Low-temperature silver paste for FPC (Flexible Printed Circuit) conductive circuit and preparation method of low-temperature silver paste

Abstract

The invention discloses low-temperature silver paste for an FPC (Flexible Printed Circuit) conductive circuit and a preparation method of the low-temperature silver paste. The silver paste comprises the following components in parts by weight: 7-15 parts of an epoxy resin composition, 30-40 parts of an organic solvent, 2-5 parts of a polyether polyamine curing agent, 0.5-2 parts of a curing accelerator, 0.5-2 parts of a silane coupling agent and 40-55 parts of micron-sized flaky silver powder, the epoxy resin composition is a combination of high-elongation epoxy resin and hydrogenated bisphenol A epoxy resin, and the organic solvent is an ester mixed solvent with the boiling point of 180-230 DEG C; the polyether polyamine curing agent is a bifunctional or trifunctional polyether amine curing agent; and the curing accelerator is an imidazole accelerator. The low-temperature silver paste has the advantages that the defects that a conventional epoxy system cured material is hard, brittle and not resistant to bending are overcome, the low-temperature silver paste has good flexibility so as to adapt to application of an FPC conductive circuit, the low-temperature silver paste can be completely cured under the condition of 130 DEG C for 30 min, the preparation process is simple and easy to implement, the use amount of silver powder is low while the requirement for the conductivity of the FPC circuit is met, and the cost of the silver paste is effectively reduced.

Description

Technical Field

[0001] The invention relates to a low-temperature silver paste for an FPC conductive circuit and a preparation method thereof, belonging to the technical field of paste preparation. Background Art

[0002] Flexible printed circuit (FPC) is a high-reliability, high-flexibility, high-flexibility, and high-integration electronic component made of flexible insulating materials such as polyimide as the substrate. Compared with traditional rigid circuit boards, FPC has incomparable flexibility, lightness, windability, and foldability. It is suitable for the design scenarios of various complex electronic devices and meets the development needs of modern electronic fields such as new automotive electronics, medical devices, mobile devices, and industrial equipment.

[0003] In FPC, the conductive circuit is usually formed by chemical etching of the copper-plated layer or by printing low-temperature silver paste and then heating and curing. Low-temperature silver paste has good conductivity and current carrying capacity. It does not need to use high-cost and high-pollution processes such as exposure, development, and chemical etching required for copper circuits. It can form large-area high-density circuits through printing processes and cure into films below 150°C. It does not damage the substrate and is easy to use. It has become a key material for the manufacture of FPC conductive circuits.

[0004] The bonding phase in the existing low-temperature silver paste components is usually a thermoplastic resin such as polyester resin, polyurethane resin, acrylic resin, chlorovinyl resin, etc., which has good flexibility but often has low adhesion to the polyimide substrate used in FPC. The resin crosslinking density is low, resulting in a lack of adhesion of the low-temperature silver paste, which leads to insufficient reliability of the conductive circuit. Conventional thermosetting epoxy resins have a high crosslinking density and adhesion after curing, but they are usually rigid hard materials after curing, and are not bendable, making it difficult to meet the needs of FPC conductive circuits. Therefore, researching a low-temperature silver paste for FPC conductive circuits that has both flexibility and adhesion is a technical problem that needs to be solved urgently. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a low-temperature silver paste for FPC conductive circuits which has high adhesion to a polyimide substrate used in FPC circuit boards and has flexible and bendable properties, and a preparation method thereof.

[0006] In order to solve the above technical problems, the low-temperature silver paste for FPC conductive circuits of the present invention comprises the following components in parts by weight:

[0007] Epoxy resin composition: 7-15 parts Organic solvent: 30-40 parts Polyether polyamine curing agent: 2-5 parts Curing accelerator: 0.5-2 parts Silane coupling agent: 0.5-2 parts Micron-sized flaky silver powder: 40-55 parts;

[0008] Among them, the epoxy resin composition is a combination of a high-extension epoxy resin and a hydrogenated bisphenol A epoxy resin, and the organic solvent is an ester mixed solvent with a boiling point of 180-230°C; the polyether polyamine curing agent is a bifunctional or trifunctional polyetheramine curing agent; the curing accelerator is an imidazole accelerator.

[0009] Further, by weight ratio, the ratio of the high-extension epoxy resin to the hydrogenated bisphenol A epoxy resin is 3-6:1.

