Multi-layer structure high-shielding electromagnetic shielding film for flexible circuit board

Abstract

The invention discloses a multilayer-structure high-shielding electromagnetic shielding film for a flexible circuit board, which relates to the technical field of electromagnetic shielding materials and sequentially comprises an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer and a base material layer from top to bottom, the base material layer is prepared from a polymer comprising a structural unit introduced by the following monomers: biphenyl dicarboxylic acid, 4, 8-tricyclo [5.2. 1. O2, 7] decane dimethyl carbinol, and 2, 3, 5, 6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are mutually independently prepared by uniformly mixing the following components: amino-terminated hyperbranched polyimide, 3, 3 '-diamino-4, 4'-difluorodiphenyl sulfone, melamine, amino-terminated hyperbranched polyamide, 1, 3, 5-triglycidyl-S-triazinetrione, a solvent and a conductive filler. The shielding film is high in electromagnetic shielding effectiveness and tensile strength, good in high-temperature and high-humidity resistance and high in bending resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic shielding materials, and particularly to a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards. Background Art

[0002] In recent years, with the development of electronic devices towards miniaturization, light weight, and high performance, flexible printed circuit boards (FPCs) have been widely used in fields such as aerospace, military, mobile communications, laptops, computer peripherals, PDAs, digital cameras, etc. or on electronic products. Flexible printed circuit boards (FPCs), also known as flexible circuit boards, are printed circuits made of polyester film or polyimide as the substrate, with highly reliable and excellent flexural properties formed by etching circuits on copper foil. It can be freely bent, wound, and folded, arranged arbitrarily according to the spatial layout requirements, and moved and stretched arbitrarily in three-dimensional space, thus achieving the integration of component assembly and wire connection. However, flexible printed circuit boards (FPCs) are prone to electromagnetic interference (EMI) during high-speed signal transmission, affecting the normal operation of the device. Therefore, it is necessary to attach an electromagnetic shielding film to the surface of the FPC to suppress EMI.

[0003] Traditional electromagnetic shielding films are mostly made of single-layer metal foil or conductive coatings, with problems such as low shielding efficiency, poor flexibility, and easy peeling. To improve the electromagnetic shielding efficiency, multi-layer electromagnetic shielding films have emerged, which have attracted wide attention in the industry. However, the multi-layer electromagnetic shielding films on the market still have more or less technical defects such as insufficient interlayer bonding force, unstable shielding efficiency, poor high-temperature and high-humidity resistance, and the need to further improve the bending resistance and tensile strength.

[0004] To solve the above problems, the Chinese invention patent with the authorization announcement number CN114932734B discloses an electromagnetic shielding multi-layer composite film and its processing technology. The electromagnetic shielding multi-layer composite film includes a substrate, a dielectric layer, and a waterproof layer laminated in sequence; introducing fiber nanocrystalline composite MXene to effectively solve problems such as the dispersion of MXene in epoxy resin, and improving the conductivity, wave absorption performance, and mechanical properties of the electromagnetic shielding multi-layer composite film; modifying carbonyl iron with silica and controlling the ball milling conditions to make carbonyl iron composite silica have easy-plane anisotropy, and introducing carbonyl iron composite silica to achieve thin-layer high-efficiency absorption of the electromagnetic shielding composite film. When the thickness of the composite film is 0.4 - 0.6 mm, it still maintains high-efficiency absorption performance for electromagnetic waves; in the preparation of the waterproof layer, while toughening epoxy resin with carbonyl iron composite silica, a hydrophobic and anti-corrosion micro-structure is prepared on the waterproof layer through micro-milling, picosecond laser processing, and fluorination treatment, making it have anti-corrosion, self-cleaning, and drag reduction effects, and improving the service life of the composite film. However, the high-temperature and high-humidity resistance, bending resistance, and tensile strength of this film still need to be further improved.

[0005] Therefore, it is particularly important to develop a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards with high electromagnetic shielding efficiency, high tensile strength, good high-temperature and high-humidity resistance, and strong bending resistance. Summary of the Invention

[0006] The main object of the present invention is to provide a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards with high electromagnetic shielding efficiency, high tensile strength, good high-temperature and high-humidity resistance, and strong bending resistance.

