Lead-free halogen-free soldering paste and preparation process thereof

Abstract

The invention relates to a lead-free and halogen-free soldering paste and a preparation process thereof, and belongs to the technical field of soldering paste production, through the combination of nano-silver-loaded carbon nanotubes, organically coated solder alloy powder and soldering flux, the surface tension of the lead-free and halogen-free soldering paste can be reduced by carbon nanotube surface carboxyl in the nano-silver-loaded carbon nanotubes, so that the lead-free and halogen-free soldering paste is obtained. Nano-silver and tin form an Ag3Sn strengthening phase which synergistically promotes wetting and spreading of the solder on a substrate, meanwhile, the carbon nanotubes loaded with the nano-silver are uniformly dispersed in the soldering paste to play a dispersion strengthening role, and the brittleness caused by bismuth segregation is effectively inhibited in cooperation with the grain refinement effect of copper and indium in the solder alloy powder; and the organic coating layer hydrogenated rosin-terpene resin and the soldering flux alcohol ether solvent form a slight dissolution-fusion effect, so that the welding flux alloy powder is prevented from being oxidized, an organic acid active agent can be released to remove a substrate oxidation film, the metallurgical bonding quality of a welding interface is further guaranteed, and the defects of pseudo soldering, cavities and the like are reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of solder paste production and relates to a lead-free and halogen-free solder paste and a preparation process thereof. BACKGROUND

[0002] In the process of the rapid advancement of the electronic industry towards automation and intelligence, the performance of solder paste as a core assembly material is directly related to the quality and reliability of electronic products. However, traditional solder paste is gradually difficult to adapt to the development needs of the industry and environmental protection standards. The lead element in traditional lead-containing solder paste not only seriously damages the nervous system of the human body, but also is difficult to degrade in the natural environment. The halogen-containing solder paste produces corrosive gas in the welding process, which not only corrodes electronic components and equipment, but also reduces the insulation performance of electronic products due to the residual halide, which easily causes short circuit failure. Even though some traditional lead-free solders (such as Sn-Ag-Cu solder) avoid the harm of lead, they still have problems such as poor wettability, high surface insulation resistance, and large dielectric loss, which easily leads to the presence of holes in the welding interface and greatly reduces the reliability of the welding point.

[0003] An invention patent application with the publication number CN116833614B discloses a lead-free and halogen-free solder paste and a preparation process thereof, which includes 82-87% of solder powder, 1-2% of a wrapping agent, and 11-17% of a fluxing agent. Hydrogen doping is used to apply first short-term oxidation protection to the solder powder. The wrapping agent provides long-term oxidation protection for the solder powder, and the two make the metal quality of the solder powder remain and reduce the interface effect between the solder powder and other organic components. In combination with the improved fluxing agent formula, the obtained solder paste product has low bubble rate, high anti-settling stability, high oxidation resistance, and is easy to store. After use, the solder paste has low welding defect rate, low organic residue rate, and high welding quality.

[0004] The lead-free and halogen-free solder paste in the above scheme performs excellently in terms of oxidation resistance and anti-settling stability, but in the scenarios of high-density electronic packaging and fine-pitch welding, the mechanical strength and wettability of the welding point are insufficient, the spreading speed and coverage uniformity of the molten solder on the substrate surface are limited, which easily causes the risk of bridging and virtual welding in fine-pitch welding, affects the stability of the welding process and the product yield, and is not sufficient to adapt to the long-term service requirements of high-reliability electronic devices. SUMMARY

[0005] The purpose of the present application is to provide a lead-free and halogen-free solder paste and a preparation process thereof. The solder paste is obtained by mixing carbon nanotubes loaded with nano-silver, a fluxing agent, and an organic-coated solder alloy powder in a certain proportion, which achieves the beneficial effects of good wettability and high welding strength.

