Preparation method and application of conductive silver paste suitable for flexible circuits

Abstract

A method for preparing and applying a conductive silver paste suitable for flexible circuits belongs to the field of flexible electronics and low-temperature conductive pastes. The conductive silver paste suitable for flexible circuits prepared by the present invention relies on the formation of a cross-linked network internally after mixing a first type of functional mixture with a second type of functional mixture. The interlacing and mutual entanglement of the resin molecular chains enhance the deformation ability of the conductive paste, improve the ductility and impact resistance of the material, and ultimately enhance toughness. Compared with traditional conductive pastes, this conductive paste has better conductivity, flexibility, and mechanical strength, and meets the harsh environmental conditions that flexible circuits may face during application. In addition, by optimizing the organic carrier, the amount of conductive phase can be reduced, greatly reducing the production cost.

Description

Technical Field

[0001] The present invention belongs to the field of flexible electronics and low-temperature conductive pastes, and particularly relates to a preparation method and application of a conductive silver paste suitable for flexible circuits. Background Art

[0002] With the rapid development of the information electronics industry, various electronic devices are constantly shifting towards miniaturization and lightweighting, which has also led to the increasingly important position of flexible electronic technology in the modern electronics industry. Flexible printed circuits (FPCs), as the core components of flexible electronic technology, are widely used in wearable devices, OLED displays, flexible sensors, and automotive electronics due to their stretchability and bendability. Flexible circuits are mainly composed of three parts: a flexible substrate, a conductive material, and electronic components. Among them, the conductive material plays a key role in forming the conductive path, and mainly includes metal nanowires, carbon nanotubes, graphene, and conductive pastes. Traditional conductive materials such as copper foil and metal films, although they have good conductivity, face many problems in their application in flexible circuits, such as poor ductility, easy cracking or falling off during bending and stretching, and ultimately leading to decreased conductivity or even failure. In addition, traditional conductive materials usually require high-temperature processes, which is not conducive to their compatibility with flexible substrates suitable for low temperatures.

[0003] In order to overcome these problems, conductive silver paste has received widespread attention as an important alternative material. Conductive silver paste has good conductivity, chemical stability and flexibility, and is suitable for low-temperature processes. However, existing conductive silver pastes still have some problems in practical applications, such as insufficient adhesion, easy cracking or breakage due to bending, which leads to a decrease in conductive performance and affects the stability of the device. These problems limit the large-scale application of conductive silver paste in flexible circuits. Therefore, developing a conductive silver paste suitable for flexible circuits, and improving bending resistance and adhesion while ensuring excellent conductivity, has become an important research direction in the current technical field. Summary of the Invention

[0004] The purpose of the present invention is to solve the problems of poor durability and low conductivity of conductive silver paste in flexible circuits, and to provide a preparation method and application of a conductive silver paste suitable for flexible circuits. By optimizing the paste formula, this method can not only achieve low-temperature curing, but also maintain stable conductive properties under conditions of repeated bending and stretching. The development of this new conductive silver paste will provide strong support for the further development of flexible electronic devices.

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

[0006] A method for preparing a conductive silver paste suitable for flexible circuits, the method comprising:

[0007] Step 1: Preparation of the first type of functional mixture: Weigh 65% to 70% of thermoplastic resin, 2% to 5% of dispersant, and 25% to 33% of organic cosolvent, and add the weighed substances into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant. Stir at 75°C at a speed of 1000r / min to 2000r / min for 30min to 45min, and after cooling to room temperature, a uniform and stable mixture without solid particles is obtained, which is the first type of functional mixture; in step 1, the organic cosolvent is added first, and then the thermoplastic resin and dispersant are added, so that the thermoplastic resin and the organic cosolvent can be better mixed, the wettability of the thermoplastic resin can be improved, the subsequent preparation of the organic carrier can be facilitated, and the silver powder in the conductive paste can be more evenly dispersed.

[0008] Step 2: Preparation of the second type of functional mixture: weigh 35% to 45% of thermosetting resin, 35% to 45% of the first type of functional mixture, 2% to 5% of coupling agent, 2% to 5% of thixotropic agent, and 10% to 15% of organic cosolvent. Add the weighed substances into a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent. Stir at 1000 r / min to 2000 r / min for 30 min to 45 min at 75 ° C. After cooling to room temperature, add the first type of functional mixture. The functional mixture and thixotropic agent are added to a beaker and stirred at 75°C at a speed of 1000r / min to 2000r / min for 30min to 45min to obtain a uniform and stable mixture without solid particles, which is the second type of functional mixture; in step 2, an organic cosolvent is added first, and then a thermosetting resin and a coupling agent are added, which can make the thermosetting resin and the organic cosolvent better mixed, improve the wettability of the resin, facilitate the subsequent preparation of the organic carrier and allow the silver powder in the conductive paste to be more evenly dispersed. After cooling, the first type of functional mixture and thixotropic agent prepared in step 1 are added again, which can make the first type of functional mixture better dispersed in the thermosetting resin in step 2. The two resins (thermosetting resin and thermoplastic resin) combine with each other to form an internal network structure, improve the wettability to the silver powder, and have better rheological properties.

