Halogen-free soldering flux for solder paste and preparation method of halogen-free soldering flux

Abstract

The invention relates to the technical field of solder paste, in particular to halogen-free soldering flux for solder paste and a preparation method of the halogen-free soldering flux. The halogen-free soldering flux for the solder paste comprises the following raw materials in parts by mass: 20-40 parts of hydrogenated rosin, 1-5 parts of maleic anhydride, 0.1-0.4 part of p-toluenesulfonic acid, 1-5 parts of activated mesoporous silica, 10-20 parts of a composite organic acid active agent, 5-10 parts of a thixotropic agent, 1-3 parts of a tackifier, 1-2 parts of a pH regulator, 1-2 parts of a surfactant, 1-2 parts of a chelating agent, 1-2 parts of an antioxidant and 50-80 parts of an organic solvent. Through the innovative component design and the preparation process, the technical contradiction existing in the field of halogen-free scaling powder for a long time is effectively solved, and an ideal material solution is provided for high-reliability electronic packaging.

Description

Technical Field

[0001] The present invention relates to the technical field of solder pastes, and in particular to a halogen-free flux for solder paste and a preparation method thereof. Background Art

[0002] With the development of the electronic packaging industry towards high density and miniaturization, the performance requirements for fluxes used in solder pastes are becoming increasingly stringent. Traditional halogen-containing fluxes are gradually being replaced by halogen-free fluxes due to the easy corrosion of circuit boards caused by the residue of halogen ions. However, there are still many technical bottlenecks in existing halogen-free fluxes that need to be broken through.

[0003] The flux for solder paste usually consists of a solvent, a resin, a surfactant, an activator, an antioxidant, and a thixotropic agent. Different components play different roles, and these components form an overall system that influences each other. In the traditional system, although halogen-based activators represented by organic amine halides (such as diethylamine hydrochloride, triethylamine hydrobromide, etc.) have excellent soldering activity and can achieve rapid tinning and good soldering effects, the residual halogen ions they leave can cause double hazards: on the one hand, during the service life of electronic products, the residues can easily lead to the corrosion of circuit boards; on the other hand, during waste treatment, halogenated substances such as polybrominated diphenyl ethers are difficult to degrade, can pollute the ecological chain through groundwater, and ultimately accumulate in human adipose tissue, causing persistent health damage.

[0004] Organic acids, as components of activators, are used in the preparation of halogen-free fluxes due to their advantages such as low corrosiveness and high reliability. However, the acidic components remaining after soldering of organic acid activators may still corrode the circuit board in a humid and hot environment. At the same time, their activity is significantly different from that of traditional halogen-based activators, resulting in the electrical performance and reliability after soldering being difficult to reach an ideal state. Summary of the Invention

[0005] The purpose of the present invention is to solve the disadvantages existing in the prior art, and to propose a halogen-free flux for solder paste and a preparation method thereof.

[0006] A halogen-free flux for solder paste, the raw materials of which by mass include: 20 - 40 parts of hydrogenated rosin, 1 - 5 parts of maleic anhydride, 0.1 - 0.4 parts of p-toluenesulfonic acid, 1 - 5 parts of activated mesoporous silica, 10 - 20 parts of composite organic acid activator, 5 - 10 parts of thixotropic agent, 1 - 3 parts of tackifier, 1 - 2 parts of pH regulator, 1 - 2 parts of surfactant, 1 - 2 parts of chelating agent, 1 - 2 parts of antioxidant, and 50 - 80 parts of organic solvent.

[0007] Preferably, the activated mesoporous silica is mesoporous silica activated by silane coupling agent KH550.

[0008] Among them, the BET specific surface area of the mesoporous silica is 500 - 800m 2 / g.

[0009] Preferably, the composite organic acid activator includes: 2-ethylhexanoic acid, sebacic acid, nano cerium oxide; the mass ratio of 2-ethylhexanoic acid, sebacic acid, nano cerium oxide is 10:1 - 5:0.1 - 1.

[0010] Preferably, the thixotropic agent is polyethylene oxide wax and / or hydrogenated castor oil.