[0006] The purpose of the present application can be achieved by the following technical solutions: A preparation process of a lead-free and halogen-free solder paste, comprising the following steps: Step 1: The solder alloy powder is pretreated with a compound acid solution and then coated with a composite coating solution to obtain organically coated solder alloy powder.

[0007] Step 2: Mix the organic solvent, activator, film-forming agent, pH adjuster, surfactant, antioxidant, corrosion inhibitor and thixotropic agent evenly to obtain the flux.

[0008] Step 3: Mix carbon nanotubes loaded with silver nanoparticles, flux, and organically coated solder alloy powder to obtain lead-free and halogen-free solder paste.

[0009] Furthermore, the preparation process of the organic-coated solder alloy powder is as follows: The 1.5wt% composite coating solution is dispersed at 1000-1200 r / min for 10-15 min, then pretreated solder alloy powder is added, and the mixture is stirred at 30-35℃ and 500-700 r / min for 15-20 min, ultrasonically vibrated for 15-20 min, filtered, and dried to obtain organically coated solder alloy powder.

[0010] Furthermore, the ratio of composite coating liquid to pretreated solder alloy powder is 200-350mL: 90-120g.

[0011] Furthermore, the mass ratio of hydrogenated rosin, terpene resin and anhydrous ethanol in the composite coating solution is 1:0.5:98.5.

[0012] Furthermore, the preparation process of the pretreated solder alloy powder is as follows: Solder alloy powder and a 1 wt% compound acid solution are added to a reaction vessel, ultrasonically vibrated for 5-10 minutes, filtered, washed, and dried to obtain pretreated solder alloy powder.

[0013] Furthermore, the ratio of solder alloy powder to compound acid solution is 100-150g: 300-500mL.

[0014] Furthermore, the compound acid solution is prepared by mixing citric acid and phosphoric acid in a volume ratio of 1:1.

[0015] Furthermore, the mass ratio of tin, bismuth, copper, and the fourth metal indium in the solder alloy powder is 60:38.5:1:0.5.

[0016] Furthermore, the mass ratio of organic solvent, surfactant, film-forming agent, pH adjuster, surfactant, antioxidant, corrosion inhibitor and thixotropic agent is 38.6-40.6: 10.8-11.8: 35-43: 6.0-8.0: 1.0-2.0: 0.1-0.3: 0.5-0.7: 8.0-10.0.

[0017] Furthermore, the preparation process of carbon nanotubes loaded with silver nanoparticles is as follows: Acidified carbon nanotubes and distilled water were added to a reaction vessel and ultrasonically dispersed for 10-15 min. Then, a 0.1 mol / L silver nitrate solution was added, followed by a 10 g / L sodium citrate solution at 75-80℃ and 300-400 r / min. The mixture was stirred for 20-30 min, cooled to room temperature, filtered, washed, and dried to obtain carbon nanotubes loaded with silver nanoparticles.

[0018] Furthermore, the ratio of acidified carbon nanotubes, distilled water, silver nitrate solution, and sodium citrate solution is 0.5-0.7g: 20-30mL: 80-100mL: 50-60mL.

[0019] Furthermore, the mass ratio of silver-loaded carbon nanotubes, flux, and organically coated solder alloy powder is 0.041-0.087:13-18:82-87.

[0020] The beneficial effects of this invention are: 1. This invention combines carbon nanotubes loaded with silver nanoparticles, organically coated solder alloy powder, and flux. The carboxyl groups on the surface of the carbon nanotubes loaded with silver nanoparticles can reduce the surface tension of lead-free and halogen-free solder paste. The silver nanoparticles and tin form an Ag3Sn reinforcing phase, which synergistically promotes the wetting and spreading of the solder on the substrate. At the same time, the carbon nanotubes loaded with silver nanoparticles are uniformly dispersed in the solder paste, playing a dispersion strengthening role. Combined with the grain refinement effect of copper and indium in the solder alloy powder, it effectively suppresses the brittleness caused by bismuth segregation. The organic coating layer of hydrogenated rosin-terpene resin and the flux alcohol ether solvent form a slight dissolution-fusion effect, which not only avoids the oxidation of the solder alloy powder, but also releases organic acid activators to remove the oxide film on the substrate, further ensuring the metallurgical bonding quality of the welding interface and reducing defects such as cold solder joints and voids.

