A washable low-temperature solder paste flux, its preparation method and application

Abstract

This invention belongs to the field of solder paste technology, specifically relating to a washable low-temperature solder paste flux, its preparation method, and its application. The washable low-temperature solder paste flux comprises the following components by mass percentage: 18-22% sulfonic acid-based aromatic copolyester, 8.8-13.1% polyalkyl alcohol amine, 0.5-2.0% lauryl glucoside, 0.2-0.6% corrosion inhibitor, 1%-3% thixotropic agent, 5-10% polyethylene glycol diacrylate, 3-7% citric acid, 2-3% succinic acid, 2-3% lactic acid, with the balance being solvent; the solvent is ethanol and water. This invention's washable low-temperature solder paste flux, through the synergistic effect of its components, reduces the corrosiveness of the sulfonic acid groups, improves the low-temperature soldering activity of the flux, and is easily washed away and dissociated in warm water.

Description

Technical Field

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

[0002] With the rapid development of electronic devices towards miniaturization, high density, and multifunctionality, surface mount technology (SMT) is becoming increasingly important in PCB assembly processes. Solder paste, as a core material in SMT processes, directly affects soldering quality, reliability, and production efficiency.

[0003] Conventional lead-free solder paste (such as SAC305) typically requires a reflow soldering peak temperature of 240-250℃, which can easily cause thermal damage to heat-sensitive components (such as flexible substrates, LEDs, ceramic capacitors, plastic connectors, etc.), leading to deformation, delamination, or performance degradation, thus limiting its application in advanced electronic packaging. Therefore, low-temperature solder paste with a temperature of <200℃ is needed to meet the soldering requirements of these heat-sensitive components.

[0004] While existing low-temperature solder pastes (such as bismuth-based and indium-based alloys) can reduce the peak temperature of reflow soldering (peak temperature 180~220℃), their flux systems are mostly based on activators that are difficult to dissolve in water. The residues are difficult to completely remove with gentle water washing, which can easily lead to electrochemical migration or insulation failure.

[0005] Chinese patent CN118106659A discloses a washable lead-free solder paste, comprising 80-100 parts water-soluble modified rosin, 70-80 parts organic solvent, 30-35 parts organic acid, 1-10 parts surfactant, and 10-20 parts thixotropic agent. The organic acid is sulfonic acid, which solves the technical problem of flux being difficult or impossible to clean thoroughly from solder joints. This Chinese patent uses sulfonic acid as the active ingredient to ensure good soldering performance. However, due to the significant hygroscopic nature of sulfonic acid, this type of solder paste is prone to moisture absorption during production, storage, and use, which can corrode the metal solder powder inside the paste. Simultaneously, it may corrode the circuit board during the soldering process, ultimately adversely affecting the soldering performance of the paste itself and the reliability of components. Furthermore, this washable flux requires hot water (above 60°C) or mechanical rinsing, which may damage precision components. In addition, sulfonic acid does not have film-forming properties, making it difficult to protect the solder from secondary oxidation during the soldering process, thus reducing the stability of the solder paste. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects and shortcomings of existing washable solder paste fluxes, where sulfonic acid lacks film-forming properties, leading to easy oxidation of the solder. This invention provides a sulfonic acid-based aromatic copolyester prepared by esterification and polycondensation reactions of 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid, and ethylene glycol. This sulfonic acid-based aromatic copolyester exhibits good film-forming properties, protecting the solder from secondary oxidation during the soldering process and improving the stability of the solder paste.

[0007] The above-mentioned objective of the present invention is achieved through the following technical solution:

[0008] A sulfonic acid-based aromatic copolyester is prepared by esterification and polycondensation reactions of 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol.

[0009] The molar ratio of 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol is (5~8):(30~32):(62~65):100;

[0010] The sulfonic acid-based aromatic copolyester has a number-average relative molecular mass of 9000-11000 g / mol, a degree of polymerization of 40-50, and a density of 1.2-1.3 g / cm³. 3 Its melting point is 115~120℃.

