High-tensile FPC (Flexible Printed Circuit) and preparation method thereof

Abstract

The invention discloses a high-stretching FPC (Flexible Printed Circuit) and a preparation method thereof, and the method comprises the steps: coating conductive silver paste on a flexible substrate film in a printing, dispensing or coating mode on the back of the flexible substrate film, and forming a metal circuit layer through low-temperature curing; the printing thickness of the conductive silver paste is controlled to be 8-20 [mu] m; pasting a protective layer on the metal circuit layer, and pressing; after the supporting film is torn off, shape punching is conducted, and the product is obtained. The initial paste of the conductive silver paste comprises the following components: 50 wt%-70 wt% of mixed silver powder, 10 wt%-15 wt% of acrylic resin, 10 wt%-15 wt% of dimethyl glutarate, 5 wt%-10 wt% of dimethyl adipate and 5 wt%-10 wt% of dimethyl succinate. The high-stretching FPC and the preparation process thereof have the characteristics of high efficiency and low cost, and large-scale production can be realized. And on the other hand, the operability is high, and the operability problems that flexible base materials such as TPU are prone to displacement in the silk-screen printing process, and printing is poor are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of flexible electronics and relates to a high-stretch FPC and a preparation method thereof. Background Art

[0002] In recent years, with the continuous development of science and technology and its wide application in various industries, the application scenarios have higher and higher requirements for circuit boards. Flexible circuit boards (FPC) can no longer fully meet the needs of scientific and technological progress. In some special application fields, especially medical applications, there is a demand for elastic circuit boards with thermoplastic polyurethane (TPU) as the substrate. Flexible circuit boards or flexible conductive materials are required to have properties such as stretchability or bendability and high conductivity. Traditional flexible circuit boards are prepared by adding metal and carbon conductive particles to PDMS or TPU, and then the resistance of most materials increases rapidly with the increase of tensile deformation.

[0003] According to literature reports, the stretching behavior of human skin is generally within 20%, and the stretching deformation of the limb joints is about 60%. This stretching ratio has exceeded the long-term use range of conventional flexible circuit boards. TPU is a plastic material made of polymer materials with excellent physical properties, chemical stability and biocompatibility. It has a wide range of applications in medical, military and other fields. TPU has good softness and elasticity. However, to realize the circuit graphics with TPU film as the substrate, the production process is much more difficult than the traditional PI (polyimide) and PET (polyester) as the substrate to form the circuit graphics. The core is to solve the problems of poor printing workability of TPU, light material body and elastic stretching causing excessive resistance or fracture; there is no mature case of this process in the industry, so the above problems need to be verified by process development. For example, "Coatings" (2023, Vol. 13, p. 1499) reported that the use of flaky silver powder conductive paste to print flexible circuits on the surface of polyurethane film substrates showed good bending resistance, but the printed circuit had high resistivity and poor conductivity, which was difficult to meet the requirements of practical applications.

[0004] In the prior art, conductive glue, conductive ink and other electronic pastes are directly printed on the surface of flexible substrates by screen printing, inkjet printing and other methods to prepare flexible thin film circuits, which have outstanding advantages such as energy saving and high efficiency, and can meet the needs of digitalization and personalized customization. However, due to the relatively soft and thin texture of TPU film, it is difficult to adsorb it flatly on the equipment table during the screen printing process, which can easily cause printing deviation. In addition, the material is relatively light, and it is very easy to transfer and stick to the screen, causing "sticking" problems. In addition, in the baking and curing process after screen printing, the TPU film will be blown up and moved by the circulating hot air in the oven, causing poor printing.

[0005] Further tests have found that, because TPU has certain stretchability, if the TPU with silver powder conductive paste circuit is dynamically stretched, the silver powder conductive paste circuit will break and become non-conductive. Therefore, how to ensure that the silver powder conductive paste circuit remains conductive after the TPU substrate is dynamically stretched is the problem that the present invention aims to solve. Summary of the invention

[0006] In view of the existing problems, the present invention provides a method for preparing a high-stretch FPC, one of the purposes of which is to solve the operability problems of flexible substrates such as TPU moving during screen printing, poor printing, etc. The second purpose of the present invention is to ensure that the silver paste circuit remains conductive after the TPU with the silver paste circuit is dynamically stretched.

