Water-soluble soldering paste and preparation method thereof

Abstract

The invention discloses water-soluble soldering paste and a preparation method thereof, and the water-soluble soldering paste comprises the following components in percentage by mass: 85-92% of soldering powder and 8-15% of C soldering flux, the soldering flux C comprises the following components in percentage by mass: 60-95% of soldering flux A and 40-5% of soldering flux B; the soldering flux A comprises a solvent A, a rosin derivative A, a thixotropic agent A, an active agent A and a surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydroabietinol; the solvent A is a hydrophobic solvent; the mass percent of the rosin derivative A in the soldering flux A is at least 29%; the soldering flux B comprises a solvent B, a rosin derivative B, an active agent B, a thixotropic agent B and a corrosion inhibitor; the solvent B is a hydrophilic solvent; the rosin derivative B comprises polyoxyethylene rosin amine; the soldering flux A and the soldering flux B are used for being compounded into a soldering flux C of an emulsion structure; and the soldering flux C is used for being mixed with soldering powder to form the water-soluble soldering paste.

Description

Technical Field

[0001] The invention belongs to the technical field of solder paste, and particularly relates to an ultrafine tin powder water-soluble lead-free solder paste and a preparation method thereof. Background Art

[0002] Solder paste (especially soft solder paste) is primarily used in the manufacture of electronic circuits. The reflow process melts the solder powder in the paste, where it reacts with the metal on component pins and the metal on the pads of the circuit board (or substrate) to form intermetallic compounds. This creates the mechanical and electrical connection between the electronic component and the circuit board (or substrate), ultimately forming an effective circuit system.

[0003] Recently, due to the bottleneck of quantum effects encountered in chip manufacturing during miniaturization, CMOS feature sizes have been difficult to reduce further near 1-2 nanometers. Consequently, electronic system design has evolved from system-on-chip (SoC) to system-in-package (SIP) (known in the industry as "More-than-Moore") to meet the practical needs of scenarios such as edge computing in the Internet of Things. A system-in-package (SiP) integrates several different types of chips and electronic components (resistors, capacitors, etc.) into a single package module to form complex and independent functional modules.

[0004] In the electronic packaging field, the ever-decreasing miniaturization of chips and components within SiP modules requires soldering to shrinking pad sizes and pitches, while maintaining high reliability throughout the SiP's lifecycle. A classic example of this type of package is copper pillar bumping technology, where pad sizes have been reduced to 55 microns and pad pitches to 35 microns. Traditional no-clean solder residues are difficult to meet reliability requirements, leading the industry to adopt water-soluble solder paste.

[0005] However, prior to 1991, conventional cleaning water-soluble solder pastes (such as disclosed in U.S. Patent 5,069,730) used water-soluble PEG (polyethylene glycol) polymers as carriers. PEG polymers are water-soluble polymers formed by the polymerization of ethylene oxide. Prior to 2016, the industry still relied on water-soluble polymers for water-soluble solder pastes, as exemplified by the technologies disclosed in CN103909358A and CN105290650A. Without the protective effect of rosin, the flux required a high amount of activator, resulting in poor stability. This was primarily manifested in a rapid increase in solder paste viscosity during the printing process.

[0006] However, a common drawback of water-soluble solder pastes currently on the market is their poor printing performance. Stencil life on printed boards typically does not exceed eight hours (after eight hours of continuous printing, the paste viscosity increases so much that noticeable printing defects occur).

[0007] The technology disclosed in this invention provides a comprehensive solution with superior performance. It not only offers an excellent lifespan on printed boards, but also maintains normal print quality with a viscosity increase of less than 15% after 12 hours of continuous printing. The resulting solder paste also exhibits excellent heat collapse resistance and residue cleaning performance. This solder paste is achieved primarily by inhibiting solder powder corrosion. Summary of the Invention

[0008] In response to the problem of insufficient life of existing solder paste on printed boards, the present invention provides a solder paste that alleviates solder powder corrosion to extend the life of the printed board, has excellent other comprehensive properties, such as solder joint brightness, easy cleaning of residue, and smooth tin printing.

[0009] The technical solution of the present application for solving the above-mentioned technical problems is a water-soluble solder paste, which comprises, by mass percentage, 85-92% of solder powder and 8-15% of C flux; C flux comprises, by mass percentage, 60-95% of A flux and 40-5% of B flux; A flux comprises A solvent, A rosin derivative, A thixotropic agent, A activator, and surfactant; the A rosin derivative comprises at least one of rosin amine and tetrahydroabietin; A solvent is a hydrophobic solvent; the mass percentage of A rosin derivative in A flux is at least 29%; B flux comprises B solvent, B rosin derivative, B activator, B thixotropic agent, and corrosion inhibitor; B solvent is a hydrophilic solvent; B rosin derivative comprises polyoxyethylene rosin amine; A flux and B flux are compounded to form C flux with an emulsion structure; C flux is mixed with solder powder to form a water-soluble solder paste.

[0010] The flux A may include, by mass percentage, 30-50% of solvent A, 30-50% of rosin derivative A, 4-12% of thixotropic agent A, 3-6% of active agent A, and 0.5-3% of surfactant; the solvent A may include any one or more of hydrophobic organic alcohol ethers, alkanes, and rosin modifications.

[0011] The organic alcohol ether substance in solvent A may include any one or more of diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tripropylene glycol n-butyl ether.

[0012] The alkane in solvent A may include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes; and the boiling point of the alkane is 215-270°C.

[0013] Alternatively, the rosin modification in solvent A may include methyl hydrogenated abietic acid.

[0014] It may be that the A activator in the A flux includes an organic acid or a salt formed by an organic acid and an organic amine; the organic acid includes any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; and the salt formed by an organic acid and an organic amine includes rosin amine.

[0015] The thixotropic agent A of the flux A may include any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent.

[0016] The surfactant of flux A may include any one or more of Span 60 and Span 80.