[0011] The sulfonic acid-based aromatic copolyester of this invention is prepared by esterification and polycondensation reactions of 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid, and ethylene glycol. The 5-sulfonic acid isophthalate derivative unit provides the active sites for the sulfonic acid groups. The sulfonic acid groups have strong activity and water solubility, which can improve the soldering activity and washability of solder paste. The norbornene-2,3-dicarboxylic acid derivative unit is used to improve the rigidity of the long-chain sulfonic acid-based aromatic copolyester, and the sebacic acid derivative unit can serve as a flexible segment of the long-chain sulfonic acid-based aromatic copolyester to improve film toughness.

[0012] The sulfonic acid-based aromatic copolyester of the present invention enables the flux to have excellent low-temperature welding activity and good film-forming properties. It can protect the solder from secondary oxidation during the welding process, improve the stability of the solder paste, and also has the characteristics of easy cleaning and dissociation in warm water.

[0013] Preferably, the 5-sulfonic acid isophthalate is one or more of sodium 5-sulfonic acid isophthalate, ammonium 5-sulfonic acid isophthalate, or silver 5-sulfonic acid isophthalate.

[0014] The present invention also protects a method for preparing the sulfonic acid-based aromatic copolyester, comprising the following steps: mixing 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol, first esterifying at 180-200°C for 2.5-3 hours, and then polycondensing at 250-270°C and 50-100 Pa for 4-4.5 hours.

[0015] This invention also protects the use of sulfonic acid-based aromatic copolyesters in the preparation of washable low-temperature solder paste flux.

[0016] This invention also protects a washable low-temperature solder paste flux, comprising the following components by weight percentage:

[0017] The above-mentioned sulfonic acid-based aromatic copolyesters contain 18-22%.

[0018] Polyalkyl alcohol amines: 8.8%–13.1%

[0019] Lauryl glucoside 0.5~2.0%,

[0020] Corrosion inhibitor 0.2~0.6%,

[0021] Thixotropic agent 1%~3%,

[0022] 5-10% polyethylene glycol diacrylate

[0023] Citric acid 3-7%,

[0024] Succinic acid 2-3%,

[0025] Lactic acid 2~3%,

[0026] The remainder is solvent;

[0027] The solvent is ethanol and water, and the volume ratio of ethanol to water is (2~3):1;

[0028] The chemical structural formula of the polyalkyl alcoholamine is any one of the following formulas I or II:

[0029]

[0030] Formula I

[0031]

[0032] Formula II

[0033] In Formula I or Formula II, R is an alkyl group having 12 to 16 carbon atoms.

[0034] In a specific embodiment, sulfonic acid-based aromatic copolyester and polyalkyl alcohol amine can be mixed first, and then mixed evenly with the remaining components to obtain a water-washable low-temperature solder paste flux.

[0035] In the specific preparation process of the water-washable low-temperature solder paste flux of the present invention, sulfonic acid aromatic copolyester and polyalkyl alcohol amine are first mixed, so that sulfonic acid aromatic copolyester and polyalkyl alcohol amine can be neutralized to obtain sulfonic acid aromatic copolyester-amine neutralized product.

[0036] This invention utilizes the neutralization of sulfonic acid groups in this long-chain sulfonic acid aromatic copolyester with polyalkyl alcohol amines to generate sulfonic acid aromatic copolyester-amine neutralizers. This reduces the corrosiveness of sulfonic acid groups to metals, preventing them from easily absorbing moisture and causing corrosion during production, storage, and use, thus accelerating electrochemical corrosion. Furthermore, it maintains the low-temperature welding activity of the flux, giving it good film-forming properties and easy cleanability and dissociation in warm water. This invention leverages the difference in dissociation behavior of sulfonic acid aromatic copolyester-amine neutralizers in solvent environments to achieve stable film formation during welding, enhanced activity in ultra-low temperature zones, and easy post-weld water washing removal.

