of an organic amine and an organic acid. The organic amine is at least one selected from ethylenediamine, monoethanolamine, triethanolamine and ethylamine. The organic acid is at least one selected from 2-pyridine carboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid and dodecanedioic acid. The boiling point of the organic solvent is 190-280 DEG C. The solder paste provided by the present application not only has low-temperature property and is suitable for applications with more low-temperature welding requirements, solves the problem that the medium and high-temperature solder paste is not widely used, but also has the characteristics of less solder beads after welding and low residue.

Description

Technical Field

[0001] This invention belongs to the field of solder paste materials for surface mount technology, specifically relating to a solder paste suitable for low-temperature tin-bismuth based alloys and its preparation method. Background Technology

[0002] With the advent of the information age, electronic products have gradually shifted from manual soldering to lightweight, small, and thin microelectronic devices. In particular, device packaging has evolved towards higher precision and density. Surface Mount Technology (SMT) has thus emerged as a modern electronic assembly technology.

[0003] SMT (Surface Mount Technology) eliminates the need for drilling through-holes in printed circuit boards (PCBs). Instead, solder paste is applied to the pads on the PCB, and surface mount components (without leads or short leads) are precisely placed onto these pads. The PCB is then heated to melt the solder paste, which cools and solidifies to form solder joints between the components and the PCB, achieving interconnection and metallurgical bonding. Electronic products manufactured using SMT technology are characterized by their small size, comprehensive functionality, high performance, and low cost. Therefore, SMT technology has a wide range of applications, including aerospace, computer communications, medical electronics, home appliances, and automotive microelectronics. In these electronics manufacturing fields, electronic packaging technology is trending towards low-temperature soldering. Low-temperature soldering reduces energy loss and component damage during production, lowering the requirements for soldering equipment and components. Therefore, finding high-performance solder paste with low temperature, high wettability, and no residue after soldering is of great significance.

[0004] CN110202295A discloses a low-temperature aluminum solder paste, which includes solder alloy powder and flux. The flux includes modified rosin, indium halide, zinc hydroxy acid salt, triethanolamine borate, polyaniline, thixotropic agent, and solvent. The indium halide is selected from at least one of indium chloride, indium fluoride, and indium bromide. The zinc hydroxy acid salt is selected from zinc salts of tartaric acid, salicylic acid, lactic acid, malic acid, glycolic acid, or citric acid. This low-temperature aluminum solder paste, through the synergistic effect of indium halide and zinc hydroxy acid salt, significantly removes aluminum oxide film, thereby promoting the peeling of aluminum oxide film and exhibiting good electrochemical corrosion resistance and storage stability. CN111015021A discloses a low-temperature lead-free solder paste, which includes a low-temperature solder and a flux. The low-temperature solder includes both a tin-bismuth alloy and carbon nanotubes. 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, their compatibility with other components is enhanced, and the strength and toughness of the solder joints during soldering are increased. Although the above solder pastes each have their own advantages, the low-temperature characteristics, the number of solder beads after soldering, and the residue-free properties still need further improvement. Summary of the Invention

[0005] The first objective of this invention is to provide a low-temperature solder paste that has low temperature, low number of solder balls after soldering, and low residue.

[0006] The second objective of this invention is to provide a method for preparing the aforementioned low-temperature solder paste.

[0007] Specifically, the low-temperature solder paste provided by the present invention contains tin-bismuth-based alloy powder and flux. The flux contains rosin, activator, thixotropic agent, surfactant, corrosion inhibitor and organic solvent. The activator is a complex of organic amine and organic acid in a mass ratio of (2-3):1. The organic amine is selected from at least one of ethylenediamine, monoethanolamine, triethanolamine and ethylamine. The organic acid is selected from at least one of 2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid and dodecanoic acid. The boiling point of the organic solvent is 190-280°C.

[0008] The method for preparing low-temperature solder paste provided by the present invention includes stirring and mixing flux paste with tin-bismuth-based alloy powder until uniform.

