High performance low silver content silver paste and method of making same

Abstract

The application discloses a kind of high-performance low-silver content silver paste and preparation method thereof, belong to printed circuit technical field.The preparation method includes: preparation epoxy-polyurethane copolymer resin;Epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether active diluent and mixed organic solvent are stirred and reacted, to obtain organic carrier;Organic carrier, 3-amino propyl triethoxysilane coupling agent and BYK-180 dispersant are stirred at low speed, then carboxylated single-walled carbon nanotube is added, stirred at high speed, batched silver flake is added, continue to stir, to obtain mixed slurry;Thickening agent is added to mixed slurry, stir, then curing agent and accelerator are added, continue to stir, to obtain uniform paste;Uniform paste is repeatedly ground, then vacuum degassing and filtration are carried out in turn.The silver content of high-performance low-silver content silver paste prepared by the application is low, curing temperature is low, conductivity is good and flexural resistance is good, suitable for low-temperature fast drying requirement's membrane switch or flexible printed circuit.

Description

Technical Field

[0001] This invention belongs to the field of printed circuit technology, and particularly relates to a high-performance low-silver-content silver paste and its preparation method. Background Technology

[0002] As core components for human-computer interaction and internal connection in electronic devices, membrane switches and flexible printed circuits (FPCs) directly affect product competitiveness due to the performance and cost of their conductive layer materials (mainly conductive silver paste). With the upgrading of the electronics industry's demand for lightweight, flexible, and low-cost equipment, the market has put forward three core requirements for conductive silver paste: (1) Low silver content: Silver, as a precious metal, accounts for more than 70% of the cost of silver paste. Reducing the silver content can significantly improve the product's price advantage and adapt to fierce market competition; (2) Low temperature and fast drying: Traditional silver paste curing temperature is mostly above 150℃ and curing time exceeds 3 minutes, which cannot be adapted to flexible substrates with limited heat resistance such as PET (PET has a long-term temperature resistance of ≤150℃) and restricts production efficiency; (3) High performance: It needs to simultaneously meet the requirements of high conductivity (low sheet resistance), high flexibility (bending resistance), strong adhesion, and high hardness to adapt to the application scenarios of frequent pressing of membrane switches and dynamic bending of FPCs. However, the silver content of existing silver pastes is too high, the curing temperature is high, the conductivity is low, or the bending resistance is poor. Summary of the Invention

[0003] One of the objectives of this invention is to provide a method for preparing high-performance low-silver-content silver paste. The high-performance low-silver-content silver paste prepared by this method has low silver content, low curing temperature, good conductivity and good flexural strength, and is suitable for thin-film switches or flexible printed circuits that require low temperature and fast drying.

[0004] The second objective of this invention is to provide a high-performance silver paste with low silver content.

[0005] To achieve one of the above objectives, the present invention employs the following technical solution: A method for preparing a high-performance, low-silver-content silver paste, the method comprising the following steps: Step S1: Under the catalysis of dibutyltin dilaurate, epoxy resin and polyurethane prepolymer are subjected to catalytic reaction to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds. Step S2: The epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent are stirred and reacted to obtain an organic carrier; Step S3: After stirring the organic carrier, 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant at low speed, add carboxylated single-walled carbon nanotubes and stir at high speed. Then add flake silver powder in batches and continue stirring to obtain a mixed slurry. Step S4: At a temperature of 20~30℃, add thickener to the mixed slurry, stir, then add latent curing agent and accelerator, and continue stirring to obtain a uniform slurry; Step S5: Feed the uniform slurry into a three-roll mill for repeated grinding until the slurry fineness is ≤15μm. Then, perform vacuum degassing and filtration in sequence to obtain high-performance low-silver-content silver paste.

[0006] Further, in step S1, the mass ratio of the epoxy resin to the polyurethane prepolymer is 7~8:2~3; In step S1, the amount of dibutyltin dilaurate used is 0.3-0.7% of the sum of the mass of the epoxy resin and the polyurethane prepolymer; In step S1, the epoxy resin is a bisphenol A epoxy resin with an epoxy value of 0.48~0.56 eq / 100g; In step S1, the polyurethane prepolymer is a toluene diisocyanate type polyurethane prepolymer with an NCO content of 5-8%. In step S1, the temperature of the catalytic reaction is 55~65℃ and the time is 1.5~2.5 hours.

[0007] Furthermore, the bisphenol A epoxy resin is one of E-51, E-54, and E-55.

[0008] Further, in step S2, the mass ratio of the epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is 13.0~15.0:3.5~4.5:47.5~52.6; In step S2, the stirring reaction is carried out at a temperature of 45-55°C, a speed of 450-550 rpm, and a time of 0.5-1.5 hours.

