A water-washing soldering flux for soldering paste, a water-washing soldering paste and application thereof

Abstract

The present application belongs to the technical field of soldering paste, and particularly relates to a water-washing soldering paste flux, a water-washing soldering paste and application thereof. The water-washing soldering paste flux comprises the following components in percentage by mass: modified epoxy resin 15-30%, non-ionic surfactant 0.5-3%, organic amine 0.5-2%, antioxidant 1-3%, composite organic acid 3-8%, composite thixotropic agent 1-4%, anti-sedimentation agent 1-2%, and the rest is organic solvent. The water-washing soldering paste flux of the present application, through the synergistic cooperation of the modified epoxy resin, the composite thixotropic agent and the anti-sedimentation agent, enables the water-washing soldering paste to simultaneously have excellent welding performance, water-washability, anti-collapse property and anti-settling property, realizes thorough cleaning of post-welding residues, ensures the precision of fine-pitch printing and the stability of product storage, and ensures the whole-process reliability of the product from storage to use.

Description

Technical Field

[0001] This invention relates to the field of solder paste, and more specifically, to a flux for water-washable solder paste, water-washable solder paste, and its application. Background Technology

[0002] In surface mount technology (SMT), solder paste is a key material, composed of solder alloy powder and flux. The core function of flux is to remove oxides from the surfaces of pads and component leads during reflow soldering, ensuring a smooth soldering process and reliable solder joints.

[0003] Traditional solder pastes typically use rosin or its derivatives as film-forming agents and fluxing components. While rosin-based fluxes possess some fluxing activity, their post-soldering residues are highly hydrophobic, requiring cleaning with organic solvents (such as chlorofluorocarbons or alcohols). This process is not only costly and hazardous but also environmentally unfriendly. With increasingly stringent environmental regulations and the trend towards higher density and reliability in electronic products, there is an urgent market demand for water-based cleaning solder pastes.

[0004] Existing water-washable solder paste fluxes often fail to simultaneously solve the following problems:

[0005] 1. The residue after welding is difficult to clean thoroughly with pure water.

[0006] 2. Collapse and bridging are prone to occur during printing and reflow.

[0007] 3. During storage, high-density solder alloy powder is prone to settling and clumping in the flux system, resulting in poor product uniformity, short service life, and affecting printing weight and welding reliability.

[0008] Chinese patent CN119525815B discloses a high-precision nano solder paste and its preparation method, comprising solder powder, flux, glucomannan, and polyethylene glycol-modified aliphatic epoxy resin. The weight ratio of the solder powder, flux, glucomannan, and polyethylene glycol-modified aliphatic epoxy resin is 85~95:10~15:1~3:1~3. The flux, by mass percentage, comprises: 6.3%~10% activator, 0.25%~1.5% surfactant, 4%~10% thixotropic agent, 1%~5% antioxidant, 1%~3% corrosion inhibitor, 1%~5% film-forming agent, with the balance being solvent. This solder paste still suffers from poor washability and poor anti-collapse properties. Summary of the Invention

[0009] The purpose of this invention is to overcome the defects and shortcomings of existing solder pastes, such as poor washability and poor anti-collapse properties, and to provide a flux for washable solder paste. Through the synergistic effect of each component, the washable solder paste simultaneously possesses excellent soldering performance, washability, anti-collapse properties and anti-settling properties, achieving thorough cleaning of post-soldering residues.

[0010] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0011] A flux for water-washable solder paste, comprising the following components by weight percentage:

[0012] Modified epoxy resin 15~30%,

[0013] Nonionic surfactant 0.5~3%,

[0014] Organic amines 0.5-2%,

[0015] Antioxidant 1-3%,

[0016] Complex organic acids 3-8%,

[0017] Composite thixotropic agent 1~4%,

[0018] Anti-settling agent 1~2%,

[0019] The remainder is organic solvent;

[0020] The modified epoxy resin is obtained by reacting bisphenol A type epoxy resin with polyethylene glycol;

[0021] The weight ratio of polyethylene glycol to bisphenol A epoxy resin is (0.5~1):1;

[0022] The epoxy equivalent of the bisphenol A type epoxy resin is 710-875 g / eq;

[0023] The number-average molecular weight of the polyethylene glycol is 400-8000;

[0024] The viscosity of the modified epoxy resin is 28000~42000 mPa·s, and the test condition is 25℃.