[0009] The key to this invention lies in using a specific compound of organic amines and organic acids as a flux, and on this basis, combining organic solvents with specific boiling points, rosin, thixotropic agents, surfactants and corrosion inhibitors. The resulting solder paste not only has low-temperature properties, making it suitable for applications with high requirements for low-temperature soldering, thus solving the problem of the limited application of medium and high temperature solder paste, but also has the characteristics of fewer solder balls and less residue after soldering. The reasons for this are speculated to be as follows: Firstly, using organic solvents with boiling points of 190–280℃ can provide a good carrier and ionization environment, which can prevent solder ball splashing and non-melting, thus improving the phenomena of voids and solder balls. Secondly, specific organic amines (ethylenediamine, monoethanolamine, triethanolamine, and ethylamine) and specific organic acids (2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid, and dodecanoic acid) are compounded as activators. These specific organic amines can form copper-amine complexes with copper plates, decompose into metallic Cu at the soldering temperature, promote solder wetting and spreading, and can also adjust the pH of the flux with specific organic acids to improve the stability of the flux. In addition, rosin, thixotropic agents, surfactants, and corrosion inhibitors are added. The synergistic effect of these components makes the flux paste finer, improves its wettability, and reduces soldering defects and the number of solder balls. On the other hand, the carboxyl groups in the organic acids effectively remove oxides from the solder substrate and solder surface, assist the solder in wetting the pads, reduce the surface tension between the solder and the material, and improve the soldering properties of the solder paste. Simultaneously, the organic acids and organic amines can combine to form amide neutralization products. These neutralization products maintain the flux paste's inertness at low temperatures, reducing the reaction between the activator and the solder and other substances. At soldering temperatures, the generated neutralization products rapidly decompose into organic amines and organic acids, each exerting its soldering activity. After soldering, the remaining organic acids continue to react with organic amines to form neutralization products, resulting in low residue, low corrosivity, and good solder paste storage. Furthermore, the low-temperature solder paste provided by this invention does not contain halogens or lead, making it environmentally friendly, safe, and reliable, with less solder joint corrosion. Detailed Implementation

[0010] The low-temperature solder paste provided by this invention contains tin-bismuth-based alloy powder and flux. Specifically, based on the total weight of the low-temperature solder paste, the content of the tin-bismuth-based alloy powder is preferably 85% to 90%, such as 85%, 86%, 87%, 88%, 89%, 90%, or any value between therewith; the content of the flux is preferably 10% to 15%, such as 10%, 11%, 12%, 13%, 14%, 15%, or any value between therewith.

[0011] In this invention, the tin-bismuth-based alloy powder may include other doping elements besides tin (Sn) and bismuth (Bi), such as silver (Ag), antimony (Sb), selenium (Ce), zinc (Zn), magnesium (Mg), and thorium (Th). In a preferred embodiment, the doping elements are Ag and Sb, that is, the tin-bismuth-based alloy powder is Sn-Bi-Ag-Sb, which can refine the grains and weaken the precipitation of bismuth. In the tin-bismuth-based alloy powder, the mass percentage of Sn is 35-45%, the mass percentage of Bi is 50-60%, the mass percentage of Ag is 0.5-2%, and the mass percentage of Sb is 0.1-2%. Furthermore, the particle size of the tin-bismuth-based alloy powder is preferably 20–60 μm, such as 20 μm, 22 μm, 25 μm, 28 μm, 30 μm, 32 μm, 35 μm, 38 μm, 40 μm, 42 μm, 45 μm, 48 μm, 50 μm, 52 μm, 55 μm, 58 μm, 60 μm or any value between them.

[0012] In this invention, the flux contains rosin, activator, thixotropic agent, surfactant, corrosion inhibitor and organic solvent. Based on the total weight of the flux, the rosin content is preferably 30% to 50%, such as 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, or any value between them; the activator content is preferably 4% to 7%, such as 4%, 5%, 6%, 7%, or any value between them; the thixotropic agent content is preferably 4% to 6%, such as 4%, 5%, 6%, or any value between them; the surfactant content is 12% to 14%, such as 12%, 13%, 14%, or any value between them; the corrosion inhibitor content is 4% to 6%, such as 4%, 5%, 6%, or any value between them; and the organic solvent content is 15% to 35%, such as 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, or any value between them.