[0037] The specific principle is as follows:

[0038] The hydrophobic polyalkylolamine and the sulfonic acid groups in the sulfonic acid aromatic copolyester form a sulfonic acid aromatic copolyester-amine neutralizer. This neutralizer not only significantly reduces the corrosiveness of the sulfonic acid groups, but the hydrophobic chains of the polyalkylolamine also form a tightly packed structure, making the film resistant to solder corrosion and non-hygroscopic. During post-soldering water washing, the sulfonic acid aromatic copolyester-amine neutralizer dissociates in the high dielectric constant water environment, causing the entire membrane structure to disintegrate, thus enabling it to be removed with warm water. The polyalkylolamine neutralizes most of the sulfonic acid groups in the sulfonic acid aromatic copolyester. Even if a small amount of unneutralized sulfonic acid groups remain, they are encapsulated within the membrane constructed by the hydrophobic ends of the polymer, preventing the sulfonic acid groups from reacting with water. Simultaneously, during welding, it synergistically acts with other activators to enhance the activity of the flux system, thereby overcoming the deficiency of insufficient activity of low-temperature activators and improving the welding performance of the solder joints.

[0039] Among them, long-chain alkyl groups with C12 to C16 carbon atoms are selected from polyalkylolamines. If the alkyl chain is too short, the hydrophobic effect will be insufficient, and the welded film will easily absorb water in a humid environment. At the same time, the intermolecular forces of short-chain alkyl groups are weak, resulting in poor film-forming performance and failure to form a continuous protective layer. Long alkyl chains lead to excessive hydrophobicity, increased molecular chain rigidity, poor dispersibility in solvents, and easy formation of agglomerates, resulting in uneven film thickness. In addition, the melt viscosity is high, the fluidity is poor, it is difficult to uniformly cover the weld joints, and post-weld cleaning is difficult.

[0040] Lauryl glucoside can be used as a nonionic surfactant in soldering flux. This surfactant possesses an amphiphilic structure with hydrophobic chains and hydrophilic sugar groups, enabling it to orient itself at the interface and reduce interfacial tension. The hydrophilic glucose units of lauryl glucoside are highly polar and have significant steric hindrance, allowing them to bind with solvents (water / ethanol) or polar components in the system via hydrogen bonds. This breaks the spontaneous aggregation tendency of long-chain polymers, reduces system viscosity, and improves dispersion uniformity.

[0041] Polyethylene glycol diacrylate (PEG) possesses excellent flexibility and polarity, enabling it to form a synergistic viscosity-regulating network with sulfonic acid aromatic copolyester-amine neutralizers, ensuring the printability and moldability of solder paste. The hydrogen bonding between PEG and sulfonic acid aromatic copolyester-amine neutralizers enhances the integrity of the film structure during the soldering stage, preventing secondary oxidation of the solder. Simultaneously, during the post-soldering cleaning stage, the slight cross-linking between PEG and sulfonic acid aromatic copolyester-amine neutralizers at low temperatures depolymerizes in warm water due to hydrogen bond breakage, leading to rapid hydration of the molecular chains. This disrupts the dense structure of the residual film, making it easier for water molecules to contact the sulfonate ammonium salt ion pairs and form mixed micelles with the dissociated long-chain alkyl ammonium ions, enhancing the emulsifying and dispersing ability of residues. Therefore, the addition of PEG has a dual function: assisting film formation and enhancing the washing effect.

[0042] In this invention, a specific volume ratio of ethanol-water mixture is used as the solvent for the flux. Sulfonic acid aromatic copolyester contains rigid aromatic ring units, which have better solubility in ethanol with lower polarity. Polyalkyl alcoholamine contains hydrophilic ethanolamine groups, which have a certain solubility in water. The specific volume ratio of ethanol to water balances the solubility requirements of both. Ethanol ensures that the molecular chains of sulfonic acid aromatic copolyester are fully extended, while the aqueous phase promotes the dissociation and dispersion of alcoholamine, so that the neutralization reaction can be carried out efficiently in a homogeneous system. The sulfonic acid aromatic copolyester-amine neutralizer generated by the neutralization reaction remains stable in the ethanol-water mixture without significant dissociation, but it can rapidly dissociate and dissolve during water washing. The addition of ethanol significantly reduces the overall polarity of the solvent system, weakening the hydrogen bonding between the sulfonic acid group and water molecules. At the same time, the hydroxyl group (-OH) in the ethanol molecule can form a weak hydrogen bond with the sulfonic acid group. This "bridging effect" can both prevent the sulfonic acid group from absorbing too much water and forming a hydration layer, and limit the number of water molecules binding to the sulfonic acid group through steric hindrance, so that the nucleophilic attack of the amino group becomes the dominant reaction and thus does not produce hydrolysis.