[0010] Further, the high-extension rate epoxy resin is one or a combination of more than one of Dow DER 732, Dow DER 736, Dow DER 791, Dow DER 9313, Dow XZ92465.00, Dow XZ92466.00, Complex High-Tech EPU-133L, Complex High-Tech EPU-133S, Complex High-Tech EPU-253, Complex High-Tech EPU-300A, or Complex High-Tech EPU-300S.

[0011] Further, the hydrogenated bisphenol A epoxy resin is one of ADK EP-4080E, Tohto Kasei ST-3000, Hexion EPONEX1510, Mitsubishi YX8000, Mitsubishi YX8034, or Baling Petrochemical CYDH3000.

[0012] Further, the organic solvent is one or a combination of more than one of ethylene glycol diacetate, ethylene glycol butyl ether acetate, dimethyl adipate, diethylene glycol ethyl ether acetate, and DBE.

[0013] Further, the bifunctional polyetheramine curing agent is one of Huntsman D-205, Huntsman D-230, Huntsman D-400, or Huntsman D-2000; the trifunctional polyetheramine curing agent is one of Huntsman T-403 or Huntsman T-5000.

[0014] Further, the imidazole accelerator is one of 2-MZ, 2-PZ, 2P4MZ, 2E4MZ, 2E4MZ-CN, C11Z, C11Z-A, or C17Z imidazole accelerators.

[0015] Further, the silane coupling agent is one or a combination of more than one of silane coupling agent KH171, silane coupling agent KH172, silane coupling agent KH-550, silane coupling agent KH560, silane coupling agent KH570, or silane coupling agent KH792.

[0016] Further, the micron-sized flaky silver powder has a particle size of 5.0-8.0 μm and a tapped density of 1.2-1.8 g / cm 3 .

[0017] A preparation method of a low-temperature silver paste for FPC conductive circuits as described above, comprising the following steps:

[0018] (1) Weigh an epoxy resin composition, an organic solvent, and a silane coupling agent according to a weight ratio, mix them evenly in a planetary disperser, with a dispersion speed of 1200 - 1600 rpm and a time of 30 - 60 min;

[0019] (2) Weigh a polyether polyamine curing agent and a curing accelerator according to a weight ratio. Keep the mixture prepared in step (1) in a low-speed stirring state at a speed of 200 - 500 rpm in a planetary disperser, add the polyether polyamine curing agent and the curing accelerator. After the addition is completed, raise the speed to 1200 - 1600 rpm and mix evenly for 45 - 90 min. After mixing, use a filter screen to filter out impurities;

[0020] (3) Weigh the mixture prepared in step (2) and flaky silver powder according to a weight ratio, mix them evenly in a planetary disperser, with a dispersion speed of 1200 - 1600 rpm and a time of 45 - 90 min, and then put them into a three-roll mill for grinding. After grinding to a fineness of less than 10 μm, the low-temperature silver paste for FPC circuit boards is obtained.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] Through the cooperation of the resin system with the curing agent and the curing accelerator, the advantages of high adhesion of epoxy resin to the substrate and stable adhesion are exerted, overcoming the defect that the cured product of the conventional epoxy system is hard, brittle, and not resistant to bending. The low-temperature silver paste has good flexibility to adapt to the application of FPC conductive circuits and can be completely cured under the conditions of 130°C for 30 min. Among them, micron-scale flaky silver powder is used as the conductive filler, which has the advantages of easy dispersion, high filling, and good conductivity. Its preparation process is simple and easy to implement. While meeting the conductivity requirements of FPC circuits, the amount of silver powder used is low, effectively reducing the cost of the silver paste. Specific embodiments

[0023] The following combines specific embodiments to further elaborate in detail on the low-temperature silver paste for FPC conductive circuits of the present invention and its preparation method.

[0024] Example 1:

[0025] The low-temperature silver paste for FPC conductive circuits in this example, in parts by weight, comprises the following components:

[0026] Epoxy resin composition: 7 - 15 parts, organic solvent: 30 - 40 parts, polyether polyamine curing agent: 2 - 5 parts, curing accelerator: 0.5 - 2 parts, silane coupling agent: 0.5 - 2 parts, micron-scale flaky silver powder: 40 - 55 parts;