[0013] In this invention, the activator is a complex of an organic amine and an organic acid, which helps to regulate the acidity, alkalinity, and stability of the solder paste, thereby improving the activity of the activator and the wettability of the solder joints, and reducing soldering defects such as solder balls. The preferred mass ratio of the organic amine to the organic acid is (2-3):1, such as 2:1, 2.2:1, 2.5:1, 2.8:1, 3:1, or any value between them. The organic amine is selected from at least one of ethylenediamine, monoethanolamine, triethanolamine, and ethylamine. The organic acid is selected from at least one of 2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid, and dodecanoic acid. The organic amine and organic acid can be compounded to form a neutralization product. At low temperatures, this keeps the solder paste inert, reducing the reaction between the activator and the solder and other substances. At soldering temperatures, the neutralization product rapidly decomposes into organic amine and organic acid, each exerting its soldering activity. After soldering, the remaining organic acid reacts with the organic amine to form another neutralization product, resulting in low residue and low corrosivity after soldering, ensuring good storage of the solder paste. The specific reaction process is shown below:

[0014] R-COOH + CuO → Cu(OCOR)₂ + H₂O

[0015] R-COOH+Cu2O→Cu(OCOR)2+Cu+H2O

[0016] R-COOH+R'-NH2→R-COHN-R'→R-COOH+R'-NH2

[0017] In this invention, the rosin can form a dense organic film on the surface of the solder during the soldering process, preventing oxidation of the solder and substrate, providing good thermal stability and corrosion and moisture resistance, while also regulating the viscosity of the solder paste, providing an active effect, and promoting wetting. Specific examples of the rosin include, but are not limited to, at least one of: hydrogenated rosin, KE-604, disproportionated rosin, polymerized rosin, maleic rosin, and Ax-E rosin.

[0018] In this invention, the thixotropic agent is used to ensure the stability and printability of the solder paste. Specific examples of the thixotropic agent include, but are not limited to, at least one of: hydrogenated castor oil, modified hydrogenated castor oil, unsaturated fatty amides, saturated fatty amides, polyamide waxes, aromatic polyamides, and Thixcin R thixotropic agent.

[0019] In this invention, the surfactant is used to provide wettability to the solder paste. Specific examples of the surfactant include, but are not limited to, at least one of OP-10, glyceryl stearate, hydrogenated dimer acid, and sodium dodecylbenzenesulfonate.

[0020] In this invention, the corrosion inhibitor is used to prevent corrosion of metal alloys, extend the shelf life of solder paste, and extend the lifespan of electronic products. Specific examples of the corrosion inhibitor include, but are not limited to, at least one of 2-phenylimidazole, 2-methylbenzimidazole, benzotriazole, butylated hydroxytoluene, and 2-ethylimidazole.

[0021] In this invention, the boiling point of the organic solvent is 190–280°C, such as 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, or any value between them. Organic solvents with a boiling point of 190–280°C provide a good carrier and ionization environment, preventing solder ball splashing and non-melting, improving the phenomena of voids and solder balls, and these organic solvents have excellent evaporation rates, which can well ensure the storage stability of solder paste and low residue after soldering. Specific examples of the organic solvent include, but are not limited to, at least one selected from: diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol octyl ether, tetraethylene glycol dimethyl ether, tetrahydrofurfuryl alcohol, isopropanol, glycerol, ethylene glycol, and ethanol.

[0022] The method for preparing low-temperature solder paste provided by the present invention includes stirring and mixing flux with tin-bismuth-based alloy powder until uniform, thereby obtaining low-temperature solder paste.

[0023] In a preferred embodiment, the flux is prepared according to the following steps:

[0024] S1. Mix some rosin and some organic solvent and heat until completely dissolved. Then add thixotropic agent at 130-150°C. Cool the temperature to 100-110°C and stir until completely dissolved. Then cool the resulting liquid and seal it in cold water at 9-13°C for 3-4 hours to obtain a paste.

[0025] S2. Stir the remaining rosin, remaining organic solvent and corrosion inhibitor at 90-100°C until completely dissolved to obtain the intermediate;

[0026] S3. Stir and disperse the paste and organic acid at 42-65℃ for 1-2 hours. Then, control the temperature of the resulting dispersion at 35-45℃ and add the intermediate dropwise. After the dropwise addition is complete, add the organic amine and stir and disperse for 2-3 hours. Then, control the temperature of the resulting mixture at 25-30℃ and add the surfactant and stir and disperse for 1-2 hours to obtain the flux paste.