[0009] Furthermore, in step S2, the melting point of the mixed organic solvent is 150~200℃; In step S2, the mixed organic solvent includes propylene glycol methyl ether acetate and diethylene glycol butyl ether; The mass ratio of propylene glycol methyl ether acetate to diethylene glycol butyl ether is 35~38:12.5~14.6.

[0010] Further, in step S3, the mass ratio of the organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes, and flake silver powder is 64~70:0.5~0.7:0.3~0.4:0.2~0.8:26.6~27.8; In step S3, the low-speed stirring rate is 550~650 rpm, and the time is 25~35 minutes; In step S3, the high-speed stirring rate is 1450~1550 rpm, and the time is 40~50 minutes; In step S3, the stirring rate is 950-1050 rpm and the time is 55-65 minutes.

[0011] Furthermore, in step S3, the particle size of the thin-film silver powder is 10~15μm, the aspect ratio is >100, and the bulk density is ≤0.6g / cm³. 3 ; In step S3, the carboxylated single-walled carbon nanotubes have a diameter of 1-2 nm, an aspect ratio of 500-1000, and a carboxyl content ≥2.0 wt%. In step S3, the batch is 3 to 4 times.

[0012] Further, in step S4, the mass ratio of the mixed slurry, thickener, latent curing agent, and accelerator is 95.8~96.4:0.5~0.7:2.8~3.2:0.3; In step S4, the thickener is one of fumed silica and organobentonite; In step S4, the latent curing agent is one of 2-phenylimidazole and dicyandiamide latent curing agent; In step S4, the accelerator is one of salicylic acid and 3-phenyl-1,1-dimethylurea.

[0013] Furthermore, in step S5, the roller spacing of the three-roll mill is 10~15μm; In step S5, the grinding is repeated 3 to 4 times. In step S5, the vacuum degree of the vacuum degassing is ≤-0.09MPa, and the time is 15~25 minutes; In step S5, the mesh size of the filter screen is 150 to 250 mesh.

[0014] To achieve the second objective mentioned above, the present invention employs the following technical solution: A high-performance, low-silver-content silver paste, wherein the high-performance, low-silver-content silver paste is prepared by the preparation method described above.

[0015] In summary, the solution proposed in this invention has the following technical effects: This invention utilizes the spread contact area of ​​thin-film silver powder to achieve efficient bonding with low silver content under the same mass conditions. Furthermore, it does not rely on high silver content; it achieves rapid and tight bonding through "surface-to-surface contact," forming a continuous and stable main conductive network. The contact resistance can be as low as ≤10 Ω·cm. -5 With a high conductivity of Ω·cm, this invention achieves moderate printing viscosity with low silver content by reducing the loose packing density of thin-film silver powder. The invention utilizes carboxylated single-walled carbon nanotubes to fill gaps, ensuring complete pathways. These carboxylated single-walled carbon nanotubes precisely fill the micron-level gaps between large sheets of thin silver powder, constructing a "silver powder-carbon nanotube-silver powder" inter-gap conductive bridge, avoiding the problem of conductive pathway breakage in low-silver-content scenarios. This invention resolves the contradiction between "low silver content and high conductivity" through a composite conductive system of "thin-film silver powder + carboxylated single-walled carbon nanotubes." Through the synergistic effect of thin-film silver powder and carboxylated single-walled carbon nanotubes, the sheet resistance of the system can still be stably controlled at ≤20mΩ / □ when the silver content is reduced to ≤28%.

[0016] This invention improves adhesion by using epoxy groups in the epoxy resin to form covalent bonds with the hydroxyl groups on the surface of the PET substrate. It also enhances flexibility by using flexible urethane segments in the polyurethane prepolymer to absorb bending stress. Furthermore, this invention's "epoxy-polyurethane copolymer resin" combines high adhesion (strong interfacial bonding with the PET substrate, adhesion reaching 5B), high flexibility (elongation at break ≥200%), and low-temperature reactivity. Combined with a latent curing agent, it can rapidly crosslink at 130℃, thus solving the defects of "low-temperature curing and flexibility / adhesion".

[0017] This invention utilizes the hydrogen bond network formed by the silanol groups on the surface of nanoscale particles of fumed silica thickener to construct a temporary three-dimensional structure in the slurry system, significantly improving the system's viscosity and thixotropy. This achieves moderate viscosity with low silver content, improving printability and avoiding the problems of insufficient slurry viscosity caused by reduced conductive filler content in low-silver-content applications, which can lead to defects such as line diffusion and missing prints during printing.

[0018] This invention provides basic interfacial bonding through epoxy-polyurethane copolymer resin, while introducing 3-aminopropyltriethoxysilane as a coupling agent. The amino group at one end of 3-aminopropyltriethoxysilane reacts with the epoxy groups in the resin to form a chemical bond, while the ethoxy group at the other end hydrolyzes to generate silanol groups, which condense with the hydroxyl groups on the PET substrate surface, constructing a chemical bridging layer of "resin-coupling agent-substrate". This not only stabilizes the adhesion at level 5B, but also prevents moisture and oxygen from penetrating the interface. After the cured film is placed in an environment of 85℃ / 85% RH for 500 hours, the sheet resistance change rate is ≤10%, which is significantly better than the damp heat resistance performance of the prior art. In other words, this invention achieves a dual improvement in adhesion and environmental stability through epoxy-polyurethane copolymer resin and coupling agent.