[0009] Step 3: Preparation of organic carrier: weigh 60% to 70% organic solvent, 15% to 25% second type functional mixture, 0% to 2% photoinitiator, 1% to 5% curing agent, 2% to 5% surfactant, 2% to 5% defoaming agent, put the weighed substances into a beaker in the order of organic solvent and second type functional mixture, stir at 1500r / min to 2000r / min for 45min to 55min at 75℃, cool to room temperature, put the weighed substances into a beaker in the order of photoinitiator, curing agent and surfactant, and stir at 1500r / min to 2000r / min for 45min to 55min. / min and stir for 60min~70min, add the defoamer into the beaker and stir at 1000r / min~1500r / min for 30min~45min, and obtain a uniform and stable mixed substance without solid particles, which is the organic carrier; in step three, first add the organic solvent and then add the second type of functional mixture (equivalent to the resin in steps one and two) to make the internal resin better dispersed in the organic solvent, and the outside of the resin has been wetted with an organic co-solvent in advance in steps one and two, which will make the dispersion of the resin in the solvent better and more uniform than directly adding the resin to the solvent. After cooling, relevant additives (photoinitiator, curing agent, surfactant) are added to prevent these types of additives from reacting with each other or becoming ineffective under high temperature conditions. The defoamer is finally added to eliminate bubbles generated during high-speed stirring.

[0010] Step 4: Preparation of low-temperature conductive silver paste: Weigh 25% to 35% of the organic vehicle and 65% to 75% of the silver powder, mix them evenly, and use a three-roll mill to roughly roll them once (the speed of the three-roll mill is 200rpm-250rpm, and the gap is 50um-100um), and fine roll them twice (the speed of the three-roll mill is 400rpm-450rpm, and the gap is 15um-35um) to obtain the silver paste. After sieving, defoam it in a defoamer to obtain a uniform, stable, viscous mixture, which is the conductive silver paste.

[0011] Furthermore, in step one, the thermoplastic resin is one or a combination of at least two of polyvinyl chloride, polyetheretherketone, polystyrene, polyetherester, polytetrafluoroethylene, polyurethane, polyvinyl alcohol, polyethylene, polystyrene-butadiene copolymer, polycarbonate, polyvinyl acetal, modified thermoplastic epoxy resin, thermoplastic acrylic resin, and polyimide ester, among which polyurethane resin or polyethylene resin is preferred. In particular, if polyurethane resin is selected, the following requirements should be met: molecular weight between 100,000 and 200,000. A higher molecular weight is beneficial to enhancing the mechanical properties and wear resistance of the slurry, and the hardness is between Shore A50 and Shore D80; correspondingly, if polyethylene resin is selected, the following requirements should be met: molecular weight between 150,000 and 300,000 g / mol, density between 0.910 and 0.945 g / cm3 The melt index is between 0.5 and 15 g / 10 min.

[0012] Furthermore, in step 1, the dispersant is one or a combination of at least two of BYK-155 / 35, BYK-154, BYK-220SN, BYK-155 / 50, BYK-329, and BYK-1165; the organic cosolvent is one or a combination of at least two of toluene diisocyanate, ethyl acetate, toluene, dimethylformamide, a urethane solution, N-methylpyrrolidone, tetrahydrofuran, ethylene glycol, and propiophenone. Specifically, if the thermoplastic resin is a polyurethane resin, one or a combination of at least two of N-methylpyrrolidone, ethyl acetate, dimethylformamide, and propiophenone should be selected; specifically, if a polyethylene resin is selected, one or a combination of at least two of tetrahydrofuran, toluene, ethyl acetate, and ethylene glycol should be selected. The cosolvent used when selecting a polyurethane or polyethylene resin needs to be selected from these solutions. The choice of cosolvent for resins other than these two resins depends on the solubility, volatility, and process requirements of the resin. Commonly used cosolvents such as alcohols, ketones or esters can dissolve these resins well and optimize their processing properties. They must be selected according to the chemical structure of the resin and the application scenario to ensure uniform dispersion and optimal curing effect.