[0011] Preferably, the tackifier includes at least one of hydrogenated rosin glyceride, phenylmelamine, and erucic acid amide.

[0012] Preferably, the pH regulator is triisopropanolamine.

[0013] Preferably, the antioxidant is antioxidant 1010 and / or antioxidant 246.

[0014] Preferably, the organic solvent is at least one of diethylene glycol diethyl ether, hexyl diethylene glycol, isodecanol, and isophorone.

[0015] Preferably, the surfactant is fatty alcohol polyoxyethylene ether.

[0016] Preferably, the chelating agent is EDTA disodium salt.

[0017] The preparation method of the halogen-free soldering flux for the above solder paste includes the following steps:

[0018] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to toluene, reflux and react at 120 - 140°C for 2 - 4 h under nitrogen protection, cool to 50 - 60°C, add deionized water and stir for 10 - 20 min, separate and remove the lower aqueous phase, distill off toluene under reduced pressure, and dry in vacuum; add activated mesoporous silica and mix, ball mill for 1 - 2 h under nitrogen protection, and add to the organic solvent, heat and stir to dissolve to obtain a mixed material;

[0019] S2. Adjust the temperature of the mixed material to 90 - 100°C, add the composite organic acid activator and antioxidant and stir evenly, adjust the temperature to 60 - 70°C, and add the thixotropic agent, tackifier, pH regulator, surfactant, and chelating agent thereto and stir evenly.

[0020] Beneficial effects:

[0021] The present invention uses the compounding of 2-ethylhexanoic acid and sebacic acid, which not only forms a eutectic system at the initial stage to reduce the overall melting point, but also gradually releases activity above 210°C and provides excellent oxide removal ability. Nano cerium oxide forms a coordination bond with the carboxylate through surface oxygen vacancies, delays the volatilization of organic acids at low temperatures, and releases active components at high temperatures, realizing staged activation, greatly enhancing the activity of the soldering flux, and simultaneously reducing the content of ionizable acidic substances.

[0022] The present invention utilizes the combination of hydrogenated rosin and maleic anhydride to generate a modified rosin with a three-dimensional network structure, and embeds mesoporous silica into the gaps between rosin molecular chains. This not only creates exhaust channels during soldering, promotes the rapid escape of volatile substances, reduces the accumulation of volatile residues, and ensures a clean surface after soldering; but also cooperates with a composite organic acid activator, making the system have good wettability, low corrosivity to solder joints, and can significantly improve the surface insulation resistance; further cooperating with triisopropanolamine and a chelating agent, it can further neutralize residual acidity, chelate metal ions, and reduce electrochemical corrosion in a humid and hot environment.

[0023] The present invention not only does not contain halogen compounds, is green and environmentally friendly, but also the exhaust channels of mesoporous silica promote wetting. The residues after soldering are light in color and clear, and at the same time, the electrical performance meets the requirements of high reliability, effectively improving the soldering performance of the solder paste.

[0024] Through innovative component design and preparation processes, the present invention effectively solves the long-existing technical contradictions in the field of halogen-free soldering fluxes, providing an ideal material solution for high-reliability electronic packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a comparison chart of the expansion rates of the soldering fluxes obtained in Example 5 and Comparative Examples 1-2.

[0026] Figure 2 It is a comparison chart of the surface insulation resistance and organic residue rate of the solder pastes in Example 5 group and Comparative Examples 1-2 group. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be further illustrated below in conjunction with specific embodiments.

[0028] Example 1

[0029] A halogen-free soldering flux for solder paste, the raw materials of which include: 20 g of hydrogenated rosin, 1 g of maleic anhydride, 0.1 g of p-toluenesulfonic acid, 1 g of activated mesoporous silica, 10 g of composite organic acid activator, 5 g of hydrogenated castor oil, 1 g of hydrogenated rosin glyceride, 1 g of triisopropanolamine, 1 g of AEO-9, 1 g of EDTA disodium salt, 1 g of antioxidant 1010, and 50 g of diethylene glycol diethyl ether.