[0027] Among them, the said epoxy resin composition is a combination of a high-extension epoxy resin and a hydrogenated bisphenol A epoxy resin. In terms of the weight parts ratio, the ratio of the high-extension epoxy resin to the hydrogenated bisphenol A epoxy resin is 3 - 6:1. Specifically, in this embodiment, the said high-extension rate epoxy resin is one or a combination of more than one of Dow DER 732, Dow DER 736, Dow DER 791, Dow DER 9313, Dow XZ92465.00, Dow XZ92466.00, Complex High-Tech EPU-133L, Complex High-Tech EPU-133S, Complex High-Tech EPU-253, Complex High-Tech EPU-300A or Complex High-Tech EPU-300S; the hydrogenated bisphenol A epoxy resin is one of ADK EP-4080E, Toagosei ST-3000, Hexion EPONEX 1510, Mitsubishi YX8000, Mitsubishi YX8034 or Yueyang Petrochemical CYDH3000. The combination of the high-extension rate epoxy resin and the hydrogenated bisphenol A epoxy resin is used as the bonding phase of the low-temperature silver paste. Among them, the high-extension rate resin is the main bonding phase, which has the characteristic of high bonding force of conventional epoxy resins to the substrate, and its flexible chain segments enable the low-temperature silver paste to have the characteristics of being stretchable, bendable and impact-resistant. The hydrogenated bisphenol A epoxy resin is inserted into the cross-linked network of the high-extension rate epoxy resin as an auxiliary bonding phase, and provides hardness and mechanical strength for the flexible silver paste film layer under the preferred addition ratio, forming a low-temperature silver paste bonding phase system with good comprehensive mechanical properties. At the same time, the hydrogenated bisphenol A epoxy resin does not contain easily broken unsaturated carbon-carbon double bonds, improving the tolerance of the low-temperature silver paste to corrosion factors such as water vapor, high temperature, ultraviolet light, chemicals, etc.

[0028] The said organic solvent is one or a combination of more than one of ethylene glycol diacetate, ethylene glycol butyl ether acetate, dimethyl adipate, diethylene glycol ethyl ether acetate and DBE. The present invention uses an ester mixed solvent with a boiling point of 180 - 230 °C. The ester solvent is a non-reactive solvent, which is used to adjust the fluidity of the low-temperature silver paste to match the printing process, so that the low-temperature silver paste remains in a wet state during the printing process, volatilizes rapidly during heating and curing, does not affect the reaction activity of the curing agent and meets the environmental protection requirements.

[0029] In this embodiment, the polyether polyamine curing agent is a bifunctional or trifunctional polyetheramine curing agent; preferably, the bifunctional polyetheramine curing agent is one of Huntsman D-205, Huntsman D-230, Huntsman D-400 or Huntsman D-2000; the trifunctional polyetheramine curing agent is one of Huntsman T-403 or Huntsman T-5000; the curing accelerator mentioned is an imidazole accelerator. Preferably, the imidazole accelerator is one of 2-MZ, 2-PZ, 2P4MZ, 2E4MZ, 2E4MZ-CN, C11Z, C11Z-A or C17Z imidazole accelerators. The main chain of the polyether polyamine is a highly flexible polyether structure, which provides a flexible cross-linking network for the low-temperature silver paste system after curing with epoxy resin. The imidazole accelerator is used to improve the reaction activity and cross-linking density of the system, shorten the curing time, thereby reducing the process difficulty and saving production costs.

[0030] The silane coupling agent mentioned is one or a combination of more than one of silane coupling agent KH171, silane coupling agent KH172, silane coupling agent KH-550, silane coupling agent KH560, silane coupling agent KH570, silane coupling agent KH792. The silane coupling agent is used to further enhance the adhesion between the low-temperature silver paste and the polyimide substrate.

[0031] The micron-sized flaky silver powder mentioned has a particle size of 5.0 - 8.0 μm and a tapped density of 1.2 - 1.8 g / cm 3 , at the same added weight, the micron-sized flaky silver powder with a large particle size and a low tapped density has a larger filling volume ratio and a better dispersion effect, and can be stably dispersed in the silver paste system without adding a dispersant separately, avoiding the agglomeration and sedimentation phenomena of conventional micron-sized silver powder. The large-particle-size flake morphology realizes the large-area lap between silver powder flakes, and achieves a dense stacking layer by layer after curing, forming more conductive paths, and meeting the FPC circuit conductivity requirements at a low silver powder filling weight.