[0013] Furthermore, in step 2, the thermosetting resin is one or a combination of at least two of epoxy resin, unsaturated polyester resin, phenolic resin, amino resin, acrylic resin, maleic acid resin, polysulfone resin, polyvinyl fluoride, epoxy ester resin, polysulfide resin, styrene resin, melamine resin, and epoxy acrylic resin, among which epoxy resin or acrylic resin is preferred. In particular, if epoxy resin is selected, the following conditions should be met: molecular weight between 1500 and 2500, and curing temperature between 30°C and 150°C; in particular, if acrylic resin is selected, the following conditions should be met: molecular weight between 2000 and 3500.

[0014] Furthermore, in step 2, the coupling agent is one or a combination of at least two selected from KH-550, KH-560, KH-570, SILANEA-187, MPTS, and TMOS; the thixotropic agent is one or a combination of at least two selected from CMC, PVA, polyamide wax, hydrogenated castor oil, fumed silica, and polyvinyl alcohol; and the organic cosolvent is one or a combination of at least two selected from acetone, methyl ethyl ketone, xylene, n-butanol, N,N-dimethylformamide, propylene glycol methyl ether acetate, ethylene glycol ethyl ether, methoxypropanol, cyclohexanone, and isobutyl acetate. Specifically, if an epoxy resin is selected, one or a combination of at least two selected from methyl ethyl ketone, N,N-dimethylformamide, ethylene glycol ethyl ether, and methoxypropanol should be selected; specifically, if an acrylic resin is selected, one or a combination of at least two selected from isobutyl acetate, cyclohexanone, and methyl ethyl ketone should be selected. The cosolvent used when selecting an epoxy resin or acrylic resin must be specifically selected. Other thermosetting resins also have cosolvent requirements, which must be selected based on the resin's chemical structure and application scenario to ensure uniform dispersion and optimal curing. The choice of cosolvent depends on the resin's solubility, volatility, and process requirements. Common cosolvents such as alcohols, ketones, and esters can effectively dissolve resins and optimize their processing properties.

[0015] Furthermore, in step 3, the organic solvent is one or a combination of at least two of isophorone, triethylamine, ethyl acetate, butyl acetate, butanone, dimethylformamide, diethylene glycol propyl ether, alcohol ester 12, alcohol ester 16, dimethyl sulfoxide, dimethyl glutarate, butyl carbitol acetate, butyl carbitol, dipropylene glycol methyl ether, methyl isobutyl ketone, and γ-butyrolactone; the photoinitiator is α-hydroxy ketones (Irgacure 184, Darocur 1173, Irgacure 127, Irgacure2959, Irgacure 500), acylphosphine oxides (Irgacure TPO, Irgacure 2100), benzoylformate (Darocur MBF, Irgacure 754); the curing agent is one or a combination of at least two of diethylenetriamine, 1,3-dimethylimidazolium hexafluoroantimonate, boron trifluoride ethylamine complex, maleic acid, diethylaminoethanol, and DICY; the surfactant is one or a combination of at least two of lecithin, fatty diamine organic salt TDO, polyvinyl alcohol-ethylene copolymer, BYK-3450, BYK-3480, BYK-3550, and BYK-3558; the defoaming agent is one or a combination of at least two of BYK-017, BYK-023, BYK-028, BYK-085, BYK-077SG, BYK-141SG, BYK-1610, and BYK-1618.

[0016] Furthermore, in step 4, the silver powder is one or a combination of at least two of spherical silver powder, flaky silver powder, nano silver powder, needle-shaped silver powder, and ultrafine silver powder. In particular, if spherical silver powder is used, the particle size of the silver powder should be between 1um and 4um; in particular, if flaky silver powder is used, the thickness of the silver powder should be between 20nm and 80nm; in particular, if nano silver powder is used, the particle size of the silver powder should be between 1nm and 40nm; the needle-shaped silver powder must meet the following requirements: have a suitable aspect ratio between 30:1 and 50:1; the ultrafine silver powder must meet the following requirements: have a particle size between 20 and 80nm, and a specific surface area of ​​0.5-3.5m 2 / g.

[0017] A conductive silver paste suitable for flexible circuits prepared by the above preparation method is used in the preparation of flexible circuit boards. The flexible circuit board includes a flexible substrate and the conductive silver paste printed thereon. The flexible substrate is any one of a PET substrate, a PAI substrate, a PI substrate, a PE substrate, a PTFE substrate, a PEN substrate, a PES substrate, a PC substrate, and a paper substrate. The preparation method is as follows: in an air atmosphere, heating the substrate to 120°C to 200°C at a heating rate of 3°C / min to 4°C / min, and drying and curing the substrate for 10min to 15min to obtain a flexible circuit board.