[0019] This invention utilizes an imidazole-based latent curing agent, enabling the slurry to be stored at 25°C for more than 6 months. When the temperature rises to 130°C, the curing agent is rapidly activated, undergoing a crosslinking reaction with the epoxy and urethane groups in the resin, and curing to form a film can be completed within 120 seconds. This invention also utilizes ethylene glycol diglycidyl ether as an active diluent, which not only participates in the curing reaction, increasing the crosslinking density, but also reduces the porosity defects in the film layer caused by solvent evaporation, resulting in a film hardness of 4H or higher, thus achieving a balance between low-temperature rapid curing and storage stability.

[0020] The silver content of the low-silver-content high-performance membrane switch / flexible printed circuit silver paste of the present invention is ≤28%. Under the curing conditions of 130℃ / 120" the silver paste has a sheet resistance ≤20mΩ / □, a flexural strength ≥10 times (180° reciprocating bend, resistance change rate ≤30%), an adhesion of 5B, and a hardness ≥4H. The product is suitable for membrane switches and flexible printed circuits that require low temperature and fast drying. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] This embodiment provides a method for preparing a high-performance, low-silver-content silver paste, which includes the following steps: Step S1: Under the catalysis of dibutyltin dilaurate, epoxy resin and polyurethane prepolymer are subjected to a catalytic reaction to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds.

[0023] Because the epoxy groups in epoxy resins (especially bisphenol A epoxy E-51 and E-54 types) can undergo ring-opening reactions with the hydroxyl groups on the surface of polyethylene terephthalate (PET) substrates to form stable covalent bonds, this is the core factor in improving the interfacial adhesion between the silver paste and the substrate. However, epoxy resins themselves are brittle; if the proportion is too high, it will lead to insufficient flexibility, while if the proportion is too low, the number of covalent bonds will decrease, resulting in reduced adhesion. Therefore, in this embodiment, the mass ratio of epoxy resin to polyurethane prepolymer is controlled at 7~8:2~3. This ensures that the interfacial adhesion reaches grade 5B (no peeling during cross-cut test) and avoids the accumulation of interfacial brittleness caused by excessive epoxy resin, preventing the film layer from peeling off from the substrate when bending due to excessive interfacial rigidity.

[0024] Meanwhile, the flexible urethane segments in the polyurethane prepolymer can absorb bending stress, inhibit crack propagation, and are key to improving the flexural strength of the silver paste; however, the adhesion and hardness of polyurethane itself are insufficient, requiring epoxy resin to supplement structural strength. This embodiment controls the mass ratio of epoxy resin and polyurethane prepolymer to provide sufficient flexible segments, ensuring that the elongation at break of the copolymer resin is ≥200%, guaranteeing that the silver paste can withstand ≥10 180° reciprocating bends after curing (resistance change rate ≤30%), adapting to the application scenarios of FPC dynamic bending and frequent pressing of membrane switches. At the same time, a rigid skeleton is formed through cross-linking reaction, avoiding the film layer becoming soft and lacking hardness due to excessive polyurethane content (hardness ≥4H is required), and supporting the structural stability of the conductive network to prevent the conductive path from breaking during bending.

[0025] In this embodiment, the mass ratio of epoxy resin to polyurethane prepolymer is controlled at 7~8:2~3. This ensures that the epoxy resin (epoxy value 0.48~0.56eq / 100g) and the latent curing agent have moderate reactivity at 130℃, guaranteeing the crosslinking density and ensuring that curing and film formation are completed within 120 seconds. It also ensures that the polyurethane prepolymer (NCO content 5~8%) undergoes a co-crosslinking reaction with the hydroxyl groups of the epoxy resin, which does not hinder the low-temperature curing process of the epoxy resin, and can also reduce the porosity and crack defects of the film layer during low-temperature curing by adjusting the internal stress of the crosslinking network through the flexibility of the urethane bonds.

[0026] In this embodiment, the quality control of epoxy resin and polyurethane prepolymer is 7~8:2~3, which avoids the defects of a single resin system and achieves the following results for silver paste with low silver content (≤28%): sheet resistance ≤20mΩ / □, adhesion 5B, hardness ≥4H and flexural strength ≥10 times.

