A low-temperature lead-free formic acid solder paste and its preparation method and application

Abstract

The present invention relates to a low-temperature lead-free formic acid solder paste, its preparation method and application, belonging to the technical field of welding materials. The low-temperature lead-free formic acid solder paste is made of raw materials in the following mass percentages: 85-90% of alloy solder powder and 10-15% of flux; the flux includes raw materials in the following parts by weight: 10-15 parts of activator, 1-5 parts of rheological agent, 20-30 parts of organic solvent, 20-30 parts of pasting agent, 3-5 parts of surfactant and 0.1-0.5 part of corrosion inhibitor. The activator is a composition of dimerized fatty acid, glycolic acid and ethylenediaminetetraacetic acid. The organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol. The pasting agent is polyethylene glycol. The solder paste of the present invention is suitable for low-temperature welding processes at 120-150 °C, can reduce welding energy consumption, and has a low residue rate after welding, without subsequent cleaning, which can improve the production environment and reduce production costs.

Description

Technical Field

[0001] The present invention relates to the technical field of welding materials, and particularly relates to a low-temperature lead-free formic acid solder paste, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous development of electronic technology, the trend of miniaturization and high performance of electronic devices has become increasingly obvious, posing higher requirements for the welding quality of electronic components. At the same time, the enhancement of environmental awareness has made lead-free welding an inevitable trend in the industry development. Low-temperature lead-free solder paste can effectively avoid damage to heat-sensitive components by achieving welding at a lower temperature, and can be used for special chip interconnection, as well as for chips of medical / special detectors.

[0003] Solder paste, as a key material in electronic welding, is composed of components such as alloy powder and flux. Traditional tin-lead solder poses a serious threat to the environment and human health due to the toxicity of lead, and is gradually being replaced by lead-free solder. In the lead-free solder system, low-temperature lead-free solder paste has become a research and application hotspot due to its unique advantages, such as reducing welding temperature, reducing energy consumption, protecting sensitive components, etc. However, existing low-temperature lead-free solder paste still has some problems in practical applications, such as insufficient welding strength, poor wettability, easy generation of voids, etc., which limit its further application in the manufacture of high-end electronic products. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a low-temperature lead-free formic acid solder paste, a preparation method thereof, and an application thereof.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] In the first aspect, the present invention provides a low-temperature lead-free formic acid solder paste, which is made from the following raw materials by mass percentage: 85 - 90% of alloy solder powder and 10 - 15% of flux;

[0007] The flux includes the following raw materials by weight: 10 - 15 parts of activator, 1 - 5 parts of rheological agent, 20 - 30 parts of organic solvent, 20 - 30 parts of pasting agent, 3 - 5 parts of surfactant, and 0.1 - 0.5 part of corrosion inhibitor. The activator is a composition of dimerized fatty acid, glycolic acid, and ethylenediaminetetraacetic acid. The organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol. The pasting agent is polyethylene glycol.

[0008] The activators of the flux are hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid. Among them, hydrogenated dimer acid has a long-chain structure and can still maintain a flowing state at low temperatures (such as 150-180°C). It can quickly penetrate the pores of the oxide film. The dicarboxylic acid groups (-COOH) it contains can adsorb metal oxides at multiple points, with a high local concentration at low temperatures, accelerating the metal saponification reaction and effectively improving the activity of the flux at low temperatures. Glycolic acid has a low boiling point (112°C) and can quickly vaporize and release H + , without high-temperature thermal activation. The six-dentate coordination structure of ethylenediaminetetraacetic acid (EDTA) can still efficiently chelate metal ions at low temperatures, forming stable water-soluble complexes, overcoming the insufficient low-temperature reaction rate. At the same time, the EDTA complex can be washed with water, reducing the residue risk. Through the synergistic mechanism of low-temperature volatile activation, molecular structure adaptability and efficient metal ion chelation of hydrogenated dimer acid, glycolic acid and EDTA, the oxide layer removal ability and wettability of the flux in a low-temperature environment are significantly improved. This system breaks through the limitation of traditional organic acids relying on high-temperature reactions and provides key technical support for low-temperature lead-free soldering and precision electronic packaging.