[0027] The inventors of this invention unexpectedly discovered that the flux paste prepared using the above-mentioned preferred method has advantages in physical properties such as fine paste texture, good resistance to cold collapse, full solder joints after soldering, and less solder black.

[0028] The present invention will be described in detail below through embodiments.

[0029] In the following examples and comparative examples:

[0030] The CAS number for hydrogenated rosin is 65997-06-0; the CAS number for disproportionated rosin is 8050-09-7; and the CAS number for polymerized rosin is 65997-05-9.

[0031] The CAS number for hydrogenated castor oil is 8001-78-3; the CAS number for modified hydrogenated castor oil is 8001-79-4; the CAS number for aromatic polyamide is 5892-11-5; and the CAS number for Thixcin R thixotropic agent is 8001-79-45.

[0032] OP-10 CAS number is 9041-29-6; glyceryl stearate CAS number is 11099-07-3; hydrogenated dimer acid CAS number is 61788-89-4; sodium dodecylbenzenesulfonate CAS number is 27176-87-0.

[0033] Example 1

[0034] (1) Raw materials:

[0035] The alloy powder is Sn 42 -Bi 58 Alloy powder with a particle size of 25-56μm.

[0036] The flux ingredients include 384g rosin, 32g activator, 36g thixotropic agent, 104g surfactant, 40g corrosion inhibitor, and 200mL organic solvent. The rosin contains hydrogenated rosin, disproportionated rosin, and polymerized rosin in a 3:1:1 mass ratio. The activator is a mixture of organic amines and organic acids in a 2:1 mass ratio. The organic amines include ethylenediamine and monoethanolamine in a 1:1 mass ratio, and the organic acids include glutaric acid, malonic acid, phthalic acid, and dodecanoic acid in a 2:1:1:1 mass ratio. The thixotropic agent contains hydrogenated castor oil, modified hydrogenated castor oil, aromatic polyamide, and Thixcin R thixotropic agent in a 2:1:1.5:1 mass ratio. The surfactant contains OP-10, glyceryl stearate, hydrogenated dimeric acid, and sodium dodecylbenzenesulfonate in a 1:2:1:1 mass ratio. The corrosion inhibitor contains 2-phenylimidazole, 2-methylbenzimidazole, benzotriazole, dibutylhydroxytoluene, and 2-ethylimidazole in a mass ratio of 1:1:2:1:1. The organic solvent has a boiling point of 190–280 °C and contains diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol octyl ether, tetraethylene glycol dimethyl ether, tetrahydrofurfuryl alcohol, isopropanol, glycerol, ethylene glycol, and ethanol in a volume ratio of 1.5:2:1:1:2:1:1:2:1.

[0037] (2) Preparation of low-temperature solder paste:

[0038] ① Preparation of flux paste:

[0039] S1. Mix 1 / 4 rosin and 1 / 3 organic solvent at 130°C until completely dissolved. Raise the temperature to 140°C, add the thixotropic agent, and continue stirring until completely dissolved. Cool the temperature to 105°C and seal the cooled liquid in cold water at 11°C for 4 hours to obtain paste 1. Heat and stir the remaining rosin, remaining organic solvent, and corrosion inhibitor at 90-100°C until completely dissolved to obtain the intermediate.

[0040] S2. Add paste 1 and organic acid to an emulsifier and stir and disperse at 42°C for 2 hours. Add intermediate at 42°C, and stir and disperse for 1 hour after the addition is complete. Add organic amine and stir and disperse for 1 hour. Add surfactant at 25°C and stir and disperse for 2 hours to obtain flux paste.

[0041] ②Preparation of low-temperature solder paste:

[0042] solder paste and Sn 42 -Bi 58 Alloy powders are mixed evenly at a mass ratio of 11:89 to obtain low-temperature solder paste.

[0043] Example 2

[0044] (1) Raw materials:

[0045] The alloy powder is Sn 41 -Bi 57.5 -Ag 1.0 -Sb 0.5 Alloy powder with a particle size of 25-56μm.