[0025] The modified epoxy resin of this invention is prepared by reacting bisphenol A type epoxy resin with polyethylene glycol. This modified epoxy resin can be used as a film-forming agent in flux for water-washable solder paste. Bisphenol A type epoxy resin contains polyfunctional epoxy groups in its molecular structure, exhibiting high reactivity and high ring-opening reaction efficiency with polyethylene glycol (PEG). The resulting modified epoxy resin possesses a stronger three-dimensional network structure and higher film strength. Hydrophilic PEG segments are introduced into the epoxy resin molecular backbone through a ring-opening reaction between the hydroxyl groups at the PEG terminals and the epoxy groups. This modified epoxy resin inherits the excellent film-forming properties, temporary tack, and electrical insulation of epoxy resin. It not only imparts hydrophilicity, but the PEG segments also facilitate the dissolution and emulsification of post-soldering residues by deionized water, achieving thorough washing. Furthermore, it provides synergistic thickening, and its molecular structure can form a stronger network with subsequent thixotropic agents, improving anti-collapse properties.

[0026] This invention incorporates a composite organic acid that decomposes and removes metal oxides at welding temperatures. Due to its complete water solubility, any residue remaining after welding can be completely dissolved and carried away by water, posing no risk of corrosion. Simultaneously, the composite organic acid forms a synergistic mechanism with the organic amine in the formulation: on the one hand, the organic amine neutralizes any remaining acidic substances after welding, completely eliminating the risk of corrosion; on the other hand, the two form a buffer system during storage and use, maintaining the chemical stability of the flux and synergistically releasing their activity only at high welding temperatures, achieving efficient welding and ensuring post-weld cleanliness and long-term reliability.

[0027] Preferably, the preparation method of the modified epoxy resin includes the following steps: mixing bisphenol A type epoxy resin and polyethylene glycol in an organic solvent, heating to 110~130℃ under an inert gas atmosphere, and holding at that temperature for 1.5~2.5h to obtain the modified epoxy resin.

[0028] Preferably, the number average molecular weight of the bisphenol A type epoxy resin is 1420~1750 g / mol.

[0029] Preferably, the number average molecular weight of the polyethylene glycol is 1000-2000.

[0030] Preferably, the composite thixotropic agent is a mixture of fumed silica and hydroxyethyl cellulose (HEC), with a weight ratio of hydroxyethyl cellulose to fumed silica of (0.2~1):1.

[0031] Preferably, the specific surface area of ​​the fumed silica is 150-400 m². 2 / g.

[0032] Preferably, the hydroxyethyl cellulose has a viscosity of 4500-5500 cps, and the test conditions are a 2% aqueous solution at 25°C.

[0033] Both the PEG chains of the modified epoxy resin and the water-soluble cellulose chains of hydroxyethyl cellulose (HEC) in the composite thixotropic agent are polar molecules, tightly bound together through intermolecular interactions. Simultaneously, the linear HEC molecular chains can wrap around the modified epoxy resin molecular chains, further enhancing the structural strength of the system. Fumed silica can fill the gaps between the modified epoxy resin and HEC molecular chains, restricting the free movement of the molecular chains through steric hindrance, reducing flow deformation of the system during rest, and forming a more stable and elastic gel structure with significantly better anti-collapse performance than using a single thixotropic agent. The three-dimensional network structure formed by the composite thixotropic agent can physically "encapsulate" and support solder powder, greatly increasing the viscosity of the system in a static state, thus significantly slowing down the settling rate. This structure is rapidly destroyed under printing shearing without affecting its use.

[0034] Preferably, the composite organic acid is a first organic acid, a second organic acid, and a third organic acid in a weight ratio of 1:(0.4~0.6):(0.1~0.4), wherein the first organic acid is at least one of adipic acid or malic acid, the second organic acid is at least one of citric acid or salicylic acid, and the third organic acid is at least one of succinic acid or sebacic acid.

[0035] Preferably, the nonionic surfactant is alkylphenol polyoxyethylene ether OP-10.

[0036] Preferably, the organic amine is an alcoholamine compound.

[0037] The organic amine is triethanolamine.

[0038] Preferably, the anti-settling agent is polyamide wax.