[0018] The beneficial effects of the present invention compared to the prior art are:

[0019] The conductive silver paste developed by this research is suitable for flexible circuits. The first functional compound and the second functional compound form a cross-linked network within the paste after mixing. The interlacing and intertwining of the resin molecular chains enhances the paste's deformability, improving its ductility and impact resistance, and ultimately enhancing its toughness. Compared to traditional conductive pastes, this paste exhibits improved conductivity, flexibility, and mechanical strength, meeting the harsh environmental conditions that flexible circuits may face during their application. Furthermore, by optimizing the organic vehicle, the amount of conductive phase used can be reduced, significantly reducing production costs.

[0020] Compared with traditional conductive pastes, the present invention optimizes the performance of organic carriers, forms a network through the mutual cross-linking of different types of functional mixtures, and ultimately improves the deformation ability of the conductive paste, which can reduce the occurrence of cracks under the action of external forces. The cured conductive silver paste undergoes 100 bends in both directions without cracks and has a resistivity change of less than 300%. Due to the optimization of the organic carrier, the conductive phase can be better dispersed in the carrier, and only a small amount of the conductive phase is required to form more conductive paths. This not only improves the conductive performance of the conductive silver paste but also reduces the amount of conductivity used, saving costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 3D microscopic diagram of the silver grid line morphology of Example 1;

[0022] Figure 2 3D microscopic diagram of the silver grid line morphology of Example 2;

[0023] Figure 3 3D microscopic diagram of the silver grid line morphology of Example 3;

[0024] Figure 4 3D microscopic diagram of the silver grid line morphology of Example 4;

[0025] Figure 5 Schematic diagram of the appearance of the silver grid line of Example 1 after being bent forward and backward 100 times;

[0026] Figure 6 Schematic diagram of the appearance of the silver grid line of Example 2 after being bent forward and backward 100 times;

[0027] Figure 7 Schematic diagram of the appearance of the silver grid line of Example 3 after being bent forward and backward 100 times;

[0028] Figure 8 Schematic diagram of the appearance of the silver grid line of Example 4 after being bent forward and backward 100 times;

[0029] Figure 9 This is the resistivity change data after bending. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0031] The present invention prepares a conductive silver paste suitable for flexible circuits by mixing the formula according to a certain ratio, then subjecting it to high-speed stirring and multiple slurrying on a three-roll mill. The conductive paste has a simple preparation process and a short curing time. Furthermore, through this dispersion and compounding method, the conductive silver paste exhibits excellent conductivity, adhesion, and flexibility after curing when printed on a flexible substrate, making it widely applicable to various flexible electronic components.

[0032] Example 1

[0033] A method for preparing a flexible circuit board containing a conductive silver paste suitable for a flexible circuit is as follows:

[0034] Step 1, Preparation of the First Type Functional Mixture: Add the weighed materials into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant. Heat in a 75°C water bath and stir at 1500 rpm for 40 minutes. Cool to room temperature to obtain a uniform, stable, and solid particle-free first type functional mixture. The raw material composition is as follows: (1) Thermoplastic resin: 57.9% polyurethane resin, 10.9% polyethylene resin; (2) Dispersant: 4% BYK-154; (3) Organic cosolvent: 18.3% ethyl acetate, 8.9% propiophenone.

[0035] Step 2: Prepare the second type of functional mixture: Place the weighed materials in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heat in a constant temperature water bath at 75°C and stir at 1500 r / min for 40 minutes, cool to room temperature, add the first type of functional mixture and thixotropic agent polyamide wax to the beaker, stir at 75°C at 2000 r / min for 45 minutes, and cool to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is as follows: (1) thermosetting resin: acrylic resin 40%; (2) first type functional mixture 43.2%; (3) coupling agent: γ-aminopropyltriethoxysilane (KH-550) 2.3%; (4) thixotropic agent: polyamide wax 4%; (5) organic cosolvent: isobutyl acetate 4.8%, cyclohexanone 5.7%.