[0007] To achieve the above object, the present invention provides a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards, which sequentially includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer from top to bottom; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 15-25 parts of amino-terminated hyperbranched polyimide, 1-3 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 3-5 parts of melamine, 20-30 parts of amino-terminated hyperbranched polyamide, 5-8 parts of 1,3,5-triglycidyl-S-triazine trione, 40-50 parts of solvent, and 5-8 parts of conductive filler.

[0008] Preferably, the thickness of the insulating layer is 5-25 μm; the thickness of the first adhesive layer is 5-15 μm; the thickness of the conductive layer is 1-10 μm; the thickness of the second adhesive layer is 5-15 μm; the thickness of the substrate layer is 10-30 μm.

[0009] Preferably, the insulating layer is made of any one of polyimide, polyethylene terephthalate, and polyethylene naphthalate.

[0010] Preferably, the polyimide is DuPont KAPTON® polyimide film; the polyethylene terephthalate is SKC-PET-V7610 polyester film; the polyethylene naphthalate is Teonex® TN8050SC polyethylene naphthalate film.

[0011] Preferably, the preparation method of the functional polymer comprises the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, and a catalyst into a high-boiling solvent, stirring at 110-125°C for 1-3 hours to obtain a reaction solution, then adding the reaction solution into a high-pressure reaction kettle, closing the feeding port after the feeding is completed, purging the air in the reaction kettle with an inert gas, then stirring and reacting at 130-150°C under normal pressure for 3-5 hours, then reducing the pressure to 20-100 Pa, raising the temperature to 250-270°C, maintaining the temperature and pressure and stirring and reacting for 10-16 hours, cooling to room temperature after the reaction is completed, precipitating in water, then washing the precipitated polymer with ethanol 3-6 times, and finally drying to a constant weight at 85-95°C in a vacuum dryer to obtain the functional polymer.

[0012] Preferably, the molar ratio of the biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, the catalyst, and the high-boiling solvent is 1:0.7:0.3:(0.8-1.2):(9-15).

[0013] Preferably, the high-boiling solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate in a mass ratio of (1-2):(0.8-1.2):1; the inert gas is any one of nitrogen, helium, neon, and argon.

[0014] Preferably, there is no special requirement for the source of the amino-terminated hyperbranched polyimide. In one embodiment of the present invention, the amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B.

[0015] Preferably, the amino-terminated hyperbranched polyamide is amino-terminated hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.

[0016] Preferably, the solvent is any one of toluene, ethyl acetate, and methyl ethyl ketone.

[0017] Preferably, the conductive filler is any one of copper powder, carbon nanotubes, and graphene.

[0018] Preferably, the particle size of the copper powder is 1300-1800 mesh.

[0019] Preferably, the conductive layer is made of a conductive metal, and the conductive metal is copper, aluminum, nickel, or an alloy thereof; it is a network structure, the grid line width is 10-50 μm, and the grid spacing is 100-500 μm.

[0020] Another object of the present invention is to provide a method for preparing a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board, comprising the following steps: Step S1: Corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: Roughen the conductive metal foil, and then form a grid-like structure through a lithography process to form a conductive layer; Step S3: Thermocompression bond the lower surface of the conductive layer and the first adhesive layer; then coat a second adhesive layer on the other side of the conductive layer; Step S4: Melt and extrude the functional polymer at 235-260 °C, then obtain a polyester film through casting, sheet casting, and biaxial stretching in sequence. Then corona-treat the polyester film, and then thermocompression bond it with the second adhesive layer to form a substrate layer; Step S5: Carry out curing treatment to obtain a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board.

[0021] Preferably, the curing treatment is specifically: harden at 60 °C - 80 °C for 30 - 50 minutes, and then cure at room temperature for more than 20 hours.

[0022] Due to the application of the above technical solution, the present invention has the following beneficial effects: (1) The method for preparing a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board disclosed by the present invention has a simple process, convenient operation control, high preparation efficiency and product qualification rate, low dependence on equipment, is suitable for continuous large-scale production, and has high popularization and application value.

