Formic acid tin paste with ultra-low porosity, preparation method and application thereof

Abstract

The application relates to a formic acid tin paste with ultra-low porosity, a preparation method and application thereof, the tin paste is prepared from the following raw materials in percentage by mass: 85-90% of alloy welding powder and 10-15% of a fluxing agent; the fluxing agent comprises the following raw materials in parts by weight: 5-10 parts of an activator, 1-5 parts of a rheological agent, 30-40 parts of an organic solvent, 20-30 parts of a paste forming agent, 3-5 parts of a surfactant and 0.1-0.5 parts of a corrosion inhibitor, the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isomeric tridecanol polyoxyethylene ether and N-vinyl caprolactam, and the activator is a composition of castor oil acid, ethylenediaminetetraacetic acid and succinic acid amine salt. The surfactant and the activator in the fluxing agent of the application synergistically act, ensure the welding surface and the solder cleanliness, improve the wettability and the flowability, reduce the porosity, make the solder uniformly spread and fill, form a high-quality welding spot, and significantly reduce the tin paste porosity.

Description

Technical Field

[0001] This invention relates to the technical field of formic acid solder paste, specifically to an ultra-low void ratio formic acid solder paste, its preparation method, and its application. Background Technology

[0002] With the booming development of the machinery and electronics industries and increasingly fierce market competition, companies are paying more and more attention to product quality control. Against this backdrop, the solderability of metal materials and electronic component leads, as well as post-soldering cleaning, have become pressing technical and quality challenges in the production process.

[0003] Early solder paste production often used rosin-based flux, which suffered from numerous problems, including high viscosity, poor soldering effect, excessive residue on the PCB surface, difficulty in cleaning, and unsatisfactory electrical performance, severely impacting product quality. Residue not only affected the PCB's appearance but also led to increased contact resistance, decreased insulation performance, and PCB corrosion. Furthermore, the chlorofluorocarbon (CFC) cleaning agents used in traditional cleaning processes are environmentally harmful and have been banned in many countries.

[0004] To meet market demand, water-soluble solder paste was developed. This type of solder paste leaves easy-to-clean flux residue after soldering, requiring only water or warm water for thorough washing, without causing corrosion or environmental pollution. However, void ratio is a crucial indicator in the quality control of soldered products. The presence of voids weakens the mechanical properties of the solder joint, and with prolonged use, the enlarged voids may lead to solder joint breakage. Furthermore, voids formed during soldering can affect interfacial thermal conductivity, causing localized overheating and reducing solder joint reliability.

[0005] Most water-soluble solder pastes currently on the market are non-rosin-based, with poor activity and a tendency to produce large voids. Although adding halogens can significantly improve soldering activity and reduce void ratio, halogens can severely corrode electronic components, affecting product reliability and lifespan. Therefore, developing a halogen-free, low-voidity formic acid solder paste has become an urgent need to solve these problems. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a formate solder paste with ultra-low void ratio, its preparation method and application.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, the present invention provides a formate solder paste with ultra-low void ratio, made from the following raw materials in weight percentages: 85-90% alloy solder powder and 10-15% flux; the flux comprises the following raw materials in parts by weight: 5-10 parts activator, 1-5 parts rheology modifier, 30-40 parts organic solvent, 20-30 parts paste-forming agent, 3-5 parts surfactant and 0.1-0.5 parts corrosion inhibitor, wherein the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam, and the activator is a composition of ricinoleic acid, ethylenediaminetetraacetic acid and succinic acid amine salt.

[0008] The surfactant in the flux of this invention is a compound of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether (nonionic), and N-vinylcaprolactam (polar polymer). The lauryl amphoteric imidazoline surfactant contains both anionic and cationic groups, exhibiting both hydrophilic and lipophilic properties. It adsorbs onto the surface of solder particles through electrostatic interaction, reducing the solid-liquid interfacial tension. During the soldering process, the amphoteric groups react with metal oxides, assisting the activator in removing the oxide film and improving solder spreadability. The isotridecyl alcohol polyoxyethylene ether, as a nonionic surfactant, provides hydrophilicity through its polyoxyethylene segments and hydrophobicity through its alkyl segments. It can form stable micelles in organic solvents, encapsulating and uniformly dispersing solder powder particles, preventing solder powder sedimentation. N-vinylcaprolactam contains a polar cyclic amide structure, which combines hydrophilicity and certain surface activity. This enhances the compatibility of the surfactant system with the flux-forming agent, improves the overall rheological properties of the flux, and ensures the solder paste maintains a stable form during printing and reflow, reducing bubble trapping caused by viscosity unevenness. Simultaneously, its polar amide groups interact with the Sn in the solder. 2+ Complexation reduces the flux evaporation rate and prolongs the activator's action time.