[0043] Preferably, the polyalkyl alcoholamine is one or more of N-dodecyl diethanolamine, N-tridecyl diethanolamine, N-tetradecyl diethanolamine, N-pentadedecyl diethanolamine, or N-hexadecyl diethanolamine.

[0044] Preferably, the corrosion inhibitor is at least one of benzotriazole or triethanolamine.

[0045] Preferably, the thixotropic agent is at least one of hydrogenated castor oil or polyamide wax.

[0046] Preferably, the average molecular weight of polyethylene glycol diacrylate is 575-700.

[0047] The present invention also protects a method for preparing the water-washable low-temperature solder paste flux according to any of the above claims, comprising the following steps: mixing sulfonic acid-based aromatic copolyester and polyalkyl alcohol amine, and then mixing them evenly with the remaining components to obtain the water-washable low-temperature solder paste flux.

[0048] This invention also protects a washable low-temperature solder paste, comprising, by weight, 88-92 wt% solder and 8-12 wt% flux, wherein the flux is any of the fluxes described above, the solder is a tin-based alloy, and the reflow soldering peak temperature of the solder is less than 200°C.

[0049] Preferably, the solder is a tin-bismuth alloy (Bi58Sn42) or a tin-indium alloy (In52Sn48) alloy.

[0050] This invention also protects the application of washable low-temperature solder paste in electronic packaging, wherein components are soldered using the aforementioned washable low-temperature solder paste, and after soldering, the components are cleaned with water at a temperature of 40~50°C.

[0051] Compared with the prior art, the beneficial effects of the present invention are:

[0052] This invention discloses a sulfonic acid-based aromatic copolyester prepared by esterification and polycondensation reactions of 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol. The sulfonic acid-based aromatic copolyester has good film-forming properties, which can protect the solder from secondary oxidation during the soldering process and improve the stability of the solder paste.

[0053] This invention discloses a washable low-temperature solder paste flux. Through the synergistic effect of its components, the corrosiveness of the sulfonic acid group is reduced, and the low-temperature soldering activity of the flux is improved. It also has excellent soldering performance at reflow soldering peak temperatures below 200°C and is easily cleaned and dissociated in warm water. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the chemical structure of a sulfonic acid-based aromatic copolyester.

[0055] Figure 2 The DSC diagrams are for sulfonate aromatic copolyester 1#, sulfonate aromatic copolyester 2#, and sulfonate aromatic copolyester 3#. Detailed Implementation

[0056] 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.

[0057] Sodium 5-sulfonic acid isophthalate, CAS No. 6362-79-4.

[0058] Norbornene-2,3-dicarboxylic acid, CAS number 1200-88-0.

[0059] Sulfonate-based aromatic copolyester 1# is prepared by esterification and polycondensation reaction of sodium 5-sulfonate isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol.

[0060] The molar ratio of sodium 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol is 5:30:65:100.

[0061] The preparation method of sulfonate-based aromatic copolyester 1# is as follows:

[0062] (1) Raw material pretreatment: Sodium 5-sulfonic acid isophthalate was vacuum dried at 105°C for 4 hours to remove crystal water; all monomers were mixed in proportion, and 0.4% of tetrabutyl titanate catalyst and 0.1% of phenothiazine anti-high temperature oxidant were added. The mixture was stirred for 10 minutes under nitrogen protection to form a uniform slurry.