[0017] The flux B may include, by mass percentage, 30-50% of the solvent B, 30-50% of the rosin derivative B, 4-18% of the active agent B, 4-12% of the thixotropic agent B, and 0.2-3% of the corrosion inhibitor.

[0018] Alternatively, solvent B may be an organic alcohol solvent containing a hydroxyl group.

[0019] Solvent B may be a hydroxyl-containing organic alcohol solvent; solvent B includes any one or more of 2,5-dimethyl-2,5-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monooctyl ether, and glycerol.

[0020] It can be a B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20.

[0021] Alternatively, the thixotropic agent B may be a water-soluble amide thixotropic agent.

[0022] The active agent B may be any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole.

[0023] The corrosion inhibitor may be any one or more of 2-bromo-1-methyl-1H-imidazole, ethylimidazole, and 1,2,3-benzotriazole.

[0024] The solder powder is an alloy composed of Sn as a base and a small amount of metal elements added. The solder powder includes any one or more of: Ag 0-4%, Cu 0-1%, Sb 0-10%, Bi 0-3%, Ni 0-0.15%, and In 0-10%. The particle size of the solder powder is 2-25 μm, and the particle size types include T5 / T6 / T7 / T8.

[0025] The technical solution of the present application to solve the above-mentioned technical problems can also be a flux A, comprising a solvent A, a rosin derivative A, a thixotropic agent A, an active agent A, and a surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydroabietin; the solvent A is a hydrophobic solvent; the flux A comprises, by mass percentage, 30-50% of solvent A, 30-50% of rosin derivative A, 4-12% of thixotropic agent A, 3-6% of active agent A, and 0.5-3% of surfactant; the solvent A comprises a hydrophobic organic alcohol ether, an alkane, and a rosin modification; the boiling point of the alkane is 215-270°C.

[0026] The organic alcohol ether substance in solvent A may include any one or more of diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tripropylene glycol n-butyl ether.

[0027] The alkanes in solvent A may include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes.

[0028] Alternatively, the rosin modification in solvent A may include methyl hydrogenated abietic acid.

[0029] It may be that the A activator in the A flux includes an organic acid or a salt formed by an organic acid and an organic amine; the organic acid includes any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; and the salt formed by an organic acid and an organic amine includes rosin amine.

[0030] The thixotropic agent A of the flux A may include any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent.

[0031] The surfactant of flux A may include any one or more of Span 60 and Span 80.

[0032] The technical solution for solving the above-mentioned technical problem can also include solvent B, rosin derivative B, active agent B, thixotropic agent B, and corrosion inhibitor. Solvent B is a hydrophilic solvent. The rosin derivative B includes polyoxyethylene rosin amine. In terms of mass percentage, flux B includes: 30-50% solvent B, 30-50% rosin derivative B, 4-18% active agent B, and 4-12% thixotropic agent B. The corrosion inhibitor is 0.2-3%.

[0033] Alternatively, solvent B may be an organic alcohol solvent containing a hydroxyl group.

[0034] Solvent B may be a hydroxyl-containing organic alcohol solvent; solvent B includes any one or more of 2,5-dimethyl-2,5-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monooctyl ether, and glycerol.

[0035] It can be a B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20.

[0036] Alternatively, the thixotropic agent B may be a water-soluble amide thixotropic agent.

[0037] The active agent B may be any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole.

[0038] The corrosion inhibitor may be any one or more of 2-bromo-1-methyl-1H-imidazole, ethylimidazole, and 1,2,3-benzotriazole.

[0039] The technical solution of the present application to solve the above-mentioned technical problems can also be a method for preparing flux A, which is used to prepare the above-mentioned flux A; step A includes, step A10: mixing, stirring and heating A rosin derivative, A solvent, and A activator, the heating temperature of which is 70°C-90°C; step A20: after mixing, heating and dissolving, lowering the temperature to 60°C-75°C, step A30: adding thixotropic agent A, applying high-speed stirring, the stirring speed of which is between 2000-3000 rpm, and activating for 20-30 minutes; step A40: reducing the stirring speed, slowly stirring and cooling, and cooling to 30°C; step A50: adding surfactant at 30°C and stirring evenly; step A60: cooling to room temperature, and then cooling and grinding; step A70: after grinding, placing in a 2-10°C refrigerator for more than 10 hours for standby use.

[0040] The technical solution of the present application to solve the above-mentioned technical problems can also be a method for preparing a B flux, which is used to prepare the above-mentioned B flux; step B includes: step B10: mixing, stirring, and heating the B solvent, rosin modifier B, organic acid B, corrosion inhibitor, and B activator, and slowly stirring and dissolving them at a heating temperature of 60-80°C; step B20: lowering the temperature to 50-70°C, adding the B thixotropic agent, and activating them with high-speed stirring for 20-30 minutes; step B30: slowly stirring and cooling to 30°C; step B40: discharging the material, grinding it, and placing it in a refrigerator at 2-10°C for more than 10 hours for standby use.

[0041] The technical solution of the present application for solving the above-mentioned technical problem can also be a method for preparing a water-soluble solder paste, which is used for preparing the above-mentioned water-soluble solder paste, comprising step A: preparing flux A, and refrigerating and placing it after preparation; step B: preparing flux B, and refrigerating and placing it after preparation; step C: removing the refrigerated flux A and flux B, returning them to room temperature, and mixing and stirring the two in a set mass percentage to emulsify to obtain flux C, wherein the set mass percentage is 60-95% flux A and 40-5% flux B; after emulsification, placing the flux in a refrigerator at 2-10°C for more than 10 hours for standby use; step D: removing the above-mentioned flux C, returning it to room temperature, and then adding solder powder with a weight percentage of between 85-92% to 8-15% of flux C, and mixing them uniformly under vacuum at a stirring speed of 20-40 rpm to obtain a water-soluble solder paste.