[0009] The organic solvents selected in the present invention are tetrahydrofurfuryl alcohol and cyclohexanol. Tetrahydrofurfuryl alcohol remains liquid in the low-temperature range of 80-150°C, ensuring the uniform dispersion of the activator. Tetrahydrofurfuryl alcohol (boiling point 178°C) and glycolic acid (boiling point 112°C) form a boiling point gradient, releasing the activator in stages in the range of 80-150°C to avoid premature depletion of the activator at low temperatures. Cyclohexanol has a cyclic structure, which can reduce the crystallization tendency, enabling the flux to still maintain fluidity at -20°C and avoiding flux stratification caused by low-temperature crystallization. Therefore, the organic solvents selected in the present invention can improve the spreading rate of the solder paste at low temperatures, reduce the residue amount of the flux at low temperatures, and at the same time can increase the storage period of the solder paste, reduce the precipitation and crystallization situation, and improve the storage stability.

[0010] Preferably, the mass ratio of the hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid is 1:(5-8):(2-4).

[0011] Preferably, the mass ratio of the tetrahydrofurfuryl alcohol and cyclohexanol is 1:(1-3).

[0012] Preferably, the rheological agent is a polyamide rheological agent.

[0013] Preferably, the hydrogenated dimer acid is C36 hydrogenated dimer acid.

[0014] Preferably, the surfactant is a fluorocarbon surfactant.

[0015] Preferably, the corrosion inhibitor is an imidazoline-based corrosion inhibitor.

[0016] Preferably, the alloy solder powder comprises raw materials in the following mass percentages: 25-35% of indium, 1-2% of silver, 0.8-1% of aluminum, and 0.5-1% of antimony, with the balance being tin. Indium and tin form a Sn-In eutectic phase (melting point 120-135°C), which is lower than the Sn-Bi eutectic (139°C). Adding silver can increase the surface tension of the molten alloy and improve the wettability of the solder paste. Aluminum and antimony can improve the mechanical properties of the alloy.

[0017] Specifically, the preparation method of the alloy solder powder comprises the following steps:

[0018] Mix all the metal elements and melt them, then make solder and perform centrifugal sieving to obtain the alloy solder powder.

[0019] Preferably, in the preparation method of the alloy solder powder, the melting temperature is 1000-1500°C and the melting time is 1-3 h.

[0020] Preferably, the particle size of the alloy solder powder is the size of powder No. 4-6.

[0021] Second, the present invention provides a preparation method of the low-temperature lead-free formic acid solder paste described in the first aspect, comprising the following steps:

[0022] S1. Place the formula amount of activator, rheological agent, organic solvent, pasting agent, surfactant, and corrosion inhibitor in a reaction kettle, heat to 40-60°C, start stirring, raise the temperature to 90-95°C while stirring and then keep warm, and continue to stir for 5-10 min to obtain a mixture. Cool the mixture, grind it, refrigerate it at 0-10°C, and then warm it back to room temperature to obtain the flux;

[0023] S2. Gradually add the alloy solder powder to the flux and stir evenly to obtain the low-temperature lead-free formic acid solder paste.

[0024] Preferably, the stirring speed in step S1 is 5000-6000 rpm and the temperature rising time is 3-5 min.

[0025] Preferably, in step S1, cool the mixture to 25-30°C, then grind it to less than 2 μm, refrigerate it at 0-10°C for 12-18 h, and then warm it back to room temperature.

[0026] Third, the present invention provides an application of the low-temperature lead-free formic acid solder paste described in the first aspect in chip encapsulation.

[0027] Specifically, the chip encapsulation includes the following processes:

[0028] Coating process, coating the formic acid solder paste for chip encapsulation on the pads of the substrate to form a solder layer;

[0029] The soldering process involves mounting the chip on the solder layer and introducing formic acid gas for reflow soldering, so that the chip is soldered onto the pad.