[0007] In order to achieve the above object, the technical solution adopted by the present invention comprises the following steps:

[0008] Step 1: Drill holes on the adhesive surface of the support film on the back of the flexible substrate film.

[0009] Step 2, prepare conductive silver paste, apply the conductive silver paste on the flexible substrate film by printing, dispensing or coating, and cure at low temperature to form a metal circuit layer; the thickness of the conductive silver paste printing is controlled to be 8um-20um. Preferably, before applying the silver paste, the surface of the flexible substrate film is subjected to corona treatment, and the high-frequency AC voltage of the corona treatment is 5000-15000V / m 2 The preferred low temperature curing temperature is 80-100°C.

[0010] Step 3, attach a protective layer on the metal circuit layer and press it. The protective layer is preferably a TPU film. Preferably, the pressing is performed using a fast pressing machine at a temperature of 80°C, a pressure of 80kg, and a pressing time of 120s.

[0011] Step 4, after tearing off the support film, perform shape punching to obtain the product.

[0012] Wherein, the initial paste of the conductive silver paste in step 2 includes the following components: 50wt%-70wt% of mixed silver powder, 10wt%-15wt% of acrylic resin, 10wt%-15wt% of dimethyl glutarate, 5wt%-10wt% of dimethyl adipate, and 5wt%-10wt% of dimethyl succinate.

[0013] Preferably, the mixed silver powder contains high-conductive silver powder with a particle size of 5um-15um and nano silver powder with a particle size of 10nm-100nm. The high-conductive silver powder accounts for 70% of the mixed silver powder, and the nano silver powder accounts for 30%.

[0014] Preferably, the solid content of the initial conductive silver paste is in the range of 63-67 wt %, the viscosity is 13-15 Pa·s, and the specific gravity is 1.6-2.0 g / cc.

[0015] Furthermore, the initial slurry of the conductive silver paste is mixed with ethanol and isopropanol in a ratio of 15:10:7 and stirred evenly.

[0016] Preferably, the support film is a low-viscosity PET film with a thickness of 0.1 mm, a peel strength of 5-10 g / cm, and a temperature resistance of 120° C. without softening or deformation.

[0017] Preferably, before applying the silver paste in step 2, the surface of the flexible substrate film is subjected to corona treatment, and the high-frequency AC voltage of the corona treatment is 5000-15000V / m 2 .

[0018] Preferably, the flexible substrate film includes, but is not limited to, one or more of thermoplastic polyurethane, fluororubber, silicone resin, and natural polymer protein.

[0019] The high-stretch FPC prepared by the present invention can be applied to flexible electronic products, wearable clothing, flexible circuits, flexible electrodes, etc.

[0020] By means of the above scheme, the present invention has at least the following advantages:

[0021] 1. The high-stretch FPC provided by the present invention has a resistance of about 110Ω when the elongation is 200%, and the circuit is not broken. It is suitable for applications in flexible electronic products, wearable clothing, flexible circuits, and flexible electrodes.

[0022] 2. The present invention has strong operability and solves the problem of poor printing operability of flexible substrate films such as TPU, especially the easy displacement during screen printing, resulting in operability problems such as poor printing.

[0023] 3. The conductive silver paste of the present invention has the following technical characteristics: (1) the silver powder content after drying is 90.3wt%; (2) the resistivity is as low as 4.9*10 -6 Ωm, the resistivity can be as low as 3.2*10 at an ambient temperature of 150°C -6 Ωm, showing extremely low resistivity. The metal circuit layer produced has excellent conductivity and tensile resistance.

[0024] 4. The present invention uses silver powders of different particle sizes so that the pores between large particles can be filled with particles of smaller particle sizes, thereby increasing the adsorption amount of silver powder on the surface of the flexible substrate.

[0025] 5. The conductive silver paste independently created and formulated by the present invention not only meets the requirements of conductivity, but also meets the requirements of stretchability, thus overcoming the problem of breakage and non-conduction in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the layer structure of the present invention;

[0027] Figure 2 It is a schematic diagram of the process flow of the present invention;

[0028] Figure 3 The resistance change during stretching of Example 1.