[0032] Example 2

[0033] The low-temperature silver paste for FPC conductive circuits in this embodiment is prepared by the following steps:

[0034] (1) Weigh 64 g of high-elongation epoxy resin Dow DER 732, 16 g of hydrogenated bisphenol A epoxy resin ADK EP-4080E, 359 g of ethylene glycol monobutyl ether acetate and 10 g of silane coupling agent KH-560, and mix them evenly in a planetary disperser at a dispersion speed of 1500 rpm for 45 min;

[0035] (2) Weigh 25 g of polyetheramine curing agent Huntsman D-230 and 6 g of imidazole accelerator 2E4MZ. Keep the mixture prepared in step (1) under low-speed stirring at a speed of 400 rpm in a planetary disperser, and slowly add the polyetheramine curing agent Huntsman D-230 and the imidazole accelerator 2E4MZ. After the addition is completed, increase the speed to 1500 rpm and mix evenly for 60 min. After mixing is completed, use a 400-mesh filter screen to filter out impurities;

[0036] (3) Mix the mixture prepared in step (2) with 520 g of micron-sized flaky silver powder (particle size 5 - 8 μm, tapped density 1.5 g / cm 3 ), and mix evenly in a planetary disperser at a dispersion speed of 1500 rpm for 60 min. Then put it into a three-roll grinder for grinding. After grinding to a fineness of less than 10 μm, the low-temperature silver paste for FPC conductive circuits of the present invention is obtained.

[0037] Example 3

[0038] The low-temperature silver paste for FPC conductive circuits in this example is prepared by the following steps:

[0039] (1) Weigh 80 g of high elongation epoxy resin Dow DER 732, 20 g of hydrogenated bisphenol A epoxy resin ADK EP-4080E, 345 g of DBE, and 10 g of silane coupling agent KH-560, and mix evenly in a planetary disperser at a dispersion speed of 1500 rpm for 45 min;

[0040] (2) Weigh 35 g of polyetheramine curing agent Huntsman D-230 and 10 g of imidazole accelerator 2E4MZ. Keep the mixture prepared in step (1) under low-speed stirring at a speed of 400 rpm in a planetary disperser, and slowly add the polyetheramine curing agent Huntsman D-230 and the imidazole accelerator 2E4MZ. After the addition is completed, increase the speed to 1500 rpm and mix evenly for 60 min. After mixing is completed, use a 400-mesh filter screen to filter out impurities;

[0041] (3) Mix the mixture prepared in step (2) with 500 g of micron-sized flaky silver powder (particle size 5 - 8 μm, tapped density 1.5 g / cm 3 ), and mix evenly in a planetary disperser at a dispersion speed of 1500 rpm for 60 min. Then put it into a three-roll grinder for grinding. After grinding to a fineness of less than 10 μm, the low-temperature silver paste for FPC conductive circuits of the present invention is obtained.

[0042] Example 4

[0043] The low-temperature silver paste for FPC conductive circuits in this example is prepared by the following steps:

[0044] (1) Weigh 100 g of high elongation epoxy resin complexed with high - new EPU - 133S, 20 g of hydrogenated bisphenol A epoxy resin Dongdu Chemical ST - 3000, 113 g of ethylene glycol diacetate, 225 g of diethylene glycol ethyl ether acetate, and 15 g of silane coupling agent KH - 570. Mix them evenly in a planetary disperser at a dispersion speed of 1500 rpm for 45 min.

[0045] (2) Weigh 40 g of polyetheramine curing agent Huntsman T - 403 and 12 g of imidazole accelerator 2E4MZ - CN. Keep the mixture prepared in step (1) in a low - speed stirring state at a speed of 400 rpm in a planetary disperser, and slowly add the polyetheramine curing agent Huntsman T - 403 and imidazole accelerator 2E4MZ - CN. After the addition is completed, increase the speed to 1500 rpm and mix evenly for 60 min. After mixing is completed, use a 400 - mesh filter screen to filter out impurities.

[0046] (3) Mix the mixture prepared in step (2) with 475 g of micron - sized flaky silver powder (particle size 5 - 8 μm, tapped density 1.5 g / cm 3 ) evenly in a planetary disperser at a dispersion speed of 1300 rpm for 60 min, and then put it into a three - roll mill for grinding. After grinding to a fineness of less than 10 μm, the low - temperature silver paste for FPC conductive circuits of the present invention is obtained.

[0047] Example 5

[0048] The low - temperature silver paste for FPC conductive circuits of this example is prepared by the following steps:

[0049] (1) Weigh 117 g of high elongation epoxy resin complexed with high - new EPU - 300A, 23 g of hydrogenated bisphenol A epoxy resin Dongdu Chemical ST - 3000, 111 g of ethylene glycol diacetate, 222 g of diethylene glycol ethyl ether acetate, and 15 g of silane coupling agent KH - 570. Mix them evenly in a planetary disperser at a dispersion speed of 1500 rpm for 45 min.

