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

Abstract

The invention discloses a preparation method and application of conductive silver paste suitable for a flexible circuit, and belongs to the field of flexible electronics and low-temperature conductive paste. According to the prepared conductive silver paste suitable for the flexible circuit, a cross-linked network is formed inside after the first type of functional mixture and the second type of functional mixture are mixed, the deformability of the conductive paste is enhanced through staggering and mutual winding of resin molecular chains, the ductility and impact resistance of the material are improved, and finally the toughness is enhanced. Compared with the traditional conductive paste, the conductive paste has better conductivity, flexibility and mechanical strength, and meets the harsh environment condition possibly faced by a flexible circuit in the application process. And by optimizing the organic carrier, the use amount of the conductive phase can be reduced, and the cost required by production is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the field of flexible electronics and low-temperature conductive paste, and specifically 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 lightweight, which has also led to the flexible electronic technology occupying an increasingly important position in the modern electronic industry. As the core component of flexible electronic technology, flexible printed circuit (FPC) is widely used in wearable devices, OLED displays, flexible sensors, and automotive electronics due to its stretchable and bendable characteristics. Flexible circuits are mainly composed of three parts: flexible substrates, conductive materials, and electronic components. Among them, conductive materials play a key role in forming conductive paths, mainly 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 the application of 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, but this 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, the existing conductive silver paste still has some problems in practical applications, such as insufficient adhesion, easy cracking or breakage due to bending, resulting in decreased conductivity, affecting the stability of the device, etc. 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. The method can not only achieve low-temperature curing but also maintain stable conductive properties under conditions of repeated bending and stretching by optimizing the paste formula. 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 purpose, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a conductive silver paste suitable for a flexible circuit, 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, 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 the 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 75°C at a speed of 1000r / min to 2000r / min for 30min to 45min, cool to room temperature, and then 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 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 functional mixture and thixotropic agent prepared in step 1 are added, so that the first functional mixture can be better dispersed in the thermosetting resin in step 2, and the two resins (thermosetting resin and thermoplastic resin) are combined with each other to form an internal network structure, which improves the wettability to the silver powder and has better rheological properties.

[0009] Step 3: Preparation of organic carrier: weigh 60% to 70% organic solvent, 15% to 25% second functional mixture, 0% to 2% photoinitiator, 1% to 5% curing agent, 2% to 5% surfactant, and 2% to 5% defoaming agent, put the weighed substances into a beaker in the order of organic solvent and second functional mixture, stir at 1500r / min to 2000r / min for 45min to 55min at 75°C, 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 to 70min, add the defoamer into the beaker and stir at 1000r / min to 1500r / min for 30min to 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, add relevant additives (photoinitiator, curing agent, surfactant) 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 carrier 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] Further, in step 1, 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: the molecular weight is between 100,000 and 200,000, and a higher molecular weight is conducive 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: the molecular weight is between 150,000 and 300,000 g / mol, and the density is between 0.910 and 0.945 g / cm3 The melt index is between 0.5 and 15 g / 10 min.

[0012] Further, in step 1, the dispersant is one or at least two combinations of BYK-155 / 35, BYK-154, BYK-220SN, BYK-155 / 50, BYK-329, and BYK-1165; the organic cosolvent is one or at least two combinations of toluene diisocyanate, ethyl acetate, toluene, dimethylformamide, carbamate solution, N-methylpyrrolidone, tetrahydrofuran, ethylene glycol, and propiophenone. In particular, if the thermoplastic resin is a polyurethane resin, one or at least two combinations of N-methylpyrrolidone, ethyl acetate, dimethylformamide, and propiophenone should be selected; in particular, if a polyethylene resin is selected, one or at least two combinations of tetrahydrofuran, toluene, ethyl acetate, and ethylene glycol should be selected. The cosolvent used when selecting polyurethane or polyethylene resin needs to be selected from these solutions. The selection of resin cosolvents other than these two resins depends on the solubility, volatility, and process requirements of the resin. Commonly used co-solvents 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: the molecular weight is between 1500 and 2500, and the curing temperature is between 30°C and 150°C; in particular, if acrylic resin is selected, the following conditions should be met: the molecular weight is between 2000 and 3500.