[0036] Step 3, preparation of organic carrier: put the weighed substances into a beaker in the order of organic solvent and second type functional mixture and stir at 1800r / min at 75°C for 50min. After cooling to room temperature, add the weighed substances into the beaker in the order of curing agent and surfactant and stir at 1500r / min for 70min. Then add the defoaming agent to the beaker and stir at 1200r / min for 35min to obtain a stable organic carrier without solid particles. The raw material composition is as follows: (1) organic solvent: 19.7% butyl carbitol acetate, 5.6% isopropyl alcohol, 16.5% butyl acetate, 7.4% propylene glycol methyl ether acetate, 19.1% dimethyl glutarate; (2) 19.5% second type functional mixture; (3) curing agent: 2.8% boron trifluoride ethylamine complex, 1.2% 1,3-dimethylimidazole hexafluoroantimonate; (4) surfactant: 2.6% TDO and 1% BYK-3558; (5) defoaming agent: 4.6% BYK-023.

[0037] Step 4: Preparation of low-temperature conductive paste: The organic vehicle prepared in step 3 was mixed and ground with the silver powder using a three-roll mill. The slurry was then rolled three times (one rough roll at 200 rpm, 80 μm clearance, and two fine rolls at 450 rpm, 20 μm clearance). The fully ground slurry was passed through a 500-mesh sieve and the particle size was measured using a scraper fineness meter. The resulting fluid with a particle size of <5 μm was the low-temperature conductive paste. The raw material composition was: 66.1% silver powder, 33.9% organic vehicle.

[0038] Step 5, preparation of flexible circuit board: print the low-temperature conductive paste prepared in step 4 onto a flexible substrate by screen printing with a printing thickness of 0.15um, heat it to 150°C at a heating rate of 3°C / min in an air atmosphere, and dry and cure it for 10 minutes to obtain a flexible circuit board.

[0039] Example 2:

[0040] A method for preparing a flexible circuit board containing a conductive silver paste suitable for a flexible circuit is as follows:

[0041] Step 1: Prepare the first functional mixture: Add the weighed materials into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heat in a 75°C constant temperature water bath, and stir at 1800 rpm for 35 minutes. After cooling to room temperature, a uniform and stable first functional mixture free of solid particles is obtained. The raw material composition is as follows: (1) thermoplastic resin: polyurethane resin 46.8%, polyvinyl chloride resin 10.8%, polyvinyl alcohol 6.1%; (2) dispersant: BYK-329 content 4.5%; (3) organic cosolvent: dimethylformamide 16%, toluene 8.6%, ethylene glycol 7.2%.

[0042] Step 2: Prepare the second type of functional mixture: Place the weighed materials in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heat in a constant temperature water bath at 75°C and stir at 1800 r / min for 35 minutes, cool to room temperature, add the first type of functional mixture and thixotropic agent to the beaker, stir at 75°C at 2000 r / min for 45 minutes, and cool to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is as follows: (1) thermosetting resin: epoxy resin 33.6%, phenolic resin 10.6%; (2) first type functional mixture 36%; (3) coupling agent: 3.8% KH-570; (4) thixotropic agent: polyamide wax 1.1%, hydrogenated castor oil 2.6%; (5) organic cosolvent: methyl ethyl ketone 7.9%, ethylene glycol ethyl ether 4.4%.

[0043] Step 3, preparation of organic carrier: weighed materials are placed in a beaker in the order of organic solvent and second functional mixture, stirred at 2000 r / min for 45 minutes at 75°C, cooled to room temperature, weighed materials are added to the beaker in the order of curing agent and surfactant, stirred at 1800 r / min for 60 minutes, and then defoamer is added to the beaker and stirred at 1500 r / min for 35 minutes to obtain a stable organic carrier without solid particles. The raw material composition is as follows: (1) organic solvent: 36.7% butyl carbitol acetate, 15.7% alcohol ester dodecahydrate, 7.7% alcohol ester hexahydrate, 5.8% butanone; (2) 24.7% second functional mixture; (3) curing agent: 1.8% boron trifluoride ethylamine complex; (4) surfactant: 4.7% lecithin; (5) defoamer: 2.9% BYK-085.

[0044] Step 4: Preparation of low-temperature conductive paste: The organic vehicle prepared in step 3 is mixed and ground with the silver powder using a three-roll mill. The slurry is then rolled three times (one rough roll at 200 rpm, 80 μm gap, and two fine rolls at 450 rpm, 20 μm gap). The fully ground slurry is passed through a 500-mesh sieve and the particle size is measured using a scraper fineness meter. The resulting fluid with a particle size of <5 μm is the low-temperature conductive paste. The raw material composition is: 70.7% silver powder, 29.3% organic vehicle.

[0045] Step 5, preparation of flexible circuit board: print the low-temperature conductive paste prepared in step 4 onto a flexible substrate by screen printing with a printing thickness of 0.15um, heat it to 200℃ at a heating rate of 4℃ / min in an air atmosphere, and dry and cure it for 10 minutes to obtain a flexible circuit board.