[0027] To ensure the activation of the copolymerization reaction and avoid insufficient reaction leading to resin structural defects and decreased silver paste performance, while also preventing excessive catalysis that could cause "explosive polymerization" or metal ion residue, and avoiding uneven resin structure, unstable silver paste storage, and deterioration in environmental resistance, the amount of dibutyltin dilaurate used in this embodiment is 0.3~0.7% of the sum of the mass of epoxy resin and polyurethane prepolymer. This balances the reaction rate with the resin mass, ensuring the conductivity and flexibility of the subsequent silver paste.

[0028] In this embodiment, the epoxy resin is a bisphenol A epoxy resin with an epoxy value of 0.48~0.56 eq / 100g, and the bisphenol A epoxy resin is one of E-51, E-54 and E-55. The polyurethane prepolymer is a toluene diisocyanate type polyurethane prepolymer with an NCO content of 5~8%.

[0029] In this embodiment, the catalytic reaction is carried out at a temperature of 55~65℃ for 1.5~2.5 hours.

[0030] Step S2: The epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent are stirred and reacted to obtain an organic carrier.

[0031] This embodiment uses epoxy-polyurethane copolymer resin to ensure adhesion and flexibility; ethylene glycol diglycidyl ether to adjust crosslinking density and reduce film defects; and mixed organic solvents to ensure uniform dissolution of each component. The combination of these three components allows the organic carrier to adapt to the printing and curing requirements of low-silver-content silver paste. In this embodiment, the mass ratio of epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is controlled at 13.0~15.0:3.5~4.5:47.5~52.6, thus balancing the performance of the organic carrier.

[0032] In this embodiment, the stirring reaction temperature is controlled at 45~55℃ to activate the molecular motion of the resin and reactive diluent, accelerate dissolution, and avoid excessive temperature leading to solvent boiling or premature resin cross-linking. The stirring rate is controlled at 450~550 rpm to generate suitable shear force, breaking up resin agglomerates and preventing system stratification. The stirring time is controlled at 0.5~1.5 hours to ensure complete resin dissolution and uniform diluent dispersion, preventing incomplete dissolution and the resulting microparticles from affecting the subsequent dispersion of conductive fillers. This ensures the uniformity of the organic carrier from a process perspective, prevents excessive temperature from causing solvent evaporation or premature resin reaction, and also prevents uneven mixing caused by improper stirring rate, thus ensuring the stable quality of the organic carrier.

[0033] In summary, this embodiment reduces the interfacial tension of each component by controlling the above-mentioned mass ratio and process, ensuring the formation of a stable continuous phase organic carrier, and producing a uniform, transparent organic carrier with suitable viscosity. When mixed with conductive silver powder and carbon nanotubes, it can uniformly coat the filler, avoiding the breakage of the conductive network due to uneven carrier, thus helping the final silver paste to achieve core properties such as low sheet resistance and high flexural strength, and ultimately ensuring the comprehensive performance of the silver paste.

[0034] This embodiment controls the amount of ethylene glycol diglycidyl ether reactive diluent to avoid insufficient carrier strength due to excessive diluent or excessive viscosity due to excessive resin.

[0035] The mixed organic solvent in this embodiment has a melting point of 150~200℃ and includes propylene glycol methyl ether acetate and diethylene glycol butyl ether in a mass ratio of 35~38:12.5~14.6.

[0036] Step S3: After stirring the organic carrier, 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant at low speed, add carboxylated single-walled carbon nanotubes and stir at high speed. Then, add flake silver powder in batches and continue stirring to obtain a mixed slurry.

[0037] 3-Aminopropyltriethoxysilane coupling agent and BYK-180 dispersant were added sequentially to the above organic carrier and stirred. Then, carboxylated single-walled carbon nanotubes were added, the rotation speed was increased, and high-speed dispersion was carried out to ensure uniform dispersion of carbon nanotubes. Then, flake silver powder was added in batches, the rotation speed was kept constant, and stirring was continued to obtain a preliminarily dispersed mixed slurry.

[0038] In this embodiment, 3-aminopropyltriethoxysilane serves as the core coupling agent. Through a bidirectional bridging effect of "chemical bonding between the amino group at one end and the epoxy group of the organic carrier, and condensation between the silanol group at one end and the hydroxyl group of the PET substrate," a stable interface layer is constructed, significantly improving the adhesion of the silver paste to level 5B. At the same time, it blocks moisture intrusion and ensures environmental stability. Its dosage is precisely matched to the reaction sites of the carrier and the substrate to avoid insufficient or excessive dosage affecting performance.

[0039] In this embodiment, the mass ratio of organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes, and flake silver powder is controlled at 64~70:0.5~0.7:0.3~0.4:0.2~0.8:26.6~27.8. This significantly reduces the cost of precious metals (more than 50% lower than traditional silver paste) by using carboxylated single-walled carbon nanotubes to fill the gaps between silver powder particles and construct a continuous conductive path using the tunneling effect. This solves the problem of insufficient conductivity at low silver content and avoids the defects of excessive carbon nanotubes causing silver powder overlap resistance and failure of the tunneling effect, or insufficient carbon nanotubes failing to form an effective conductive bridge.

