Water-soluble solder paste and method for producing the same

Abstract

The water-soluble soldering paste and a preparation method thereof, comprising 85-92% of soldering powder and 8-15% of C fluxing agent by mass percentage; the C fluxing agent comprises 60-95% of A fluxing agent and 40-5% of B fluxing agent by mass percentage; the A fluxing agent comprises A solvent, A rosin derivative, A thixotropic agent, A active agent and a surfactant; the A rosin derivative comprises at least one of rosin amine and tetrahydroabietinol; the A solvent is a hydrophobic solvent; the mass percentage of the A rosin derivative in the A fluxing agent is at least 29%; the B fluxing agent comprises B solvent, B rosin derivative, B active agent, B thixotropic agent and corrosion inhibitor; the B solvent is a hydrophilic solvent; the B rosin derivative comprises polyoxyethylene rosin amine; the A fluxing agent and the B fluxing agent are used to compound the C fluxing agent in an emulsion structure; and the C fluxing agent is used to mix with the soldering powder to form the water-soluble soldering paste.

Description

Technical Field

[0001] This invention belongs to the field of solder paste technology, specifically relating to an ultrafine tin powder water-soluble lead-free solder paste and its preparation method. Background Technology

[0002] Solder paste (especially flexible solder paste) is mainly used in the manufacture of electronic circuits. Through the reflow process, the solder powder in the solder paste melts and reacts with the metal on the pins of components and the metal on the pads of the circuit board (or substrate) to form intermetallic compounds, thereby completing the mechanical and electrical connection between electronic components and the circuit board (or substrate), thus forming an effective circuit system.

[0003] Recently, due to the bottleneck of quantum effects encountered in the miniaturization of chip manufacturing processes, the feature size of CMOS has become difficult to further reduce around 1-2 nanometers. As a result, the design of electronic systems has shifted from System on Chip (SiP) to System in Packaging (SiP), which is known in the industry as evolving in a way that goes beyond Moore's Law, to meet the practical needs of scenarios such as edge computing in the Internet of Things (IoT). SiP integrates several different types of chips and electronic components (resistors, capacitors, etc.) into a single package module to form a complex and independent functional module.

[0004] In the field of electronic packaging, there is a demand for increasingly miniaturized chips and components in SiP modules to be soldered with ever-reducing pad sizes and pitches, while maintaining high reliability throughout the entire application lifecycle of the SiP. A classic application of this type of packaging is copper pillar bump technology, for which the pad size has now been reduced to 55 micrometers and the pad pitch to 35 micrometers. The residue from traditional no-clean solder is insufficient to meet reliability requirements, so the industry demands the use of water-soluble solder paste.

[0005] However, traditional cleaning methods for water-soluble solder paste (such as those disclosed in US Patent 5069730) before 1991 used water-soluble PEG (polyethylene glycol) polymers as carriers; PEG polymers are water-soluble polymers polymerized from ethylene oxide. Prior to 2016, the industry continued to use water-soluble polymers for water-soluble solder pastes, as illustrated by technologies disclosed in CN103909358A and CN105290650A. Due to the lack of rosin protection, the flux activator required was relatively large, resulting in poor stability of these products, primarily manifested in a rapid increase in solder paste viscosity during the printing process.

[0006] However, a common drawback of water-soluble solder paste currently on the market is that its printability is not good enough, and its stencil life on the printed circuit board is usually no more than 8 hours (after 8 hours of continuous printing, the viscosity of the paste increases too much to cause significant printing defects).

[0007] The technology disclosed in this invention provides a superior overall performance solution. It not only boasts excellent lifespan on printed circuit boards, with the viscosity increase remaining within 15% after 12 hours of continuous printing while maintaining normal printing quality, but also exhibits excellent resistance to thermal collapse and residual cleaning properties. This solder paste is primarily obtained by inhibiting solder powder corrosion. Summary of the Invention

[0008] To address the problem of insufficient lifespan of existing solder pastes on printed circuit boards, this invention provides a solder paste that alleviates solder powder corrosion, thereby achieving a longer lifespan on printed circuit boards, excellent overall performance, high solder joint brightness, easy residue cleaning, and smooth solder application.

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

[0010] It can be that, by mass percentage, flux A includes 30-50% solvent A, 30-50% rosin derivative A, 4-12% thixotropic agent A, 3-6% activator A, and 0.5-3% surfactant; solvent A includes any one or more of hydrophobic organic alcohol ethers, alkanes, and rosin modifiers.

[0011] It can be that 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.

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

[0013] Yes, the rosin modifier in solvent A includes methyl hydrogenated rosinate.

[0014] It is possible that the A activator in flux A includes organic acids or salts formed by organic acids and organic amines; organic acids include any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; and salts formed by organic acids and organic amines include rosin amine.