[0030] Preferably, the time for introducing formic acid gas is 120 - 180 s.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] In the solder paste of the present invention, a rosin-free flux formulation is used to avoid the generation of smoke and harmful substances during soldering. The selection of specific activators and organic solvents enables gradient activation in a wide temperature range (120 - 150 °C). When reaching 112 °C (the boiling point of glycolic acid), the oxidation layer removal is initiated to meet the requirements of lead-free low-temperature solder. At 140 - 160 °C, the azeotrope of tetrahydrofurfuryl alcohol and cyclohexanol dissolves the sub-oxide layer, ensuring the cleaning effect and wettability, and increasing the spreading area of the alloy solder; when the temperature is above 160 °C, the remaining tetrahydrofurfuryl alcohol forms a film to prevent solder joints from having voids and reduce residual ion contamination. Therefore, the solder paste of the present invention is suitable for low-temperature soldering processes at 120 - 150 °C, can reduce soldering energy consumption, has a low residue rate after soldering, does not require subsequent cleaning, improves the production environment, and reduces production costs. Detailed implementation manners

[0033] To better illustrate the purpose, technical solutions, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0034] The sources of the materials used in the following examples and comparative examples are as follows:

[0035] C36 hydrogenated dimer acid: The manufacturer is Jiexike Commercial Trade (Shanghai) Co., Ltd., and the model is dimer acid IPU22;

[0036] Formic acid: The manufacturer is Shanghai Macklin Biochemical Co., Ltd.;

[0037] Polyamide rheology modifier: The manufacturer is Shenzhen Xinzhihe New Materials Co., Ltd., and the model is 6500;

[0038] Fluorocarbon surfactant: The manufacturer is Shenzhen Xinzhihe New Materials Co., Ltd., and the brand is FS - 3100;

[0039] Imidazoline corrosion inhibitor: 2 - methylimidazole, the manufacturer is Shenzhen Jintenglong Industry Co., Ltd.

[0040] Other materials, reagents, etc. used in the detailed implementation manners can be obtained from commercial channels without special instructions.

[0041] Example 1

[0042] A low-temperature lead-free formic acid solder paste is made of raw materials in the following mass percentages: 87% alloy solder powder and 13% flux;

[0043] The flux includes raw materials in the following parts by weight: 12 parts activator, 2 parts rheological agent, 28 parts organic solvent, 23 parts pasting agent, 4 parts surfactant, and 0.2 part corrosion inhibitor; the activator is a composition of hydrogenated dimer acid, glycolic acid, and ethylenediaminetetraacetic acid with a mass ratio of 1:6:3; the organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol with a mass ratio of 1:2; the pasting agent is polyethylene glycol PEG-1000; the rheological agent is a polyamide rheological agent, the hydrogenated dimer acid is C36 hydrogenated dimer acid, the surfactant is a fluorocarbon surfactant, and the corrosion inhibitor is an imidazoline-based corrosion inhibitor;

[0044] The alloy solder powder includes raw materials in the following mass percentages: 30% indium, 1.5% silver, 1% aluminum, and 0.6% antimony, with the balance being tin.

[0045] Specifically, the preparation method of the alloy solder powder includes the following steps:

[0046] Mix and melt all the metal elements at 1200 °C for 2 h, make solder, then centrifuge and screen to obtain alloy solder powder with a size of No. 4 powder.

[0047] The preparation method of the low-temperature lead-free formic acid solder paste includes the following steps:

[0048] S1. Place the formula amount of activator, rheological agent, organic solvent, pasting agent, surfactant, and corrosion inhibitor in a reaction kettle, heat to 50 °C, start stirring at a speed of 5500 rpm, while stirring, heat up to 95 °C at a speed of 5 °C / min and keep warm, then continue stirring for 8 min to obtain a mixture. Cool the mixture to 25 °C, grind it to 2 μm, refrigerate it at 5 °C for 12 h, and then warm it to room temperature to obtain the flux;

[0049] S2. Gradually add the alloy solder powder to the flux and stir evenly to obtain the low-temperature lead-free formic acid solder paste.