[0040] From the perspective of system stability and process compatibility, the above-mentioned mass ratio control enables the organic carrier to maintain the target viscosity of the slurry, and the BYK-180 dispersant ensures the uniform dispersion of silver powder and carbon nanotubes. Thus, under the premise of low silver content, high conductivity with sheet resistance ≤20mΩ / □ is achieved. At the same time, it is compatible with subsequent grinding, printing and curing processes, and finally achieves the comprehensive performance goal of silver paste of "low cost, high conductivity, strong adhesion and bending resistance".

[0041] In this embodiment, the flake silver powder has a particle size of 10-15 μm, an aspect ratio > 100, and a bulk density ≤ 0.6 g / cm³. 3 By controlling the particle size of the flake silver powder, a low-content "surface-to-surface contact" conductive network is formed, saving silver costs while ensuring conductivity and meeting printing viscosity requirements. Controlling the aspect ratio of the flake silver powder avoids the problem of excessively small aspect ratios, which would significantly reduce the contact area for the same mass, making it difficult to form a continuous conductive network with low silver content, leading to a significant increase in sheet resistance. It also avoids the problem of "point-to-point contact" between silver powder particles, resulting in unstable conductive paths, easy breakage during bending, and compromised flexural strength. Controlling the loose packing density of the silver powder allows printing viscosity to be maintained without increasing the silver content, meeting the requirements of low silver and low cost.

[0042] In this embodiment, the carboxylated single-walled carbon nanotubes have a diameter of 1~2nm and an aspect ratio of 500~1000 to precisely fill the gaps between silver powders, improve dispersibility, and supplement conductivity through tunneling effect, avoiding pathway breakage under low silver content. The parameter boundaries of the two types of conductive fillers are precisely matched, and they synergistically achieve high conductivity (sheet resistance ≤20mΩ / □) under low silver content (≤28%), while adapting to subsequent processes and ensuring the overall performance of the silver paste.

[0043] In this embodiment, the batches are 3-4 times, with a low-speed stirring rate of 550-650 rpm for 25-35 minutes, a high-speed stirring rate of 1450-1550 rpm for 40-50 minutes, and a continued stirring rate of 950-1050 rpm for 55-65 minutes.

[0044] Step S4: At a temperature of 20~30℃, add thickener to the mixed slurry, stir, then add latent curing agent and accelerator, and continue stirring to obtain a uniform slurry.

[0045] After cooling the mixed slurry, add a thickener (such as one with a specific surface area of ​​200~300 m²). 2 After adding / g of fumed silica or organobentonite, stir until the viscosity of the system reaches the target range; then add a latent curing agent (such as 2-phenylimidazole or dicyandiamide latent curing agent) and an accelerator (such as salicylic acid or 3-phenyl-1,1-dimethylurea), and continue stirring to obtain a uniform slurry.

[0046] In this embodiment, the mass ratio of the mixed slurry, thickener, latent curing agent, and accelerator is 95.8~96.4:0.5~0.7:2.8~3.2:0.3. The fumed silica thickener in this embodiment has a specific surface area of ​​200~300 m². 2 / g. The layered structure of organobentonite can form a thixotropic network in organic systems, which can adjust the viscosity of the slurry to the target range.

[0047] This embodiment controls the amount of mixed paste to ensure synergistic effects of functional components and achieve performance balance. The mixed paste (containing conductive functional phases, organic carriers, and other core components) serves as the main component, ensuring that thickeners, curing agents, and accelerators are uniformly dispersed in the system. This prevents the conductive network from becoming sparse (increased sheet resistance) due to an excessively low proportion of the main component, while also preventing the system's compatibility from being compromised due to an excessively high proportion of functional components (such as thickener agglomeration or localized overdose of curing agent). Ultimately, under the premise of low silver content (≤28%), the core performance targets of silver paste, namely "sheet resistance ≤20mΩ / □, adhesion 5B, and flexural strength ≥10 times," are achieved, while also being compatible with industrial printing and curing processes.

[0048] This embodiment precisely controls the rheological properties of the slurry by controlling the amount of thickener used, forming a hydrogen bond network with the silanol groups on the surface of nanoparticles, and constructing a temporary three-dimensional structure in the mixed slurry. This stabilizes the viscosity at 10,000~20,000 mPa·s (25℃), avoiding both low viscosity due to insufficient use (leading to line diffusion and missing prints) and excessive viscosity (preventing smooth screen printing) caused by excessive use. At the same time, its thixotropic properties ensure rapid viscosity recovery after printing to maintain the line shape, laying the foundation for subsequent film quality.