[0042] The step C includes: step C10: a mixing and emulsification process includes: emulsifying for 5-10 minutes at a stirring speed of more than 1000 rpm to obtain C flux; step C20: after the C flux obtained in step C10 is subjected to three-roller grinding, it is placed in a 2-10°C refrigerator for more than 10 hours for standby use.

[0043] The beneficial effects of the present application are as follows: flux A and flux B are used to compound into flux C with an emulsion structure; solvent A of flux A is a hydrophobic solvent, so flux A acts as a continuous phase, thereby effectively blocking the process of water vapor in the air penetrating into the paste, inhibiting tin powder corrosion caused by water during the printing process or the waiting process, thereby extending the life of the solder paste printed board.

[0044] The beneficial effect of the present application is that by reacting a strong organic acid with an organic amine to form a salt, the salt is confined in a low hydrogen bond density, hydrophobic A flux continuous phase, thereby preventing and alleviating the corrosion reaction caused by the contact between the organic acid and the solder powder during the storage and printing process of the solder paste, thereby maintaining the stability of the solder paste.

[0045] The beneficial effects of this application are as follows: during the reflow process, due to the increased temperature, solvent volatilization and enhanced molecular thermal motion increase the chance of organic acid migrating to the solder powder surface, thereby removing the oxide film on the solder powder surface and promoting cold soldering between the tin powders. The presence of a large amount of rosin-like substances and hydrophobic solvent in the continuous phase effectively prevents water vapor in the heated gas from reoxidizing and corroding the tin powder during the preheating stage of reflow soldering, thereby ensuring excellent heat collapse resistance.

[0046] The beneficial effects of this application are as follows: The rosin amine in flux A, as the amine group in the rosin amine molecule has a weak alkalinity, can react with metal oxides (such as SnO and SnO2) at high soldering temperatures to form soluble salts, thereby removing the oxide layer on the metal surface and enhancing solder wettability. Rosin amine can react with organic acids to form more stable rosin amine salts, which inhibit the chemical reaction rate between organic acids and solder powder during storage and use, improving the stability of solder paste. Intermediate products generated during the soldering process can coat the metal surface of the solder joint, preventing the formation of new oxides at high temperatures.

[0047] The beneficial effects of the present application are as follows: tetrahydroabietin in flux A acts as a thickener to adjust the rheological properties of the flux; its hydroxyl group (-OH) can weakly coordinate with metal oxides to assist rosin amine in exerting its deoxidation effect; these two substances form a uniform covering film on the surface of the solder paste and solder joints, isolating air and moisture, and providing thermal stability and physical film-forming properties.

[0048] The beneficial effects of this application are as follows: The hydrophilicity of the polyoxyethylene rosin amine in the B flux gives it excellent surface activity, significantly reducing the contact angle between the solder and the metal surface, promoting solder spreading on PCB pads and pins, and reducing false and cold solder joints. The amino groups in the structure also participate in the deoxidation process at high temperatures. In addition, the flexible structure of the polyoxyethylene chain can increase the viscoelasticity of the flux, allowing the solder paste to maintain good thixotropy and anti-collapse properties during the screen printing or stencil printing process, preventing excessive solder paste diffusion or clogging of the stencil opening, adjusting the solder paste's rheological properties and demolding properties, and improving printing accuracy and efficiency.

[0049] The beneficial effects of this application are as follows: the composition ratio of the various components in flux A, with solvent A and rosin derivative A as the main components, can provide flux A with hydrophobic properties, making flux A the continuous phase that occupies a larger mass percentage, thereby exerting better hydrophobicity, that is, a water-repellent effect, thereby improving the life of the solder paste on the printed board. At the same time, the components in flux B also give the solder paste excellent printing and demolding properties, thereby maintaining excellent oxide removal ability, enhancing wettability, and improving the printing performance and stability of the solder paste. It is a key component of high-performance water-soluble solder paste suitable for high-density, high-reliability electronic packaging.

[0050] The beneficial effects of this application are as follows: The organic alcohol ether in solvent A is hydrophobic and contains an amphiphilic structure with ether bonds and hydroxyl groups, which reduces the surface tension of the solder, enhances wettability to the metal, ensures uniform solder spreading, and reduces cold solder joints. During the preheating stage of welding, some of the alcohol ether volatilizes, forming an inert gas that protects the solder joint surface and prevents high-temperature oxidation. The remaining portion combines with water after welding to form a soluble complex, facilitating cleaning.

[0051] The beneficial effects of this application are as follows: The long-chain alkane in solvent A acts as a thickener, adjusting the viscosity of the solder paste and preventing printing collapse. At high soldering temperatures, it transforms into a low-surface-energy protective film, reducing solder oxidation. During solder paste application, it coats the metal surface, forming a hydrophobic temporary protective film that delays oxidation and prevents absorption of moisture and oxygen from the environment.

[0052] The beneficial effects of the present application are as follows: the carboxylic acid groups contained in the rosin modified substance in solvent A, such as methyl hydrogenated abietic acid, can react with metal oxides to form soluble salts. The saturated structure can reduce high-temperature carbonization, and the residue can be more easily cleaned by water saponification after methyl esterification, thereby improving the post-weld cleaning performance.

[0053] The beneficial effects of the present application are as follows: the activator A is an organic acid or a salt formed by an organic acid and an organic amine, which provides soldering activity to the solder paste and efficiently removes the oxide film on the surface of the solder powder. With the development of advanced packaging technology, solder joints are becoming more and more refined. As the particle size of ultrafine solder powder decreases, its surface area increases exponentially, and its oxides also increase exponentially, which increases the deoxidation load of the flux. The activator A has strong activity and can quickly remove the oxide film on the surface of the ultrafine solder powder.

[0054] The beneficial effects of the present application are: the thixotropic agent A includes any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent, which provides rheological properties for the solder paste, maintains good thixotropy, and enables the solder paste to have good shape-retaining ability.