[0063] (2) Esterification reaction: Add the mixed slurry to a four-necked flask with a water separator, purge with nitrogen (flow rate 0.5 L / min), heat to 180℃ and keep warm for 1 hour to dehydrate, then gradually heat to 200℃ and react for 2.5-3 hours until the water output reaches more than 95% of the theoretical value. Measure the acid value every 30 minutes during the reaction. When the acid value At≤50mg KOH / g, a low molecular weight prepolymer is obtained, which is convenient for the next step of polycondensation reaction.

[0064] (3) Polycondensation reaction: Remove the water separator, connect the high vacuum system to reduce the pressure to 50-100 Pa, raise the temperature to 250℃ and react for 2 hours, then raise the temperature to 260-270℃ and react for 4-4.5 hours to form a polymer. During the process, measure the acid value every 60 minutes. When the acid value reaches the target value (5.5-6.0 mg KOH / g), stop the reaction immediately.

[0065] (4) Post-processing: After breaking the vacuum and purging with nitrogen, the molten polymer was poured into a mold and cooled to room temperature after cooling to 150°C. After crushing, it was boiled and washed three times with deionized water and dried under vacuum at 80°C for 12 hours to obtain a white granular product.

[0066] A schematic diagram of the chemical structure of a sulfonic acid-based aromatic copolyester is shown below. Figure 1 As shown.

[0067] The degree of polymerization of sulfonic acid-based aromatic copolyesters was determined by gel permeation chromatography (GPC). Standard calibration curve (PMMA): log 10 (M) = -0.1472t + 6.6256.

[0068] The melting point of the sulfonic acid-based aromatic copolyester was tested using a DSC-600 differential scanning calorimeter at a scanning rate of 10℃ / min.

[0069] The density of sulfonic acid-based aromatic copolyesters was tested using the density gradient column method, and the calibration curve was ρ = -0.02h + 1.50.

[0070] The DSC chart of sulfonate-based aromatic copolyester 1# is shown below. Figure 2 As shown in (a), the number-average relative molecular mass of sulfonate aromatic copolyester 1# is 9350 g / mol, the degree of polymerization is 42, and the density is 1.23 g / cm³. 3 Its melting point is 117.7℃.

[0071] Sulfonate-based aromatic copolyester 2# is prepared by esterification and polycondensation reaction of sodium 5-sulfonate isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol.

[0072] The molar ratio of sodium 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol is 6:32:62:100.

[0073] The preparation method of sulfonate aromatic copolyester 2# is the same as that of sulfonate aromatic copolyester 1#.

[0074] The DSC chart of sulfonate-based aromatic copolyester 2# is shown below. Figure 2 As shown in (b), the sulfonic acid-based aromatic copolyester 2# has a relative molecular mass of 9842 g / mol, a degree of polymerization of 44, and a density of 1.25 g / cm³. 3 Its melting point is 118.3℃.

[0075] Sulfonate-based aromatic copolyester 3# is prepared by esterification and polycondensation reaction of sodium 5-sulfonate isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol.

[0076] The molar ratio of sodium 5-sulfonic acid isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol is 8:30:62:100.

[0077] The preparation method of sulfonate aromatic copolyester 3# is the same as that of sulfonate aromatic copolyester 1#.

[0078] The DSC chart of sulfonate-based aromatic copolyester 3# is shown below. Figure 2 As shown in (c), the sulfonic acid aromatic copolyester 3# has a relative molecular mass of 10200 g / mol, a degree of polymerization of 45, and a density of 1.26 g / cm³. 3 Its melting point is 119.2℃.

[0079] Polyalkyl alcoholamine 1#: N-dodecyl diethanolamine, CAS number 1541-67-9.

[0080] Polyalkyl alcoholamine 2#: N-hexadecyl diethanolamine, CAS number 18924-67-9.

[0081] Polyalkyl alcohol amine 3#: Diethanolamine stearate, CAS number 10213-78-2.

[0082] Polyalkyl alcohol amine 4#: N-Undecyldiethanolamine, CAS number 112919-11-6.

[0083] Lauryl glucoside: CAS number 110615-47-9.

[0084] Corrosion inhibitor #1: Benzotriazole.