[0039] The antioxidant is a benzotriazole compound. Specifically, the benzotriazole compound is benzotriazole (BTA).

[0040] Preferably, the organic solvent includes high-boiling-point solvents and low-boiling-point solvents.

[0041] More preferably, the high-boiling-point solvent is glycerol, and the low-boiling-point solvent is diethylene glycol monobutyl ether.

[0042] More preferably, the weight ratio of the low-boiling-point solvent to the high-boiling-point solvent is 1:(3~4).

[0043] The preparation method of the above-mentioned flux for water-washable solder paste includes the following steps: mixing the components to obtain the flux for water-washable solder paste.

[0044] Preferably, the mixing temperature is 60~120℃.

[0045] This invention also protects a water-washable solder paste, comprising the following components in parts by weight: 8-12 parts of flux for the above-mentioned water-washable solder paste, and 88-92 parts of tin alloy powder.

[0046] The preparation method of the above-mentioned water-washable solder paste includes the following steps: mixing the water-washable solder paste with flux and tin alloy powder to obtain the water-washable solder paste.

[0047] Preferably, the water-washed solder paste is prepared by first stirring the flux and tin alloy powder under conditions of -30~-50 kPa and 70~90 rpm for 0.5~1 hour, and then stirring under conditions of 150~180 rpm for 1~2 hours to obtain the water-washed solder paste.

[0048] More preferably, the particle size distribution of the tin alloy powder is 20~38μm.

[0049] Preferably, the tin alloy powder is Sn90Sb10 alloy, Sn95Sb5 alloy, Sn63Pb37 alloy, and SAC305 alloy.

[0050] This invention also protects the application of the above-mentioned water-washable solder paste in surface mount technology.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows: The flux for water-washable solder paste of the present invention comprises the following components: modified epoxy resin, nonionic surfactant, organic amine, antioxidant, composite organic acid, composite thixotropic agent, anti-settling agent, and organic solvent. Through the synergistic effect of modified epoxy resin, composite thixotropic agent, and anti-settling agent, the flux for water-washable solder paste of the present invention enables the water-washable solder paste to simultaneously possess excellent soldering performance, water washability, anti-collapse, and anti-settling properties, achieving thorough cleaning of post-soldering residues, ensuring the accuracy of fine-pitch printing, and the stability of product storage. This guarantees the reliability of the product throughout the entire process from storage to use. Attached Figure Description

[0052] Figure 1 This is a schematic diagram of the chemical structure of the modified epoxy resin. Detailed Implementation

[0053] To more clearly and completely describe the technical solution of the present invention, the present invention will be further described in detail below through specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention. Various changes can be made within the scope of the claims of the present invention.

[0054] The raw materials used in each embodiment and proportion are as follows. Unless otherwise specified, all reagents used are commercially available products.

[0055] Epoxy resin #1, bisphenol A type epoxy resin, manufactured by Nan Ya, brand name NPES-904, molecular weight 1560~1700 g / mol, epoxy equivalent 780-850 g / eq.

[0056] Epoxy resin #2, bisphenol A type epoxy resin, manufactured by Nan Ya, brand name NPES-901, molecular weight 900~1000g / mol, epoxy equivalent 450-500 g / eq;

[0057] Epoxy resin #3, bisphenol A type epoxy resin, manufactured by Nan Ya, brand name NPES-906H, molecular weight 2500~2800 g / mol, epoxy equivalent 1250-1400 g / eq;

[0058] Epoxy resin #4, bisphenol A type epoxy resin, manufactured by Baling Petrochemical, grade CYD-014U, molecular weight 1420~1750 g / mol, epoxy equivalent 710-875 g / eq.