[0055] The beneficial effects of the present application are: the surfactant of flux A, including any one or more of Span 60 and Span 80, has unique hydrophobicity and emulsification stabilization effect, which regulates the rheological properties of the paste, reduces interfacial tension, stabilizes the emulsion structure, and significantly improves the printing accuracy, welding reliability and residue controllability of the solder paste.

[0056] The beneficial effects of this application are as follows: the hydroxyl-containing organic alcohol solvent in the flux B can form hydrogen bonds or other interactions with certain active ingredients in the flux. During the heating process, these interactions are destroyed, thereby promoting the release of the active ingredients and improving the activity of the flux. At the same time, it can adjust the rheological properties of the solder paste, control the viscosity, dissolve other solid particulate components in the flux, reduce the viscosity of the post-soldering residue, and facilitate cleaning.

[0057] The beneficial effects of the present application are: the B active agent is any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole; these substances provide the solder paste with welding activity at high temperatures, maintain the solder paste's continuous activity throughout the entire soldering process, and at the same time reduce the surface tension of the solder, facilitate spreading and wetting, and achieve metallurgical connection between the solder and the pad.

[0058] The beneficial effects of the present application are as follows: the active agent B includes 2-bromo-1-methyl-1H-imidazole, the corrosion inhibitor includes 2-bromo-1-methyl-1H-imidazole, and the same substance acts as both the active agent and the corrosion inhibitor.

[0059] The beneficial effects of the present application are: the particle size of the solder powder is 2~25um, and the particle size models include T5 / T6 / T7 / T8. The water-soluble solder paste of the present application has good adaptability and is suitable for a variety of particle sizes.

[0060] The beneficial effects of the present application are: A soldering flux is separately packaged and prepared, which is convenient for storage and use. B soldering flux is separately packaged and prepared, which is convenient for storage and use.

[0061] The beneficial effects of the present application are: the C flux is ready for use, and the ratio of the A flux and the B flux can be adjusted according to the chemical composition, particle size model and oxide level of the solder powder to prepare the C flux that matches different solder powders and is suitable for different process windows and characteristic requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 Table 1 shows the formula of flux A in Examples 1 to 5; Figure 2 Table 2 shows the formulations of flux B in Examples 1 to 5; Figure 3 is Table 3, showing the formulation of the comparative example; Figure 4 is Table 4, showing the continuous printing test results of Examples and Comparative Examples; Figure 5 Table 5 shows the thermal collapse test results of the embodiments and comparative examples; Figure 6 This is a comparison diagram before and after solder joint cleaning; Figure 7 This is a schematic diagram of Raman spectroscopy analysis of residual materials before solder joint cleaning; Figure 8 This is a schematic diagram of Raman spectroscopy analysis of residual materials after solder joint cleaning. DETAILED DESCRIPTION

[0063] The content of this application is further described in detail below with reference to the accompanying drawings.

[0064] Glossary: Rosin: It is a natural product extracted from the resin of pine trees. Its main component is abietic acid. It has good chemical stability and is a complex mixture, but it has low activity. It is suitable for welding scenarios that do not require high flux activity.

[0065] Rosin Modifiers: Rosin modifiers are typically made by treating rosin through physical mixing or mild chemical reactions (such as hydrogenation, esterification, and polymerization), retaining the primary structure but improving performance. These modifications are intended to improve performance or impart new functionalities. Modification methods include esterification, hydrogenation, and polymerization. For example, the hydrogenated rosin methyl ester described in this patent is suitable for applications requiring high soldering temperatures and quality. Rosin derivatives: Rosin is further processed through chemical reactions to produce compounds with different functional groups or structures. These compounds typically possess different chemical properties and applications than rosin, enhancing their activity and making them suitable for soldering applications requiring high flux activity and reliability.

[0066] Rosin derivatives, through more complex chemical reactions, change their structure to form new compounds with different functional groups or molecular skeletons, such as polyoxyethylene rosin amine, polyoxyethylene polymerized rosin, rosin amine, and tetrahydroabietin alcohol in this patent.

[0067] HLB value (Hydrophile-Lipophile Balance) is a quantitative indicator of the balance in size and strength between the hydrophilic and lipophilic groups in a surfactant molecule. A higher value indicates greater hydrophilicity, while a lower value indicates greater lipophilicity.

[0068] In this application, mass percentage % means weight percentage wt%. In this application, flux C means C flux; flux A means A flux; and flux B means B flux.

[0069] Solvent A is solvent A, rosin derivative A is rosin derivative A, thixotropic agent A is thixotropic agent A, and activator A is activator A.

[0070] Solvent B is solvent B, rosin derivative B is rosin derivative B, activator B is activator B, and thixotropic agent B is thixotropic agent B.

[0071] Although the surface of the solder powder is covered with a tin oxide film, the oxide layer still has defects, and the tin atoms still have the opportunity to be exposed to the flux, causing the solder paste to corrode during storage and printing. The process can be described by the following reaction formula: 4Sn+2H2O+4H + +O2=2H2+2Sn(OH - )2+2Sn 2+ As shown in the above reaction formula, the simultaneous presence of water and oxygen is necessary for the corrosion of solder powder.

[0072] In modern packaging processes, devices are becoming smaller and smaller, and packaging density is increasing. The waiting time before reflow soldering on circuit boards or substrates after solder paste printing is increasing, placing higher demands on solder paste stability. Solder paste absorbs water both during the printing and rolling process and while it remains stationary on the board; this water absorption can exacerbate corrosion reactions.

[0073] The solder paste of this application maintains a hydrophobic, continuous-phase A flux on its outer surface during printing and storage, effectively preventing atmospheric moisture from penetrating the paste. This inhibits tin powder corrosion caused by the aforementioned mechanism, thereby extending the life of the solder paste printed on the board. The technology of this invention effectively mitigates the water absorption effect of the solder paste during the printing and rolling process and while the solder paste remains stationary on the board. Therefore, the present invention improves the stability of the solder paste by limiting the flux's absorption of water.