[0085] Corrosion inhibitor #2: Triethanolamine (TEA).

[0086] Thixotropic agent 1#: Hydrogenated castor oil, Nippon Fine Chemical, HCO.

[0087] Thixotropic agent #2: Polyamide wax, Haimings, THIXATROL® P2100W.

[0088] Polyethylene glycol diacrylate: CAS number 26570-48-9, Maclean, 437441, average molecular weight 575.

[0089] Solvent 1#: prepared by mixing ethanol and water in a volume ratio of 7:3.

[0090] Solvent 2#: prepared by mixing ethanol and water in a volume ratio of 6:4.

[0091] Solvent 3#: is obtained by mixing ethanol and water in a volume ratio of 8:2.

[0092] Examples 1-9

[0093] A washable low-temperature solder paste flux comprises the following components by weight percentage: sulfonic acid aromatic copolyester, polyalkyl alcohol amine, lauryl glucoside, corrosion inhibitor, thixotropic agent, polyethylene glycol diacrylate, citric acid, succinic acid, lactic acid, and the balance being solvent; the solvent is ethanol and water;

[0094] The components and their mass percentages for each embodiment are shown in Table 1 below.

[0095] Table 1. Components and their mass percentages (%) for each embodiment

[0096]

[0097] The preparation method of the above flux includes the following steps:

[0098] 1) Add the prescribed amount of mixed solvent to the reactor and start stirring and heating to (68±2) ℃. While maintaining the temperature, slowly add the weighed amount of sulfonic acid aromatic copolyester to the solvent in batches. Stir continuously at 300-400 rpm until the polymer is completely dissolved, forming a homogeneous and transparent solution.

[0099] 2) After melting the prescribed amount of polyalkylolamine into a homogeneous liquid in an oil bath at 55-60℃, slowly and evenly add the prescribed amount of polyalkylolamine dropwise to the polymer solution over 20-30 minutes using a dropping funnel, maintaining mechanical stirring at 300-400 rpm throughout the process. After the addition is complete, continue to maintain the temperature at 68℃ and stir for 1.5 hours. Take samples at 30 minutes, 1 hour, and 1.5 hours after the start of the reaction to measure the pH value, ensuring that the pH value of the reaction system is stabilized between 7.0 and 7.5.

[0100] 3) Add the prescribed amounts of polyethylene glycol diacrylate and lauryl glucoside in sequence. Stir for at least 10-15 minutes after each addition to ensure complete and uniform mixing.

[0101] 4) Add the corrosion inhibitor and thixotropic agent in the formula amount in sequence, and further increase the stirring speed to 500-600 rpm. Continue stirring under this high-intensity shear for about 1 hour. Then add citric acid, succinic acid and lactic acid in sequence, and continue stirring at this speed for 1-2 hours to ensure that all components are fully mixed and evenly dispersed, and the system achieves a high degree of homogeneity and stability.

[0102] 5) Cooling and post-treatment: Stop heating and allow the resulting mixture to cool naturally or with circulating water to room temperature (25±2℃). Finally, pass the mixture through a filter with a pore size of 5~10μm to remove undissolved particulate impurities, thus obtaining a clear and uniform liquid flux product.

[0103] A washable low-temperature solder paste comprises, by weight, 89 parts solder and 11 parts flux, wherein the flux is the aforementioned flux and the solder is a Bi58Sn42 tin-bismuth alloy with a particle size distribution of 25~38μm.

[0104] Comparative Examples 1-7

[0105] A solder paste flux comprises the following components by weight percentage: sulfonic acid-based aromatic copolyester, polyalkyl alcohol amine, lauryl glucoside, corrosion inhibitor, thixotropic agent, polyethylene glycol diacrylate, citric acid, succinic acid, lactic acid, and the balance being solvent.

[0106] The components and their mass percentages for each comparative example are shown in Table 2 below.

[0107] Table 2. Components and their mass percentages (%) for each comparative example.

[0108]

[0109] The preparation method of the above flux is the same as that in the examples.

[0110] A solder paste, differing from the examples only in the flux.