[0030] The activated mesoporous silica is obtained by mixing silane coupling agent KH550 and mesoporous silica in a mass ratio of 0.05:10. The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano-cerium oxide in a mass ratio of 10:1:0.1.

[0031] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0032] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 100 g of toluene. Under nitrogen protection, reflux and react at 120 °C for 2 h. Cool to 50 °C, add 50 g of deionized water, stir for 10 min, separate and remove the lower aqueous phase by liquid separation, remove toluene by vacuum distillation, and dry under vacuum; add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 200 r / min for 1 h, with a ball-to-material ratio of 10:1. Add to diethylene glycol diethyl ether, heat and stir to dissolve to obtain a mixed material;

[0033] S2. Adjust the temperature of the mixed material to 90 °C, add a composite organic acid activator and antioxidant 1010, stir evenly, adjust the temperature to 60 °C, and add hydrogenated castor oil, hydrogenated rosin glyceride, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0034] Example 2

[0035] A halogen-free soldering flux for solder paste, the raw materials of which include: 40 g of hydrogenated rosin, 5 g of maleic anhydride, 0.4 g of p-toluenesulfonic acid, 5 g of activated mesoporous silica, 20 g of composite organic acid activator, 10 g of hydrogenated castor oil, 3 g of phenylmelamine, 2 g of triisopropanolamine, 2 g of AEO-9, 2 g of disodium EDTA, 2 g of antioxidant 246, and 80 g of hexyl diethylene glycol.

[0036] The activated mesoporous silica is obtained by mixing silane coupling agent KH550 and mesoporous silica in a mass ratio of 0.15:10. The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano-cerium oxide in a mass ratio of 10:5:1.

[0037] The preparation method of the above halogen-free soldering flux for solder paste includes the following steps:

[0038] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 200 g of toluene. Under nitrogen protection, reflux and react at 140 °C for 4 h. Cool to 60 °C, add 100 g of deionized water, stir for 20 min, separate and remove the lower aqueous phase by liquid separation, remove toluene by vacuum distillation, and dry under vacuum; add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 300 r / min for 2 h, with a ball-to-material ratio of 10:2. Add to hexyl diethylene glycol, heat and stir to dissolve to obtain a mixed material;

[0039] S2. Adjust the temperature of the mixed material to 100 °C, add a composite organic acid activator and antioxidant 246, stir evenly, adjust the temperature to 70 °C, and add hydrogenated castor oil, phenylmelamine, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0040] Example 3

[0041] A halogen-free soldering flux for solder paste, the raw materials of which include: 25 g of hydrogenated rosin, 4 g of maleic anhydride, 0.2 g of p-toluenesulfonic acid, 4 g of activated mesoporous silica, 12 g of composite organic acid activator, 9 g of oxidized polyethylene wax, 1.5 g of erucamide, 1.8 g of triisopropanolamine, 1.3 g of AEO-9, 1.8 g of EDTA disodium salt, 1.3 g of antioxidant 1010, and 70 g of isodecanol.

[0042] The activated mesoporous silica is obtained by mixing silane coupling agent KH550 and mesoporous silica in a mass ratio of 0.08:10. The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano cerium oxide in a mass ratio of 10:4:0.3.

[0043] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0044] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 180 g of toluene. Under nitrogen protection, reflux and react at a temperature of 125 °C for 3.5 h. Cool to 52 °C, add 90 g of deionized water and stir for 12 min. Separate and remove the lower aqueous phase, remove toluene by vacuum distillation, and dry in vacuum; add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 280 r / min for 80 min, with a ball-to-material ratio of 10:1.7, and add to isodecanol, heat and stir to dissolve to obtain a mixed material;

[0045] S2. Adjust the temperature of the mixed material to 92 °C, add the composite organic acid activator and antioxidant 1010 and stir evenly. Adjust the temperature to 68 °C, and add oxidized polyethylene wax, erucamide, triisopropanolamine, AEO-9, and EDTA disodium salt and stir evenly.