[0059] Epoxy resin #5, aliphatic epoxy resin, manufactured by Dow Chemical Company, USA, grade DER 331, molecular weight 364-384 g / mol, epoxy equivalent 182-192 g / eq;

[0060] Polyethylene glycol 1#, manufactured by Aladdin, with a number average molecular weight of 1000;

[0061] Polyethylene glycol 2#, manufactured by Aladdin, with a number average molecular weight of 2000;

[0062] Polyethylene glycol 3#, manufactured by Aladdin, with a number average molecular weight of 400;

[0063] Polyethylene glycol 4#, manufactured by Aladdin, with a number average molecular weight of 8000;

[0064] Polyethylene glycol 5#, manufactured by Aladdin, with a number average molecular weight of 300;

[0065] Polyethylene glycol 6#, manufactured by Aladdin, with a number average molecular weight of 20,000;

[0066] Modified epoxy resin 1# was prepared as follows: Epoxy resin 1 and polyethylene glycol 1 (weight ratio 1:0.5) and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser. Under nitrogen protection, the temperature was slowly raised to 120°C, and the reaction was continuously stirred at this temperature for 2 hours to obtain modified epoxy resin 1#. The viscosity was 33810 mPa·s, and the test condition was 25°C.

[0067] Modified epoxy resin 2# was prepared as follows: Epoxy resin 1# and polyethylene glycol 2#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 38800 mPa·s, and the test condition was 25℃.

[0068] Modified epoxy resin 3# was prepared as follows: Epoxy resin 1# and polyethylene glycol 3#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 29700 mPa·s, and the test condition was 25℃.

[0069] Modified epoxy resin 4# was prepared as follows: Epoxy resin 1# and polyethylene glycol 4#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 41600 mPa·s, and the test condition was 25℃.

[0070] Modified epoxy resin 5# was prepared as follows: Epoxy resin 4# and polyethylene glycol 1#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 29270 mPa·s, and the test condition was 25℃.

[0071] Modified epoxy resin 6# was prepared as follows: Epoxy resin 2# and polyethylene glycol 1#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 31300 mPa·s, and the test condition was 25℃.

[0072] Modified epoxy resin 7# was prepared as follows: Epoxy resin 3# and polyethylene glycol 1#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 57200 mPa·s, and the test condition was 25℃.

[0073] Modified epoxy resin #8 was prepared as follows: Epoxy resin #5 and polyethylene glycol #1 (weight ratio 1:0.5) and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 32900 mPa·s, and the test condition was 25℃.

[0074] Modified epoxy resin 9# was prepared as follows: Epoxy resin 1# and polyethylene glycol 5#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 19200 mPa·s, and the test condition was 25℃.

[0075] Modified epoxy resin 10# was prepared as follows: Epoxy resin 1# and polyethylene glycol 6#, with a weight ratio of 1:0.5, and 50% organic solvent were added to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared according to the same method described above. The viscosity was 62500 mPa·s, and the test condition was 25℃.

[0076] Modified epoxy resin 11# was prepared as follows: Epoxy resin 1# and polyethylene glycol 1#, along with 50% organic solvent, were added in a weight ratio of 1:1 to a three-necked flask equipped with a stirrer, thermometer, and condenser, and prepared using the same method described above. The viscosity was 37600 mPa·s, and the test conditions were 25℃.

[0077] Nonionic surfactant: Alkylphenol polyoxyethylene ether OP-10;

[0078] Organic amines: triethanolamine;

[0079] Organic solvent 1#: composed of diethylene glycol monobutyl ether and glycerol in a weight ratio of 1:3;

[0080] Organic solvent #2: Composed of diethylene glycol monobutyl ether and glycerol in a weight ratio of 1:4;

[0081] Complex organic acid 1#: composed of malic acid, salicylic acid and succinic acid in a weight ratio of 1:0.4:0.1;

[0082] Compound organic acid 2#: composed of adipic acid, citric acid, and sebacic acid in a weight ratio of 1:0.6:0.4;

[0083] Composite thixotropic agent 1#: composed of hydroxyethyl cellulose and fumed silica in a weight ratio of 0.2:1;

[0084] Composite thixotropic agent 2#: composed of hydroxyethyl cellulose and fumed silica in a weight ratio of 1:1;

[0085] Thixotropic agent #2: Fumed silica, manufactured by McLean, with a specific surface area of ​​150-400 m². 2 / g.

[0086] Thixotropic agent #3: Hydroxyethyl cellulose, manufactured by McLean, with a viscosity of 4500-5500 cps, tested under the conditions of 2% aqueous solution at 25℃.

[0087] Anti-settling agent: polyamide wax, ITOHWAX K-630;

[0088] Antioxidant: Benzotriazole (BTA).