[0111] Performance testing

[0112] The Bi58Sn42 solder pastes prepared with the fluxes of each embodiment and comparative example had a reflow soldering peak temperature of 170°C. Performance tests were conducted according to the following test methods.

[0113] 1) Void Rate: The void rate after welding is tested using a micro-focus X-ray inspection device. The industry standard is to keep the void rate below 10%.

[0114] 2) Wettability: Tested according to GB / T 31475-2015 standard. A wettability grade of 1 or 2 is generally required; a grade of 3 or 4 is considered poor wettability.

[0115] 3) Viscosity: Tested according to GB / T 31475-2015 standard. The viscosity of solder paste should be within ±15% of the product's nominal value. The viscosity of flux will increase significantly after absorbing water; the viscosity change is used to determine whether the solder paste has absorbed moisture.

[0116] 4) Solder ball test: The test shall be conducted in accordance with the GB / T 31475-2015 standard. The solder ball test of the solder paste shall be rated according to the table, and the test result shall not be lower than level 2.

[0117] 5) Printing Performance: Tests were conducted according to the "Chip 1608, 2125, 3216 Solder Paste Printing Standards." The following four aspects were observed and recorded: ① Solder paste covers more than 90% of the pad area; ② No solder paste misalignment; ③ Uniform solder paste quantity and thickness, with a thickness of 8.31 MILS; ④ Good solder paste formation, without collapse or breakage. The evaluation criteria are: Excellent printing performance is achieved if all four aspects are met; Good printing performance is achieved if three of the four aspects are met; Poor printing performance is achieved if 0-2 of the four aspects are met.

[0118] 6) Flux homogeneity test: Physical stability test shall be conducted in accordance with GB / T 9491-2021 6.3 standard. The flux shall not have foreign matter, stratification or precipitation. This is used to determine the homogeneity of the flux system.

[0119] 7) Cleaning performance: Soak in 40℃ warm water for 5 minutes and rinse 3 times. Observe the surface under a microscope to see if there are any residues.

[0120] 8) Corrosion: The test shall be conducted in accordance with GB / T 9491-2021 6.3 standard. The copper mirror shall not have penetrating corrosion.

[0121] Test Results

[0122] The performance test results of the solder pastes prepared with the fluxes of each embodiment and comparative example are shown in Table 3.

[0123] Table 3 Performance test results of each embodiment and comparative example

[0124]

[0125] As can be seen from Table 3:

[0126] Examples 1-9 all exhibited a void ratio below 6%, wettability grade 1, viscosity within the nominal value of 230±10 Pa·s, excellent or good printability, uniform flux, and absence of foreign matter, delamination, or sedimentation. After cleaning, no residue was found on the solder joints or substrate, and the copper mirror tested showed no penetrating corrosion. This indicates that the flux of the present invention can enable solder paste to have a low void ratio, good solderability, good printability, and easy cleaning.

[0127] Comparative Example 1 used unmodified sodium 5-sulfonic acid isophthalate, resulting in solder paste with lower viscosity, inferior printability compared to Example 1, and severe corrosivity. Because the relative molecular mass of sodium 5-sulfonic acid isophthalate is much smaller than that of the sulfonic acid aromatic copolyester in Example 1, even with the same weight of sodium 5-sulfonic acid isophthalate added to Comparative Example 1 as to Example 1, the sodium 5-sulfonic acid isophthalate in Comparative Example 1 would have more sulfonic acid groups. Adding the same weight of alkanolamine cannot completely neutralize the sulfonic acid groups in the sodium 5-sulfonic acid isophthalate, thus Comparative Example 1 exhibits severe corrosivity.

[0128] Comparative Example 2 used polyalkylolamine 3#, with alkyl groups having 18 carbon atoms. Its high void ratio resulted in failed solder ball tests, and its wettability and printability were inferior to Example 1, leading to flux precipitation. This is because the excessively long alkyl chains resulted in excessive hydrophobicity, increased molecular chain rigidity, poor dispersibility in solvents, and a tendency to form aggregates, leading to uneven film thickness. Furthermore, its high melt viscosity and poor flowability made it difficult to uniformly cover solder joints, and post-soldering cleaning was challenging.