[0009] The activator consists of ricinoleic acid, ethylenediaminetetraacetic acid (EDTA), and succinate ammonium salt. Ricinoleic acid, as an organic acid activator, contains long-chain fatty acids and hydroxyl groups. When heated to the welding temperature (180~230℃), it decomposes into free carboxylic acid, which reacts with the oxide film on the metal surface (such as CuO, SnO2, etc.) to generate volatile metal carboxylate salts, thoroughly removing the oxide layer, exposing a clean metal surface, and avoiding incomplete welding and localized gas encapsulation caused by residual oxidation. EDTA, as a chelating activator, complexes metal ions on the solder pad or solder powder surface through multidentate coordination, preventing oxide film regeneration, while simultaneously promoting the penetration of ricinoleic acid into deeper oxide films, ensuring the continuity and thoroughness of the activation reaction. Ammonium succinate is a neutralizing activator. Its amine groups can neutralize the excess acidity of ricinoleic acid, forming a weakly acidic buffer system, controlling the activation reaction rate, and preventing the gas from bursting due to excessively fast reaction. At the same time, the amine decomposes to produce ammonia when heated, which can inhibit the secondary oxidation of tin at the high temperature of soldering, further ensuring the cleanliness of the solder joint surface (SnO2 + 4NH3 → SnN4 + 2H2O↑).

[0010] The surfactant of this invention reduces the surface tension of the solder paste, making the solder spread more easily on the soldering surface, while forming a protective film to prevent the solder and soldering surface from re-oxidizing at high temperatures. The activator removes oxides from the soldering surface and the solder surface, allowing the solder to better contact and bond with the metal surface, while reducing the viscosity of the solder paste. The synergistic effect of the surfactant and activator ensures the cleanliness of the soldering surface and the solder, improves the wettability and flowability of the solder, and reduces voids caused by surface tension, viscosity, and oxides. During the soldering process, this synergistic effect allows the solder to spread and fill more evenly, forming high-quality solder joints, thereby significantly reducing the void rate of the solder paste.

[0011] Preferably, the mass ratio of the lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam is 1:(3-5):(1-2). More preferably, the mass ratio of the lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam is 1:4:1.5.

[0012] Preferably, the mass ratio of ricinoleic acid, ethylenediaminetetraacetic acid and succinic acid amine salt is 1:(0.5-2):(1-2).

[0013] Preferably, the rheology modifier is a polyamide rheology modifier.

[0014] Preferably, the organic solvent is at least two of benzyl alcohol, polyethylene glycol, 2-methyl-2,4-pentanediol, terpineol, and diethylene glycol ethyl ether.

[0015] Preferably, the ointment-forming agent is hydroxyethyl cellulose and / or polyvinylpyrrolidone.

[0016] Preferably, the corrosion inhibitor is an imidazoline-based corrosion inhibitor.

[0017] Preferably, the alloy solder powder comprises the following raw materials by mass percentage: indium 10-25%, silver 0.5-2%, cobalt 0.8-1% and antimony 0.5-1%, with the balance being tin.

[0018] Specifically, the preparation method of the alloy welding powder includes the following steps: All the elemental metals are mixed and melted to obtain liquid solder. The liquid solder is then centrifuged, atomized, and sieved to obtain alloy solder powder.

[0019] Preferably, in the preparation method of the alloy welding powder, the melting temperature is 1500-1600℃, the melting time is 1-3h, and the particle size of the alloy welding powder is 20-40μm.

[0020] Secondly, the present invention provides a method for preparing formic acid solder paste with ultra-low void ratio, comprising the following steps: S1. Place the activator, rheology modifier, organic solvent, paste-forming agent, surfactant and corrosion inhibitor in a reaction vessel according to the formula amount, heat to 40-60℃ and start stirring, while stirring and raising the temperature to 90-95℃ and holding it at that temperature, then continue stirring for 5-10 minutes to obtain a mixture. Cool the mixture, grind it, refrigerate it and then warm it to obtain the flux. S2. Gradually add alloy solder powder to the flux and stir evenly to obtain the formic acid solder paste with ultra-low void ratio.