[0046] Example 4

[0047] A halogen-free soldering flux for solder paste, the raw materials of which include: 35 g of hydrogenated rosin, 2 g of maleic anhydride, 0.3 g of p-toluenesulfonic acid, 2 g of activated mesoporous silica, 18 g of composite organic acid activator, 7 g of oxidized polyethylene wax, 2.5 g of phenylmelamine, 1.2 g of triisopropanolamine, 1.7 g of AEO-9, 1.2 g of EDTA disodium salt, 1.7 g of antioxidant 2461, and 60 g of isophorone.

[0048] The activated mesoporous silica is obtained by mixing silane coupling agent KH550 and mesoporous silica in a mass ratio of 0.12:10. The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano cerium oxide in a mass ratio of 10:2:0.7.

[0049] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0050] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 120 g of toluene. Under nitrogen protection, reflux and react at 135 °C for 2.5 h. Cool to 58 °C, add 70 g of deionized water, stir for 18 min, separate and remove the lower aqueous phase, remove toluene by vacuum distillation, and dry under vacuum. Add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 220 r / min for 100 min, with a ball-to-material ratio of 10:1.3. Add to isophorone, heat and stir to dissolve to obtain a mixed material;

[0051] S2. Adjust the temperature of the mixed material to 98 °C, add a composite organic acid activator and antioxidant 246, stir evenly, adjust the temperature to 62 °C, and add polyethylene wax oxide, phenylmelamine, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0052] Example 5

[0053] A halogen-free soldering flux for solder paste, the raw materials of which include: 30 g of hydrogenated rosin, 3 g of maleic anhydride, 0.25 g of p-toluenesulfonic acid, 3 g of activated mesoporous silica, 15 g of composite organic acid activator, 8 g of polyethylene wax oxide, 2 g of hydrogenated rosin glyceride, 1.5 g of triisopropanolamine, 1.5 g of AEO-9, 1.5 g of disodium EDTA, 1.5 g of antioxidant 1010, and 65 g of diethylene glycol diethyl ether.

[0054] The activated mesoporous silica is prepared by dispersing mesoporous silica in toluene, adding silane coupling agent KH550, refluxing and treating at 80 - 100 °C for 2 - 4 h, filtering, washing, and drying under vacuum. The mass ratio of silane coupling agent KH550 to mesoporous silica is 0.1:10.

[0055] The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano-cerium oxide in a mass ratio of 10:3:0.5; during the preparation process, nano-cerium oxide, 2-ethylhexanoic acid, and sebacic acid are mixed according to the ratio and sent to a ball mill for treatment for 12 min (ball-to-material ratio 8:1, 200 r / min).

[0056] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0057] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 150 g of toluene. Under nitrogen protection, reflux and react at 130 °C for 3 h. Cool to 55 °C, add 80 g of deionized water, stir for 15 min, separate and remove the lower aqueous phase, adjust the pH value of the system to neutral, distill off toluene under reduced pressure, and dry in vacuum; add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 260 r / min for 90 min, with a ball-to-material ratio of 10:1.5. The ball milling medium is zirconia balls, and the ball milling vacuum degree is 10 - 50 Pa. Add it to diethylene glycol diethyl ether, heat and stir to dissolve to obtain a mixed material;

[0058] S2. Adjust the temperature of the mixed material to 95 °C, add a composite organic acid activator and antioxidant 1010, stir evenly, adjust the temperature to 65 °C, and add polyethylene wax oxide, hydrogenated rosin glyceride, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0059] Comparative Example 1

[0060] A halogen-free soldering flux for solder paste, whose raw materials include: 30 g of hydrogenated rosin, 3 g of maleic anhydride, 0.25 g of p-toluenesulfonic acid, 3 g of activated mesoporous silica, 15 g of composite organic acid activator, 8 g of polyethylene wax oxide, 2 g of hydrogenated rosin glyceride, 1.5 g of triisopropanolamine, 1.5 g of AEO-9, 1.5 g of disodium EDTA, 1.5 g of antioxidant 1010, and 65 g of diethylene glycol diethyl ether.

[0061] The activated mesoporous silica is prepared by dispersing mesoporous silica in toluene, adding silane coupling agent KH550, refluxing and treating at 80 - 100 °C for 2 - 4 h, filtering, washing, and drying in vacuum. The mass ratio of silane coupling agent KH550 to mesoporous silica is 0.1:10.

