A composite flux for improving stability of lead-free tin paste and a preparation method of corresponding lead-free tin paste

Abstract

This invention provides a composite additive for improving the stability of lead-free solder paste and a corresponding method for preparing lead-free solder paste, relating to the field of soldering materials technology. The composite additive prepared by this invention is composed of tert-butylhydroquinone, glycerol, jojoba oil, squalane, and lanolin. Through the strong moisturizing, strong antioxidant, high penetration, and small-molecule oil sealing of the solder paste surface, the shelf life of lead-free solder paste can be extended to 150 days. Furthermore, the lead-free solder paste formulated based on this composite additive exhibits high stability, excellent slumpability, and soldering performance.

Description

Technical Field

[0001] This invention relates to the field of soldering materials technology, and in particular to a composite additive for improving the stability of lead-free solder paste and a corresponding method for preparing lead-free solder paste. Background Technology

[0002] Solder paste is a key material in the reflow soldering process of surface mount technology (SMT) packaged electronic products. Under suitable soldering process conditions, it forms a metallurgical bond with the materials being soldered, creating reliable solder joints and enabling electrical and mechanical interconnection between electronic components and circuit boards.

[0003] Solder paste is mainly composed of solder powder and flux, and its quality directly affects the reliability of soldering and the performance of electronic products. Currently, the mainstream lead-free solder paste used in SMT belongs to the SnAgCu system (i.e., the solder powder in the paste is an alloy of Sn, Ag, and Cu, with widely used grades such as SAC305, SAC307, or SAC387). However, the stability of lead-free solder paste is a major drawback, determining its lifespan and consequently affecting the sealing, impedance and capacitance, insulation, and stability of the packaged circuit. Typical solder pastes have different lifespans, such as three months or six months, and have strict requirements for storage and use. For example, it needs to be stored at low temperature and away from light; before use, it needs to be allowed to warm up naturally for about 3-4 hours and thoroughly stirred; once opened, the solder paste should be used within 24 hours; and after substrate printing, it should be placed in a reflow oven for encapsulation within 4-6 hours. In actual production, electronics factories achieve very high yields and first-pass yields when using small batches of lead-free solder paste for trial production. However, during mass production, they find significant viscosity variations and poor consistency, primarily due to the poor stability of the solder paste quality. Improper preparation, storage, and use of solder paste can easily lead to problems such as drying and sanding, which in turn affect the printing stability of the solder paste, resulting in open circuits in the solder pattern, insufficient solder, solder spikes, edge buildup and burrs, cold solder joints, bridging, collapse, and residual solder powder.

[0004] Therefore, the preparation of lead-free solder paste with stable viscosity, excellent quality, and high stability is an inevitable requirement for the development of solder paste. Summary of the Invention

[0005] In view of this, the present invention provides a composite additive for improving the stability of lead-free solder paste and a corresponding method for preparing lead-free solder paste. By using the strong moisturizing, strong antioxidant, high penetration and small molecule oil of the composite additive to seal the surface of the solder paste, the shelf life of lead-free solder paste can be extended to 150 days. Moreover, the lead-free solder paste formulated based on this stabilizing composite additive has high stability in use, the problem of surface drying is effectively alleviated, and the solder paste has good collapse and soldering performance.

[0006] The first objective of this invention is to provide a composite additive for improving the stability of lead-free solder paste, the composite additive being composed of tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in a weight ratio of 3:6.7:(3.5-5):(1-3):(1.3-3.5).

[0007] A second objective of this invention is to provide a lead-free solder paste containing a composite additive, wherein, by weight percentage, the lead-free solder paste comprises 84wt%-87wt% tin powder and 13wt%-16wt% flux, wherein the tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder, and the flux comprises the following components: 28wt%-34wt% solvent, 6.6wt% activator, 35wt% film-forming agent, 7wt% thixotropic agent, 1.7wt% surfactant, and 16wt%-21wt% composite additive.

[0008] Preferably, the tin powder has a particle size of 25-38 μm.