[0015] It is possible that the thixotropic agent A in flux A includes any one or more of polyamide thixotropic agents and hydrogenated castor oil thixotropic agents.

[0016] It is possible that the surfactant of flux A includes any one or more of Span 60 and Span 80.

[0017] Yes, by mass percentage, flux B includes: 30-50% solvent B, 30-50% rosin derivative B, 4-18% total activator B, 4-12% thixotropic agent B, and 0.2-3% corrosion inhibitor.

[0018] Yes, solvent B is a hydroxyl-containing organic alcohol solvent.

[0019] Solvent B can 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 could be B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20.

[0021] Yes, it is possible. Thixotropic agent B is a water-soluble amide-based thixotropic agent.

[0022] It can be that the active agent B is any one or more of succinic acid, adipic acid, phenyl succinic acid, glutaric acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole.

[0023] It can be any one or more of 2-bromo-1-methyl-1H-imidazolium, ethylimidazolium, and 1,2,3-benzotriazole.

[0024] Yes, solder powder is an alloy composed of Sn as the matrix with a small amount of metal elements added. The solder powder includes any one or more of the following: 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 2~25um, and the particle size type includes T5 / T6 / T7 / T8.

[0025] The technical solution of this 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 activator A, and a surfactant; the rosin derivative A includes at least one of rosin amine and tetrahydrorosinol; the solvent A is a hydrophobic solvent; by mass percentage, flux A comprises 30-50% solvent A, 30-50% rosin derivative A, 4-12% thixotropic agent A, 3-6% activator A, and 0.5-3% surfactant; the solvent A includes hydrophobic organic alcohol ethers, alkanes, and rosin modifiers; the boiling point of the alkanes is 215-270℃.

[0026] It can be that 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.

[0027] It can be that the alkane in solvent A includes any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes.

[0028] Yes, the rosin modifier in solvent A includes methyl hydrogenated rosinate.

[0029] It is possible that the A activator in flux A includes organic acids or salts formed by organic acids and organic amines; organic acids include any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; and salts formed by organic acids and organic amines include rosin amine.

[0030] It is possible that the thixotropic agent A in flux A includes any one or more of polyamide thixotropic agents and hydrogenated castor oil thixotropic agents.

[0031] It is possible that the surfactant of flux A includes any one or more of Span 60 and Span 80.

[0032] The technical solution to the above-mentioned technical problem in this application can also be 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; by mass percentage, B flux includes: B solvent 30-50%, B rosin derivative 30-50%, B activator total amount 4-18%, B thixotropic agent 4-12%, and corrosion inhibitor 0.2-3%.

[0033] Yes, solvent B is a hydroxyl-containing organic alcohol solvent.

[0034] Solvent B can 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 could be B rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20.

[0036] Yes, it is possible. Thixotropic agent B is a water-soluble amide-based thixotropic agent.

[0037] It can be that the active agent B is any one or more of succinic acid, adipic acid, phenyl succinic acid, glutaric acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole.

[0038] It can be any one or more of 2-bromo-1-methyl-1H-imidazolium, ethylimidazolium, and 1,2,3-benzotriazole.

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

[0040] The technical solution of this application to solve the above-mentioned technical problems can also be a method for preparing flux B, used to prepare the above-mentioned flux B; step B includes: step B10: solvent B, rosin modifier B, organic acid B, corrosion inhibitor and activator B are mixed, stirred and heated, and slowly stirred and mixed to dissolve at a heating temperature of 60-80℃; step B20: the temperature is reduced to 50-70℃ and thixotropic agent B is added, and activated by high-speed stirring for 20-30 minutes; step B30: after slow stirring and cooling to 30℃; step B40: the material is discharged, ground and placed in a refrigerator at 2-10℃ for more than 10 hours for later use.

[0041] The technical solution of this application to solve the above-mentioned technical problems can also be a method for preparing water-soluble solder paste, which includes step A: preparation of flux A, and refrigeration after preparation; step B: preparation of flux B, and refrigeration after preparation; step C: taking out the refrigerated flux A and flux B, warming them to room temperature, and mixing and emulsifying them at a set mass percentage to obtain flux C; wherein the set mass percentage is 60-95% flux A and 40-5% flux B; after emulsification, it is placed in a refrigerator at 2-10℃ and left for more than 10 hours for later use; step D: taking out the above flux C, warming it to room temperature, and then adding solder powder at a weight percentage of 85-92% to 8-15% of flux C, and mixing it evenly under vacuum at a stirring speed of 20-40 rpm to obtain water-soluble solder paste.

[0042] Step C includes: Step C10: The mixing and emulsification process includes: emulsifying at a stirring speed of 1000 rpm or higher for 5-10 minutes to obtain flux C; Step C20: After the flux C obtained in step C10 is milled by three rollers, it is placed in a refrigerator at 2-10℃ and left for more than 10 hours for later use.