[0021] 2. The organic coating layer in this invention serves as a dense hydrophobic layer, which can isolate oxygen from contact with the solder alloy powder and inhibit the formation of SnO and Bi2O3 at high temperatures. The antioxidant hydroquinone and the corrosion inhibitor benzotriazole in the flux work synergistically to inhibit the oxidation of the flux itself and the corrosion of the solder alloy powder surface, respectively. The carbon nanotubes loaded with nano-silver maintain the stability of the solder microstructure and extend the shelf life by hindering the excessive growth of IMC and the segregation of bismuth atoms.

[0022] 3. This invention does not introduce harmful components such as lead and halogens. The solder alloy powder uses a lead-free system of tin, bismuth, copper, and indium, and the flux uses halogen-free components such as organic acids and alcohol ether solvents, fully complying with environmental protection standards. This avoids the environmental and health hazards of lead-containing solders and the corrosive residue problems of halogen-containing solders. At the same time, through the precise synergy of each component, it makes up for the shortcomings of traditional lead-free and halogen-free solder pastes, such as poor wetting and insufficient mechanical strength.

[0023] 4. The carbon nanotubes loaded with silver nanoparticles in this invention are produced by a mixed acid acidification and sodium citrate reduction process to ensure that the silver nanoparticles are uniformly anchored on the carbon nanotubes and avoid agglomeration. The solder alloy powder is pretreated with compound acid and coated with an organic coating layer, which not only completely removes the original oxide film, but also ensures that the coating film is dense and uniform, thereby improving the compatibility of the carbon nanotubes loaded with silver nanoparticles, the organically coated solder alloy powder and the flux, and forming a stable solder paste. Detailed Implementation

[0024] To further illustrate the technical means and effects of the present invention in achieving the intended purpose, the following detailed description of the specific implementation methods, features and effects of the present invention, in conjunction with preferred embodiments, is provided below.

[0025] Example 1: This example provides a lead-free and halogen-free solder paste, prepared through the following steps: S1: Mix 2g of carbon nanotubes with a length of 25-30μm with 120mL of mixed acid solution (sulfuric acid and nitric acid in a volume ratio of 3:1), stir until homogeneous, react at 80℃ for 4h, cool to room temperature, filter, wash the filter cake with deionized water until neutral, freeze dry to obtain acidified carbon nanotubes; add 0.5g of acidified carbon nanotubes and 20mL of distilled water to a reaction vessel, sonicate for 10min, then add 80mL of 0.1mol / L silver nitrate solution, add 50mL of 10g / L sodium citrate solution at 75℃ and 300r / min, stir for 20min, cool to room temperature, filter, wash the filter cake three times with deionized water, dry to obtain carbon nanotubes loaded with silver nanoparticles.

[0026] Carbon nanotubes are modified with strong acid to give them carboxyl groups on their surface. Silver ions in silver nitrate and carboxyl groups form coordinate bonds through strong electronic states, which enhances their interaction. Then, silver ions are reduced to silver particles by sodium citrate and uniformly anchored on the surface of acidified carbon nanotubes to obtain carbon nanotubes loaded with silver nanoparticles.

[0027] S2: Add 100g of solder alloy powder (the mass ratio of tin, bismuth, copper and the fourth metal indium is 60:38.5:1:0.5) and 300mL of a 1wt% compound acid solution (the volume ratio of citric acid and phosphoric acid is 1:1) to the reaction vessel, place it in an ultrasonic cleaner and vibrate it for 5 minutes, then filter it with filter paper. Wash the filter cake once with deionized water and once with anhydrous ethanol, filter it, and dry it at 40℃ to obtain pretreated solder alloy powder. 200 mL of a 1.5 wt% composite coating solution (hydrogenated rosin, terpene resin, and anhydrous ethanol in a mass ratio of 1:0.5:98.5) was dispersed at 1000 r / min for 10 min. Then, 90 g of pretreated solder alloy powder was added, and the mixture was stirred at 30 °C and 500 r / min for 15 min. The mixture was then ultrasonically vibrated for 15 min using an ultrasonic cleaner, filtered, and dried at 45 °C for 2 h to obtain organically coated solder alloy powder.