[0074] The present invention provides a water-soluble solder paste, which comprises, by mass percentage, 85-92% of solder powder and 8-15% of a C flux; the C flux comprises, by mass percentage, 60-95% of an A flux and 40-5% of a B flux; the A flux comprises an A solvent, an A rosin derivative, an A thixotropic agent, an A activator, and a surfactant; the A rosin derivative comprises at least one of rosin amine and tetrahydroabietin; the A solvent is a hydrophobic solvent; the mass percentage of the A rosin derivative in the A flux is at least 29%; the B flux comprises a B solvent, a B rosin derivative, a B activator, a B thixotropic agent, and a corrosion inhibitor; the B solvent is a hydrophilic solvent; the B rosin derivative comprises polyoxyethylene rosin amine; the A flux and the B flux are compounded to form a C flux with an emulsion structure; and the C flux is mixed with the solder powder to form the water-soluble solder paste.

[0075] The solder powder content is between 85-92wt%. The solder powder is composed of a Sn base alloy with small amounts of one or more of the following metal elements: Ag (0-4%), Cu (0-1%), Sb (0-10%), Bi (0-3%), Ni (0-0.15%), and In (0-10%). The particle size of this solder powder meets the requirements for #6, #7, and #8 powders specified in the electronics industry standard SJ / T11391-2019, "Solder Alloy Powder for Electronic Product Soldering," or IPCJ-STD-005A-2012. Particle sizes include T5, T6, T7, and T8. T5 represents a particle diameter range of 15-25μm, T6 represents a particle diameter range of 5-15μm, T7 represents a particle diameter range of 2-11μm, and T8 represents a particle diameter range of 2-8μm.

[0076] The C flux consists of two parts, A flux and B flux. In terms of mass percentage, the mass percentage of A flux is 60-95%; the mass percentage of B flux is 40-5%. A flux and B flux are compounded in the above proportions and an emulsion structure is formed under high-speed stirring conditions in which A flux is a continuous phase and B flux is a dispersed phase.

[0077] Solder flux A includes solvent A, a rosin derivative A, a thixotropic agent A, an active agent A, and a surfactant. The rosin derivative A includes at least one of rosin amine and tetrahydroabietin. Solder A is a hydrophobic solvent. By mass percentage, flux A includes 30-50% solvent A, 30-50% rosin derivative A, 4-12% thixotropic agent A, 3-6% active agent A, and 0.5-3% surfactant.

[0078] Solvent A includes hydrophobic organic alcohol ethers, alkanes, and rosin modifications; the alkanes have a boiling point of 215-270° C. Solvent A is a low hydrogen bond density, weakly polar, and hydrophobic solvent.

[0079] The organic alcohol ethers in solvent A include any one or more of diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tripropylene glycol n-butyl ether. The alkanes in solvent A include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes. The rosin modification in solvent A includes methyl hydrogenated abietic acid. Organic alcohol ethers have a low density of functional groups capable of forming hydrogen bonds.

[0080] The A activator in flux A includes an organic acid or a salt formed by an organic acid and an organic amine; the organic acid includes any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; the salt formed by an organic acid and an organic amine includes rosin amine. The A activator is an organic acid activator.

[0081] The thixotropic agent A of the A flux includes any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent.

[0082] The surfactant of flux A includes one or more of Span 60 and Span 80. The surfactant is a low HLB value substance such as Span 60 or Span 80, with an HLB value between 3 and 8.

[0083] The A solvent of A flux is mainly low-polarity molecules, or molecules with a low density of functional groups that can form hydrogen bonds per unit volume, or molecules that cannot form hydrogen bonds.

[0084] The B solvent of the B flux mainly includes molecules rich in hydrogen bond forming properties, and the organic acid in the B solvent is dispersed in the A flux.

[0085] Calculated by mass percentage, B flux includes: B solvent 30-50%, rosin derivative B 30-50%, B active agent total amount of 4-18%, B thixotropic agent 4-12%, corrosion inhibitor 0.2-3%.

[0086] The B solvent is an organic alcohol with high hydrogen bond density, strong polarity and strong water solubility; the B solvent is an organic alcohol solvent containing hydroxyl groups; and the B active agent includes: an organic acid and a halogen total active agent.

[0087] Solvent B includes any one or more of 2,5-dimethyl-2,5-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monooctyl ether, and glycerol.

[0088] B rosin derivatives are polyoxyethylene rosin amines with an HLB value between 8 and 20. The rosin derivatives used in B flux are high-HLB polyoxyethylene rosin amines and polyoxyethylene polymerized rosin; high HLB values ​​refer to compounds with an HLB value between 8 and 20.

[0089] Thixotropic agent B is a water-soluble amide thixotropic agent; the water-soluble polyamide thixotropic agent RE-103 produced by Japan Kyoeisha Chemical Co., Ltd. is used.

[0090] The active agent B is any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole.

[0091] The corrosion inhibitor is any one or more of 2-bromo-1-methyl-1H-imidazole, ethylimidazole, and 1,2,3-benzotriazole. 2-bromo-1-methyl-1H-imidazole is also a halogen activator. The surface corrosion inhibitor used in the B flux of the water-soluble solder paste is 2-bromo-1-methyl-1H-imidazole, which is also a halogen activator.

[0092] The water-soluble solder paste of this application comprises two types of solvents. One is a low-hydrogen-bonding-density, weakly polar, and hydrophobic organic solvent located in the continuous phase A, or flux A; the other is a high-hydrogen-bonding-density, highly polar, and relatively hydrophilic organic solvent located in the dispersed phase B, or flux B. The flux production process results in an emulsion structure. Flux A, primarily composed of the hydrophobic solvent, forms the continuous phase, while flux B, primarily composed of the hydrophilic solvent, constitutes the dispersed phase.

[0093] The technology of the present invention prevents and alleviates the corrosion reaction caused by the contact between the organic acid and solder powder during the storage and printing of the solder paste by reacting a strong organic acid with an organic amine to form a salt, and then confines the salt in a low hydrogen bond density, hydrophobic A flux continuous phase, thereby maintaining the stability of the solder paste.