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

[0044] The beneficial effects of this application are: by reacting strong organic acids with organic amines to form salts, and then confining them in a continuous phase of hydrophobic flux A with low hydrogen bond density, the corrosion reaction caused by the contact between organic acids and solder powder during the storage and printing of solder paste is prevented and mitigated, thus maintaining the stability of the solder paste.

[0045] The beneficial effects of this application are as follows: During reflow, the increased temperature leads to solvent evaporation and enhanced molecular thermal motion, increasing the chance of organic acids migrating to the surface of the solder powder. This removes the oxide film from the solder powder surface and promotes cold soldering between the solder particles. The presence of abundant rosin-like substances and hydrophobic solvents in the continuous phase effectively prevents the re-oxidation and corrosion of the solder powder by moisture carried in the heating gas during the preheating stage of reflow soldering, thus ensuring excellent resistance to thermal collapse.

[0046] The beneficial effects of this application are as follows: Rosin amine in flux A, as an amine group in the rosin amine molecule, has weak basicity and can react with metal oxides (such as SnO, SnO2) at high soldering temperatures to generate 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, inhibiting the chemical reaction rate between organic acids and solder powder during storage and use, thus improving the stability of the solder paste. Intermediate products generated during the soldering process can cover the surface of the solder joint metal, preventing the formation of new oxides at high temperatures.

[0047] The beneficial effects of this application are: In flux A, tetrahydrorosin alcohol acts as a thickener, which adjusts the rheological properties of the flux. Its hydroxyl group (-OH) can weakly coordinate with metal oxides, assisting rosin amine in playing a deoxidizing role. These two substances form a uniform covering film on the surface of solder paste and solder joints, which isolates air and moisture, and provides thermal stability and physical film-forming properties.

[0048] The beneficial effects of this application are as follows: The hydrophilicity of polyoxyethylene rosin amine in flux B gives it excellent surface activity, which can significantly reduce the contact angle between the solder and the metal surface, promote the spread of solder on PCB pads and leads, and reduce cold solder joints and poor solder joints. The amine groups in the structure also participate in the deoxide removal function at high temperatures. In addition, the flexible structure of the polyoxyethylene chain can increase the viscoelasticity of the flux, so that the solder paste maintains good thixotropy and anti-collapse properties during screen printing or stencil printing, avoids excessive diffusion of solder paste or blockage of stencil openings, adjusts the rheological properties and release properties of the solder paste, and improves printing accuracy and efficiency.

[0049] The beneficial effects of this application are as follows: The composition of flux A, with solvent A and rosin derivative as the main components, provides hydrophobic properties, making flux A a continuous phase occupying a larger mass percentage. This results in better hydrophobicity, i.e., water-blocking effect, thereby increasing the lifespan of the solder paste on the printed circuit board. Simultaneously, the components in flux B give the solder paste excellent printability and release properties. This maintains excellent deoxidizing ability, enhances wettability, and improves the printability and stability of the solder paste, making it a key component of high-performance water-soluble solder paste, suitable for high-density, high-reliability electronic packaging applications.

[0050] The beneficial effects of this application are as follows: The organic alcohol ethers in solvent A are hydrophobic and contain amphiphilic structures with ether bonds and hydroxyl groups, which reduce the surface tension of the solder, enhance its wettability to the metal, ensure uniform solder spread, and reduce cold solder joints. During the soldering preheating stage, some of the alcohol ethers volatilize, forming an inert gas to protect the solder joint surface and prevent high-temperature oxidation. The remaining portion can combine with water after soldering to form a soluble complex, which is easy to clean.

[0051] The beneficial effects of this application are: the alkane in solvent A has a long chain, which, as a thickener, can adjust the viscosity of the solder paste and prevent printing collapse. At high soldering temperatures, it can be converted into a low surface energy protective film, reducing solder oxidation. During the use of solder paste, it covers the metal surface, forming a hydrophobic temporary protective film, delaying oxidation, and preventing the absorption of moisture and oxygen from the environment.

[0052] The beneficial effects of this application are: the rosin modifier in solvent A, such as methyl hydrogenated rosin, contains a carboxylic acid group in its structure that can react with metal oxides to generate soluble salts. Its saturated structure can reduce high-temperature carbonization, and the residue is more easily cleaned by water saponification after methylation, thus increasing the post-weld cleaning performance.

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

[0054] The beneficial effects of this application are: A thixotropic agent includes any one or more of polyamide thixotropic agents and hydrogenated castor oil thixotropic agents, which provide rheological properties for solder paste, maintain good thixotropy, and enable the solder paste to have good shape retention ability.

[0055] The beneficial effects of this application are: the surfactant of flux A, including any one or more of Span 60 and Span 80, has unique hydrophobic and emulsifying stabilizing effects, which can regulate the rheological properties of the paste, reduce interfacial tension, stabilize the emulsion structure, and significantly improve the printing accuracy, soldering reliability and residue controllability of solder paste.