[0014] Further, in step 2, the coupling agent is one or at least two combinations of KH-550, KH-560, KH-570, SILANEA-187, MPTS, and TMOS, the thixotropic agent is one or at least two combinations of CMC, PVA, polyamide wax, hydrogenated castor oil, fumed silica, and polyvinyl alcohol, and the organic cosolvent is one or at least two combinations 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. In particular, if epoxy resin is selected, one or at least two combinations of methyl ethyl ketone, N, N-dimethylformamide, ethylene glycol ethyl ether, and methoxypropanol should be selected; in particular, if acrylic resin is selected, one or at least two combinations of isobutyl acetate, cyclohexanone, and methyl ethyl ketone should be selected. The cosolvent used when selecting epoxy resin or acrylic resin must be a specific selection. As for other thermosetting resins, there are also co-solvent requirements, which must be selected according to the chemical structure of the resin and the application scenario to ensure uniform dispersion and optimal curing effect. The choice of co-solvent depends on the solubility, volatility and process requirements of the resin. Commonly used co-solvents such as alcohols, ketones or esters can dissolve the resin well and optimize their processing performance.

[0015] Further, 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 twelve, alcohol ester hexadecone, 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, Irgacure 2959, Irgacure 500), acylphosphine oxides (Irgacure TPO, Irgacure 2100), benzoylformate (Darocur MBF, Irgacure 754) or a combination of at least two thereof; the curing agent is one or a combination of at least two thereof including 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 thereof including 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 thereof including BYK-017, BYK-023, BYK-028, BYK-085, BYK-077SG, BYK-141SG, BYK-1610, and BYK-1618.

[0016] Further, 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: the particle size is between 20 and 80nm, and the specific surface area is between 0.5 and 3.5m 2 / g.

[0017] The invention discloses an application of a conductive silver paste suitable for a flexible circuit prepared by the above preparation method in the preparation of a flexible circuit board, wherein the flexible circuit board comprises a flexible substrate and the conductive silver paste is printed thereon, 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, and the preparation method comprises the following steps: heating the temperature to 120°C to 200°C at a heating rate of 3°C / min to 4°C / min in an air atmosphere, and drying and curing the temperature 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 suitable for flexible circuits prepared by the present invention relies on the formation of a cross-linked network after the first type of functional mixture is mixed with the second type of functional mixture. The interlacing and intertwining 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 the 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. And by optimizing the organic carrier, the amount of conductive phase can be reduced, greatly reducing the production cost.

[0020] Compared with the traditional conductive paste, the present invention optimizes the performance of the organic carrier, forms a network through the mutual cross-linking of different types of functional mixtures, and finally improves the deformation ability of the conductive paste, can reduce the occurrence of cracks under the action of external force, and the conductive silver paste obtained by curing undergoes 100 forward and reverse bending without cracks and the resistivity change is 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 part of the conductive phase is needed to form more conductive paths, which not only improves the conductive performance of the conductive silver paste, but also reduces the amount of conductivity, thereby 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 This is a schematic diagram of the appearance of the silver grid wire of Example 1 after being bent forward and backward 100 times;

[0026] Figure 6 This is a schematic diagram of the appearance of the silver grid wire of Example 2 after being bent forward and backward 100 times;

[0027] Figure 7 This is a schematic diagram of the appearance of the silver grid wire of Example 3 after being bent forward and backward 100 times;

[0028] Figure 8 This is a schematic diagram of the appearance of the silver grid wire of Example 4 after being bent forward and backward 100 times;

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

[0030] In order to make the purpose, 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 used to limit the present invention.

[0031] After the formula of the present invention is mixed in a certain proportion, a conductive silver paste suitable for a flexible circuit can be prepared by a method of high-speed stirring and multiple pulping with a three-roller machine. The preparation process of the conductive paste is simple, the curing time is short, and through the dispersed composite method, the conductive silver paste is printed on a flexible substrate and cured, and exhibits excellent conductivity, adhesion and flexibility, and can be widely used in 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 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 a speed of 1500r / min for 40min, and cool to room temperature to obtain a uniform and stable first type of functional mixture without solid particles. The raw material composition is: (1) thermoplastic resin: polyurethane resin 57.9%, polyethylene resin 10.9%; (2) dispersant: BYK-154 content 4%; (3) organic cosolvent: ethyl acetate 18.3%, propiophenone 8.9%.