[0009] Preferably, the solvent is at least two selected from diethylene glycol hexyl ether, diethylene glycol octyl ether, 2-methyl-2,4-pentanediol, triethylene glycol butyl ether, tripropylene glycol methyl ether, triethylene glycol dimethyl ether, diethylene glycol monobutyl ether, diethylene glycol dibutyl ether, diethylene glycol diethyl ether, propylene glycol phenyl ether, glycerol, tetrahydrofurfuryl alcohol, and 2-ethyl-1,3-hexanediol.

[0010] More preferably, the solvent is composed of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a weight ratio of 1:1.4.

[0011] Preferably, the activator is at least two of the following: adipic acid, azelaic acid, succinic acid, succinic acid, salicylic acid, hydroxyethyl acid, maleic acid, malonic acid, octanoic acid, phthalic acid, oleic acid, citric acid, glycine, tartaric acid, p-tert-butylbenzoic acid, dimethylolpropionic acid, and dibromosuccinic acid.

[0012] More preferably, the activator is composed of adipic acid and succinic acid in a weight ratio of 1:1.2.

[0013] Preferably, the film-forming agent is composed of water-white hydrogenated rosin and disproportionated rosin in a weight ratio of 2.5:1.

[0014] Preferably, the thixotropic agent is hydrogenated castor oil.

[0015] Preferably, the surfactant is composed of polyethylene glycol 6000 and Pingpingjia O-10 in a weight ratio of 1:0.7.

[0016] A third objective of this invention is to provide a method for preparing lead-free solder paste containing composite additives, comprising the following steps:

[0017] S1. Measure half of the solvent, maintain a vacuum environment and a temperature of 40°C, and add glycerol, jojoba oil, squalane, tert-butylhydroquinone and lanolin in sequence according to the proportion. After mixing, let it stand at room temperature to obtain the composite additive, which is denoted as mixture A.

[0018] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0019] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After mixing, the flux for lead-free solder paste is obtained.

[0020] S4. Mix the flux and solder powder evenly in the specified ratio to obtain lead-free solder paste.

[0021] In the composite additives provided by this invention, tert-butylhydroquinone has strong antioxidant properties, which can slow down the contact between solder powder on the solder paste surface and oxygen, thereby reducing the probability of surface oxidation and drying. Glycerol, jojoba oil, squalane, and lanolin all have moisturizing effects. In addition, jojoba oil has high permeability, which can increase the dispersibility of flux in solder paste. Squalane is a small molecule moisturizer, which is more likely to float on the surface when the solder paste is standing, playing a role in protecting and lubricating the surface of the solder paste. Lanolin has a thickening effect, which can adjust the viscosity of flux and solder paste without affecting the stability of the solder paste.

[0022] The reagents and raw materials used in this invention are all commercially available.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) This invention utilizes the synergistic effect between glycerol, jojoba oil, squalane, tert-butylhydroquinone and lanolin to reduce the contact between the solder powder alloy on the surface of the solder paste and oxygen in the air, and uses small molecule oil to seal the surface of the solder paste, which greatly reduces the probability of the solder paste surface and interior drying out.

[0025] (2) The storage life of lead-free solder paste prepared by adding the composite additives formulated in this invention is greatly improved, and the storage life can reach 150 days under low temperature storage.

[0026] (3) The composite additive and lead-free solder paste preparation method provided by the present invention are simple to operate, the lead-free solder paste has excellent storage stability and use stability, and good soldering performance. Attached Figure Description

[0027] Figure 1This refers to the viscosity change of the lead-free solder paste prepared in Example 1 of this invention;

[0028] Figure 2 This is the 7-day collapse test result of the lead-free solder paste prepared in Example 1 of this invention;

[0029] Figure 3 This is the 90-day collapse test result of the lead-free solder paste prepared in Example 1 of this invention;

[0030] Figure 4 The results are the collapse test results of the lead-free solder paste prepared in Example 1 of this invention after 150 days. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0032] Unless otherwise specified, the test methods or experimental methods described in the following examples are all conventional methods; unless otherwise specified, the raw materials and additives are obtained from conventional commercial sources or prepared by conventional methods.