[0050] (2) Weigh 47 g of polyetheramine curing agent Huntsman T - 403 and 15 g of imidazole accelerator 2E4MZ - CN. Keep the mixture prepared in step (1) in a low - speed stirring state at a speed of 400 rpm in a planetary disperser, and slowly add the polyetheramine curing agent Huntsman T - 403 and imidazole accelerator 2E4MZ - CN. After the addition is completed, increase the speed to 1500 rpm and mix evenly for 60 min. After mixing is completed, use a 400 - mesh filter screen to filter out impurities.

[0051] (4) Mix the mixture prepared in step (2) with 450 g of micron - sized flaky silver powder (particle size 5 - 8 μm, tapped density 1.5 g / cm3 , uniformly mixed in a planetary disperser at a dispersion speed of 1300 rpm for 60 min, and then put into a three-roll mill for grinding. After grinding to a fineness of less than 10 μm, the low-temperature silver paste for FPC conductive circuits of the present invention is obtained.

[0052] Comparative Example 1

[0053] Basically the same as Example 1, except that the epoxy resin composition is replaced with conventional epoxy resin E44, and the other components and their weight ratios remain unchanged.

[0054] Comparative Example 2

[0055] Basically the same as Example 1, except that the epoxy resin composition is replaced with 80 g of Dow DER732, and hydrogenated bisphenol A epoxy resin ADK EP-4080E is not added, and the other components and their weight ratios remain unchanged.

[0056] Comparative Example 3

[0057] Basically the same as Example 1, except that the imidazole accelerator 2E4MZ is not added, and the other components and their weight ratios remain unchanged.

[0058] Comparative Example 4

[0059] Basically the same as Example 1, except that the micron-sized flaky silver powder has a particle size of 5 - 8 μm and a tapped density of 1.5 g / cm 3 ) is replaced with micron-sized flaky silver powder (particle size 1.0 - 2.0 μm, tapped density 3.0 g / cm 3 ), and the other components and their weight ratios remain unchanged.

[0060] Performance Test

[0061] The low-temperature silver pastes prepared in Examples 2 - 5 and Comparative Examples 1 - 4 of the present invention were printed on a polyimide film by screen printing. The screen plate was 200 mesh, and the thickness of the photosensitive resin was 10 μm. A rheometer VT-06 was used to measure the viscosity of the silver paste. Printing was performed in a 100 mm × 20 mm rectangular pattern to test the sheet resistance, hardness, and adhesion, and printing was performed in a 100 mm × 0.4 mm fine line to test the bend resistance. The curing temperature was 130 °C, and the curing time was 30 min. After curing, a four-probe tester was used to measure the sheet resistance, a pencil hardness tester was used to measure the hardness, and the cross-cut method was used to measure the adhesion. For the bend resistance test, the cured silver paste sample strip was folded back and forth with a 2 kg weight for 1 min each, which was recorded as one cycle. After 10 cycles, the change rate of the line resistance was calculated. The test results are shown in Table 1:

[0062] Table 1

[0063]

[0064] As can be seen from the above test results, the low-temperature silver pastes prepared in Examples 2-5 of the present invention have good conductivity after curing, excellent adhesion to polyimide film materials, low resistance change rate after strict bending tests, and still maintain good conductive functions, and are suitable for the preparation of FPC conductive circuits. The conventional epoxy resin used in Comparative Example 1 is relatively hard and brittle after curing, and the circuit is broken during bending, resulting in the loss of the circuit conduction function. In Comparative Example 2, hydrogenated bisphenol A epoxy resin was not used as an auxiliary binder phase, and the hardness of the low-temperature silver paste after curing was too low and the mechanical properties were poor, resulting in a large resistance change rate after bending. In Comparative Example 3, a curing accelerator was not used, and it was not completely cured under the specified curing conditions, resulting in poor performance in terms of sheet resistance, hardness, adhesion, and bend resistance. In Comparative Example 4, the conductive filler is micron-sized flaky silver powder with small particle size and medium tapped density. Compared with micron-sized thin flaky silver powder, it has low filling property, difficult dispersion, insufficient lap area between silver powders, which causes a large increase in sheet resistance, and then the bending test further reduces the lap degree of silver powders.