[0056] The beneficial effects of this application are: The flux B contains hydroxyl-containing organic alcohol solvents. These hydroxyl groups can form hydrogen bonds or other interactions with certain active components in the flux. During heating, these interactions are disrupted, thereby promoting the release of active components and enhancing the flux's activity. Simultaneously, it can adjust the rheological properties of the solder paste, control viscosity, dissolve other solid particles in the flux, reduce the viscosity of post-soldering residues, and facilitate cleaning.

[0057] The beneficial effects of this application are: Agent B is any one or more of succinic acid, adipic acid, phenylsuccinic acid, glutaric acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazolium; these substances provide soldering activity to the solder paste at high temperatures, maintain the continuous activity of the solder paste throughout the soldering process, and at the same time reduce the surface tension of the solder, making it easier to spread and wet, and achieving metallurgical connection between the solder and the pad.

[0058] The beneficial effects of this application are: the surfactant B includes 2-bromo-1-methyl-1H-imidazole, and the corrosion inhibitor includes 2-bromo-1-methyl-1H-imidazole, so that the same substance can act as both a surfactant and a corrosion inhibitor.

[0059] The beneficial effects of this 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 this application has good adaptability and is suitable for a variety of particle sizes.

[0060] The beneficial effects of this application are: A. The flux is individually packaged and prepared, facilitating storage and use. B. The flux is individually packaged and prepared, facilitating storage and use.

[0061] The beneficial effects of this application are: flux C is ready to use and can be prepared immediately. The ratio of flux A and flux B can be adjusted according to the chemical composition, particle size and oxide content of the solder powder to prepare flux C that matches different solder powders and is suitable for different process windows and characteristic requirements. Attached Figure Description

[0062] Figure 1 Table 1 shows the formulations 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 Table 3 shows the formulations for the comparative example; Figure 4 Table 4 shows the continuous printing test results for the 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 of the solder joints before and after cleaning; Figure 7 This is a schematic diagram of Raman spectroscopy analysis of residual substances before solder joint cleaning; Figure 8 This is a schematic diagram of Raman spectroscopy analysis of residual substances after solder joint cleaning. Detailed Implementation

[0063] The contents of this application will be further described in detail below with reference to the accompanying drawings.

[0064] Definitions: Rosin is a natural product extracted from the resin of pine trees. Its main component is rosin acid. It has good chemical stability and is a complex mixture, but its activity is low, making it suitable for welding scenarios where the flux activity requirements are not high.

[0065] Rosin Modifiers: Rosin modifiers are typically produced by physically mixing or mildly chemically reacting rosin (such as hydrogenation, esterification, and polymerization) to retain its main structure while improving its properties. These modifications aim to enhance its performance or impart new functional compounds. Modification methods include esterification, hydrogenation, and polymerization. For example, hydrogenated rosin methyl ester in this patent is suitable for applications requiring high soldering temperatures and quality. Rosin Derivatives: These are compounds produced through deep chemical processing of rosin, resulting in compounds with different functional groups or structures. They typically possess different chemical properties and applications than rosin, exhibiting enhanced activity and are suitable for soldering applications requiring high flux activity and reliability.

[0066] Rosin derivatives, on the other hand, undergo more complex chemical reactions to alter their structure, generating new compounds with different functional groups or molecular skeletons. Examples include polyoxyethylene rosin amine, polyoxyethylene polymerized rosin, rosin amine, and tetrahydrorosin alcohol in this patent.

[0067] HLB value: The full English name is Hydrophobic-Lipophile Balance; it is a quantitative indicator of the balance between the size and strength of the hydrophilic and lipophilic groups in a surfactant molecule. A higher value indicates stronger hydrophilicity, and vice versa.

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

[0069] Solvent A is solvent A, rosin derivative A is rosin derivative A, thixotropic agent A is thixotropic agent A, and surfactant A is surfactant 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 solder powder is coated with a tin oxide film, defects are still prevalent in this oxide layer, allowing tin atoms to remain exposed to the flux. This leads to corrosion of the solder paste during storage and printing, a process that can be described by the following reaction: 4Sn + 2H₂O + 4H₂O + +O2=2H2+2Sn(OH - )2+2Sn 2+ The reaction formula above shows that 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 packaging density is increasing. The waiting time for circuit boards or substrates after solder paste printing before reflow soldering is also increasing, placing higher demands on the stability of the solder paste. Solder paste absorbs water during the printing and rolling process and while stationary on the board; water absorption exacerbates the corrosion reaction.