[0033] The viscosity of the solder paste in the examples and comparative examples was tested using a MALCOM PCU-205 solder paste viscosity tester.

[0034] Example 1

[0035] A lead-free solder paste containing composite additives, by mass percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: 31.2 wt% solvent, 6.6 wt% activator, 35 wt% film-forming agent, 7 wt% thixotropic agent, 1.7 wt% surfactant, and 18.5 wt% composite additives.

[0036] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0037] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0038] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0039] The thixotropic agent is hydrogenated castor oil;

[0040] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0041] The composite additive is composed of tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in a weight ratio of 3:6.7:4.8:2.6:1.4.

[0042] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0043] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in sequence according to the proportion. After mixing thoroughly, let it stand to room temperature to obtain the composite additive, which is denoted as mixture A.

[0044] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0045] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0046] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0047] The lead-free solder paste prepared in Example 1 was sealed and placed in a refrigerator. After 150 days, the lead-free solder paste still maintained high gloss and the surface of the solder paste did not dry out. The solder paste had stable viscosity, excellent slump and solderability, and could reach the storage life of general commercial use.

[0048] The viscosity of the solder paste was tested using a MALCOM PCU-205 solder paste viscosity tester. The temperature was set to 24.6℃, the pre-stirring time was 6 minutes, and the measurement time was 5 minutes.

[0049] according to Figure 1 It can be seen that under low temperature storage conditions, the solder paste prepared in Example 1 can maintain high stability within 150 days. As the storage time increases, the viscosity of the solder paste is between 190-197 mPa·s.

[0050] The collapse performance of the lead-free solder paste prepared in Example 1 was tested, and the results are as follows: Figures 2-4 As shown, the solder paste exhibits relatively stable slump within 150 days, with no sticking or blemishes after slumping.

[0051] Comparative Example 1

[0052] A lead-free solder paste containing composite additives, by mass percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: 49.7 wt% solvent, 6.6 wt% activator, 35 wt% film-forming agent, 7 wt% thixotropic agent, and 1.7 wt% surfactant.

[0053] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0054] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0055] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0056] The thixotropic agent is hydrogenated castor oil;

[0057] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7.

[0058] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0059] S1. Place the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 100°C, and add the film-forming agent while stirring. When the film-forming agent is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After they are completely dissolved, let them stand to room temperature to obtain the mixture.

[0060] S2. Under vacuum and room temperature conditions, the mixture is stirred evenly and then homogenized using a homogenizer to obtain lead-free solder paste flux.

[0061] S3. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0062] The lead-free solder paste prepared in Comparative Example 1 was sealed and placed in a refrigerator. After 7 days, the gloss decreased and the surface became dry.

[0063] Comparative Example 2

[0064] A lead-free solder paste containing composite additives, by weight percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: 35.2 wt% solvent, 6.6 wt% activator, 35 wt% film-forming agent, 7 wt% thixotropic agent, 1.7 wt% surfactant, and 14.5 wt% composite additives.

[0065] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0066] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0067] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0068] The thixotropic agent is hydrogenated castor oil;

[0069] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0070] The composite additive is composed of tert-butylhydroquinone, glycerol, and jojoba oil in a weight ratio of 3:6.7:4.8.

[0071] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0072] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol and jojoba oil in sequence according to the proportion. After mixing thoroughly, let it stand to room temperature to obtain the composite additive, which is denoted as mixture A.

[0073] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0074] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0075] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0076] The lead-free solder paste prepared in Comparative Example 2 was sealed and placed in a refrigerator. After 56 days, the gloss decreased and the surface became dry.

[0077] Comparative Example 3

[0078] A lead-free solder paste containing composite additives, by mass percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: 39 wt% solvent, 6.6 wt% activator, 35 wt% film-forming agent, 7 wt% thixotropic agent, 1.7 wt% surfactant, and 10.7 wt% composite additives.