[0129] Comparative Example 3 used polyalkylolamine #4, whose alkyl group has 11 carbon atoms. This prevented it from forming a continuous protective layer. Moisture absorption by the flux led to a high void ratio, resulting in failed solder ball tests. Moisture absorption also increased the solder paste viscosity and poor printability. This is because the short alkyl chain of the polyalkylolamine resulted in insufficient hydrophobicity. The solder film easily absorbed water in humid environments, causing the sulfonic acid-based aromatic copolyester-amine neutralizer to hydrolyze, releasing the sulfonic acid groups and causing corrosion.

[0130] Comparative Example 4, which did not use lauryl glucoside, produced solder paste with excessively high void ratio, poor printability, and slight delamination.

[0131] Comparative Example 5, which did not use polyethylene glycol diacrylate, produced solder paste with high void ratio, failed solder ball test, had poor printability, and left residue after cleaning.

[0132] After changing the solvent ratio in Comparative Examples 6-7, the solder paste had a high void ratio, unsatisfactory wettability and solder ball properties, poor printability, and contained foreign matter, delamination, or sediment. After cleaning, there were residues and high corrosivity.

[0133] 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 water-washable low-temperature solder paste flux, characterized by, Comprise the following components by mass percentage: Sulfonic acid-based aromatic copolyester 18~22%, Polyalkyl alcohol amine 8.8~13.1%, Lauryl glucoside 0.5~2.0%, Corrosion inhibitor 0.2~0.6%, Thixotropic agent 1%~3%, Polyethylene glycol diacrylate 5~10%, Citric acid 3~7%, Succinic acid 2~3%, Lactic acid 2~3%, The balance is solvent; The solvent is ethanol and water, and the volume ratio of ethanol and water is (2~3):1; The polyalkyl alcohol amine is one or more of N-dodecyl diethanolamine, N-tridecyl diethanolamine, N-tetradecyl diethanolamine, N-pentadecyl ethanolamine or N-hexadecyl diethanolamine; The sulfonic acid-based aromatic copolyester is prepared by esterification and polycondensation reaction of 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol; The molar ratio of 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol is (5~8):(30~32):(62~65):100; The number average relative molecular mass of the sulfonic acid group-containing aromatic copolyester is 9000-11000 g / mol, the polymerization degree is 40-50, the density is 1.2-1.3 g / cm 3 , and the melting point is 115-120℃.

2. The water-washable low temperature solder paste flux according to claim 1, wherein The preparation method of the sulfonic acid-based aromatic copolyester comprises the following steps: mixing 5-sulfonic acid-based isophthalate, norbornene-2,3-dicarboxylic acid, sebacic acid and ethylene glycol, first esterification reaction at 180-200℃ for 2.5-3 hours, and then polycondensation reaction at 250-270℃, 50-100Pa for 4-4.5 hours.

3. The water-washable low temperature solder paste flux according to claim 1, wherein The corrosion inhibitor is at least one of benzotriazole or triethanolamine.

4. The water-washable low temperature solder paste flux according to claim 1, wherein The thixotropic agent is at least one of hydrogenated castor oil or polyamide wax.

5. The method of producing the water-washable low-temperature solder paste flux according to any one of claims 1 to 4, characterized by, Comprise the following steps: mixing sulfonic acid-based aromatic copolyester and polyalkyl alcohol amine, and then mixing with the rest of the components uniformly to obtain the water-washable low-temperature solder paste flux.

6. A water-washable low temperature solder paste characterized by, By weight, comprising 88~92wt% solder and 8~12wt% flux, the flux is the flux of any one of claims 1~4, the solder is a tin-based alloy, and the peak temperature of the solder during reflow soldering is less than 200℃.

7. Use of the water-washable low temperature solder paste according to claim 6 in electronic packaging, characterized in that, In the application, the water-washable low-temperature solder paste of claim 6 is used to weld components, and after welding is completed, the components are washed with water at a temperature of 40~50℃.

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