[0049] This embodiment balances low-temperature curing and storage stability by controlling the amount of latent curing agent, ensuring that the silver paste does not gel after being stored at 25°C for more than 6 months. When curing at 130°C, the amount of accelerator is controlled to precisely activate the latent curing agent, accelerating its crosslinking reaction with the epoxy-polyurethane copolymer resin. Curing can be completed within 120 seconds (avoiding incomplete low-temperature curing). This prevents film embrittlement caused by excessive curing agent and avoids prolonged curing time due to insufficient accelerator. At the same time, it avoids excessive acidity in the system caused by excessive accelerator (corrosion of the substrate or reduction of adhesion).

[0050] In this embodiment, the stirring rate is 750~850 rpm for 15~25 minutes, and the stirring rate is continued at 750~850 rpm for 25~35 minutes.

[0051] Step S5: Feed the uniform slurry into a three-roll mill for repeated grinding until the slurry fineness is ≤15μm. Then, perform vacuum degassing and filtration in sequence to obtain high-performance low-silver-content silver paste.

[0052] The above slurry is fed into a three-roll mill, the roller spacing is adjusted, and the slurry is repeatedly ground until the fineness of the slurry is ≤15μm; the ground slurry is degassed for 15~25 minutes under a vacuum of ≤-0.09MPa, and then filtered with a 150~250 mesh screen to obtain a low silver content high-performance membrane switch / flexible printed circuit silver paste.

[0053] In this embodiment, the roller spacing of the three-roll mill is 10~15μm, and the number of repeated grinding cycles is 3~4.

[0054] The technical solution of the present invention is described below with specific embodiments: Example 1: Step S1: Add dibutyltin dilaurate catalyst to E-55 with an epoxy value of 0.56 eq / 100g, then add toluene diisocyanate type polyurethane prepolymer with an NCO content of 8%, and catalyze the reaction at 65°C for 2.5 hours to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds.

[0055] The mass ratio of E-55 to toluene diisocyanate type polyurethane prepolymer is 7:3, and the amount of dibutyltin dilaurate is 0.7% of the sum of the mass of epoxy resin and polyurethane prepolymer.

[0056] Step S2: The epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent are added to a dispersion vessel and stirred at 55°C and 550 rpm for 1.5 hours to obtain the organic carrier. The mass ratio of the epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is 15.0:4.5:52.6. The mixed organic solvent includes propylene glycol methyl ether acetate and diethylene glycol butyl ether in a mass ratio of 38:14.6.

[0057] Step S3: Add 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant to the organic carrier. Stir at a low speed of 650 rpm for 35 minutes. Then add carboxylated single-walled carbon nanotubes with a diameter of 2 nm, an aspect ratio of 1000, and a carboxyl content of 2.1 wt%. Stir at a high speed of 1550 rpm for 50 minutes. Then add in three portions particles with a diameter of 15 μm, an aspect ratio of 120, and a bulk density of 0.58 g / cm³. 3 Thin flakes of silver powder were stirred at 1050 rpm for 65 minutes to obtain a mixed slurry.

[0058] The mass ratio of organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes and flake silver powder is 70:0.7:0.4:0.8:27.8.

[0059] Step S4: At a temperature of 30°C, add a mixture with a specific surface area of ​​300 m² to the mixed slurry. 2 A mixture of / g of fumed silica thickener was stirred at 850 rpm for 20 minutes, followed by the addition of 2-phenylimidazolium latent curing agent and salicylic acid accelerator. The mixture was then stirred at 850 rpm for another 35 minutes to obtain a homogeneous slurry. The mass ratio of the slurry, fumed silica thickener, 2-phenylimidazolium latent curing agent, and salicylic acid accelerator was 96.4:0.67:3.2:0.3.

[0060] Step S5: Feed the uniform slurry into a three-roll mill, adjust the roller spacing to 15μm, and grind repeatedly 3 times until the slurry fineness is 15μm. Then, vacuum degas it for 25 minutes under a vacuum of -0.095MPa, filter it through a 250-mesh screen, and obtain a high-performance low-silver-content silver paste.

[0061] According to the test results, the silver content of the silver paste in this embodiment is 26.6%. Under the curing conditions of 130℃ / 120" the sheet resistance is 18.6mΩ / □. After 16 180° reciprocating bends, the sheet resistance change rate is ≤15%. The adhesion is 5B, the hardness is 4H, and the storage stability at 25℃ reaches 6 months.

[0062] Example 2: Step S1: Add dibutyltin dilaurate catalyst to E-51 with an epoxy value of 0.52 eq / 100g, then add toluene diisocyanate type polyurethane prepolymer with an NCO content of 6%, and catalyze the reaction at 60°C for 2.0 hours to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds.

[0063] The mass ratio of E-51 to toluene diisocyanate type polyurethane prepolymer is 7:3, and the amount of dibutyltin dilaurate is 0.5% of the sum of the mass of epoxy resin and polyurethane prepolymer.