[0079] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0080] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0081] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0082] The thixotropic agent is hydrogenated castor oil;

[0083] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0084] The composite additive is composed of tert-butylhydroquinone, glycerol, and squalane in a weight ratio of 3:6.7:1.

[0085] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0086] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol and squalane in sequence according to the proportion. After mixing thoroughly and allowing to stand at room temperature, the composite additive is obtained and denoted as mixture A.

[0087] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0088] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0089] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0090] The lead-free solder paste prepared in Comparative Example 3 was sealed and placed in a refrigerator. After 21 days, the lead-free solder paste showed signs of decreased gloss and surface dryness.

[0091] Comparative Example 4

[0092] A lead-free solder paste containing composite additives, by mass percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: 34.2 wt% solvent, 6.6 wt% activator, 35 wt% film-forming agent, 7 wt% thixotropic agent, 1.7 wt% surfactant, and 15.5 wt% composite additives.

[0093] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0094] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0095] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0096] The thixotropic agent is hydrogenated castor oil;

[0097] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0098] The composite additive is composed of tert-butylhydroquinone, glycerol, jojoba oil, and squalane in a weight ratio of 3:6.7:4.8:1.

[0099] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0100] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol, jojoba oil and squalane in sequence according to the proportion. After mixing thoroughly and allowing to stand at room temperature, the composite additive is obtained and denoted as mixture A.

[0101] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0102] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0103] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0104] The lead-free solder paste prepared in Comparative Example 4 was sealed and placed in a refrigerator. After 84 days, the lead-free solder paste showed signs of decreased gloss and surface dryness.

[0105] Comparative Example 5

[0106] A lead-free solder paste containing composite additives, by weight percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: solvent 27.4 wt%, activator 6.6 wt%, film-forming agent 35 wt%, thixotropic agent 7 wt%, surfactant 1.7 wt%, and composite additives 22.3 wt%.

[0107] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0108] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0109] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0110] The thixotropic agent is hydrogenated castor oil;

[0111] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0112] The composite additive is composed of tert-butylhydroquinone, glycerol, jojoba oil, and squalane in a weight ratio of 3:6.7:9.6:3.

[0113] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0114] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol, jojoba oil and squalane in sequence according to the proportion. After mixing thoroughly and allowing to stand at room temperature, the composite additive is obtained and denoted as mixture A.

[0115] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0116] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0117] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0118] The viscosity of the lead-free solder paste prepared in Comparative Example 5 was 97.6 mPa·s, which was too low. Bridging occurred during the collapse test, and the shape retention was poor during printing.

[0119] Comparative Example 6

[0120] A lead-free solder paste containing composite additives, by mass percentage, comprises 85% tin powder and 15% flux. The tin powder is selected from SAC305 (Sn 96.5, Ag 3.0, Cu 0.5) alloy powder with a particle size of 25-38 μm. The flux comprises the following components: solvent 28.1 wt%, activator 6.6 wt%, film-forming agent 35 wt%, thixotropic agent 7 wt%, surfactant 1.7 wt%, and composite additives 21.6 wt%.

[0121] The solvent is selected from a mixture of 2-ethyl-1,3-hexanediol and diethylene glycol dibutyl ether in a ratio of 1:1.4;

[0122] The activator is a mixture of adipic acid and succinic acid in a ratio of 1:1.2;

[0123] The film-forming agent is a mixture of water-white hydrogenated rosin and disproportionated rosin in a ratio of 2.5:1;

[0124] The thixotropic agent is hydrogenated castor oil;

[0125] The surfactant is a mixture of polyethylene glycol 6000 and Pingping plus O-10 in a ratio of 1:0.7;

[0126] The composite additive is composed of tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in a weight ratio of 3:6.7:4.8:2.6:4.5.