[0094] During the reflow process, the increased temperature causes solvent volatilization and enhanced molecular thermal motion, increasing the chances of organic acids migrating to the solder powder surface, thereby removing the oxide film on the solder powder surface and promoting cold soldering between the tin powders. The presence of a large amount of rosin-like substances and hydrophobic solvents in the continuous phase effectively prevents the re-oxidation and corrosion of the tin powder by water vapor carried in the heated gas during the preheating stage of reflow soldering, thus ensuring excellent heat collapse resistance.

[0095] To maximize the continuous phase properties and thermal collapse resistance of flux A, flux A should account for 60-95% of the total flux dosage. To maintain the overall activity of the solder paste, the rosin content in flux A must be maintained at above 29wt%. Due to its hydrophobic nature, corrosion inhibitors are not required in the continuous phase flux A. To ensure the overall activity, rheological properties, printability, and adhesion of the solder paste, conventional flux B, with its high hydrogen bond density and high polarity, is still required as the dispersed phase, accounting for 5-40wt% of the total flux.

[0096] The emulsion structure of this flux effectively blocks the absorption and diffusion of atmospheric moisture into the flux during the solder paste printing process, on-board storage, and normal storage, thereby inhibiting solder powder corrosion and improving stencil life. The hydrophobic nature of the continuous phase effectively inhibits the corrosive effects of strong organic acids without hindering cold soldering during the early stages of reflow, resulting in excellent stencil life and thermal collapse resistance. The materials used in this technology also provide this product with a wide active process window and excellent residue cleaning performance.

[0097] The present invention provides a method for preparing the water-soluble solder paste as follows (1) Preparation of flux A: Add rosin derivative A, solvent A, and active agent A to a temperature-controlled heating device, slowly stir, mix, and dissolve at a temperature range of 70-90°C; lower the system temperature to 60-75°C, add thixotropic agent A, and stir at high speed at a speed of 2000-3000 rpm for 20-30 minutes. After the thixotropic agent is activated, cool to 30°C while slowly stirring, add surfactant, and mix evenly. After cooling to room temperature, grind, and then place in a refrigerator at 2-10°C for more than 10 hours before use.

[0098] (2) Preparation process of B flux: Add hydroxyl-containing B solvent, rosin modifier B, B active agent, and corrosion inhibitor B into a temperature-controlled heating device, slowly stir and mix to dissolve at a temperature range of 60-80°C, lower the system temperature to 40-55°C, add thixotropic agent, activate at high speed for 20-30 minutes (speed between 2000-3000 rpm), slowly stir and cool to 30°C, discharge, grind, and place in a 2-10°C refrigerator for more than 10 hours for standby use.

[0099] (3) Take out the refrigerated flux A and flux B, return them to room temperature, and then compound them in a ratio of 60-95%:40-5%. Emulsify them for 5-10 minutes under high-speed stirring at a stirring speed of more than 1000 rpm to obtain flux C. After three-roll grinding, put the flux into a 2-10℃ refrigerator and leave it for more than 10 hours for use.

[0100] (4) Take out the C flux obtained above, return it to room temperature, then add 85-92wt% of solder powder to 8-15wt% of the C flux above, and mix them evenly in a vacuum at a stirring speed of 20-40rpm to obtain solder paste.

[0101] Beneficial effects: (1) The use of two types of solvents (especially hydrophobic A solvent) and a surfactant with a low HLB value can promote the flux structure of the emulsion. The presence of the hydrophobic continuous phase inhibits the absorption of water. Since the presence of water is necessary for the electrochemical corrosion of solder powder, the present invention can significantly reduce the corrosion of solder powder and effectively prevent the viscosity of the solder paste from increasing during continuous printing, thereby improving the life of the printed board.

[0102] The present invention uses a strong acid in the hydrophobic continuous phase A, i.e., the A solvent, which can effectively inhibit its corrosion while still giving full play to its cold soldering effect in the early stage of reflow, so that the solder paste has excellent on-net life and anti-heat collapse performance. The large amount of rosin-based substances in the continuous phase A, i.e., the A solvent, and the high HLB rosin-based surfactant in the dispersed phase B, i.e., the B solvent, can provide effective protection for solder powder during the reflow process, thereby ensuring activity and residual cleaning performance; B Solvent contains halogen corrosion inhibitors that provide protection for tin powder while also increasing the activity of the paste. Example

[0103] At 75°C, add 40 parts of n-tetradecane to a container, slowly stir and mix 32 parts of tetrahydroabietin alcohol, then add a mixture of 15 parts of rosin amine and 5 parts of malonic acid and stir until uniform. Then, when the system temperature drops to 65°C, add 7 parts of polyamide thixotropic agent 6650, stir at high speed for 20-30 minutes, slowly stir and cool to 30°C, add 1 part of Span 60, mix uniformly, discharge, and after cooling to room temperature, grind and place in a refrigerator at 2-10°C for more than 10 hours for later use. At 70°C, 38 parts of diethylene glycol hexyl ether, 10 parts of succinic acid, 5 parts of sebacic acid, 40 parts of polyoxyethylene rosin amine, and 0.3 parts of 2-bromo-1-methyl-1H-imidazole were added to a container insulated with an oil bath, slowly stirred, mixed and dissolved, then the system temperature was lowered to 55°C, 6.7 parts of a water-soluble polyamide thixotropic agent RE-103 were added, and after high-speed stirring and activation for 20-30 minutes, the mixture was slowly stirred and cooled to 30°C. The mixture was discharged, ground, and placed in a refrigerator at 2-10°C for more than 10 hours for standby use.