[0064] Step S2: Add epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent to a dispersion vessel. Stir at 50°C and 500 rpm for 1 hour to obtain the organic carrier. The mass ratio of epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is 14.0:4.0:50. The mixed organic solvent includes propylene glycol methyl ether acetate and diethylene glycol butyl ether in a mass ratio of 36:13.

[0065] Step S3: Add 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant to the organic carrier. Stir at a low speed of 600 rpm for 30 minutes. Then add carboxylated single-walled carbon nanotubes with a diameter of 1.5 nm, an aspect ratio of 800, and a carboxyl content of 2.2 wt%. Stir at a high speed of 1500 rpm for 45 minutes. Then add in three portions particles with a diameter of 12 μm, an aspect ratio of 110, and a bulk density of 0.55 g / cm³. 3 Thin flakes of silver powder were stirred at 1000 rpm for 60 minutes to obtain a mixed slurry.

[0066] The mass ratio of the organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes, and flake silver powder is 68:0.6:0.3:0.5:27.

[0067] Step S4: At a temperature of 25°C, add a mixture with a specific surface area of ​​200 m² to the mixed slurry. 2The fumed silica thickener (g / g) was stirred at 800 rpm for 20 minutes, then 2-phenylimidazolium latent curing agent and salicylic acid accelerator were added, and the mixture was stirred at 800 rpm for another 30 minutes to obtain a homogeneous slurry. The mass ratio of the mixed slurry, fumed silica thickener, 2-phenylimidazolium latent curing agent, and salicylic acid accelerator was 96.0:0.6:3.0:0.3.

[0068] Step S5: Feed the uniform slurry into a three-roll mill, adjust the roller spacing to 12μm, and grind repeatedly 3 times until the slurry fineness is 15μm. Then, vacuum degas it for 20 minutes under a vacuum of -0.095MPa, filter it through a 200-mesh screen, and obtain a high-performance low-silver-content silver paste.

[0069] According to the test results, the silver content of the silver paste in this embodiment is 27.8%. Under the curing conditions of 130℃ / 120" the sheet resistance is 17.0mΩ / □. After 10 cycles of 180° bending, the sheet resistance change rate is ≤10%. The adhesion is 5B, the hardness is 4H+, and the storage stability at 25℃ reaches 6 months.

[0070] Example 3: Step S1: Add dibutyltin dilaurate catalyst to E-54 with an epoxy value of 0.48 eq / 100g, then add toluene diisocyanate type polyurethane prepolymer with an NCO content of 5%, and catalyze the reaction at 55°C for 1.5 hours to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds.

[0071] The mass ratio of E-54 to toluene diisocyanate type polyurethane prepolymer is 4:1, and the amount of dibutyltin dilaurate is 0.3% of the sum of the mass of epoxy resin and polyurethane prepolymer.

[0072] Step S2: The epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent are added to a dispersion vessel and stirred at 45°C and 450 rpm for 0.5 hours to obtain the organic carrier. The mass ratio of the epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is 13.0:3.5:47.5. The mixed organic solvent includes propylene glycol methyl ether acetate and diethylene glycol butyl ether in a mass ratio of 35:12.5.

[0073] Step S3: Add 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant to the organic carrier. Stir at a low speed of 550 rpm for 25 minutes. Then add carboxylated single-walled carbon nanotubes with a diameter of 1.0 nm, an aspect ratio of 500, and a carboxyl content of 2.0 wt%. Stir at a high speed of 1450 rpm for 40 minutes. Finally, add in four portions particles with a diameter of 10 μm, an aspect ratio of 105, and a bulk density of 0.6 g / cm³. 3 Thin flakes of silver powder were stirred at 950 rpm for 55 minutes to obtain a mixed slurry.

[0074] The mass ratio of organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes and flake silver powder is 64:0.5:0.35:0.2:26.6.

[0075] Step S4: At a temperature of 20°C, add the organobentonite thickener to the mixed slurry and stir at 750 rpm for 15 minutes. Then add the dicyandiamide latent curing agent and the 3-phenyl-1,1-dimethylurea accelerator and continue stirring at 750 rpm for 25 minutes to obtain a homogeneous slurry. The mass ratio of the mixed slurry, organobentonite thickener, dicyandiamide latent curing agent, and 3-phenyl-1,1-dimethylurea accelerator is 95.8:0.5:2.8:0.3.

[0076] Step S5: Feed the uniform slurry into a three-roll mill, adjust the roller spacing to 10μm, and grind repeatedly 4 times until the slurry fineness is 12μm. Then, vacuum degas it for 15 minutes under a vacuum of -0.09MPa, and filter it through a 150-mesh screen to obtain a high-performance low-silver-content silver paste.