[0089] Examples 1-13

[0090] A flux for water-washable solder paste comprises the following components by weight percentage: modified epoxy resin, nonionic surfactant, organic amine, antioxidant, complex organic acid, complex thixotropic agent, anti-settling agent, and the balance being organic solvent.

[0091] The components and their mass percentages for each embodiment are shown in Table 1 below.

[0092] Table 1. Components and their mass percentages (%) for each embodiment

[0093]

[0094] Continued from Table 1

[0095]

[0096] The preparation method of the above flux includes the following steps:

[0097] 1) Preparation of modified epoxy resin.

[0098] 2) Add the remaining organic solvent to the reactor. Stir at 400 rpm, then add the composite organic acid, nonionic surfactant, and antioxidant sequentially. Slowly heat to 85°C and maintain this temperature while stirring for 30 minutes until all solid components are completely dissolved and the system is a homogeneous, transparent liquid.

[0099] 3) Cool the mixture from step two to 60℃. Stir slowly and evenly at 400 rpm, adding the composite thixotropic agent and anti-settling agent. After addition, gradually increase the stirring speed to 1500-2000 rpm and perform high-speed shear dispersion for 40-60 minutes, controlling the temperature to not exceed 65℃.

[0100] 4) Slowly add the modified epoxy resin prepolymer solution (temperature approximately 80℃) prepared in step one to the paste matrix obtained in step three while stirring (speed 600~800 r / min), and then add the organic amine. After the addition is complete, maintain the temperature at 60℃ and stir at this temperature for 1 hour to ensure thorough mixing and uniform emulsification.

[0101] 5) Transfer the paste product obtained in step four to a container and cool it to room temperature.

[0102] Subsequently, the paste was transferred to a three-roll mill for grinding, and then transferred to a planetary mixer for vacuum degassing. At a vacuum degree of -0.1 MPa, it was stirred at a low speed of 200 r / min for 30 minutes until no visible bubbles remained in the paste.

[0103] A water-washable solder paste comprises the following components in parts by weight: 12 parts of flux for water-washable solder paste and 88 parts of tin alloy powder. Among them, the tin alloy powder is No. 4 powder, SAC305 alloy (3% Ag, 0.5% Cu and 96.5% Sn), and the particle size distribution of the tin alloy powder is 20 - 38 μm.

[0104] The preparation method of the water-washable solder paste comprises the following steps: Put the flux for water-washable solder paste and the tin alloy powder into a planetary mixer, stir at a speed of 80 r / min for 30 min under a vacuum degree of -30 to -50 kPa, and then stir at a speed of 160 r / min for 1 h to obtain the water-washable solder paste.

[0105] Comparative Examples 1 - 8

[0106] A flux for water-washable solder paste comprises the following components by weight percentage: modified epoxy resin, non-ionic surfactant, organic amine, antioxidant, composite organic acid, composite thixotropic agent, anti-settling agent, and the balance is organic solvent.

[0107] The components and their mass percentages of each comparative example are shown in Table 2 below.

[0108] Table 2 Components and Their Mass Percentages (%) of Each Comparative Example

[0109]

[0110] The preparation method of the above flux is the same as that of the example.

[0111] A solder paste, the difference from the example is only the flux.

[0112] Performance Test

[0113] The solder pastes of each example and comparative example were tested according to the following method:

[0114] 1) Water washability: The solder paste was printed on a PCB board, after reflow soldering, it was ultrasonically cleaned with deionized water at 50°C for 5 minutes, and then the residue area on the board surface was observed.

[0115] 2) Anti-collapse property: Test was carried out according to the IPC J-STD-005A standard. It was placed at 25°C and relative humidity of 60% for 30 minutes, and the bridging incidence rate was measured. A bridging rate lower than 3% is qualified.

[0116] 3) Anti-settling property: Put the solder paste into a sealed container and store it at 5°C for 3 months. Observe whether the solder paste separates into layers. If it does not separate into layers, it indicates that no sedimentation has occurred. If it separates into layers, it indicates that sedimentation has occurred.

[0117] The performance test results of the solder pastes prepared with the fluxes of each embodiment and comparative example are shown in Table 3.

[0118] Table 3 Performance test results of each embodiment and comparative example

[0119]

[0120] As can be seen from Table 3:

[0121] After washing, the residual area of ​​Examples 1-13 was all below 2.5%, the bridging rate was all below 2.5%, and there was no delamination. This indicates that the flux for water-washable solder paste of the present invention can make the solder paste have excellent washability, anti-collapse and anti-settling properties.