[0104] Remove the refrigerated flux A and flux B and return them to room temperature. Then, mix the two at a ratio of 65wt%:35wt% and emulsify them at a high-speed stirring speed of 1100rpm for 5 minutes to obtain flux C. Place flux C in a refrigerator at 2-10℃ and leave it for more than 10 hours before use. The C flux obtained above was taken out and returned to room temperature, and then 87 wt % of Sn96.5Ag3Cu0.5T7 solder powder was added to 13 wt % of the C flux above, and the mixture was mixed in a vacuum at a stirring speed of 25 rpm to obtain a solder paste.

[0105] Example 2. The formula of Example 2 is shown in Tables 1 and 2. The production process is the same as that of Example 1, except that the weight ratio of flux A to flux B is 60wt%:40wt%.

[0106] Example 3. The formula of Example 3 is shown in Table 1 and Table 2. Its production process is the same as that of Example 1. The difference is that the weight ratio of flux A to flux B is 70wt%:30wt% Example 4. The formula of Example 4 is shown in Table 1 and Table 2. The production process is the same as that of Example 1, except that the weight ratio of flux A to flux B is 80wt%:20wt%.

[0107] Example 5. The formula of Example 5 is shown in Table 1 and Table 2, and its preparation process is the same as that of Example 1. The difference is that the weight ratio of flux A to flux B is 90wt%:10wt%.

[0108] like Figure 1, i.e. Table 1, shows the formula of flux A in Examples 1 to 5. Figure 2 , i.e. Table 2, shows the formula of flux B in Examples 1 to 5.

[0109] In order to better illustrate the effect of the technical solution in this application, the following comparative examples are designed for effect comparison.

[0110] The formula of comparative example 1 is as follows Figure 3 As shown in Table 3, this formulation is identical to flux B in Example 1. The flux in Comparative Example 1, lacking the protection of a hydrophobic phase, suffers from a poor on-board life, as evidenced by a comparison of the printing performance of solder pastes prepared using the formulations in Example 1 and Comparative Example 1.

[0111] The formula of comparative example 2 is as follows Figure 3 As shown in Table 3, the difference between its formula and the flux of Example 2B is that malonic acid is dispersed in flux A in the example, while it is added to the only main flux in the comparative example.

[0112] Beneficial effects of the embodiment: Figure 4 Table 4 shows the results of the continuous printing test of the examples and comparative examples; the continuous printing test (viscosity unit Pa.S) conditions are: Malcom PCU02V @10rpm 25°C. The plate life of each sample is characterized by the viscosity value of continuous printing. Figure 4 As can be seen, Examples 1 to 5 not only have suitable initial viscosities, but also have smaller viscosity changes after 8 and 12 hours, indicating more stable viscosity performance. Comparative Example 1 can also test initial viscosity, viscosity after 8 and 12 hours, but the viscosity changes after 8 and 12 hours are greater than those of the Examples.

[0113] like Figure 5 Table 5 shows the thermal collapse test results for Examples and Comparative Examples. The non-bonding distances for Examples 1 to 5 are smaller than those for Comparative Example 1. The non-bonding distance for Comparative Example 2 was not measured because it could not be printed.

[0114] like Figure 6 Schematic diagram comparing solder joints before and after cleaning. Solder joint performance: Under nitrogen atmosphere, all sample solder joints were bright, free of tin beads, and exhibited good spreading and wetting. Water washability: All sample solder joint residues were completely cleaned.

[0115] like Figure 7 ,Schematic diagram of Raman spectroscopy analysis of residual materials before solder joint cleaning; Figure 8 This is a schematic diagram of Raman spectroscopy analysis of residual materials after solder joint cleaning. The residual materials are mainly rosin-like substances, with visible wavelengths of 2000-3000 cm -1 The Raman shift of the hydroxyl peak near 1450 cm-1 The characteristic peaks near the Raman shift disappeared after cleaning.

[0116] The above are merely embodiments of the present application and are not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made using the contents of the application specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A water-soluble solder paste, characterized in that: In terms of mass percentage, it includes 85-92% solder powder and 8-15% C flux; C flux includes 60-95% A flux and 40-5% B flux; The flux A comprises a solvent A, a rosin derivative A, a thixotropic agent A, an active agent A, and a surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydroabietin; the solvent A is a hydrophobic solvent; and the mass percentage of the rosin derivative A in the flux A is at least 29%; B flux includes B solvent, B rosin derivative, B activator, B thixotropic agent, and corrosion inhibitor; B solvent is a hydrophilic solvent; B rosin derivative includes polyoxyethylene rosin amine; Flux A and flux B are used to compound into flux C with an emulsion structure; flux C is used to mix with solder powder to form a water-soluble solder paste.

2. The water-soluble solder paste according to claim 1, wherein In terms of mass percentage, flux A includes: A solvent 30-50%, A rosin derivative 30-50%, A thixotropic agent 4-12%, A active agent 3-6%, surfactant 0.5-3%; Solvent A includes any one or more of hydrophobic organic alcohol ethers, alkanes, and rosin modifications.

3. The water-soluble solder paste according to claim 2, wherein: Include any one or more of the following technical features: TA1: The organic alcohol ether substances in solvent A include any one or more of diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tripropylene glycol n-butyl ether; TA2: The alkane in solvent A includes any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes; the boiling point of the alkane is 215-270°C; TA3: Rosin modifications in solvent A include methyl hydrogenated abietic acid; TA4: The activator A in the flux A includes an organic acid or a salt formed by an organic acid and an organic amine; the organic acid includes any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; the salt formed by an organic acid and an organic amine includes rosin amine; TA5: The thixotropic agent A of the flux A includes any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent; A6: The surfactant of flux A includes any one or more of Span 60 and Span 80.

4. The water-soluble solder paste according to claim 1, wherein Calculated by mass percentage, the B flux includes: 30-50% of B solvent, 30-50% of B rosin derivative, 4-18% of B active agent in total, 4-12% of B thixotropic agent, and 0.2-3% of corrosion inhibitor.