[0028] S3: Add 38.6g of organic solvent (the ratio of alcohol ether A to alcohol B is 2:3) and 10.8g of activator (organic acid A and organic acid B) to the reaction vessel, and stir at 500r / min and 50℃ until the activator dissolves. Then add 35g of film-forming agent (the ratio of rosin R1 to rosin R2 is 6:4), and stir at 70℃ until a clear solution is obtained. Cool down to 50℃, and add 6.0g of pH adjuster (organic amine Am1), 1.0g of surfactant (OP-10), 0.1g of antioxidant (hydroquinone), and 0.5g of corrosion inhibitor (benzotriazole) in sequence. Stir at a constant temperature for 20min until a pale yellow emulsion is obtained. Then add 8.0g of thixotropic agent (hydrogenated castor oil), and stir at a constant temperature for 20min at 1000r / min and 50℃. Finally, stir at 20℃ and 1400r / min until a white emulsion is obtained to obtain the flux.

[0029] S4: Add 0.041g of carbon nanotubes loaded with silver nanoparticles and 13g of flux to a high-speed disperser and disperse for 15min at 3000r / min. Then add 82g of organically coated solder alloy powder and stir for 30min at 1500r / min under nitrogen protection until homogeneous to obtain lead-free and halogen-free solder paste.

[0030] Example 2: This example provides a lead-free and halogen-free solder paste, prepared through the following steps: S1: Mix 3g of carbon nanotubes with a length of 25-30μm with 160mL of mixed acid solution (sulfuric acid and nitric acid in a volume ratio of 3:1), stir until homogeneous, react at 85℃ for 4h, cool to room temperature, filter, wash the filter cake with deionized water until neutral, freeze-dry to obtain acidified carbon nanotubes; add 0.6g of acidified carbon nanotubes and 25mL of distilled water to a reaction vessel, sonicate for 12min, then add 90mL of 0.1mol / L silver nitrate solution, add 55mL of 10g / L sodium citrate solution at 77℃ and 350r / min, stir for 25min, cool to room temperature, filter, wash the filter cake with deionized water 4 times, dry to obtain carbon nanotubes loaded with silver nanoparticles.

[0031] S2: Add 125g of solder alloy powder (the mass ratio of tin, bismuth, copper and the fourth metal indium is 60:38.5:1:0.5) and 400mL of a 1wt% compound acid solution (the volume ratio of citric acid and phosphoric acid is 1:1) to the reaction vessel, place it in an ultrasonic cleaner and vibrate it for 7 minutes, then filter it with filter paper. Wash the filter cake once with deionized water and once with anhydrous ethanol, filter it, and dry it at 45℃ to obtain pretreated solder alloy powder. 275 mL of a 1.5 wt% composite coating solution (hydrogenated rosin, terpene resin, and anhydrous ethanol in a mass ratio of 1:0.5:98.5) was dispersed at 1100 r / min for 12 min. Then, 105 g of pretreated solder alloy powder was added, and the mixture was stirred at 32 °C and 600 r / min for 17 min. The mixture was then ultrasonically vibrated for 17 min using an ultrasonic cleaner, filtered, and dried at 50 °C for 2 h to obtain organically coated solder alloy powder.