[0029] Label: 1. Support film; 2. Flexible substrate film; 3. Metal circuit layer; 4. Protective layer. DETAILED DESCRIPTION

[0030] In order to better understand the technical solution of the present invention, the embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. It should be clear that the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Unless otherwise specified, the test materials and equipment used in the examples are common commercial products and can be purchased in the market. Figure 1 It is a structural schematic diagram of the product of the present invention during production, which comprises a support film 1, a flexible substrate film 2, a metal circuit layer 3, and a protective layer 4 in sequence.

[0032] In the prior art, after the flexible substrate film 2 is stretched, the metal circuit layer 3 will break, resulting in poor circuit conduction. During the research and development process, the present invention analyzed and found that this is because the silver powder particles are not densely adsorbed on the surface of the flexible substrate film 2 after the silver paste is printed, and there are tiny gaps. When the flexible substrate film 2 is stretched by a certain external force, gaps will appear between the silver powder particles, resulting in circuit disconnection and affecting conductivity. On the other hand, the printing workability of the flexible substrate film 2, especially the TPU film, is not good. The present invention improves the prior art by the above two points, such as Figure 2 The process flow shown in Example 1 includes the following steps:

[0033] Step 1, the flexible substrate film 2 is used as the base film. The present invention is suitable for the production of extremely thin and light film flexible substrates according to the process flow, including but not limited to flexible substrates such as TPU, fluororubber, silicone resin, and natural polymer protein. For the convenience of description, the flexible substrate film 2 in the embodiment is a TPU film.

[0034] Step 2, the flexible substrate film 2 is attached to the adhesive surface of the support film 1, and holes are drilled. First, the support film 1 and the flexible substrate film 2 are fixed on the laminating equipment respectively, and the flexible substrate film 2 is rolled on the support film by the roller of the equipment, so as to ensure that the laminated flexible substrate film 2 and the support film 1 will not deviate and are consistent. Then, the flexible substrate film 2 after being backed can be cut into single sheets according to the size of different products. The purpose of drilling holes is to prevent the printing deviation during the subsequent printing of silver paste and the pin holes of the mold sleeve required for the punching of the outer mold.

[0035] The support film 1 is preferably a low-viscosity PET film with a peel strength of 5-10 g / cm, a thickness of 0.1 mm, and a temperature resistance of 120° C. It can be torn off with a relatively light pulling force, and no glue residue will remain on the flexible substrate film 2. The PET film used must maintain good stability and performance within a temperature range below 120° C. to meet the temperature requirements of subsequent curing and avoid softening or deformation of the PET film.

[0036] Step 3, prepare conductive silver paste, and apply the conductive silver paste on the flexible substrate film 2 by printing, dispensing or coating; the thickness of the silver paste printing is controlled to be 10um-20um, and the circuit has good conductivity and low resistance. The conductive silver paste can be cured at low temperature.

[0037] Step 4, low temperature curing. The method of baking is adopted, and the baking parameters are: temperature 80-100°C, time: 15 minutes. Because of the support of the support film 1, the flexible substrate film 2 will not be blown up by the hot air in the oven when it is baked in the oven, avoiding the silver paste circuit failure.

[0038] Step 5, affix the protective layer 4 and then press together. The protective layer 4 is preferably a TPU film. Print conductive silver paste on the surface of the flexible substrate film 2, and then cover the silver paste circuit with a layer of protective layer 4 after baking to protect the circuit. Use a fast laminating machine with a temperature of 80°C, a pressure of 80kg, and a time of 120s to make the upper and lower layers of TPU film adhere tightly together with hot melt adhesive, and at the same time, discharge the bubbles in the middle to enhance the bonding strength. Because the flexible substrate film 2 is supported by a layer of support film 1, it is easy to operate and align when the protective layer 4 is affixed, and will not cause lamination deviation.

[0039] Step 6, tear off the support film 1 to obtain a high-stretch FPC. Because the tensile force of the support film 1 is only about 5g, the support film 1 can be easily torn off manually after the flexible substrate film 2 is hot-pressed, and no foreign matter such as residual glue will remain on the surface of the flexible substrate film 2. The conductivity of the silver paste circuit is not affected after the support film 1 is torn off. Because the silver paste used in the present invention has the property of resisting stretching, even if the flexible substrate film 2 is stretched to a certain extent when the support film 1 is torn off, it will not affect the conductivity of the silver paste circuit. Ordinary silver paste will break the silver paste circuit after stretching, and there will be a phenomenon of non-conduction.