[0050] Example 2

[0051] A low-temperature lead-free formic acid solder paste is made of raw materials in the following mass percentages: 85% alloy solder powder and 15% flux;

[0052] The soldering flux comprises raw materials in the following parts by weight: 10 parts of activator, 1 part of rheological agent, 20 parts of organic solvent, 20 parts of pasting agent, 3 parts of surfactant and 0.1 part of corrosion inhibitor; the activator is a composition of hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid with a mass ratio of 1:5:2; the organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol with a mass ratio of 1:1; the pasting agent is polyethylene glycol PEG-1000; the rheological agent is a polyamide rheological agent, the hydrogenated dimer acid is C36 hydrogenated dimer acid, the surfactant is a fluorocarbon surfactant, and the corrosion inhibitor is an imidazoline-based corrosion inhibitor;

[0053] The alloy solder powder comprises raw materials in the following mass percentages: indium 25%, silver 1%, aluminum 0.8% and antimony 0.5%, and the balance is tin.

[0054] Specifically, the preparation method of the alloy solder powder comprises the following steps:

[0055] Mix and melt all the metal elements at 1000 °C for 3 h, make solder, then centrifuge and screen to obtain alloy solder powder with a size of No. 4 powder.

[0056] The preparation method of the low-temperature lead-free formic acid tin paste comprises the following steps:

[0057] S1. Put the formula amount of activator, rheological agent, organic solvent, pasting agent, surfactant and corrosion inhibitor into a reaction kettle, heat to 40 °C, start stirring at a speed of 6000 rpm, while stirring, heat up to 90 °C at a speed of 5 °C / min and then keep warm, and continue to stir for 3 min to obtain a mixture; cool the mixture to 20 °C, grind it to 1 μm, refrigerate it at 0 °C for 12 h, and then warm it to room temperature to obtain the soldering flux;

[0058] S2. Gradually add the alloy solder powder to the soldering flux and stir evenly to obtain the low-temperature lead-free formic acid tin paste.

[0059] Example 3

[0060] A low-temperature lead-free formic acid tin paste is made of the following raw materials in mass percentages: 90% of alloy solder powder and 10% of soldering flux;

[0061] The soldering flux comprises the following raw materials by weight: 15 parts of activator, 5 parts of rheological agent, 30 parts of organic solvent, 30 parts of pasting agent, 5 parts of surfactant and 0.5 part of corrosion inhibitor; the activator is a composition of hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid with a mass ratio of 1:8:4; the organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol with a mass ratio of 1:3; the pasting agent is polyethylene glycol PEG-1000; the rheological agent is a polyamide rheological agent, the hydrogenated dimer acid is C36 hydrogenated dimer acid, the surfactant is a fluorocarbon surfactant, and the corrosion inhibitor is an imidazoline-based corrosion inhibitor;

[0062] The alloy solder powder comprises the following raw materials by mass percentage: 35% of indium, 2% of silver, 1% of aluminum and 1% of antimony, and the balance is tin.

[0063] Specifically, the preparation method of the alloy solder powder comprises the following steps:

[0064] Mix and melt all the metal elements at 1500 °C for 1 h to make solder, then perform centrifugal sieving to obtain alloy solder powder with a particle size of 30-40 μm.

[0065] The preparation method of the low-temperature lead-free formic acid tin paste comprises the following steps:

[0066] S1. Place the formula amount of activator, rheological agent, organic solvent, pasting agent, surfactant and corrosion inhibitor in a reaction kettle, heat to 60 °C, start stirring at a speed of 5000 rpm, while stirring, raise the temperature to 95 °C at a speed of 5 °C / min and keep warm, then continue stirring for 5 min to obtain a mixture; cool the mixture to 30 °C, grind it to 2 μm, refrigerate it at 10 °C for 18 h, and then warm it to room temperature to obtain the soldering flux;

[0067] S2. Gradually add the alloy solder powder to the soldering flux and stir evenly to obtain the low-temperature lead-free formic acid tin paste.

[0068] Example 4

[0069] The difference between Example 4 and Example 1 is that: the addition amount of the activator remains unchanged, and the mass ratio of the hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid is 3:6:1.

[0070] Example 5

[0071] The difference between Example 5 and Example 1 is that: the addition amount of the activator remains unchanged, and the mass ratio of the hydrogenated dimer acid, glycolic acid and ethylenediaminetetraacetic acid is 1:3:6.

[0072] Comparative Example 1

[0073] The difference between Comparative Example 1 and Example 1 is that; the addition amount of the activator remains unchanged, hydrogenated dimer acid is not added, and glycolic acid and ethylenediaminetetraacetic acid with a mass ratio of 2:1 are used to make up the missing amount.