[0021] Preferably, the stirring speed in step S1 is 5000-6000 rpm, and the heating time is 3-10 min.

[0022] Preferably, in step S1, the mixture is cooled to 25-30°C, then ground to a particle size ≤2μm, refrigerated at 0-10°C for 12-18h, and then warmed to room temperature.

[0023] Thirdly, the present invention provides the application of the ultra-low void ratio formate paste in the first aspect in the packaging of chips.

[0024] Specifically, chip packaging includes the following steps: In the coating process, the formate solder paste for chip packaging is applied to the pads of the substrate to form a solder layer; In the soldering process, the chip is mounted on the solder layer, and formic acid gas is introduced for reflow soldering, so that the chip is soldered onto the pad.

[0025] Preferably, the formic acid gas is introduced for 120-180 seconds.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: The surfactant in the flux of this invention is a compound of lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam. This compound enhances solder powder dispersibility, reduces interfacial tension, promotes wetting and spreading, strengthens compatibility with the solder paste, improves rheology, reduces bubble trapping, and prolongs the activator's action time. The activator, composed of ricinoleic acid, EDTA, and amine succinate, removes the oxide layer, prevents oxide film regeneration, controls the activation reaction rate, and inhibits secondary tin oxidation. The surfactant and activator in the flux work synergistically to ensure the cleanliness of the solder surface and solder, improve wettability and flowability, reduce voids, and enable uniform solder spreading and filling, forming high-quality solder joints and significantly reducing the solder paste void rate. Detailed Implementation

[0027] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0028] The sources of the raw materials used in the following examples and comparative examples are as follows: Ricinoleic acid: CAS number 61789-44-4; Ethylenediaminetetraacetic acid (EDTA): CAS number 60-00-4; Ammonium succinate: CAS number 15574-09-1; Hydroxyethyl cellulose: Manufacturer: Daicel, Japan; Model: SP500; Polyvinylpyrrolidone: CAS No. 9003-39-8; Lauryl amphoteric imidazoline surfactant: disodium lauroyl amphoteric diacetate, CAS number 14350-97-1; Isotridecyl alcohol polyoxyethylene ether: CAS No. 9043-30-5; N-Vinylcaprolactam: CAS No. 2235-00-9; Polyamide rheology modifier: Manufacturer: Shenzhen Xinzhihe New Materials Co., Ltd., Model: 6500; Imidazolyl corrosion inhibitor: 2-methylimidazole, CAS number 693-98-1.

[0029] Unless otherwise specified, all other materials, reagents, etc. used in the examples and comparative examples are commercially available.

[0030] Example 1 A formate solder paste with ultra-low void ratio is made from the following raw materials in weight percentages: 86% alloy solder powder and 14% flux. The flux comprises the following raw materials in parts by weight: 8 parts activator, 3 parts rheology modifier, 35 parts organic solvent, 25 parts paste-forming agent, 4 parts surfactant and 0.3 parts corrosion inhibitor; The activator is a composition of ricinoleic acid, ethylenediaminetetraacetic acid and succinic acid amine salt in a mass ratio of 1:1.2:1.6; the rheology modifier is a polyamide rheology modifier; the organic solvent is terpineol and diethylene glycol ethyl ether in a mass ratio of 1:1; the paste-forming agent is polyvinylpyrrolidone; the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam in a mass ratio of 1:4:1.5; and the corrosion inhibitor is an imidazoline corrosion inhibitor. The alloy solder powder comprises the following raw materials by mass percentage: 20% indium, 1% silver, 0.9% cobalt and 0.6% antimony, with the balance being tin; The method for preparing the alloy welding powder includes the following steps: All metal elements were mixed and melted at 1550℃ for 2 hours to obtain liquid solder. The liquid solder was gradually and slowly added dropwise to the high-speed rotating disk of a centrifugal atomizing device. Under nitrogen protection, the liquid solder was rotated and ejected at high speed and cooled into granular solder. The granular solder was then sieved to obtain alloy solder powder with a particle size of 30μm. The method for preparing the ultra-low void ratio formic acid solder paste includes the following steps: S1. Place the activator, rheology modifier, organic solvent, paste-forming agent, surfactant, and corrosion inhibitor in a reaction vessel according to the formula amount, heat to 50°C, start stirring at 5500 rpm, while stirring, raise the temperature to 93°C at a rate of 8°C / min and hold at that temperature, then continue stirring for 10 min to obtain a mixture. Cool the mixture to 25°C, grind it to a particle size of 2 μm, refrigerate at 5°C for 16 h, and then warm it to room temperature to obtain the flux. S2. Gradually add alloy solder powder to the flux and stir evenly to obtain the formic acid solder paste with ultra-low void ratio.