[0046] Example 3:

[0047] A method for preparing a flexible circuit board containing a conductive silver paste suitable for a flexible circuit is as follows:

[0048] Step 1: Prepare the first functional mixture: Add the weighed materials into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heat in a 75°C water bath, and stir at 1500 rpm for 35 minutes. After cooling to room temperature, a uniform, stable, and solid particle-free first functional mixture is obtained. The raw material composition is as follows: (1) thermoplastic resin: polyethylene resin 47.2%, polystyrene resin 21.3%; (2) dispersant: BYK-1165 content 2.9%; (3) organic cosolvent: toluene 12.7%, ethylene glycol 4.6%, and urethane 11.3%.

[0049] Step 2: Prepare the second type of functional mixture: Place the weighed materials in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heat in a constant temperature water bath at 75°C and stir at 1500 r / min for 40 minutes, cool to room temperature, add the first type of functional mixture and thixotropic agent to the beaker, stir at 75°C at 2000 r / min for 45 minutes, and cool to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is as follows: (1) thermosetting resin: epoxy resin 24.5%, unsaturated polyester resin 12.4%; (2) first type functional mixture 39.8%; (3) coupling agent: 4.2% KH-550; (4) thixotropic agent: hydrogenated castor oil 2.3%; (5) organic cosolvent: ethylene glycol ethyl ether 14.4%, methyl ethyl ketone 2.4%.

[0050] Step 3, preparation of organic carrier: put the weighed substances into a beaker in the order of organic solvent and second type functional mixture and stir at 1800r / min for 50min at 75°C. After cooling to room temperature, add the weighed substances into the beaker in the order of photoinitiator, curing agent and surfactant and stir at 1800r / min for 60min. Then add the defoaming agent into the beaker and stir at 1500r / min for 35min to obtain a stable organic carrier without solid particles. The raw material composition is as follows: (1) organic solvent: 15.7% ethyl acetate, 27.9% dimethyl glutarate, 20.4% dimethylformamide; (2) photoinitiator: 1.7% Irgacure 127; (3) second type functional mixture 20.9%; (4) curing agent: 3.8% maleic acid, 0.7% diethylaminoethanol; (5) surfactant: 4.7% BYK-3450; (6) defoaming agent: 4.2% BYK-023.

[0051] Step 4: Preparation of low-temperature conductive paste: The organic vehicle prepared in step 3 is mixed and ground with the silver powder using a three-roll mill. The slurry is then rolled three times (one rough roll at 250 rpm with a 100 μm gap, and two fine rolls at 400 rpm with a 15 μm gap). The fully ground slurry is passed through a 500-mesh sieve and the particle size is measured using a scraper fineness meter. The resulting fluid with a particle size of <5 μm is the low-temperature conductive paste. The raw material composition is: 73.2% silver powder, 26.8% organic vehicle.

[0052] Step 5, preparation of flexible circuit board: print the low-temperature conductive paste prepared in step 4 onto a flexible substrate by screen printing with a printing thickness of 0.15um, heat it to 150°C at a heating rate of 3°C / min in an air atmosphere, and dry and cure it for 15 minutes to obtain a flexible circuit board.

[0053] Example 4:

[0054] A method for preparing a flexible circuit board containing a conductive silver paste suitable for a flexible circuit is as follows:

[0055] Step 1: Prepare the first type of functional mixture: Add the weighed materials into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heat in a constant temperature water bath at 75°C and stir at 1200 rpm for 40 minutes, and cool to room temperature to obtain a uniform, stable, and solid particle-free first type functional mixture. The raw material composition is as follows: (1) Thermoplastic resin: 44.8% polyethylene resin, 24.9% polyvinyl acetal; (2) Dispersant: 1.4% BYK-154, 2.5% BYK-329; (3) Organic cosolvent: 9.4% ethylene glycol, 12.7% toluene diisocyanate, and 4.3% dimethylformamide.

[0056] Step 2: Prepare the second type of functional mixture: Place the weighed substances in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heat in a constant temperature water bath at 75°C and stir at 1200 r / min for 40 minutes, cool to room temperature, add the first type of functional mixture and thixotropic agent to the beaker, stir at 1500 r / min for 45 minutes at 75°C, and cool to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is as follows: (1) thermosetting resin: acrylic resin 17.9%, amino resin 8.6%, polysulfone resin 13.3%; (2) first type functional mixture 40.2%; (3) coupling agent: 2.7% KH-570 and 1.1% MPTS; (4) thixotropic agent: polyvinyl alcohol 2.4%, PVA 1.7%; (5) organic cosolvent: N, N-dimethylformamide 6.3%, methoxypropanol 2.4%, n-butanol 3.4%.