[0040] Step 7: Punching the outer shape, positioning the high tensile FPC sleeve hole on the mold for punching and forming.

[0041] Further, the preparation steps of the conductive silver paste in step 3 are as follows:

[0042] Step A, mixed silver powder 50wt%-70wt%, acrylic resin 10wt%-15wt%, dimethyl glutarate 10wt%-15wt%, dimethyl adipate 5wt%-10wt%, dimethyl succinate 5wt%-10wt%. Preferably, the mixed silver powder contains high-conductive silver powder with a particle size of 5um-15um and nano silver powder with a particle size of 10nm-100nm, and the ratio of the two is 7:3. According to the above mass percentage, the acrylic resin is added to the mixed organic solvent, and after it is completely dissolved, the mixed silver powder is added and evenly dispersed to obtain the initial slurry of conductive silver paste. The solid content range of the initial slurry of conductive silver paste is 63-67wt%, the viscosity is 13-15Pa·s, and the specific gravity is 1.6-2.0g / cc.

[0043] Step B, mix the conductive silver paste initial slurry obtained in step A + ethanol + isopropanol in a ratio of 15:10:7, and stir until uniform to obtain the conductive silver paste. Isopropanol has a certain dilution function, which prevents the conductive silver paste from being semi-cured due to exposure to the air during screen printing. The amount of ethanol and isopropanol added can be adjusted according to actual conditions. Subsequently, the conductive silver paste is coated on the flexible substrate film 2 by printing, dispensing or coating, and after being placed in an oven for baking, a high-stretch FPC is obtained.

[0044] The conductive silver paste of the present invention has the following technical characteristics: (1) the content of silver powder after drying is 90.3wt%; (2) the resistivity is as low as 4.9*10 -6 Ωm, the resistivity can be as low as 3.2*10 at an ambient temperature of 150°C -6Ωm, showing extremely low resistivity. (3) The percentage of acrylic resin is between 5% and 15%, which can ensure that the metal circuit layer has good conductivity after curing while having a certain degree of stretchability, so that the silver powder has good adhesion on the flexible substrate film 2, and can achieve low-temperature curing while ensuring conductivity. It is made of stretchable resin, high-conductivity silver powder and nano silver powder in a blending ratio, and has good conductivity, adhesion and stretchability. (4) The present invention uses silver powder of different particle sizes so that the pores generated between large particles can be filled with particles of smaller particle sizes, thereby increasing the adsorption amount of silver powder on the surface of the flexible substrate. The silver powder content in the conductive silver paste must not only take into account the stable resistance value, but also be restricted by factors such as curing characteristics, bonding strength, and economy. If the silver powder content is too high, the probability of being coated by the bonding resin is low, and the bonding force of the silver conductor after curing and film formation decreases, and there is a risk of silver particles falling off. The selection of nano silver powder particle size can effectively improve the density of the conductive silver paste after film formation and improve its conductivity.

[0045] Figure 3 It is a resistance test of the entire FPC product under different stretching states. 100% elongation means no stretching, and the resistance is zero at this time. As the tensile strength increases, the resistance rises. When the elongation is 200%, that is, stretched to twice the original length, the resistance is about 110Ω, and the circuit is not broken. The thickness of the silver paste printed in the field of technology is generally 15um-20um, and the present invention still has good tensile conductivity at a thickness of 8um-15um.

[0046] Comparative Example 1 was carried out according to the steps of Example 1, except that no 10nm-100nm nano silver powder was added to the conductive silver paste. From the comparison of the results of the comparative example and Examples 1-3, it can be seen that the pores between the large-size silver powder can be filled with particles of smaller size, thereby improving the conductivity and tensile strength of the FPC product.