[0046] The flux ingredients include 360g rosin, 44g activator, 40g thixotropic agent, 100g surfactant, 48g corrosion inhibitor, and 224mL organic solvent. The rosin contains hydrogenated rosin, disproportionated rosin, and polymerized rosin in a mass ratio of 3:2:1. The activator is a mixture of organic amines and organic acids in a mass ratio of 2:1. The organic amines include ethylenediamine, monoethanolamine, triethanolamine, and ethylamine in a mass ratio of 1:2:1:1, and the organic acids include 2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid, and dodecanoic acid in a mass ratio of 2:1:1.5:1:2:1:1. The thixotropic agent contains hydrogenated castor oil, modified hydrogenated castor oil, aromatic polyamide, and Thixcin R thixotropic agent in a mass ratio of 2:3:2:1. The surfactant contains OP-10, glyceryl stearate, hydrogenated dimer acid, and sodium dodecylbenzenesulfonate in a mass ratio of 1:2:1:1. The corrosion inhibitor contains 2-phenylimidazole, 2-methylbenzimidazole, benzotriazole, dibutylhydroxytoluene, and 2-ethylimidazole in a mass ratio of 1:1:2:1:1. The organic solvent has a boiling point of 190–280°C and contains diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol octyl ether, tetraethylene glycol dimethyl ether, tetrahydrofurfuryl alcohol, isopropanol, glycerol, ethylene glycol, and ethanol in a volume ratio of 1.5:2:1:1:2:1:1:2:1.

[0047] (2) Preparation of low-temperature solder paste:

[0048] ① Preparation of flux paste:

[0049] S1. Mix 1 / 4 rosin and 1 / 3 organic solvent at 130°C until completely dissolved. Raise the temperature to 140°C, add the thixotropic agent, and continue stirring until completely dissolved. Cool the temperature to 105°C and seal the cooled liquid in cold water at 11°C for 3 hours to obtain paste 1. Heat and stir the remaining rosin, remaining organic solvent, and corrosion inhibitor at 90-100°C until completely dissolved to obtain the intermediate.

[0050] S2. Add paste 1 and organic acid to an emulsifier and stir and disperse at 45°C for 1.5 hours. Add intermediate at 45°C, and stir and disperse for 1 hour after the addition is complete. Add organic amine and stir and disperse for 1 hour. Add surfactant at 25°C and stir and disperse for 2 hours to obtain flux paste.

[0051] ②Preparation of low-temperature solder paste:

[0052] solder paste and Sn 41 -Bi 57.5 -Ag 1.0 -Sb 0.5 Alloy powders were mixed evenly at a mass ratio of 15:85 to obtain low-temperature solder paste.

[0053] Example 3

[0054] (1) Raw materials:

[0055] The alloy powder is Sn 41 -Bi 57.5 -Ag 1.0 -Sb 0.5 Alloy powder with a particle size of 25-63μm.

[0056] The flux ingredients include 336g rosin, 52g activator, 40g thixotropic agent, 112g surfactant, 44g corrosion inhibitor, and 256mL organic solvent. The rosin contains hydrogenated rosin, disproportionated rosin, and polymerized rosin in a 3:1:1 mass ratio. The activator is a mixture of organic amines and organic acids in a 3:1 mass ratio. The organic amines include ethylenediamine, monoethanolamine, triethanolamine, and ethylamine in a 1:2:1:1 mass ratio, and the organic acids include 2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid, and dodecanoic acid in a 2:1:1.5:1:2:1:1 mass ratio. The thixotropic agent contains hydrogenated castor oil, modified hydrogenated castor oil, aromatic polyamide, and Thixcin R thixotropic agent in a 2:1:2:1 mass ratio. The surfactant contains OP-10, glyceryl stearate, hydrogenated dimer acid, and sodium dodecylbenzenesulfonate in a mass ratio of 1:2:1:1. The corrosion inhibitor contains 2-phenylimidazole, 2-methylbenzimidazole, benzotriazole, dibutylhydroxytoluene, and 2-ethylimidazole in a mass ratio of 1:1:2:1:1. The organic solvent has a boiling point of 190–280°C and contains diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol octyl ether, tetraethylene glycol dimethyl ether, tetrahydrofurfuryl alcohol, isopropanol, glycerol, ethylene glycol, and ethanol in a volume ratio of 1.5:2:1:1:2:1:1:2:1.