[0074] Comparative Example 2

[0075] The difference between Comparative Example 2 and Example 1 is that: the addition amount of the activator remains unchanged, glycolic acid is not added, and hydrogenated dimer acid and ethylenediaminetetraacetic acid with a mass ratio of 1:3 are used to make up the missing amount.

[0076] Comparative Example 3

[0077] The difference between Comparative Example 3 and Example 1 is that: the addition amount of the activator remains unchanged, ethylenediaminetetraacetic acid is not added, and hydrogenated dimer acid and formic acid with a mass ratio of 1:6 are used to make up the missing amount.

[0078] Comparative Example 4

[0079] The difference between Comparative Example 4 and Example 1 is that: the addition amount of the organic solvent remains unchanged, tetrahydrofurfuryl alcohol is not added, and an equal amount of cyclohexanol is used to make up the missing amount.

[0080] Comparative Example 5

[0081] The difference between Comparative Example 5 and Example 1 is that: the addition amount of the organic solvent remains unchanged, cyclohexanol is not added, and an equal amount of tetrahydrofurfuryl alcohol is used to make up the missing amount.

[0082] Performance Test

[0083] The following tests were respectively carried out on the solder pastes obtained from Examples 1-5 and Comparative Examples 1-5:

[0084] 1. Wettability test: The test was carried out with reference to the standard "IPC-TM-650 Solder Paste - Wettability Test";

[0085] 2. Thermal collapse: According to the IPC-TM-650 2.4.35 test method, a thermal collapse test was carried out using a 0.1 mm thick stencil. The test conditions were to place the solder paste on a 150 °C metal hot plate for 10 minutes. The qualified standard was that the spread of the solder paste edge did not exceed 20-25% of the original printing width, and there was no obvious collapse or deformation of the solder paste shape.

[0086] 3. Flux residue rate: Weigh solder paste with a mass of m1. In the specific example, the proportion of flux in the solder paste is ρ. Coat the solder paste on an aluminum substrate pad with a mass of m2 and dimensions of 30 mm × 30 mm × 0.1 mm to form a solder layer. Then place the chip on a heating plate at a constant temperature of 120°C and preheat it for 15 s. Next, attach the chip to the solder layer on the pad surface, and then place it in a formic acid furnace for soldering. Control the oxygen content in the formic acid furnace to be below 50 ppm, and introduce formic acid gas for 180 s for reflow soldering. The soldering temperature is 150°C, and the solder spreads and forms solder joints, so that the chip is soldered to the pad. After holding for 60 s, remove it, and then directly weigh it to obtain its mass m3. The post-soldering residue rate f is calculated according to the following formula: f = [m3 - m2 - (1 - ρ)m1)] / (ρm1). The specific data is shown in Table 1.

[0087] 4. Test methods for spread rate and black spots: After taking the solder paste of each group through SMT automatic solder paste printing, chip mounting, and reflow soldering, conduct spread rate tests according to the JIS Z 3197 standard. And determine whether black spots are generated based on whether black contamination appears around the solder joints.

[0088] 5. Stability test: Viscosity change is the core characterization parameter of solder paste stability. Since it directly reflects the component uniformity, chemical stability, and anti-environmental interference ability, the smaller the viscosity change, the more stable the solder paste. After storing each group of solder paste at 30°C for 90 days, use a viscometer to detect the viscosity before and after storage, and compare the viscosity values at 10 rpm to characterize the viscosity change. Among them, viscosity change % = (viscosity value at 10 rpm after storage - viscosity value at 10 rpm before storage) / viscosity value at 10 rpm before storage. Then, according to the aforementioned wettability test, detect the wettability data after storing for 90 days. The results are shown in Table 1.

[0089] Table 1 Performance test data of each group of solder paste

[0090]

[0091] As can be seen from Table 1, combined with the data of Examples 1 - 5, when the mass ratio of hydrogenated dimer acid, glycolic acid, and ethylenediaminetetraacetic acid is 1: (5 - 8): (2 - 4), the solder paste has better wettability, flux residue rate, and viscosity change performance, that is, the better the low-temperature resistance of the solder paste.