[0047] Table 1 Effect of different thickness of printed silver paste

[0048] Serial number Printing silver paste thickness Conductivity test after stretching determination Example 1-1 Less than 8um Metal circuit layer fracture NG Example 1-2 8-10um The metal circuit layer is not broken OK Examples 1-3 10-15um The metal circuit layer is not broken OK Examples 1-4 15-20um The metal circuit layer is not broken OK Examples 1-5 Greater than 20um Metal circuit layer fracture NG Comparative Example 1 10-15um Metal circuit layer fracture NG

[0049] In one preferred embodiment, since the surface of TPU material is relatively smooth, if the silver paste particles are to be more tightly and densely attached to the surface of TPU film, it is possible to consider performing corona treatment on the surface of TPU film before printing silver paste. Corona discharge is performed on the treated TPU film surface using high-frequency voltage (high-frequency AC voltage is up to 5000-15000V / m 2), and low-temperature plasma is generated, and free radicals are generated on the surface to cross-link the polymer, rearrange the molecular structure of the TPU substrate surface, and produce more polar sites, which are conducive to the attachment of foreign objects, thereby increasing the adsorption force between the silver powder particles and the TPU film surface. The unit of surface energy is dyne. To determine whether the surface energy after corona is up to standard, the dyne value of the substrate should be 10 dynes higher than the dyne value of all inks. After corona treatment, the silver powder particles in the silver paste are more densely and densely attached to the surface of the TPU substrate.

[0050] The above description is only a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any slight modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for preparing a high tensile FPC, characterized in that: The following steps are involved: Step 1, drilling holes on the adhesive surface of the support film on the back of the flexible substrate film; Step 2, preparing a conductive silver paste, coating the conductive silver paste on a flexible substrate film by printing, dispensing or coating, and curing at low temperature to form a metal circuit layer; the thickness of the conductive silver paste printing is controlled to be 8um-20um; the temperature of the low temperature curing is 80-100°C; Step 3, attaching a protective layer on the metal circuit layer and pressing it; Step 4, after tearing off the support film, perform shape punching to obtain the product; Wherein, the initial slurry of the conductive silver paste in step 2 comprises the following components: 50wt%-70wt% of mixed silver powder, 10wt%-15wt% of acrylic resin, 10wt%-15wt% of dimethyl glutarate, 5wt%-10wt% of dimethyl adipate, and 5wt%-10wt% of dimethyl succinate; the mixed silver powder contains high conductive silver powder with a particle size of 5um-15um and nano silver powder with a particle size of 10nm-100nm.

2. The method for preparing a high tensile FPC according to claim 1, characterized in that: The high-conductive silver powder accounts for 70% of the mixed silver powder, and the nano silver powder accounts for 30%.

3. The method for preparing a high tensile FPC according to claim 1, characterized in that: The solid content of the initial conductive silver paste is in the range of 63-67 wt %, the viscosity is 13-15 Pa·s, and the specific gravity is 1.6-2.0 g / cc.

4. The method for preparing a high tensile FPC according to claim 1, characterized in that: The conductive silver paste initial slurry is mixed with ethanol and isopropanol in a ratio of 15:10:7 and stirred evenly.

5. The method for preparing a high tensile FPC according to claim 1, characterized in that: The support film is a low-viscosity PET film with a thickness of 0.1 mm, a peel strength of 5-10 g / cm, and a temperature resistance of 120° C. without softening or deformation.

6. The method for preparing a high tensile FPC according to claim 1, characterized in that: Step 2: Before coating the silver paste, the surface of the flexible substrate film is subjected to corona treatment, wherein the high-frequency AC voltage of the corona treatment is 5000-15000 V / m 2 .

7. The method for preparing a high tensile FPC according to claim 1, characterized in that: The flexible substrate film includes, but is not limited to, one or more of thermoplastic polyurethane, fluororubber, silicone resin, and natural polymer protein.

8. The method for preparing a high tensile FPC according to claim 1, characterized in that: The protective layer is a TPU film.

9. The method for preparing a high tensile FPC according to claim 1, characterized in that: The pressing in step 3 is performed using a fast pressing machine at a temperature of 80° C., a pressure of 80 kg, and a pressing time of 120 s.

10. A high-stretch FPC prepared by the method according to any one of claims 1 to 9, which is used in flexible electronic products, wearable clothing, flexible circuits, and flexible electrodes.

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