[0077] According to the test results, the silver content of the silver paste in this embodiment is 27.2%. Under the curing conditions of 130℃ / 120" the sheet resistance is 17.8mΩ / □. After 12 cycles of 180° reciprocating bending, the sheet resistance change rate is ≤10%. The adhesion is 5B, the hardness is 4H+, and the storage stability at 25℃ reaches 6 months.

[0078] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A method for preparing a high-performance, low-silver-content silver paste, characterized in that, The preparation method includes the following steps: Step S1: Under the catalysis of dibutyltin dilaurate, epoxy resin and polyurethane prepolymer are subjected to catalytic reaction to obtain epoxy-polyurethane copolymer resin with alternating epoxy groups and urethane bonds. Step S2: The epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent are stirred and reacted to obtain an organic carrier; Step S3: After stirring the organic carrier, 3-aminopropyltriethoxysilane coupling agent and BYK-180 dispersant at low speed, add carboxylated single-walled carbon nanotubes and stir at high speed. Then add flake silver powder in batches and continue stirring to obtain a mixed slurry. Step S4: At a temperature of 20~30℃, add thickener to the mixed slurry, stir, then add latent curing agent and accelerator, and continue stirring to obtain a uniform slurry; Step S5: Feed the uniform slurry into a three-roll mill for repeated grinding until the slurry fineness is ≤15μm. Then, perform vacuum degassing and filtration in sequence to obtain high-performance low-silver-content silver paste.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of the epoxy resin to the polyurethane prepolymer is 7~8:2~3; In step S1, the amount of dibutyltin dilaurate used is 0.3-0.7% of the sum of the mass of the epoxy resin and the polyurethane prepolymer; In step S1, the epoxy resin is a bisphenol A epoxy resin with an epoxy value of 0.48~0.56 eq / 100g; In step S1, the polyurethane prepolymer is a toluene diisocyanate type polyurethane prepolymer with an NCO content of 5-8%. In step S1, the temperature of the catalytic reaction is 55~65℃ and the time is 1.5~2.5 hours.

3. The preparation method according to claim 2, characterized in that, The bisphenol A epoxy resin is one of E-51, E-54 and E-55.

4. The preparation method according to any one of claims 1 to 3, characterized in that, In step S2, the mass ratio of the epoxy-polyurethane copolymer resin, ethylene glycol diglycidyl ether reactive diluent, and mixed organic solvent is 13.0~15.0:3.5~4.5:47.5~52.6; In step S2, the stirring reaction is carried out at a temperature of 45-55°C, a speed of 450-550 rpm, and a time of 0.5-1.5 hours.

5. The preparation method according to claim 4, characterized in that, In step S2, the melting point of the mixed organic solvent is 150~200℃; In step S2, the mixed organic solvent includes propylene glycol methyl ether acetate and diethylene glycol butyl ether; The mass ratio of propylene glycol methyl ether acetate to diethylene glycol butyl ether is 35~38:12.5~14.

6.

6. The preparation method according to claim 5, characterized in that, In step S3, the mass ratio of the organic carrier, 3-aminopropyltriethoxysilane coupling agent, BYK-180 dispersant, carboxylated single-walled carbon nanotubes, and flake silver powder is 64~70:0.5~0.7:0.3~0.4:0.2~0.8:26.6~27.

8. In step S3, the low-speed stirring rate is 550~650 rpm, and the time is 25~35 minutes; In step S3, the high-speed stirring rate is 1450~1550 rpm, and the time is 40~50 minutes; In step S3, the stirring rate is 950-1050 rpm and the time is 55-65 minutes.

7. The preparation method according to claim 6, characterized in that, In step S3, the particle size of the flake silver powder is 10~15μm, the aspect ratio is >100, and the bulk density is ≤0.6g / cm³. 3 ; In step S3, the carboxylated single-walled carbon nanotubes have a diameter of 1-2 nm, an aspect ratio of 500-1000, and a carboxyl content of ≥2.0%. In step S3, the batch is 3 to 4 times.

8. The preparation method according to claim 7, characterized in that, In step S4, the mass ratio of the mixed slurry, thickener, latent curing agent, and accelerator is 95.8~96.4:0.5~0.7:2.8~3.2:0.3; In step S4, the thickener is one of fumed silica and organobentonite; In step S4, the latent curing agent is one of 2-phenylimidazole and dicyandiamide latent curing agent; In step S4, the accelerator is one of salicylic acid and 3-phenyl-1,1-dimethylurea.

9. The preparation method according to claim 8, characterized in that, In step S5, the roller spacing of the three-roll mill is 10~15μm; In step S5, the grinding is repeated 3 to 4 times. In step S5, the vacuum degree of the vacuum degassing is ≤-0.09MPa, and the time is 15~25 minutes; In step S5, the mesh size of the filter screen is 150 to 250 mesh.

10. A high-performance, low-silver-content silver paste, characterized in that, The high-performance, low-silver-content silver paste is prepared using the preparation method described in any one of claims 1 to 9.