[0073] The solder paste of this application has a hydrophobic, continuous phase A flux on its outer surface during printing and storage, which effectively blocks moisture from penetrating the paste and inhibits solder powder corrosion caused by the above mechanism, thereby extending the solder paste's lifespan on the printed circuit board. This invention effectively mitigates the water absorption effect of solder paste during printing and during its stationary state on the board. Therefore, this invention improves the stability of the solder paste by limiting the flux's absorption of moisture.

[0074] This invention provides a water-soluble solder paste, comprising, by weight percentage, 85-92% solder powder and 8-15% flux C; flux C comprises, by weight percentage, 60-95% flux A and 40-5% flux B; flux A comprises solvent A, rosin derivative A, thixotropic agent A, activator A, and surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydrorosinol; solvent A is a hydrophobic solvent; the rosin derivative A constitutes at least 29% of flux A by weight; flux B comprises solvent B, rosin derivative B, activator B, thixotropic agent B, and corrosion inhibitor; solvent B is a hydrophilic solvent; the rosin derivative B comprises polyoxyethylene rosin amine; flux A and flux B are used to compound flux C into an emulsion structure; flux C is used to mix with solder powder to form a water-soluble solder paste.

[0075] The solder powder content is between 85-92 wt%. The solder powder composition is an alloy with Sn as the base and small amounts of metallic elements such as 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 conforms to the particle size requirements of #6, #7, and #8 powders specified in the electronic industry standard SJ / T11391-2019 Solder Alloy Powder for Soldering Electronic Products or IPCJ-STD-005A-2012. Particle size grades include T5 / T6 / T7 / T8. T5 indicates a particle diameter range of 15-25 μm, T6 indicates a particle diameter range of 5-15 μm, T7 indicates a particle diameter range of 2-11 μm, and T8 indicates a particle diameter range of 2-8 μm.

[0076] Flux C consists of two parts: Flux A and Flux B. By mass percentage, Flux A accounts for 60-95% and Flux B accounts for 40-5%. Flux A and Flux B are compounded in the above proportions and, under high-speed stirring conditions, form an emulsion structure in which Flux A is the continuous phase and Flux B is the dispersed phase.

[0077] Flux A comprises solvent A, rosin derivative A, thixotropic agent A, activator A, and surfactant; the rosin derivative A includes at least one of rosin amine and tetrahydrorosinol; solvent A is a hydrophobic solvent. By mass percentage, flux A comprises 30-50% solvent A, 30-50% rosin derivative A, 4-12% thixotropic agent A, 3-6% activator A, and 0.5-3% surfactant.

[0078] Solvent A includes hydrophobic organic alcohol ethers, alkanes, and rosin modifiers; the boiling point of the alkanes is 215-270℃. Solvent A is a solvent with low hydrogen bond density, weak polarity, and hydrophobicity.

[0079] 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. Alkanes in solvent A include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes. Rosin modifiers in solvent A include hydrogenated rosin acid methyl ester. Organic alcohol ethers have a low density of functional groups capable of forming hydrogen bonds.

[0080] The activator A in flux A includes organic acids or salts formed from organic acids and organic amines; organic acids include any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; salts formed from organic acids and organic amines include rosin amine. Activator A is an organic acid activator.

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

[0082] Flux A contains surfactants including any one or more of Span 60 and Span 80. The surfactants are low HLB values ​​such as Span 60 and Span 80. These are low HLB surfactants with an HLB value between 3 and 8.

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

[0084] The B solvent of flux B mainly consists of molecules rich in hydrogen bonds, and the organic acids in the B solvent are dispersed in flux A.

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

[0086] Solvent B is an organic alcohol with high hydrogen bond density, strong polarity, and high water solubility; solvent B is an organic alcohol solvent containing hydroxyl groups; surfactant B includes: organic acids and halogen total surfactants.

[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 HLB values ​​between 8 and 20. The rosin derivatives used in B flux are high-HLB-value polyoxyethylene rosin amines, which are polyoxyethylene polymerized rosin; high HLB value refers to compounds with HLB values ​​in the range of 8-20.

[0089] Thixotropic agent B is a water-soluble amide thixotropic agent; it uses RE-103, a water-soluble polyamide thixotropic agent from Kyoei Chemical Co., Ltd. of Japan.

[0090] The active agent B is any one or more of succinic acid, adipic acid, phenyl succinic acid, glutaric acid, pimelic acid, octanoic 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-imidazolium, ethylimidazolium, and 1,2,3-benzotriazole. 2-bromo-1-methyl-1H-imidazolium is also a halogen activator. The surface corrosion inhibitor used in flux B of the water-soluble solder paste is 2-bromo-1-methyl-1H-imidazolium, which is also a halogen activator.