[0032] S3: Add 39.6g of organic solvent (the ratio of alcohol ether A to alcohol B is 2:3) and 11.3g of activator (organic acid A and organic acid B) to the reaction vessel, and stir at 600r / min and 52℃ until the activator dissolves. Then add 39g of film-forming agent (the ratio of rosin R1 to rosin R2 is 6:4), and stir at 72℃ until a clear solution is obtained. Cool down to 52℃, and add 7.0g of pH adjuster (organic amine Am1), 1.5g of surfactant (OP-10), 0.2g of antioxidant (hydroquinone), and 0.6g of corrosion inhibitor (benzotriazole) in sequence. Stir at a constant temperature for 25min until a pale yellow emulsion is obtained. Then add 9.0g of thixotropic agent (hydrogenated castor oil), and stir at a constant temperature for 25min at 1050r / min and 52℃. Finally, stir at 22℃ and 1500r / min until a white emulsion is obtained to obtain the flux.

[0033] S4: Add 0.064g of carbon nanotubes loaded with silver nanoparticles and 15.5g of flux to a high-speed disperser and disperse for 17min at 3100r / min. Then add 84.5g of organically coated solder alloy powder and stir for 35min at 1600r / min under nitrogen protection until homogeneous to obtain lead-free and halogen-free solder paste.

[0034] Example 3: This example provides a lead-free and halogen-free solder paste, prepared through the following steps: S1: Mix 4g of carbon nanotubes with a length of 25-30μm with 200mL of mixed acid solution (sulfuric acid and nitric acid in a volume ratio of 3:1), stir until homogeneous, react at 90℃ for 5h, cool to room temperature, filter, wash the filter cake with deionized water until neutral, freeze dry to obtain acidified carbon nanotubes; add 0.7g of acidified carbon nanotubes and 30mL of distilled water to a reaction vessel, sonicate for 15min, then add 100mL of 0.1mol / L silver nitrate solution, add 60mL of 10g / L sodium citrate solution at 80℃ and 400r / min, stir for 30min, cool to room temperature, filter, wash the filter cake 5 times with deionized water, dry to obtain carbon nanotubes loaded with silver nanoparticles.

[0035] S2: Add 150g of solder alloy powder (the mass ratio of tin, bismuth, copper and the fourth metal indium is 60:38.5:1:0.5) and 500mL of a 1wt% compound acid solution (the volume ratio of citric acid and phosphoric acid is 1:1) to the reaction vessel, place it in an ultrasonic cleaner and vibrate it for 10min, then filter it with filter paper. Wash the filter cake once with deionized water and once with anhydrous ethanol, filter it, and dry it at 50℃ to obtain pretreated solder alloy powder. 350 mL of a 1.5 wt% composite coating solution (hydrogenated rosin, terpene resin, and anhydrous ethanol in a mass ratio of 1:0.5:98.5) was dispersed at 1200 r / min for 15 min. Then, 120 g of pretreated solder alloy powder was added, and the mixture was stirred at 35 °C and 700 r / min for 20 min. The mixture was then ultrasonically vibrated for 20 min using an ultrasonic cleaner, filtered, and dried at 55 °C for 3 h to obtain organically coated solder alloy powder.

[0036] S3: Add 40.6g of organic solvent (the ratio of alcohol ether A to alcohol B is 2:3) and 11.8g of activator (organic acid A and organic acid B) to the reaction vessel, and stir at 700r / min and 55℃ until the activator dissolves. Then add 43g of film-forming agent (the ratio of rosin R1 to rosin R2 is 6:4), and stir at 75℃ until a clear solution is obtained. Cool down to 55℃, and add 8.0g of pH adjuster (organic amine Am1), 2.0g of surfactant (OP-10), 0.3g of antioxidant (hydroquinone), and 0.7g of corrosion inhibitor (benzotriazole) in sequence. Stir at a constant temperature for 30min until a pale yellow emulsion is obtained. Then add 10.0g of thixotropic agent (hydrogenated castor oil), and stir at a constant temperature for 30min at 1100r / min and 55℃. Finally, stir at 25℃ and 1600r / min until a white emulsion is obtained to obtain the flux.