[0031] Example 2 A formate solder paste with ultra-low void ratio is made from the following raw materials in weight percentages: 85% alloy solder powder and 15% flux. The flux comprises the following raw materials in parts by weight: 5 parts activator, 1 part rheology modifier, 30 parts organic solvent, 20 parts paste-forming agent, 3 parts surfactant and 0.1 parts corrosion inhibitor; The activator is a composition of ricinoleic acid, ethylenediaminetetraacetic acid and succinic acid amine salt in a mass ratio of 1:0.5:1; the rheology modifier is a polyamide rheology modifier; the organic solvent is benzyl alcohol and polyethylene glycol in a mass ratio of 1:1; the paste-forming agent is hydroxyethyl cellulose; the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam in a mass ratio of 1:3:1; and the corrosion inhibitor is an imidazoline corrosion inhibitor. The alloy solder powder comprises the following raw materials by mass percentage: 10% indium, 0.5% silver, 0.8% cobalt and 0.5% antimony, with the balance being tin; The method for preparing the alloy welding powder includes the following steps: All metal elements were mixed and melted at 1500℃ for 3 hours to obtain liquid solder. The liquid solder was gradually and slowly added dropwise to the high-speed rotating disk of a centrifugal atomizing device. Under nitrogen protection, the liquid solder was rotated and ejected at high speed and cooled into granular solder. The granular solder was then sieved to obtain alloy solder powder with a particle size of 20μm. The method for preparing the ultra-low void ratio formic acid solder paste includes the following steps: S1. Place the activator, rheology modifier, organic solvent, paste-forming agent, surfactant, and corrosion inhibitor in a reaction vessel according to the formula amount, heat to 40°C, start stirring at 5000 rpm, while stirring, raise the temperature to 90°C at a rate of 5°C / min and hold at that temperature, then continue stirring for 10 min to obtain a mixture. Cool the mixture to 25°C, grind it to a particle size of 2 μm, refrigerate at 0°C for 12 h, and then warm it to room temperature to obtain the flux. S2. Gradually add alloy solder powder to the flux and stir evenly to obtain the formic acid solder paste with ultra-low void ratio.

[0032] Example 3 A formate solder paste with ultra-low void ratio is made from the following raw materials in weight percentages: 90% alloy solder powder and 10% flux. The flux comprises the following raw materials in parts by weight: 10 parts activator, 5 parts rheology modifier, 40 parts organic solvent, 30 parts paste-forming agent, 5 parts surfactant and 0.5 parts corrosion inhibitor; The activator is a composition of ricinoleic acid, ethylenediaminetetraacetic acid and succinic acid amine salt in a mass ratio of 1:2:2; the rheology modifier is a polyamide rheology modifier; the organic solvent is 2-methyl-2,4-pentanediol and terpineol in a mass ratio of 1:1; the paste-forming agent is polyvinylpyrrolidone; the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam in a mass ratio of 1:5:2; and the corrosion inhibitor is an imidazoline corrosion inhibitor. The alloy solder powder comprises the following raw materials by mass percentage: 25% indium, 2% silver, 1% cobalt and 1% antimony, with the balance being tin; The method for preparing the alloy welding powder includes the following steps: All metal elements were mixed and melted at 1600℃ for 1 hour to obtain liquid solder. The liquid solder was gradually and slowly added dropwise to the high-speed rotating disk of a centrifugal atomizing device. Under nitrogen protection, the liquid solder was rotated and ejected at high speed and cooled into granular solder. The granular solder was then sieved to obtain alloy solder powder with a particle size of 40μm. The method for preparing the ultra-low void ratio formic acid solder paste includes the following steps: S1. Place the activator, rheology modifier, organic solvent, paste-forming agent, surfactant, and corrosion inhibitor in a reaction vessel according to the formula amount, heat to 60°C, start stirring at 6000 rpm, while stirring, raise the temperature to 95°C at a rate of 10°C / min and hold at that temperature, then continue stirring for 5 min to obtain a mixture. Cool the mixture to 25°C, grind it to a particle size of 2 μm, refrigerate at 10°C for 18 h, and then warm it to room temperature to obtain the flux. S2. Gradually add alloy solder powder to the flux and stir evenly to obtain the formic acid solder paste with ultra-low void ratio.