[0092] The water-soluble solder paste of this application contains two types of solvents. One type is a low-hydrogen-bond-density, weakly polar, hydrophobic organic solvent located in the continuous phase A, i.e., flux A; the other type is a high-hydrogen-bond-density, strongly polar, relatively hydrophilic organic solvent located in the dispersed phase B, i.e., flux B. Through the flux manufacturing process, an emulsion structure is obtained. Flux A, dominated by the hydrophobic solvent, forms the continuous phase, while flux B, dominated by the hydrophilic solvent, constitutes the dispersed phase.

[0093] This invention uses a strong organic acid to react with an organic amine to form a salt, which is then confined within a hydrophobic A flux continuous phase with low hydrogen bond density. This prevents and mitigates corrosion caused by contact between the organic acid and solder powder during the storage and printing of solder paste, thus maintaining the stability of the solder paste.

[0094] During reflow, the increased temperature leads to solvent evaporation and enhanced molecular thermal motion, increasing the chance of organic acids migrating to the solder powder surface. This removes the oxide film from the solder powder surface and promotes cold soldering between the solder particles. The presence of abundant rosin-like substances and hydrophobic solvents in the continuous phase effectively prevents the re-oxidation and corrosion of the solder powder by moisture carried in the heating gas during the preheating stage of reflow soldering, thus ensuring excellent resistance to thermal collapse.

[0095] To leverage the continuous phase characteristics and anti-slump properties of flux A, its dosage should be 60-95% of the total flux. To maintain the overall activity of the solder paste, the rosin content in flux A must be maintained at over 29 wt%. Due to its hydrophobic properties, no corrosion inhibitor is needed in the continuous phase flux A. To ensure the overall activity, rheological properties, printability, and adhesion of the solder paste, a conventional flux B with high hydrogen bond density and high polarity is still required as the dispersed phase, accounting for 5-40 wt% of the total flux.

[0096] The emulsion structure of this invention's flux effectively blocks the absorption and diffusion of moisture from the air during solder paste printing, board resting, and conventional storage, thereby inhibiting solder powder corrosion and improving board life. The hydrophobic properties of this continuous phase effectively suppress the corrosive effects of strong organic acids without hindering its cold soldering function in the early stages of reflow soldering, thus simultaneously achieving excellent board life and resistance to thermal collapse. The materials used in this technology also give the product a wide active process window and excellent residual cleaning performance.

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

[0098] (2) Preparation process of flux B: Add hydroxyl-containing solvent B, rosin modifier B, B activator and corrosion inhibitor B to a temperature-controlled heating device, stir slowly and mix to dissolve under the condition of 60-80℃, reduce the system temperature to 40-55℃ and add thixotropic agent, stir at high speed for 20-30 minutes (speed between 2000-3000rpm), stir slowly and cool to 30℃, discharge the material, grind it and put it in a refrigerator at 2-10℃ for more than 10 hours for later use.

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

[0100] (4) Take out the C flux obtained above, bring it back to room temperature, and then add solder powder with a weight percentage of 85-92wt% to the above C flux 8-15wt%. Mix it evenly under 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 solvent A) and surfactants with low HLB values ​​can promote the flux structure of the emulsion. The presence of a hydrophobic continuous phase inhibits the absorption of moisture. Since the presence of moisture is necessary for the electrochemical corrosion of solder powder, this invention can significantly reduce the corrosion of solder powder, thereby effectively preventing the increase of solder paste viscosity during continuous printing and thus improving the life of the printed circuit board.

[0102] This invention employs a strong acid in the hydrophobic continuous phase A, i.e., solvent A, which can effectively suppress its corrosive effect while exerting its cold soldering effect in the early stage of reflow, thus enabling the solder paste to simultaneously achieve excellent solder life and anti-heat collapse performance. The continuous phase A, i.e., solvent A, contains a large amount of rosin-like substances, and the dispersed phase B, i.e. solvent B, contains high HLB rosin-like surfactants, which can effectively protect the solder powder during reflow, thereby ensuring activity and residual cleaning performance. Solvent B, containing halogenated corrosion inhibitors, not only provides protection for the tin powder but also enhances 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 tetrahydrorosin alcohol, then add 15 parts of a mixture of rosin amine and 5 parts of malonic acid and stir until uniform. Then, when the system temperature is reduced to 65°C, add 7 parts of polyamide thixotropic agent 6650, stir at high speed for 20-30 minutes, then slowly stir and cool to 30°C, add 1 part of Span 60, mix evenly, discharge the material, and after cooling to room temperature, grind it. Then, put it in a refrigerator at 2-10°C and leave it for more than 10 hours for later use. At 70℃, 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-imidazolium were added to an oil bath-insulated container. The mixture was slowly stirred and dissolved. Then, the system temperature was lowered to 55℃, and 6.7 parts of water-soluble polyamide thixotropic agent RE-103 were added. After high-speed stirring and activation for 20-30 minutes, the mixture was slowly stirred and cooled to 30℃. The mixture was then discharged, ground, and placed in a refrigerator at 2-10℃ for at least 10 hours for later use.