[0065] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A low-temperature silver paste for FPC conductive circuits, comprising the following components in parts by weight: Epoxy resin composition: 7-15 parts Organic solvent: 30-40 parts Polyether polyamine curing agent: 2-5 parts Curing accelerator: 0.5-2 parts Silane coupling agent: 0.5-2 parts Micron-sized flaky silver powder: 40-55 parts; in, The epoxy resin composition is a combination of high-extension epoxy resin and hydrogenated bisphenol A epoxy resin; the organic solvent is an ester mixed solvent with a boiling point of 180 to 230° C.; the polyether polyamine curing agent is a difunctional or trifunctional polyether amine curing agent; and the curing accelerator is an imidazole accelerator.

2. The low-temperature silver paste for FPC conductive circuit according to claim 1, characterized in that: Calculated by weight ratio, the ratio of high extension epoxy resin to hydrogenated bisphenol A epoxy resin is 3-6:

1.

3. The low-temperature silver paste for FPC conductive circuit according to claim 1 or 2, characterized in that: The high elongation epoxy resin is a combination of one or more of Dow DER 732, Dow DER 736, Dow DER 791, Dow DER 9313, Dow XZ92465.00, Dow XZ92466.00, Huahe Gaoxin EPU-133L, Huahe Gaoxin EPU-133S, Huahe Gaoxin EPU-253, Huahe Gaoxin EPU-300A or Huahe Gaoxin EPU-300S.

4. The low-temperature silver paste for FPC conductive circuit according to claim 3, characterized in that: The hydrogenated bisphenol A epoxy resin is one of ADK EP-4080E, Tohto Chemical ST-3000, Hanson EPONEX 1510, Mitsubishi YX8000, Mitsubishi YX8034 or Baling Petrochemical CYDH3000.

5. The low-temperature silver paste for FPC conductive circuit according to claim 1, 2 or 4, characterized in that: The organic solvent is a combination of one or more of ethylene glycol diacetate, ethylene glycol butyl ether acetate, dimethyl adipate, diethylene glycol ethyl ether acetate and DBE.

6. The low-temperature silver paste for FPC conductive circuit according to claim 5, characterized in that: The difunctional polyetheramine curing agent is one of Huntsman D-205, Huntsman D-230, Huntsman D-400 or Huntsman D-2000; the trifunctional polyetheramine curing agent is one of Huntsman T-403 or Huntsman T-5000.

7. The low-temperature silver paste for FPC conductive circuit according to claim 1, 2, 4 or 6, characterized in that: The imidazole accelerator is one of 2-MZ, 2-PZ, 2P4MZ, 2E4MZ, 2E4MZ-CN, C11Z, C11Z-A or C17Z imidazole accelerators.

8. The low-temperature silver paste for FPC conductive circuit according to claim 7, characterized in that: The silane coupling agent is a combination of one or more of silane coupling agent KH171, silane coupling agent KH172, silane coupling agent KH-550, silane coupling agent KH560, silane coupling agent KH570, and silane coupling agent KH792.

9. The low-temperature silver paste for FPC conductive circuit according to claim 1, 2, 4, 6 or 8, characterized in that: The micron-sized flaky silver powder has a particle size of 5.0 to 8.0 μm and a tap density of 1.2 to 1.8 g / cm 3 .

10. A method for preparing a low-temperature silver paste for FPC conductive circuits according to any one of claims 1 to 9, characterized in that: The following steps are involved: (1) Weigh the epoxy resin composition, organic solvent and silane coupling agent according to the weight ratio, and mix them evenly in a planetary disperser at a dispersing speed of 1200 to 1600 rpm for 30 to 60 minutes; (2) Weigh the polyether polyamine curing agent and the curing accelerator according to the weight ratio, keep the mixture prepared in step (1) in a planetary disperser at a low speed stirring state of 200 to 500 rpm, add the polyether polyamine curing agent and the curing accelerator, increase the speed to 1200 to 1600 rpm after the addition is completed, and mix evenly for 45 to 90 minutes. After mixing, filter with a filter to remove impurities; (3) The mixture prepared in step (2) and flaky silver powder are weighed according to a weight ratio, and uniformly mixed in a planetary disperser at a dispersion speed of 1200 to 1600 rpm for 45 to 90 min. The mixture is then put into a three-roll grinder for grinding until the fineness is less than 10 μm, thereby obtaining the low-temperature silver paste for FPC circuit boards.

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