[0057] (2) Preparation of low-temperature solder paste:

[0058] ① Preparation of flux paste:

[0059] S1. Mix 1 / 4 rosin and 1 / 3 organic solvent at 130°C until completely dissolved. Raise the temperature to 140°C, add the thixotropic agent, and continue stirring until completely dissolved. Cool the temperature to 105°C and seal the cooled liquid in cold water at 11°C for 4 hours to obtain paste 1. Heat and stir the remaining rosin, remaining organic solvent, and corrosion inhibitor at 90-100°C until completely dissolved to obtain the intermediate.

[0060] S2. Add paste 1 and organic acid to an emulsifier and stir and disperse at 65°C for 1 hour; add intermediate at 42°C, and stir and disperse for 1 hour after the addition is complete; add organic amine and stir and disperse for 1 hour; add surfactant at 25°C and stir and disperse for 2 hours to obtain flux paste.

[0061] ②Preparation of low-temperature solder paste:

[0062] solder paste and Sn 41 -Bi 57.5 -Ag 1.0 -Sb 0.5 Alloy powders were mixed evenly at a mass ratio of 11:89 to obtain low-temperature solder paste.

[0063] Comparative Example 1

[0064] Low-temperature solder paste was prepared according to the method of Example 1, except that all organic amines were replaced with the same mass of glutaric acid, and the other conditions were the same as in Example 1, to obtain a reference low-temperature solder paste.

[0065] Comparative Example 2

[0066] Low-temperature solder paste was prepared according to the method of Example 1, except that the organic acid was replaced by the same mass of ethylenediamine, and the other conditions were the same as in Example 1, to obtain a reference low-temperature solder paste.

[0067] Comparative Example 3

[0068] Low-temperature solder paste was prepared according to the method of Example 1, except that the organic solvent was replaced with the same mass of tetrahydrofuran (boiling point 66°C), and the other conditions were the same as in Example 1, to obtain a reference low-temperature solder paste.

[0069] Test case

[0070] The low-temperature solder paste obtained in the above embodiments and comparative examples has a soldering test temperature of 160°C, which is lower than the actual production temperature of the customer's products. In other words, it has low-temperature properties, and the copper sheet does not have solder rejection after soldering.

[0071] (1) Viscosity:

[0072] Viscosity was tested using a Brookfield viscometer, and the results are shown in Table 1.

[0073] (2) Thixotropy:

[0074] The thixotropic index was tested using a Brookfield viscometer, and the results are shown in Table 1.

[0075] (3) Number of solder balls:

[0076] Place a clean stencil on the table, and place the customized PCB board on the corresponding stencil solder joints. Apply an appropriate amount of solder paste to be tested above the solder joints of the stencil to be printed, and spread it with a scraper. Lift the stencil, remove the printed PCB board, and after the temperature stabilizes, place the printed PCB board into a reflow soldering machine. Place the soldered PCB board under a microscope to observe whether there are solder beads, and count the number of solder beads. The results are shown in Table 1.

[0077] (4) Solder joint gloss:

[0078] Place a clean stencil on a table, place a copper sheet on the corresponding solder joint of the stencil, scrape an appropriate amount of solder paste to be tested onto the solder joint of the stencil to be printed, spread it with a scraper, lift the stencil, remove the printed copper sheet, set the oven to the test alloy temperature, and after the temperature stabilizes, put the printed copper sheet into the oven. Observe the soldered copper sheet under a microscope. The results are shown in Table 1.

[0079] (5) Post-weld residue:

[0080] The copper sheet used to test the solder joint brightness was placed under a microscope to observe the solder joint residue. The results are shown in Table 1.