[0033] Example 4 The difference between Example 4 and Example 1 is that the amount of surfactant added remains the same, and the mass ratio of the lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam is 1:1.5:4.

[0034] Example 5 The difference between Example 5 and Example 1 is that the amount of surfactant added remains unchanged, and the mass ratio of the lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam is 1.5:4:1.

[0035] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the amount of surfactant added remains the same, lauryl amphoteric imidazoline surfactant is not added, and isomeric tridecyl alcohol polyoxyethylene ether and N-vinyl caprolactam in a mass ratio of 4:1.5 are used to make up the missing amount.

[0036] Comparative Example 2 The difference between Comparative Example 1 and Example 1 is that the amount of surfactant added remains the same, but isomeric tridecyl alcohol polyoxyethylene ether is not added. Instead, lauryl amphoteric imidazoline surfactant and N-vinylcaprolactam in a mass ratio of 1:1.5 are used to make up for the missing amount.

[0037] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the amount of surfactant added remains the same, N-vinylcaprolactam is not added, and lauryl amphoteric imidazoline surfactant and isomeric tridecyl alcohol polyoxyethylene ether in a mass ratio of 1:4 are used to make up the missing amount.

[0038] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the amount of activator added remains the same, ricinoleic acid is not added, and the missing amount is made up by ethylenediaminetetraacetic acid and succinic acid amine salt in a mass ratio of 1.2:1.6.

[0039] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that the amount of activator added remains the same, ethylenediaminetetraacetic acid is not added, and ricinoleic acid and succinic acid amine salt with a mass ratio of 1:1.6 are used to make up the missing amount.

[0040] Comparative Example 6 The difference between Comparative Example 6 and Example 1 is that the amount of activator added remains the same, but succinic acid amine salt is not added, and the missing amount is made up by ricinoleic acid and ethylenediaminetetraacetic acid in a mass ratio of 1:1.2.

[0041] Performance testing The solder pastes from Examples 1-5 and Comparative Examples 1-6 were tested as follows: 1. Testing of void ratio and expansion rate After SMT automatic solder paste printing, component placement, and reflow soldering, the void ratio was tested; the spread rate was tested according to JIS Z 3197 standard. The results are shown in Table 1.

[0042] 2. Stability Test After storing each group of solder paste samples at 25℃ for 3 months, the viscosity values ​​were measured. Compared with the initial viscosity value, a viscosity increase of less than 10 Pa·s was considered normal, a viscosity increase of 13-16 Pa·s was considered deteriorated, and a viscosity increase of 29-32 Pa·s was considered scrap. The results are shown in Table 1.

[0043] Table 1 Performance test results of each group of solder paste samples Example 1 2.1 91.6 normal Example 2 3.0 90.8 normal Example 3 2.6 91.1 normal Example 4 4.2 87.8 normal Example 5 5.0 87.0 normal Comparative Example 1 10.1 73.8 Deterioration Comparative Example 2 10.5 71.5 scrapped Comparative Example 3 9.7 71.5 scrapped Comparative Example 4 10.0 72.8 Deterioration Comparative Example 5 10.6 71.6 Deterioration Comparative Example 6 9.3 73.6 scrapped As shown in Table 1, and in conjunction with the data from Examples 1-5, when the mass ratio of lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam is 1:(3-5):(1-2), the solder paste exhibits better performance in terms of void ratio, spread rate, and stability.

[0044] As can be seen from the data in Example 1 and Comparative Examples 1-3 in Table 1, when any one of the surfactants lauryl amphoteric imidazoline surfactant, isomeric tridecyl alcohol polyoxyethylene ether, and N-vinylcaprolactam is missing, the void ratio, spreading rate, and stability of the solder paste decrease significantly. This may be because the three components of the activator have a synergistic effect, which can improve the dispersibility of solder powder, reduce interfacial tension, promote wetting and spreading, enhance compatibility with the paste-forming agent, improve rheology, reduce bubble retention, and prolong the action time of the activator, thereby significantly improving the performance of the solder paste.