5. The water-soluble solder paste according to claim 1, wherein Include any one or more of the following technical features: TB1: Solvent B is an organic alcohol solvent containing hydroxyl groups; TB2: Solvent B is a hydroxyl-containing organic alcohol solvent; Solvent B includes any one or more of 2,5-dimethyl-2,5-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monooctyl ether, and glycerol; TB3: B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8-20; TB4:B thixotropic agent is a water-soluble amide thixotropic agent; TB5: B active agent is any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole; TB6: The corrosion inhibitor is any one or more of 2-bromo-1-methyl-1H-imidazole, ethylimidazole, and 1,2,3-benzotriazole.

6. The water-soluble solder paste according to claim 1, wherein Solder powder is an alloy composed of Sn as the base and a small amount of metal elements added. The solder powder includes any one or more of: Ag 0~4%, Cu 0~1%, Sb 0~10%, Bi 0~3%, Ni 0~0.15%, and In 0~10%; The particle size of solder powder is 2~25um, and the particle size models include T5 / T6 / T7 / T8.

7. A soldering flux A, characterized in that, The invention comprises a solvent A, a rosin derivative A, a thixotropic agent A, an active agent A, and a surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydroabietin; and the solvent A is a hydrophobic solvent; In terms of mass percentage, flux A includes: A solvent 30-50%, A rosin derivative 30-50%, A thixotropic agent 4-12%, A active agent 3-6%, surfactant 0.5-3%; Solvent A includes any one or more of hydrophobic organic alcohol ethers, alkanes, and rosin modifications; the boiling point of the alkanes is 215-270°C; Include any one or more of the following technical features: TA1: The organic alcohol ether substances in solvent A include any one or more of diethylene glycol diethyl ether, diethylene glycol dibutyl ether, and tripropylene glycol n-butyl ether; TA2: The alkanes in solvent A include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes; TA3: Rosin modifications in solvent A include methyl hydrogenated abietic acid; TA4: The activator A in the flux A includes an organic acid or a salt formed by an organic acid and an organic amine; the organic acid includes any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; the salt formed by an organic acid and an organic amine includes rosin amine; TA5: The thixotropic agent A of the flux A includes any one or more of a polyamide thixotropic agent and a hydrogenated castor oil thixotropic agent; The surfactant of TA6:A soldering flux includes any one or more of Span 60 and Span 80.

8. A B flux, characterized in that, B solvent, B rosin derivative, B active agent, B thixotropic agent, corrosion inhibitor; B solvent is a hydrophilic solvent; B rosin derivative includes polyoxyethylene rosin amine; In terms of mass percentage, B flux includes: B solvent 30-50%, rosin derivative B 30-50%, total amount of B active agent 4-18%, B thixotropic agent 4-12%; corrosion inhibitor 0.2-3%; Include any one or more of the following technical features: TB1: Solvent B is an organic alcohol solvent containing hydroxyl groups; TB2: Solvent B is a hydroxyl-containing organic alcohol solvent; Solvent B includes any one or more of 2,5-dimethyl-2,5-hexanediol, 2-ethyl-1,3-hexanediol, 2-methyl-2,4-pentanediol, diethylene glycol monobutyl ether, diethylene glycol monohexyl ether, diethylene glycol monooctyl ether, and glycerol; TB3: B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8-20; TB4:B thixotropic agent is a water-soluble amide thixotropic agent; TB5: B active agent is any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole; TB6: The corrosion inhibitor is any one or more of 2-bromo-1-methyl-1H-imidazole, ethylimidazole, and 1,2,3-benzotriazole.

9. A method for preparing flux A, characterized in that: Used to prepare the A soldering flux according to claim 7; Step A includes, Step A10: Mix and stir the rosin derivative A, solvent A, and active agent A, and heat at a temperature of 70° C. to 90° C.; Step A20: After mixing and heating to dissolve, lower the temperature to 60°C-75°C; Step A30: Add thixotropic agent A, apply high-speed stirring at a stirring speed between 2000-3000 rpm, and activate for 20-30 minutes; Step A40: Reduce the stirring speed, slowly stir and cool to 30°C; Step A50: Add surfactant at 30°C and stir evenly; Step A60: Cooling to room temperature, and then cooling and grinding; Step A70: After grinding, place in a 2-10℃ refrigerator for more than 10 hours and set aside.

10. A method for preparing B flux, characterized in that: Used to prepare the B flux according to claim 8; Step B includes, Step B10: Solvent B, rosin modification B, organic acid B, corrosion inhibitor and active agent B are mixed, stirred and heated, and slowly stirred and mixed to dissolve at a heating temperature of 60-80°C. Step B20: Lower the temperature to 50-70℃, add thixotropic agent B, and activate with high-speed stirring for 20-30 minutes. Step B30: After slowly stirring and cooling to 30°C, Step B40: Discharge the material, grind it, and place it in a 2-10°C refrigerator for more than 10 hours for later use.

11. A method for preparing a water-soluble solder paste, used for preparing the water-soluble solder paste according to any one of claims 1 to 6, characterized in that: include: Step A: Preparation of flux A, which is then refrigerated and placed; Step B: Preparation of B flux, which is then refrigerated and placed; Step C: Take out the refrigerated flux A and flux B, return them to room temperature, mix and emulsify them at a set mass percentage to obtain flux C; the set mass percentage is 60-95% flux A and 40-5% flux B; after emulsification, place in a refrigerator at 2-10°C for more than 10 hours for standby use; Step D: Take out the above-mentioned C flux and return it to room temperature. Then, add 85-92% by weight of solder powder to 8-15% by weight of C flux, and mix them evenly under vacuum at a stirring speed of 20-40 rpm to obtain a water-soluble solder paste.

12. The water-soluble solder paste according to claim 11, characterized in that: The step C comprises: Step C10: the mixing, stirring and emulsifying process includes: emulsifying at a stirring speed of more than 1000 rpm for 5-10 minutes to obtain C flux; Step C20: The C soldering flux obtained in step C10 is subjected to three-roller grinding and then placed in a 2-10°C refrigerator for more than 10 hours for later use.

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