[0057] Step 3, preparation of organic carrier: put the weighed substances into a beaker in the order of organic solvent and second type functional mixture and stir at 1800r / min for 45min at 75°C. After cooling to room temperature, add the weighed substances into the beaker in the order of photoinitiator, curing agent and surfactant and stir at 1800r / min for 60min. Then add the defoaming agent into the beaker and stir at 1800r / min for 35min to obtain a stable organic carrier without solid particles. The raw material composition is as follows: (1) organic solvent: 22.5% isophorone, 7.4% hexadecyl alcohol ester, 20.6% dimethyl sulfoxide, 16.7% methyl isobutyl ketone; (2) 19.6% second type functional mixture; (3) photoinitiator: 0.8% Irgacure TPO and 0.4% Irgacure 184; (4) curing agent: 2.8% diethylenetriamine, 1.4% 1,3-dimethylimidazolium hexafluoroantimonate; (4) surfactant: 1.7% BYK-3480 and 1.9% TDO; (5) defoaming agent: 4.2% BYK-077SG.

[0058] Step 4: Preparation of low-temperature conductive paste: The organic vehicle prepared in step 3 is mixed and ground with the silver powder using a three-roll mill. The slurry is then rolled three times (one rough roll at 250 rpm with a 100 μm gap, and two fine rolls at 400 rpm with a 15 μm gap). The fully ground slurry is passed through a 500-mesh sieve and the particle size is measured using a scraper fineness meter. The resulting fluid with a particle size of <5 μm is the low-temperature conductive paste. The raw material composition is: 73.6% silver powder, 26.4% organic vehicle.

[0059] Step 5, preparation of flexible circuit board: print the low-temperature conductive paste prepared in step 4 onto a flexible substrate by screen printing with a printing thickness of 0.15um, heat it to 200℃ at a heating rate of 4℃ / min in an air atmosphere, and dry and cure it for 10 minutes to obtain a flexible circuit board.

[0060] In the embodiment of the present invention, the cured flexible circuit board is bent 100 times in both directions with a bending angle of 180 degrees. Figures 1 to 8 By comparison, there is no obvious breakage of the silver grid line on the flexible circuit before and after bending 100 times, and the resistivity change during bending indicates that the silver grid line on the flexible circuit has not broken. Figure 9 It can be seen that the final resistivity change of the silver paste is less than 300%.

Claims

1. A method for preparing a conductive silver paste suitable for flexible circuits, characterized in that: The method is: Step 1: Preparation of the first type of functional mixture: Weigh 65% to 70% of a thermoplastic resin, 2% to 5% of a dispersant, and 25% to 33% of an organic cosolvent, add the weighed substances into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, stir at 75°C at a speed of 1000 r / min to 2000 r / min for 30 min to 45 min, and after cooling to room temperature, obtain a uniform and stable mixture without solid particles, which is the first type of functional mixture; the thermoplastic resin is one or a combination of at least two of polyvinyl chloride, polyetheretherketone, polystyrene, polyetherester, polytetrafluoroethylene, polyurethane, polyvinyl alcohol, polyethylene, polystyrene-butadiene copolymer, polycarbonate, polyvinyl acetal, modified thermoplastic epoxy resin, thermoplastic acrylic resin, and polyimide ester; Step 2: Preparation of the second type of functional mixture: weigh 35%~45% of thermosetting resin, 35%~45% of the first type of functional mixture, 2%~5% of coupling agent, 2%~5% of thixotropic agent, and 10%~15% of organic cosolvent. Add the weighed substances into a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent. Stir at 1000r / min~2000r / min for 30min~45min at 75℃. After cooling to room temperature, add the first type of functional mixture and The thixotropic agent is added to a beaker and stirred at 75° C. at a speed of 1000 r / min to 2000 r / min for 30 min to 45 min to obtain a uniform and stable mixture without solid particles, which is the second type of functional mixture; the thermosetting resin is one or a combination of at least two of epoxy resin, unsaturated polyester resin, phenolic resin, amino resin, acrylic resin, maleic acid resin, polysulfone resin, polyvinyl fluoride, epoxy ester resin, polysulfide resin, styrene resin, melamine resin, and epoxy acrylic resin; Step 3: Preparation of organic carrier: Weigh 60%~70% organic solvent, 15%~25% second type functional mixture, 0%~2% photoinitiator, 1%~5% curing agent, 2%~5% surfactant, and 2%~5% defoaming agent, and put the weighed substances into a beaker in the order of organic solvent and second type functional mixture. Stir at 1500r / min~2000r / min for 45min~55min at 75°C, cool to room temperature, and put the weighed substances into a beaker in the order of photoinitiator, curing agent, and surfactant, and stir at 1500r / min~2000r / min for 60min~70min. Add the defoaming agent to the beaker and stir at 1000r / min~1500r / min for 30min~45min to obtain a uniform and stable mixed substance without solid particles, which is the organic carrier; Step 4: Preparation of low-temperature conductive silver paste: Weigh 25%~35% of the organic vehicle and 65%~75% of the silver powder, mix them evenly, and then use a three-roll mill to coarsely roll them once. During the coarse rolling, the speed of the three-roll mill is 200rpm~250rpm, and the gap is 50um~100um. Fine rolling is performed twice. During the fine rolling, the speed of the three-roll mill is 400rpm~450rpm, and the gap is 15um~35um to obtain the silver paste. After sieving, it is defoamed in a defoaming machine to obtain a uniform, stable and viscous mixture, which is the conductive silver paste; the silver powder is one or a combination of at least two of spherical silver powder, flake silver powder, and needle-shaped silver powder. In particular, if spherical silver powder is used, the particle size of the silver powder should be between 1um~4um; in particular, if flake silver powder is used, the thickness of the silver powder should be between 20nm~80nm; the needle-shaped silver powder must meet the following requirements: it has a suitable aspect ratio between 30:1 and 50:

1.

2. The method for preparing a conductive silver paste suitable for flexible circuits according to claim 1, wherein: In step 1, the dispersant is one or a combination of at least two of BYK-155 / 35, BYK-154, BYK-220SN, BYK-155 / 50, BYK-329, and BYK-1165; the organic cosolvent is one or a combination of at least two of toluene diisocyanate, ethyl acetate, toluene, dimethylformamide, a carbamate solution, N-methylpyrrolidone, tetrahydrofuran, ethylene glycol, and propiophenone.

3. The method for preparing a conductive silver paste suitable for flexible circuits according to claim 1, wherein: In step 2, the coupling agent is one or a combination of at least two of KH-550, KH-560, KH-570, SILANE A-187, MPTS, and TMOS; the thixotropic agent is one or a combination of at least two of CMC, PVA, polyamide wax, hydrogenated castor oil, fumed silica, and polyvinyl alcohol; and the organic cosolvent is one or a combination of at least two of acetone, methyl ethyl ketone, xylene, n-butanol, N,N-dimethylformamide, propylene glycol methyl ether acetate, ethylene glycol ethyl ether, methoxypropanol, cyclohexanone, and isobutyl acetate.

4. The method for preparing a conductive silver paste suitable for flexible circuits according to claim 1, wherein: In step 3, the organic solvent is one or at least two combinations of isophorone, triethylamine, ethyl acetate, butyl acetate, butanone, dimethylformamide, diethylene glycol propyl ether, alcohol ester 12, alcohol ester 16, dimethyl sulfoxide, dimethyl glutarate, butyl carbitol acetate, butyl carbitol, dipropylene glycol methyl ether, methyl isobutyl ketone, and γ-butyrolactone; the photoinitiator is one or at least two combinations of α-hydroxy ketones, acylphosphine oxides, and benzoylformate; the curing agent is diethylenetriamine, 1,3-dimethylimidazolium hexafluoroantimonate, boron trifluoride ethylamine complex, cis- One or a combination of at least two of butenedioic acid, diethylaminoethanol, and DICY; the surfactant is one or a combination of at least two of lecithin, fatty diamine organic salt TDO, polyvinyl alcohol-ethylene copolymer, BYK-3450, BYK-3480, BYK-3550, and BYK-3558; the defoamer is one or a combination of at least two of BYK-017, BYK-023, BYK-028, BYK-085, BYK-077SG, BYK-141SG, BYK-1610, and BYK-1618.

5. Use of a conductive silver paste suitable for flexible circuits prepared by the preparation method according to any one of claims 1 to 4 in the preparation of flexible circuit boards, characterized in that: The flexible circuit board includes a flexible substrate and the printed conductive silver paste, wherein the flexible substrate is any one of a PET substrate, a PAI substrate, a PI substrate, a PE substrate, a PTFE substrate, a PEN substrate, a PES substrate, a PC substrate, and a paper substrate. The preparation method is as follows: in an air atmosphere, heating the substrate to 120°C~200°C at a heating rate of 3°C / min~4°C / min, and drying and curing the substrate for 10min~15min to obtain a flexible circuit board.

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