[0127] A method for preparing lead-free solder paste containing composite additives comprises the following steps:

[0128] S1. Place half of the solvent into the reaction vessel, maintain a vacuum environment and a temperature of 40°C, and add tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in sequence according to the proportion. After mixing thoroughly, let it stand to room temperature to obtain the composite additive, which is denoted as mixture A.

[0129] S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B.

[0130] S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After stirring evenly, homogenize using a homogenizer to obtain lead-free solder paste flux.

[0131] S4. Mechanically stir the flux and solder powder in proportion until they are evenly mixed to obtain lead-free solder paste.

[0132] The lead-free solder paste prepared in Comparative Example 6 has a relatively high viscosity of 267 mPa·s, making it difficult for the solder paste to pass through the stencil apertures, resulting in slightly lower printing quality.

[0133] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A composite additive for improving the stability of lead-free solder paste, characterized in that, The composite additive is composed of tert-butylhydroquinone, glycerol, jojoba oil, squalane and lanolin in a weight ratio of 3:6.7:(3.5-5):(1-3):(1.3-3.5).

2. A lead-free solder paste containing the composite additive of claim 1, characterized in that, By weight percentage, the lead-free solder paste consists of 84wt%-87wt% tin powder and 13wt%-16wt% flux. The tin powder is selected from SAC305 alloy powder. The flux includes the following components: 28wt%-34wt% solvent, 6.6wt% activator, 35wt% film-forming agent, 7wt% thixotropic agent, 1.7wt% surfactant, and 16wt%-21wt% composite additive.

3. The lead-free solder paste according to claim 2, characterized in that, The tin powder has a particle size of 25-38 μm.

4. The lead-free solder paste according to claim 2, characterized in that, The solvent is at least two selected from diethylene glycol hexyl ether, diethylene glycol octyl ether, 2-methyl-2,4-pentanediol, triethylene glycol butyl ether, tripropylene glycol methyl ether, triethylene glycol dimethyl ether, diethylene glycol monobutyl ether, diethylene glycol dibutyl ether, diethylene glycol diethyl ether, propylene glycol phenyl ether, glycerol, tetrahydrofurfuryl alcohol, and 2-ethyl-1,3-hexanediol.

5. The lead-free solder paste according to claim 2, characterized in that, The activator is at least two of the following: adipic acid, azelaic acid, succinic acid, succinic acid, salicylic acid, hydroxyethyl acid, maleic acid, malonic acid, octanoic acid, phthalic acid, oleic acid, citric acid, glycine, tartaric acid, p-tert-butylbenzoic acid, dimethylolpropionic acid, and dibromosuccinic acid.

6. The lead-free solder paste according to claim 2, characterized in that, The film-forming agent is composed of water-white hydrogenated rosin and disproportionated rosin in a weight ratio of 2.5:

1.

7. The lead-free solder paste according to claim 2, characterized in that, The thixotropic agent is hydrogenated castor oil.

8. The lead-free solder paste according to claim 2, characterized in that, The surfactant is composed of polyethylene glycol 6000 and Pingpingjia O-10 in a weight ratio of 1:0.

7.

9. A method for preparing lead-free solder paste as described in any one of claims 2-8, characterized in that, Includes the following steps: S1. Take half of the solvent, maintain a vacuum environment and a temperature of 40°C, and add glycerol, jojoba oil, squalane, tert-butylhydroquinone and lanolin in sequence according to the proportion. After mixing, let it stand at room temperature and record it as mixture A. S2. Add the film-forming agent to the remaining solvent and stir under vacuum and 100°C conditions. When the mixture is completely dissolved and the temperature drops to 55°C, add the activator, surfactant and thixotropic agent in sequence. After the mixture is completely dissolved, let it stand at room temperature to obtain mixture B. S3. Under vacuum and room temperature conditions, slowly add mixture A to mixture B while stirring. After mixing, the flux for lead-free solder paste is obtained. S4. Mix the flux and solder powder evenly in the specified ratio to obtain lead-free solder paste.

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