[0122] In Comparative Example 1, the low epoxy equivalent epoxy resin resulted in a modified epoxy resin backbone with a small molecular weight and short chain segments, leading to poor film-forming properties. The protective film formed after soldering was fragile, prone to cracking, and lacked sufficient moisture-proof and insulating properties. In Comparative Example 2, the high epoxy equivalent resin produced modified epoxy resin with excessively long chain segments, easily causing a surge in viscosity, making it unsuitable for solder paste. Furthermore, it exhibited poor washability, and the residual film after soldering was excessively hard and brittle. In Comparative Example 3, the modified epoxy resin backbone was a flexible hydrocarbon chain, resulting in a soft and sticky film lacking support and exhibiting poor anti-collapse properties, failing to provide effective physical protection. Comparative Examples 4 and 5 used either single fumed silica or hydroxyethyl cellulose as thixotropic agents, respectively, significantly reducing their anti-collapse and anti-settling properties, especially the poor anti-settling properties of single fumed silica. Comparative Example 6 did not add an anti-settling agent; although its short-term printing performance was acceptable, its storage stability was extremely poor, resulting in severe stratification. Comparative Examples 7 and 8 used polyethylene glycols with different requirements than those of the present invention. In Comparative Example 7, the number average molecular weight was too small, and the hydrophilic segments of the modified epoxy resin were too short, resulting in poor water washability. In Comparative Example 8, the number average molecular weight was too large, which reduced the compatibility and reaction efficiency with the epoxy resin, resulting in a significant decrease in anti-collapse properties.

[0123] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A flux for water-washing solder paste, characterized by comprising: Comprise the following components by weight percentage: Modified epoxy resin 15~30%, Nonionic surfactant 0.5~3%, Organic amine 0.5~2%, Antioxidant 1~3%, Composite organic acid 3~8%, Composite thixotropic agent 1~4%, Anti-settling agent 1~2%, The balance is organic solvent; The modified epoxy resin is obtained by reacting bisphenol A type epoxy resin with polyethylene glycol; The weight ratio of polyethylene glycol and bisphenol A type epoxy resin is (0.5~1) : 1; The epoxy equivalent weight of the bisphenol A type epoxy resin is 710-875 g / eq; The number average molecular weight of the polyethylene glycol is 400~8000; The viscosity of the modified epoxy resin is 28000~42000 mPa·s, and the test condition is 25℃; The composite thixotropic agent is a mixture of fumed silica and hydroxyethyl cellulose, and the weight ratio of hydroxyethyl cellulose and fumed silica is (0.2~1) :

1.

2. The soldering flux for use in the water cleaning of solder paste according to claim 1, wherein The preparation method of the modified epoxy resin comprises the following steps: mixing bisphenol A type epoxy resin and polyethylene glycol in an organic solvent, heating to 110~130℃ under inert gas atmosphere, and keeping for 1.5~2.5h to obtain modified epoxy resin; the number average molecular weight of the bisphenol A type epoxy resin is 1420~1750 g / mol.

3. The soldering flux for use in the water cleaning of solder paste according to claim 1, wherein The number average molecular weight of the polyethylene glycol is 1000~2000.

4. The soldering flux for use in the solder paste according to claim 1, wherein The composite organic acid is a mixture of first organic acid, second organic acid and third organic acid with a weight ratio of 1:(0.4~0.6):(0.1~0.4), the first organic acid is at least one of adipic acid or malic acid, the second organic acid is at least one of citric acid or salicylic acid, and the third organic acid is at least one of succinic acid or sebacic acid.

5. The solder paste cleaning agent according to claim 1, wherein The nonionic surfactant is alkyl phenol polyoxyethylene ether.

6. The solder paste cleaning agent according to claim 1, wherein The organic amine is an alcohol amine compound.

7. The solder paste cleaning agent according to claim 1, wherein The anti-settling agent is polyamide wax.

8. A water-washable solder paste, characterized by, Comprise the following components by weight percentage: the flux for water-washing solder paste 8~12 parts, and tin alloy powder 88~92 parts.

9. The application of the water-washing solder paste of claim 8 in surface mount technology.

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