[0037] S4: Add 0.087g of carbon nanotubes loaded with nano-silver and 18g of flux to a high-speed disperser and disperse for 20min at 3200r / min. Then add 87g of organically coated solder alloy powder and stir for 40min under nitrogen protection and 1700r / min until uniformly mixed to obtain lead-free and halogen-free solder paste.

[0038] Example 4: This example provides a lead-free and halogen-free solder paste. The difference from Example 1 is that the activator in step S3 is organic acid C and organic acid D.

[0039] Example 5: This example provides a lead-free and halogen-free solder paste. The difference from Example 1 is that the mixing ratio of alcohol ether A and alcohol B in step S3 is 3:2.

[0040] Example 6: This example provides a lead-free and halogen-free solder paste. The difference from Example 1 is that in step S2, the solder alloy powder is not cleaned with a compound acid solution, but is directly coated with a composite coating liquid.

[0041] Comparative Example 1: This comparative example provides a lead-free and halogen-free solder paste, which differs from Example 1 in that the carbon nanotubes loaded with silver nanoparticles are removed in step S4.

[0042] Comparative Example 2: This comparative example provides a lead-free and halogen-free solder paste. The difference from Example 1 is that in step S4, solder alloy powder is used instead of organically coated solder alloy powder.

[0043] Comparative Example 3: This comparative example provides a lead-free and halogen-free solder paste. The difference from Example 1 is that in step S4, nano-silver is used instead of carbon nanotubes loaded with nano-silver.

[0044] The performance of the lead-free and halogen-free solder pastes prepared in Examples 1-6 and Comparative Examples 1-3 was tested: Spreadability and wetting grade tests: Lead-free and halogen-free solder paste was printed onto a single-sided copper-clad board with a copper thickness of 35μm using a 0.40mm thick stainless steel stencil with an aperture diameter of Ф6.3mm. After reflow soldering, the solder melted and wetted onto the copper-clad board, spreading into a spherical crown shape. The spreadability E of the solder paste was calculated according to the following formula: E=(d﹣h) / d×100%, where d is the equivalent ball diameter of the solder / mm, and h is the height of the spherical crown of the solder joint / mm.

[0045] Viscosity Test: Following GB / T 31475-2015 standard, a Malcom PCU285 viscometer was used for testing. Before testing, the lead-free and halogen-free solder paste was removed from the refrigerator and allowed to warm to room temperature for 8 hours. The sample was then stirred, taking care not to introduce air. After thorough mixing, the rotor of the machine was placed into the solder paste container, and the equipment was started at a speed of 5 rpm. Readings were recorded after 10 minutes, with 1-minute intervals, for a total of 3 readings. The average value was taken as the final value.

[0046] High-temperature storage experiment: 100g of lead-free and halogen-free solder paste was prepared and placed in a constant temperature chamber at 50℃. Every 12 hours, the lead-free and halogen-free solder paste was taken out and cooled to room temperature. The state of the solder paste was observed. Then, solder joints were printed on a single-sided copper-clad board and reflow soldering was performed to observe the morphology and wetting of the solder joints.

[0047] Joint shear strength test: (1) Shear test specimen preparation: T2 copper sheet with size 40×10×1.0mm was wet-ground and polished with 1000 and 2000 grit sandpaper to remove oxide film; then the polished copper sheet was cleaned in an ultrasonic cleaner with anhydrous ethanol for 5 minutes to remove impurities and oil stains on the surface of the copper sheet, and dried for later use; then lead-free and halogen-free solder paste was printed on the copper sheet using a stainless steel template with an opening size of 5×5×0.2mm, and then another copper sheet was attached, and three stainless steel wires with a diameter of 200μm were used to control the spacing at the edge of the copper sheet. The overlapped joint was placed in the reflow oven and soldered according to the set reflow process curve. After the reflow was completed, the shear test specimen was obtained by cleaning with anhydrous ethanol combined with ultrasonic vibration.