[0104] Remove the refrigerated flux A and flux B and allow them to warm to room temperature. Then, mix them in a ratio of 65wt%:35wt% and emulsify at a high speed of 1100rpm for 5 minutes to obtain flux C. Place flux C in a refrigerator at 2-10℃ for at least 10 hours for later use. Take out the C flux obtained above, let it return to room temperature, and then add 87wt% Sn96.5Ag3Cu0.5T7 solder powder to 13wt% of the above C flux. Mix evenly under vacuum at a stirring speed of 25rpm to obtain solder paste.

[0105] Example 2. The formulation of Example 2 is shown in Table 1 and Table 2. Its preparation 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 formulation of Example 3 is shown in Tables 1 and 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 70wt%:30wt%. Example 4. The formulation of Example 4 is shown in Tables 1 and 2. Its preparation 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 formulation of Example 5 is shown in Tables 1 and 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 1Table 1 shows the formulations of flux A in Examples 1 through 5. Figure 2 Table 2 shows the formulations of flux B in Examples 1 to 5.

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

[0110] The formula for Comparative Example 1 is as follows Figure 3 As shown in Table 3, this formulation is exactly the same as flux B in Example 1. The flux in Comparative Example 1 has a poorer lifespan on the board due to the lack of hydrophobic phase protection, which can be seen from the comparison of the printing performance of the solder pastes prepared with the formulations of Example 1 and Comparative Example 1.

[0111] The formula for Comparative Example 2 is as follows Figure 3 As shown in Table 3, the difference between its formulation and the flux in 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 embodiments: such as Figure 4 Table 4 shows the continuous printing test results for the examples and comparative examples; the continuous printing test (viscosity unit Pa.S) conditions were: Malcom PCU02V @10rpm 25℃. The on-plate life of each sample was 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 exhibit smaller viscosity changes after 8 and 12 hours, demonstrating more stable viscosity performance. Comparative Example 1 can also test the initial viscosity, as well as the viscosity after 8 and 12 hours, but the viscosity changes after 8 and 12 hours are relatively larger compared to the Examples.

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

[0114] like Figure 6 A comparative diagram of solder joints before and after cleaning; Solder joint performance: Under nitrogen atmosphere, all sample solder joints tested were bright, without solder beads, and spread well. Water washability: Residue near the solder joints of all sample tests could be completely cleaned.

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

[0116] The above are merely embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of the application specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A water-soluble solder paste, characterized in that, By weight percentage, it includes 85-92% solder powder and 8-15% flux (C). Flux C consists of 60-95% flux A and 5-40% flux B. Flux A comprises solvent A, rosin derivative A, thixotropic agent A, activator A, and surfactant; the rosin derivative A comprises at least one of rosin amine and tetrahydrorosinol; solvent A is a hydrophobic solvent; the rosin derivative A accounts for at least 29% of the mass percentage of flux A. Flux B includes solvent B, rosin derivatives B, activator B, thixotropic agent B, and corrosion inhibitor; solvent B is a hydrophilic solvent; rosin derivatives B include polyoxyethylene rosin amine. Flux A and Flux B are used to formulate Flux C, which has an emulsion structure; Flux C is used to mix with solder powder to form water-soluble solder paste. Flux A, by weight percentage, comprises, Solvent A 30-50%, Rosin derivative A 30-50%, Thixotropic agent A 4-12%, Surfactant A 3-6%, Surfactant 0.5-3%; Solvent A includes any one or more of the following: hydrophobic organic alcohol ethers, alkanes, and rosin modifiers; By mass percentage, flux B includes: 30-50% solvent B, 30-50% rosin derivatives B, 4-18% total amount of activator B, 4-12% thixotropic agent B, and 0.2-3% corrosion inhibitor.

2. The water-soluble solder paste according to claim 1, characterized in that, Includes one or more of the following technical features: TA1: 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; 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℃; TA3: The rosin modifiers in solvent A include methyl hydrogenated rosinate; TA4: The A activator in flux A includes organic acids or salts formed by organic acids and organic amines; organic acids include any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; salts formed by organic acids and organic amines include rosin amine; TA5: The thixotropic agent in flux A includes any one or more of polyamide thixotropic agents and hydrogenated castor oil thixotropic agents; TA6:A flux contains surfactants including any one or more of Span 60 and Span 80.

3. The water-soluble solder paste according to claim 1, characterized in that, Includes one or more of the following technical features: TB1: The 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. TB2:B is a rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20. TB3:B thixotropic agent is a water-soluble amide-based thixotropic agent; TB4:B activator is any one or more of succinic acid, adipic acid, phenyl succinic acid, glutaric acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole; TB5: The corrosion inhibitor is any one or more of 2-bromo-1-methyl-1H-imidazolium, ethylimidazolium, and 1,2,3-benzotriazole.