[0092] From the data of Example 1 and Comparative Examples 1 - 3 in Table 1, when any one of hydrogenated dimer acid, glycolic acid, and ethylenediaminetetraacetic acid in the activator is missing, the wettability, spread rate, and thermal collapse performance of the solder paste significantly decrease. This shows that the three components of the activator have a synergistic effect and can significantly improve the low-temperature performance of the solder paste.

[0093] As can be seen from the data of Example 1 and Comparative Examples 4-5 in Table 1, when any one of tetrahydrofurfuryl alcohol and cyclohexanol in the organic solvent is missing, the storage stability, wettability and spread rate of the solder paste are significantly reduced, indicating that tetrahydrofurfuryl alcohol and cyclohexanol can synergistically improve the spread rate of the solder paste at low temperature, reduce the amount of residual ions at low temperature, and at the same time increase the storage period of the solder paste, reduce the crystallization precipitation, and improve the storage stability.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A low-temperature lead-free formic acid solder paste, characterized in that, It is made from the following raw materials by mass percentage: 85 - 90% alloy solder powder and 10 - 15% flux; The flux comprises the following raw materials by weight: 10 - 15 parts activator, 1 - 5 parts rheological agent, 20 - 30 parts organic solvent, 20 - 30 parts pasting agent, 3 - 5 parts surfactant, and 0.1 - 0.5 part corrosion inhibitor. The activator is a composition of hydrogenated dimer acid, glycolic acid, and ethylenediaminetetraacetic acid with a mass ratio of 1:(5 - 8):(2 - 4). The organic solvent is a composition of tetrahydrofurfuryl alcohol and cyclohexanol. The pasting agent is polyethylene glycol.

2. The low-temperature lead-free formic acid solder paste according to claim 1, wherein The mass ratio of the tetrahydrofurfuryl alcohol to the cyclohexanol is 1:(1 - 3).

3. The low-temperature lead-free formic acid tin paste according to claim 1, characterized in that, The raw materials are selected from at least one of (I)-(IV): (I) The rheological agent is a polyamide rheological agent; (II) The hydrogenated dimer acid is C36 hydrogenated dimer acid; (III) The surfactant is a fluorocarbon surfactant; (IV) The corrosion inhibitor is an imidazoline - type corrosion inhibitor.

4. The low-temperature lead-free formic acid tin paste according to claim 1, wherein The alloy solder powder comprises the following raw materials by mass percentage: 25 - 35% indium, 1 - 2% silver, 0.8 - 1% aluminum, and 0.5 - 1% antimony, with the balance being tin.

5. The low-temperature lead-free formic acid tin paste according to claim 4, wherein The preparation method of the alloy solder powder comprises the following steps: Mix and melt all the metal elements, make solder, and then perform centrifugal sieving to obtain the alloy solder powder.

6. The low-temperature lead-free formic acid solder paste according to claim 5, wherein In the preparation method of the alloy solder powder, the melting temperature is 1000 - 1500 °C, and the melting time is 1 - 3 h.

7. The preparation method of the low-temperature lead-free formic acid tin paste according to any one of claims 1-6, characterized in that, It comprises the following steps: S1. Place the formula - amount of activator, rheological agent, organic solvent, pasting agent, surfactant, and corrosion inhibitor in a reaction kettle, heat to 40 - 60 °C, start stirring, while stirring, heat up to 90 - 95 °C and then keep warm, and continue stirring for 5 - 10 min to obtain a mixture. Cool the mixture, grind it, refrigerate it, and then warm it up to obtain the flux; S2. Gradually add the alloy solder powder to the flux and stir evenly to obtain the low - temperature lead - free formic acid tin paste.

8. Application of the low - temperature lead - free formic acid tin paste according to any one of claims 1 - 6 in chip packaging.

9. The application according to claim 8, wherein The chip packaging includes the following processes: Coating process: Coat the formic acid tin paste for chip packaging on the solder pads of the substrate to form a solder layer; Soldering process: Mount the chip on the solder layer, and introduce formic acid gas for reflow soldering so that the chip is soldered to the solder pads.

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