[0035] Step 2, the second type of functional mixture is prepared: the weighed materials are placed in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heated in a constant temperature water bath at 75°C and stirred at a speed of 1500r / min for 40min, cooled to room temperature, the first type of functional mixture and thixotropic agent polyamide wax are added to the beaker, stirred at 75°C at a speed of 2000r / min for 45min, and cooled to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is: (1) thermosetting resin: acrylic resin 40%; (2) first type of 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 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 curing agent and surfactant, and stir at 1500r / min for 70min. Then add the defoaming agent into the beaker and stir at 1200r / min for 35min to obtain stable organic carriers 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: Use a three-roll mill to mix and grind the organic carrier prepared in step 3 with the silver powder, and roll the paste three times (coarse rolling once at a speed of 200 rpm and a gap of 80 um, and fine rolling twice at a speed of 450 pm and a gap of 20 um), pass the fully ground paste through a 500 mesh sieve and measure the particle size with a scraper fineness meter, and the obtained fluid with a particle size of <5 μm is the low-temperature conductive paste. The raw material composition is: 66.1% silver powder, 33.9% organic carrier.

[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, and heat to 150°C at a heating rate of 3°C / min in an air atmosphere, and dry and cure for 10 minutes to obtain a flexible circuit board.

[0039] Embodiment 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, the first type of functional mixture is prepared: the weighed materials are added into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heated in a constant temperature water bath at 75°C and stirred at a speed of 1800r / min for 35min, and cooled to room temperature to obtain a uniform and stable first type of functional mixture without solid particles. The raw material composition is: (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, the second type of functional mixture is prepared: the weighed materials are placed in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heated in a constant temperature water bath at 75°C and stirred at a speed of 1800r / min for 35min, cooled to room temperature, the first type of functional mixture and thixotropic agent are added to the beaker, stirred at 75°C at a speed of 2000r / min for 45min, and cooled to room temperature to obtain a uniform and stable second type of functional mixture without solid particles. The raw material composition is: (1) thermosetting resin: 33.6% epoxy resin, 10.6% phenolic resin; (2) 36% of the first type of functional mixture; (3) coupling agent: 3.8% KH-570; (4) thixotropic agent: 1.1% polyamide wax, 2.6% hydrogenated castor oil; (5) organic cosolvent: 7.9% methyl ethyl ketone, 4.4% ethylene glycol ether.

[0043] Step 3, preparation of organic carrier: put the weighed materials into a beaker in the order of organic solvent and second functional mixture, stir at 2000r / min for 45min at 75℃, add the weighed materials into the beaker in the order of curing agent and surfactant and stir at 1800r / min for 60min after cooling to room temperature, then add the defoamer into the beaker and stir at 1500r / min for 35min, and obtain a stable organic carrier without solid particles. The raw material composition is: (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: Use a three-roller mill to mix and grind the organic carrier prepared in step 3 with the silver powder, roll the slurry three times (coarse rolling once at a speed of 200 rpm and a gap of 80 um, and fine rolling twice at a speed of 450 rpm and a gap of 20 um), pass the fully ground slurry through a 500 mesh sieve and measure the particle size with a scraper fineness meter. The obtained 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 carrier.

[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 to 200°C at a heating rate of 4°C / min in an air atmosphere, and dry and cure for 10 minutes to obtain a flexible circuit board.

[0046] Embodiment 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, the first type of functional mixture is prepared: the weighed materials are added into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heated in a constant temperature water bath at 75°C and stirred at a speed of 1500r / min for 35min, and cooled to room temperature to obtain a uniform and stable first type of functional mixture without solid particles. The raw material composition is: (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%, carbamate 11.3%.

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

[0050] Step 3, preparation of organic carrier: put the weighed substances into a beaker in the order of organic solvent and second 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 stable organic carriers 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: Use a three-roller mill to mix and grind the organic carrier prepared in step 3 with the silver powder, and roll the slurry three times (coarse rolling once at a speed of 250 rpm and a gap of 100 um, and fine rolling twice at a speed of 400 rpm and a gap of 15 um), pass the fully ground slurry through a 500 mesh sieve and measure the particle size with a scraper fineness meter. The obtained 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 carrier.

[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, and heat to 150°C at a heating rate of 3°C / min in an air atmosphere, and dry and cure for 15 minutes to obtain a flexible circuit board.