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

5. A flux A, characterized in that, The composition includes solvent A, rosin derivative A, thixotropic agent A, surfactant A, and surfactant; the rosin derivative A includes at least one of rosin amine and tetrahydrorosinol; solvent A is a hydrophobic solvent. Flux A, by weight percentage, comprises, Solvent A 30-50%, Rosin derivative A 30-50%, Thixotropic agent A 4-12%, Surfactant A 3-6%, Surfactant 0.5-3%; Solvent A includes any one or more of hydrophobic organic alcohol ethers, alkanes, and rosin modifiers; the boiling point of the alkanes is 215-270℃. Includes one or more of the following technical features: TA1: 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; TA2: The alkanes in solvent A include any one or more of dodecane, tridecane, tetradecane, pentadecane, and cycloalkanes; TA3: The rosin modifiers in solvent A include methyl hydrogenated rosinate; TA4: The A activator in flux A includes organic acids or salts formed by organic acids and organic amines; organic acids include any one or more of oxalic acid, malonic acid, malic acid, and tartaric acid; salts formed by organic acids and organic amines include rosin amine; TA5: The thixotropic agent in flux A includes any one or more of polyamide thixotropic agents and hydrogenated castor oil thixotropic agents; TA6:A flux contains surfactants including any one or more of Span 60 and Span 80.

6. A type of flux B, characterized in that, include: B solvent, B rosin derivative, B surfactant, B thixotropic agent, corrosion inhibitor; Solvent B is a hydrophilic solvent; rosin derivatives B include polyoxyethylene rosin amine; By weight percentage, flux B comprises: 30-50% solvent B, 30-50% rosin derivative B, 4-18% total activator B, 4-12% thixotropic agent B, and 0.2-3% corrosion inhibitor. Includes one or more of the following technical features: TB1: The 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. TB2:B is a rosin derivative, which is a polyoxyethylene rosin amine with an HLB value between 8 and 20. TB3:B thixotropic agent is a water-soluble amide-based thixotropic agent; TB4:B activator is any one or more of succinic acid, adipic acid, phenyl succinic acid, glutaric acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, cyclohexylamine hydrobromide, dibromobutenediol, and 2-bromo-1-methyl-1H-imidazole; TB5: The corrosion inhibitor is any one or more of 2-bromo-1-methyl-1H-imidazolium, ethylimidazolium, and 1,2,3-benzotriazole.

7. A method for preparing flux A, characterized in that, Used to prepare the flux A of claim 5; Step A includes, Step A10: Mix and stir A rosin derivative, A solvent, and A surfactant, and heat at a temperature of 70℃-90℃; Step A20: After mixing and heating to dissolve, lower the temperature to 60℃-75℃; Step A30: Add thixotropic agent A, apply high-speed stirring at a speed between 2000-3000 rpm, and activate for 20-30 minutes; Step A40: Reduce the stirring speed and slowly stir to cool to 30°C; Step A50: Add the surfactant at 30℃ and stir until homogeneous; Step A60: Cool to room temperature, then cool again before grinding; Step A70: After grinding, place in a refrigerator at 2-10℃ for at least 10 hours before use.

8. A method for preparing flux B, characterized in that, Used to prepare the B flux as described in claim 6; Step B includes, Step B10: Mix solvent B, rosin derivative B, corrosion inhibitor and surfactant B, stir and heat at 60-80℃, stirring slowly to dissolve. Step B20: Lower the temperature to 50-70℃, add thixotropic agent B, and activate by 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 then place it in a refrigerator at 2-10℃ for more than 10 hours for later use.

9. A method for preparing water-soluble solder paste, used in the preparation of the water-soluble solder paste according to any one of claims 1 to 4, characterized in that, include: Step A: Preparation of flux A, followed by refrigeration; Step B: Preparation of flux B, followed by refrigeration. Step C: Take out the refrigerated flux A and flux B, let them warm to room temperature, and mix and emulsify them according to the set mass percentage to obtain flux C; wherein the set mass percentage is 60-95% flux A and 5-40% flux B; after emulsification, put it in a refrigerator at 2-10℃ and leave it for more than 10 hours for later use. Step D: Take out the above C flux, let it return to room temperature, and then add solder powder with a weight percentage of 85-92% to 8-15% of C flux. Mix evenly under vacuum at a stirring speed of 20-40 rpm to obtain water-soluble solder paste.

10. The water-soluble solder paste according to claim 9, characterized in that, Step C includes, Step C10: The mixing and emulsification process includes emulsifying for 5-10 minutes at a stirring speed of 1000 rpm or higher to obtain flux C; Step C20: After the flux C obtained in step C10 is subjected to three-roll milling, it is placed in a refrigerator at 2-10℃ and left for more than 10 hours for later use.

Images