[0045] As can be seen from the data in Example 1 and Comparative Examples 4-5 in Table 1, when any one of the activators, namely ricinoleic acid, ethylenediaminetetraacetic acid, and succinate ammonium salt, is missing, the void ratio, expansion rate, and stability of the solder paste decrease. This may be because the activators can synergistically remove the oxide layer, prevent oxide film regeneration, control the activation reaction rate, and inhibit secondary oxidation of tin.

[0046] In summary, the surfactants and activators in the flux work synergistically to ensure the cleanliness of the soldering surface and solder, improve wettability and flowability, reduce voids, and enable the solder to spread and fill evenly, forming high-quality solder joints and significantly reducing the void rate of solder paste. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the present invention.

Claims

1. A formic acid solder paste with ultra-low void ratio, characterized in that, It is made from the following raw materials in the following weight percentages: 85-90% alloy solder powder and 10-15% flux; the flux comprises the following raw materials in the following weight parts: 5-10 parts activator, 1-5 parts rheology modifier, 30-40 parts organic solvent, 20-30 parts paste-forming agent, 3-5 parts surfactant and 0.1-0.5 parts corrosion inhibitor, wherein the surfactant is a composition of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam, and the mass ratio of lauryl amphoteric imidazoline surfactant, isotridecyl alcohol polyoxyethylene ether and N-vinylcaprolactam is 1:(3-5):(1-2); the activator is a composition of ricinoleic acid, ethylenediaminetetraacetic acid and succinate amine salt, and the mass ratio of ricinoleic acid, ethylenediaminetetraacetic acid and succinate amine salt is 1:(0.5-2):(1-2).

2. The formic acid solder paste with ultra-low void ratio as described in claim 1, characterized in that, The raw material is selected from at least one of (I)-(IV): (I) The rheology modifier is a polyamide rheology modifier; (II) The organic solvent is at least two of benzyl alcohol, polyethylene glycol, 2-methyl-2,4-pentanediol, terpineol, and diethylene glycol ethyl ether; (III) The ointment-forming agent is hydroxyethyl cellulose and / or polyvinylpyrrolidone; (IV) The corrosion inhibitor is an imidazoline corrosion inhibitor.

3. The formic acid solder paste with ultra-low void ratio as described in claim 1, characterized in that, The alloy solder powder comprises the following raw materials by mass percentage: indium 10-25%, silver 0.5-2%, cobalt 0.8-1% and antimony 0.5-1%, with the balance being tin.

4. The formic acid solder paste with ultra-low void ratio as described in claim 3, characterized in that, The method for preparing the alloy welding powder includes the following steps: All metal elements are mixed and melted to obtain liquid solder. The liquid solder is centrifuged, atomized, and then sieved to obtain the alloy solder powder.

5. The formic acid solder paste with ultra-low void ratio as described in claim 4, characterized in that, In the preparation method of the alloy welding powder, the melting temperature is 1500-1600℃, the melting time is 1-3h, and the particle size of the alloy welding powder is 20-40μm.

6. A method for preparing the formic acid solder paste with ultra-low void ratio according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Place the activator, rheology modifier, organic solvent, paste-forming agent, surfactant and corrosion inhibitor in a reaction vessel according to the formula amount, heat to 40-60℃ and start stirring, while stirring and raising the temperature to 90-95℃ and holding it at that temperature, then continue stirring for 5-10 minutes to obtain a mixture. Cool the mixture, grind it, refrigerate it and then warm it to obtain the flux. S2. Gradually add alloy solder powder to the flux and stir evenly to obtain the formic acid solder paste with ultra-low void ratio.

7. The application of the ultra-low void ratio formate solder paste according to any one of claims 1-5 in chip packaging.

8. The application as described in claim 7, characterized in that, Chip packaging includes the following processes: In the coating process, the formate solder paste for chip packaging is applied to the pads of the substrate to form a solder layer; In the soldering process, the chip is mounted on the solder layer, and formic acid gas is introduced for reflow soldering, so that the chip is soldered onto the pad.