[0053] Embodiment 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, the first type of functional mixture is prepared: the weighed materials are added into a beaker in the order of organic cosolvent, thermoplastic resin, and dispersant, heated in a constant temperature water bath at 75°C and stirred at a speed of 1200r / min for 40min, and cooled to room temperature to obtain a uniform and stable first type of functional mixture without solid particles. The raw material composition is: (1) thermoplastic resin: polyethylene resin 44.8%, polyvinyl alcohol acetal 24.9%; (2) dispersant: BYK-154 content 1.4%, BYK-329 content 2.5%; (3) organic cosolvent: ethylene glycol 9.4%, toluene diisocyanate 12.7%, dimethylformamide 4.3%.

[0056] Step 2, the second type of functional mixture is prepared: the weighed substances are placed in a beaker in the order of organic cosolvent, thermosetting resin, and coupling agent, heated in a constant temperature water bath at 75°C and stirred at a speed of 1200r / min for 40min, and after cooling to room temperature, the first type of functional mixture and thixotropic agent are added to the beaker, stirred at a speed of 1500r / min for 45min at 75°C, and after cooling to room temperature, a uniform and stable second type of functional mixture without solid particles is obtained. 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 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 stable organic carriers 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% of the 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: Use a three-roller mill to mix and grind the organic carrier prepared in step 3 with the silver powder, and roll the slurry three times (coarse rolling once at a speed of 250 rpm and a gap of 100 um, and fine rolling twice at a speed of 400 rpm and a gap of 15 um), pass the fully ground slurry through a 500 mesh sieve and measure the particle size with a scraper fineness meter. The obtained 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 carrier.

[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 to 200°C at a heating rate of 4°C / min in an air atmosphere, and dry and cure 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°. Figures 1 to 8 By comparison, there is no obvious breakage of the silver grid line on the flexible circuit before and after 100 bends, and the resistivity change during the bends indicates that Fig. 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 a flexible circuit, characterized in that: The method is: 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, 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; 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 75°C at a speed of 1000r / min to 2000r / min for 30min to 45min, cool to room temperature, add the first type of functional mixture and thixotropic agent into a beaker, stir at 75°C at a speed of 1000r / min to 2000r / min for 30min to 45min, and obtain a uniform and stable mixture without solid particles, which is the second type of functional mixture; Step 3: Preparation of organic carrier: weigh 60% to 70% organic solvent, 15% to 25% second 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 functional mixture, stir at 1500r / min to 2000r / min for 45min to 55min at 75°C, cool to room temperature, put the weighed substances into a beaker in the order of photoinitiator, curing agent and surfactant, stir at 1500r / min to 2000r / min for 60min to 70min, add the defoaming agent into the beaker and stir at 1000r / min to 1500r / min for 30min to 45min, and 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% to 35% of the organic carrier 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.

2. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: In step 1, 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.

3. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: In step 1, the dispersant is one of BYK-155 / 35, BYK-154, BYK-220SN, BYK-155 / 50, BYK-329, BYK-1165, or a combination of at least two thereof; the organic co-solvent is one of toluene diisocyanate, ethyl acetate, toluene, dimethylformamide, carbamate solution, N-methylpyrrolidone, tetrahydrofuran, ethylene glycol, or a combination of at least two thereof.

4. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: 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.

5. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: In step 2, the coupling agent is one or a combination of at least two of KH-550, KH-560, KH-570, SILANEA-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.

6. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: 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 twelve, alcohol ester hexadecone, 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, Irgacure 2959, Irgacure 500), acylphosphine oxides (Irgacure TPO, Irgacure 2100), benzoylformate (Darocur MBF, Irgacure 754) or a combination of at least two thereof; the curing agent is one or a combination of at least two thereof including 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 thereof including 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 thereof including BYK-017, BYK-023, BYK-028, BYK-085, BYK-077SG, BYK-141SG, BYK-1610, and BYK-1618.

7. The method for preparing a conductive silver paste suitable for a flexible circuit according to claim 1, characterized in that: In step 4, the silver powder is one of spherical silver powder, flaky silver powder, nano silver powder, needle-shaped silver powder, and ultrafine silver powder, or a combination of at least two of them. 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: the particle size is between 20 and 80nm, and the specific surface area is between 0.5 and 3.5m 2 / g.

8. Use of a conductive silver paste suitable for flexible circuits prepared by the preparation method according to any one of claims 1 to 7 in the preparation of flexible circuit boards, characterized in that: 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, the temperature is increased to 120°C to 200°C at a heating rate of 3°C / min to 4°C / min, and the drying and curing time is 10min to 15min, so as to obtain a flexible circuit board.

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