[0048] (2) Shear test: The above shear specimens were subjected to a shear test using an electronic universal testing machine. The shear rate was set to 1.0 mm / min, and the breaking load F of the shear specimens was recorded. s The overlap area A of the shear specimen and the shear strength τ are calculated using the formula. Repeat the above operation three times, and take the average value as the final shear strength of the brazed joint. The calculation formula is as follows: τ = F s / A.

[0049] The test results are shown in the table below: Table 1 Performance Test Overview Item Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 Comparative Example 3 Solder joint spread rate (%) 92.5 93.1 92.1 92.8 92.9 93.1 85.6 82.3 89.2 Viscosity (Pa-s) 167.8 169.5 166.1 167.9 168.4 169.7 153.2 149.3 159.5 Spread rate after 50 days of high temperature storage (%) 90.5 91.2 90.8 90.4 91.1 90.4 80.5 75.8 83.8 Shear strength (MPa) 31.24 31.52 31.15 31.26 31.48 31.44 25.21 24.35 25.18 As shown in Table 1, the solder joint spreading rate in Examples 1-6 is greater than that in Comparative Examples 1-3. This may be because the surface carboxyl groups of the carbon nanotubes loaded with silver nanoparticles reduce the surface tension of the solder, and the silver nanoparticles can be evenly distributed in the lead-free and halogen-free solder paste. The formation of the Ag3Sn phase between the silver nanoparticles and Sn further promotes the spreading of the molten solder. The hydrogenated rosin-terpene resin coating layer avoids secondary oxidation of the alloy powder during storage and soldering, which together promotes the spreading of the solder paste during soldering.

[0050] The viscosity of Examples 1-6 is greater than that of Comparative Examples 1-3, possibly because the carbon nanotubes loaded with silver nanoparticles form a fibrous support structure in the lead-free and halogen-free solder paste. The organic coating layer has good compatibility with the alcohol ether solvent and hydrogenated castor oil thixotropic agent in the flux, which can prevent alloy powder agglomeration and maintain the uniformity of the system. The thixotropic agent and OP-10 surfactant synergistically form a stable network structure, resulting in a higher viscosity of the lead-free and halogen-free solder paste.

[0051] The spreading rates of Examples 1-6 after 50 days of high-temperature storage were higher than those of Comparative Examples 1-3, and the rate of decline in spreading rate was slower, indicating better storage performance. This may be because the organic coating layer blocks oxygen from contacting the alloy powder, inhibiting high-temperature oxidation; the carbon nanotubes loaded with silver nanoparticles can inhibit bismuth atom segregation, preventing increased solder brittleness and decreased spreading ability; and the hydroquinone in the flux inhibits its own oxidation, while benzotriazole protects the surface of the alloy powder. Comparative Example 1 lacks the micro-stabilizing effect of carbon nanotubes loaded with silver nanoparticles; Comparative Example 2 lacks coating, leading to rapid oxidation of the alloy powder; and Comparative Example 3 shows that the agglomeration of silver nanoparticles cannot uniformly inhibit oxidation and segregation, all of which accelerate the decline in spreading rate. Therefore, the storage performance of Comparative Examples 1-6 is worse.

[0052] The shear strength of Examples 1-6 is greater than that of Comparative Examples 1-3, indicating better weld bonding. This may be because the carbon nanotubes loaded with silver nanoparticles are uniformly dispersed in the lead-free and halogen-free solder paste, acting as a dispersed reinforcing phase to hinder dislocation slip, while simultaneously refining the IMC grains and improving interfacial bonding strength. The organic coating film and flux decompose simultaneously, leaving no residual interlayer, allowing the solder to form a tight metallurgical bond with the copper substrate. The copper and indium in the alloy powder can refine the matrix grains and suppress brittleness caused by bismuth segregation. Comparative Example 1 lacks the reinforcing and refining effect of the carbon nanotubes loaded with silver nanoparticles, Comparative Example 2 lacks coating, resulting in an oxide film interlayer at the weld interface, and Comparative Example 3 shows silver nanoparticle agglomeration forming stress concentration points, all of which weaken the shear strength of the solder joint.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A process for preparing a lead-free and halogen-free solder paste, characterized in that, It comprises the following steps: Step one: the solder alloy powder is pretreated by a compound acid solution, and then coated by a compound coating solution to obtain the organic coated solder alloy powder; Step two: the organic solvent, active agent, film forming agent, pH regulator, surfactant, antioxidant, corrosion inhibitor and thixotropic agent are mixed uniformly to obtain the flux; Step three: the nano silver loaded carbon nanotube, the flux and the organic coated solder alloy powder are mixed to obtain the lead-free and halogen-free solder paste.