[0062] The composite organic acid activator is composed of 2-ethylhexanoic acid and sebacic acid in a mass ratio of 10:3.

[0063] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0064] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 150 g of toluene. Under nitrogen protection, reflux and react at 130 °C for 3 h. Cool to 55 °C, add 80 g of deionized water, stir for 15 min, separate and remove the lower aqueous phase, adjust the pH value of the system to neutral, distill off toluene under reduced pressure, and dry in vacuum; add activated mesoporous silica and mix. Under nitrogen protection, ball mill at a speed of 260 r / min for 90 min, with a ball-to-material ratio of 10:1.5, and add it to diethylene glycol diethyl ether, heat and stir to dissolve to obtain a mixed material;

[0065] S2. Adjust the temperature of the mixture to 95°C, add a composite organic acid activator and antioxidant 1010, stir evenly, adjust the temperature to 65°C, and add polyethylene wax oxide, hydrogenated rosin glyceride, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0066] Comparative Example 2

[0067] A halogen-free soldering flux for solder paste, the raw materials of which include: 30 g of hydrogenated rosin, 3 g of maleic anhydride, 0.25 g of p-toluenesulfonic acid, 3 g of activated mesoporous silica, 15 g of composite organic acid activator, 8 g of polyethylene wax oxide, 2 g of hydrogenated rosin glyceride, 1.5 g of triisopropanolamine, 1.5 g of AEO-9, 1.5 g of disodium EDTA, 1.5 g of antioxidant 1010, and 65 g of diethylene glycol diethyl ether.

[0068] The activated mesoporous silica is prepared by dispersing mesoporous silica in toluene, adding silane coupling agent KH550, refluxing at 80 - 100°C for 2 - 4 h, filtering, washing, and vacuum drying. The mass ratio of silane coupling agent KH550 to mesoporous silica is 0.1:10.

[0069] The composite organic acid activator is composed of 2-ethylhexanoic acid, sebacic acid, and nano-ceria in a mass ratio of 10:3:0.5; during the preparation process, nano-ceria, 2-ethylhexanoic acid, and sebacic acid are mixed according to the ratio, and sent to a ball mill for treatment for 12 min (ball-to-material ratio 8:1, 200 r / min).

[0070] The preparation method of the above-mentioned halogen-free soldering flux for solder paste includes the following steps:

[0071] S1. Add hydrogenated rosin, maleic anhydride, and p-toluenesulfonic acid to 150 g of toluene, under nitrogen protection, reflux at 130°C for 3 h, cool to 55°C, add 80 g of deionized water, stir for 15 min, separate and remove the lower aqueous phase, adjust the pH value of the system to neutral, distill off toluene under reduced pressure, vacuum dry, add to diethylene glycol diethyl ether, and heat and stir to dissolve to obtain a mixture;

[0072] S2. Adjust the temperature of the mixture to 95°C, add a composite organic acid activator and antioxidant 1010, stir evenly, adjust the temperature to 65°C, add activated mesoporous silica thereto, ball mill at a speed of 260 r / min for 90 min, with a ball-to-material ratio of 10:1.5, and add polyethylene wax oxide, hydrogenated rosin glyceride, triisopropanolamine, AEO-9, and disodium EDTA, and stir evenly.

[0073] The properties of the soldering fluxes obtained in Example 5 and Comparative Examples 1-2 were detected. Among them, the spreading rate was tested with reference to IPC-J-STD-004B "Requirements for Soldering Fluxes", and the copper mirror corrosivity was tested with reference to IPC-TM-650.

[0074] The results are shown in Table 1 and Figure 1 as follows. The soldering flux obtained in Example 5 had the least spatter, the least smoke, and no copper mirror corrosivity; at the same time, it had the highest spreading rate, which was superior to those of Comparative Examples 1-2 (P < 0.05).

[0075] Table 1 Properties of the soldering fluxes obtained in Example 5 and Comparative Examples 1-2

[0076]

[0077]

[0078] Note: * represents the amount of smoke. The more *, the greater the smoke.