[0081] Table 1

[0082] Group Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Viscosity 540kcp 527kcp 520kcp 522kcp 496kcp 448kcp Thixotropy 0.49 0.53 0.58 0.528 0.55 0.56 Number of solder balls 30 29 5 56 44 34 Solder joint gloss bright bright bright bright bright bright Post-welding residue small amount small amount trace amounts More Moderate Moderate

[0083] As can be seen from the results in Table 1, the low-temperature solder paste provided by the present invention can significantly reduce the number of solder balls and post-soldering residue, and significantly improve the soldering effect. This indicates that the solder paste provided by the present invention has soldering performance, is environmentally friendly and non-toxic, and can be adapted to the field of electronic packaging.

[0084] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention without departing from the principles and spirit of the present invention.

Claims

1. A method for preparing low-temperature solder paste, characterized in that, The low-temperature solder paste contains tin-bismuth-based alloy powder and flux. The flux contains rosin, activator, thixotropic agent, surfactant, corrosion inhibitor, and organic solvent. The activator is a complex of organic amine and organic acid in a mass ratio of (2~3):

1. The organic amine is selected from at least one of ethylenediamine, monoethanolamine, triethanolamine, and ethylamine. The organic acid is selected from at least one of 2-pyridinecarboxylic acid, oxalic acid, succinic acid, glutaric acid, malonic acid, phthalic acid, and dodecanoic acid. The boiling point of the organic solvent is 190~280℃. This method involves mixing flux with tin-bismuth-based alloy powder until homogeneous to obtain low-temperature solder paste; The flux paste is prepared according to the following steps: S1. Mix some rosin and some organic solvent and heat until completely dissolved. Then add thixotropic agent at 130~150℃. After cooling to 100~110℃ and stirring until completely dissolved, cool the resulting liquid and seal it in cold water at 9~13℃ for 3~4 hours to obtain a paste. S2. Stir the remaining rosin, remaining organic solvent and corrosion inhibitor at 90~100℃ until completely dissolved to obtain the intermediate; S3. Stir and disperse the paste and organic acid at 42~65℃ for 1~2h, then control the temperature of the resulting dispersion at 35~45℃ and add the intermediate dropwise. After the dropwise addition is complete, add the organic amine and stir and disperse for 2~3h. Then control the temperature of the resulting mixture at 25~30℃ and add the surfactant and stir and disperse for 1~2h to obtain the flux paste. Based on the total weight of the low-temperature solder paste, the content of the tin-bismuth-based alloy powder is 85%~90%, and the content of the flux is 10%~15%. Based on the total weight of the flux, the rosin content is 30%~50%, the activator content is 4%~7%, the thixotropic agent content is 4%~6%, the surfactant content is 12%~14%, the corrosion inhibitor content is 4%~6%, and the organic solvent content is 15%~35%.

2. The method for preparing low-temperature solder paste according to claim 1, characterized in that, The tin-bismuth-based alloy powder is Sn-Bi-Ag-Sb; the particle size of the tin-bismuth-based alloy powder is 20~60 μm.

3. The method for preparing low-temperature solder paste according to claim 1 or 2, characterized in that, The rosin is selected from at least one of hydrogenated rosin, KE-604, disproportionated rosin, polymerized rosin, maleic rosin, and Ax-E rosin.

4. The method for preparing low-temperature solder paste according to claim 1 or 2, characterized in that, The thixotropic agent is selected from at least one of hydrogenated castor oil, modified hydrogenated castor oil, unsaturated fatty amide, saturated fatty amide, polyamide wax, aromatic polyamide, and Thixcin R thixotropic agent.

5. The method for preparing low-temperature solder paste according to claim 1 or 2, characterized in that, The surfactant is selected from at least one of OP-10, glyceryl stearate, hydrogenated dimer acid, and sodium dodecylbenzenesulfonate.

6. The method for preparing low-temperature solder paste according to claim 1 or 2, characterized in that, The corrosion inhibitor is selected from at least one of 2-phenylimidazole, 2-methylbenzimidazole, benzotriazole, dibutylhydroxytoluene, and 2-ethylimidazole.

7. The method for preparing low-temperature solder paste according to claim 1 or 2, characterized in that, The organic solvent is selected from at least one of diethylene glycol monohexyl ether, diethylene glycol monobutyl ether, diethylene glycol octyl ether, tetraethylene glycol dimethyl ether, tetrahydrofurfuryl alcohol, isopropanol, glycerol, ethylene glycol, and ethanol.