2. The preparation process of a lead-free and halogen-free solder paste according to claim 1, characterized in that, The preparation process of the organic coated solder alloy powder in step one is as follows: The compound coating solution with a concentration of 1.5wt% is dispersed at 1000-1200r / min for 10-15min, then the pretreated solder alloy powder is added, stirred at 30-35℃ and 500-700r / min for 15-20min, ultrasonic vibration for 15-20min, filtration, drying to obtain the organic coated solder alloy powder.

3. The process for preparing a lead-free and halogen-free solder paste according to claim 2, characterized in that, The dosage ratio of the compound coating solution to the pretreated solder alloy powder is 200-350mL:90-120g; The mass ratio of hydrogenated rosin, terpene resin and anhydrous ethanol in the compound coating solution is 1:0.5:98.

5.

4. The preparation process of a lead-free and halogen-free solder paste according to claim 2, characterized in that, The preparation process of the pretreated solder alloy powder is as follows: The solder alloy powder and the compound acid solution with a concentration of 1wt% are added into the reaction kettle, ultrasonic vibration for 5-10min, filtration, washing, drying to obtain the pretreated solder alloy powder; The dosage ratio of the solder alloy powder to the compound acid solution is 100-150g:300-500mL; The compound acid solution is compounded by citric acid and phosphoric acid in a volume ratio of 1:

1.

5. The process for preparing a lead-free and halogen-free solder paste according to claim 4, characterized in that, The mass ratio of tin, bismuth, copper and the fourth metal indium in the solder alloy powder is 60:38.5:1:0.

5.

6. The preparation process of a lead-free and halogen-free solder paste according to claim 1, characterized in that, The mass ratio of the organic solvent, active agent, film forming agent, pH regulator, surfactant, antioxidant, corrosion inhibitor and thixotropic agent in step two is 38.6-40.6:10.8-11.8:35-43:6.0-8.0:1.0-2.0:0.1-0.3:0.5-0.7:8.0-10.

0.

7. The preparation process of a lead-free and halogen-free solder paste according to claim 1, characterized in that, The preparation process of the nano silver loaded carbon nanotube in step three is as follows: The acidized carbon nanotube and distilled water are added into the reaction kettle, ultrasonic dispersion for 10-15min, then the silver nitrate solution with a concentration of 0.1mol / L is added, the sodium citrate solution with a concentration of 10g / L is added at 75-80℃ and 300-400r / min, stirring for 20-30min, cooling to room temperature, filtration, washing, drying to obtain the nano silver loaded carbon nanotube.

8. The preparation process of a lead-free and halogen-free solder paste according to claim 7, characterized in that, The dosage ratio of the acidized carbon nanotube, distilled water, silver nitrate solution and sodium citrate solution is 0.5-0.7g:20-30mL:80-100mL:50-60mL.

9. The preparation process of a lead-free and halogen-free solder paste according to claim 1, characterized in that, The mass ratio of the nano silver loaded carbon nanotube, flux and organic coated solder alloy powder in step three is 0.041-0.087:13-18:82-87.

10. A lead-free and halogen-free solder paste, characterized by, The lead-free and halogen-free solder paste is prepared by the preparation process of any one of claims 1-9.