[0079] The soldering fluxes obtained in Example 5 and Comparative Examples 1-2 were combined with tin-based alloy powder (SnCu 0.7 ) in a mass ratio of 5:95 to form solder pastes. Subsequently, the solder pastes of each group were tested with reference to SJ / T 11186-2019 "General Specification for Solder Paste". The solder ball level and wettability of the solder paste in the Example 5 group were Grade 1; while the solder ball level of the solder paste in the Comparative Example 1 group was Grade 2 and the wettability was Grade 1; the solder ball level and wettability of the solder paste in the Comparative Example 2 group were both Grade 2.

[0080] A vertical laser reflow soldering furnace was used, and a PCB aluminum substrate was set. The temperature in the laser area was controlled at 260 ± 10 °C. The solder pastes of the above Example 5 group and Comparative Examples 1-2 group were pre-coated, and then manual chip mounting and reflow soldering were carried out to weld a 1 mm × 5 mm × 50 mm wire; and the surface insulation resistance of the wire was tested with reference to the IPC-TM-650 standard. After the welding of each group was completed, it was cleaned and dried, the total weight of the wire was tested, and the organic residue rate was calculated.

[0081] Organic residue rate = (total weight of wire - mass of tin-based alloy powder) ÷ (mass of solder paste - mass of tin-based alloy powder) × 100%

[0082] As Figure 2 shown, the surface insulation resistance of the solder paste in the Example 5 group was the highest and the organic residue rate was the lowest, which was superior to those of the Comparative Examples 1-2 group (P < 0.05).

[0083] As mentioned above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A halogen-free flux for solder paste, characterized in that: The raw materials include, by mass, 20-40 parts of hydrogenated rosin, 1-5 parts of maleic anhydride, 0.1-0.4 parts of p-toluenesulfonic acid, 1-5 parts of activated mesoporous silica, 10-20 parts of composite organic acid activator, 5-10 parts of thixotropic agent, 1-3 parts of tackifier, 1-2 parts of pH regulator, 1-2 parts of surfactant, 1-2 parts of chelating agent, 1-2 parts of antioxidant and 50-80 parts of organic solvent.

2. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The activated mesoporous silica is mesoporous silica activated by using a silane coupling agent KH550.

3. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The composite organic acid activator comprises: 2-ethylhexanoic acid, sebacic acid and nano-cerium oxide; the mass ratio of 2-ethylhexanoic acid, sebacic acid and nano-cerium oxide is 10:1-5:0.1-1.

4. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The thixotropic agent is oxidized polyethylene wax and / or hydrogenated castor oil.

5. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: Tackifiers include: At least one of hydrogenated rosin glycerol ester, benzoguanamine and erucamide.

6. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The pH adjuster was triisopropanolamine.

7. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The antioxidant is antioxidant 1010 and / or antioxidant 246.

8. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The organic solvent is at least one of diethylene glycol diethyl ether, hexyl diglycol, isodecyl alcohol and isophorone.

9. The halogen-free soldering flux for solder paste according to claim 1, characterized in that: The surfactant is fatty alcohol polyoxyethylene ether; the chelating agent is EDTA disodium salt.

10. A method for preparing the halogen-free flux for solder paste according to any one of claims 1 to 9, characterized in that: The steps include: S1. Add hydrogenated rosin, maleic anhydride and p-toluenesulfonic acid to toluene, reflux at 120-140°C for 2-4h under nitrogen protection, cool to 50-60°C, add deionized water and stir for 10-20min, separate and remove the lower aqueous phase, remove toluene by vacuum distillation, and dry in vacuo; add activated mesoporous silica and mix, ball mill for 1-2h under nitrogen protection, add to an organic solvent, heat and stir to dissolve to obtain a mixture; S2. Adjust the temperature of the mixture to 90-100° C., add the composite organic acid active agent and antioxidant and stir evenly, adjust the temperature to 60-70° C., add the thixotropic agent, viscosity enhancer, pH adjuster, surfactant and chelating agent and stir evenly.

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