[0023] (2) The multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board disclosed by the present invention adopts a multi-layer structure design, and combines a grid-like conductive layer and conductive fillers, effectively improving the electromagnetic shielding effectiveness of the shielding film and being able to meet the requirements of high-frequency and high-speed signal transmission; it is made of flexible materials, and the thicknesses of each layer and the composition formula of the adhesive are optimized, so that the shielding film has good flexibility and can adapt to the bending and folding of the FPC; through measures such as corona treatment, roughening treatment, and adding conductive fillers, the bonding force between each layer is enhanced, preventing interlayer peeling, and improving the reliability and service life of the shielding film.

[0024] (3)The multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards disclosed by the present invention, wherein the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 15-25 parts of amino-terminated hyperbranched polyimide, 1-3 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 3-5 parts of melamine, 20-30 parts of amino-terminated hyperbranched polyamide, 5-8 parts of 1,3,5-triglycidyl-S-triazine trione, 40-50 parts of solvent, and 5-8 parts of conductive filler. By reasonably selecting each raw material and component, they can better play their interaction with each other, endowing the product with excellent electromagnetic shielding efficiency, tensile strength, high temperature and high humidity resistance, and bending resistance. Through the ring-opening reaction between the amino group and the epoxy group in the adhesive, the cured adhesive film contains hyperbranched polyimide, triazine trione, melamine, difluorodiphenyl sulfone, and hyperbranched polyamide structures. Under the action of various effects such as electronic effect, steric effect, and conjugation effect, and the formed interpenetrating network structure, the shielding film made has strong interlayer adhesion, excellent tensile strength, high temperature and high humidity resistance, and bending resistance; the conductive filler therein is uniformly dispersed in the interpenetrating network structure, and can cooperate with the conductive layer to effectively improve the electromagnetic shielding efficiency. Detailed Description of the Invention

[0025] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments described below are only examples, and those skilled in the art can think of other obvious variations. Example 1

[0026] A multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards, which sequentially includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer from top to bottom; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 15 parts of amino-terminated hyperbranched polyimide, 1 part of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 3 parts of melamine, 20 parts of amino-terminated hyperbranched polyamide, 5 parts of 1,3,5-triglycidyl-S-triazine trione, 40 parts of solvent, and 5 parts of conductive filler.

[0027] The thickness of the insulating layer is 15 μm; the thickness of the first adhesive layer is 10 μm; the thickness of the conductive layer is 5 μm; the thickness of the second adhesive layer is 10 μm; the thickness of the substrate layer is 20 μm; the insulating layer is made of polyimide; the polyimide is DuPont KAPTON® polyimide film.

[0028] The preparation method of the functional polymer comprises the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, and a catalyst into a high-boiling solvent, stirring at 110 °C for 1 hour to obtain a reaction solution, then adding the reaction solution into a high-pressure reaction kettle, closing the feeding port after the feeding is completed, purging the air in the reaction kettle with an inert gas, then stirring and reacting at 130 °C under normal pressure for 3 hours, then reducing the pressure to 20 Pa, raising the temperature to 250 °C, maintaining the temperature and pressure and stirring and reacting for 10 hours, cooling to room temperature after the reaction is completed, precipitating in water, washing the precipitated polymer with ethanol 3 times, and finally drying to constant weight at 85 °C in a vacuum dryer to obtain the functional polymer; the molar ratio of biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, the catalyst, and the high-boiling solvent is 1:0.7:0.3:0.8:9; the high-boiling solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate in a mass ratio of 1:0.8:1; the inert gas is nitrogen; through GPC test, the M n = 14850 g / mol, M W / M n = 1.363; through elemental analysis and weight change calculation, the molar ratio of the structural units introduced by biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, and 2,3,5,6-tetrafluoroterephthalic alcohol in the functional polymer is the same as the theoretical value.

[0029] The amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B; the terminal amino hyperbranched polyamide is terminal amino hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.; the solvent is toluene; the conductive filler is copper powder; the particle size of the copper powder is 1300 mesh; the conductive layer is made of a conductive metal, and the conductive metal is copper; it is a network structure, the grid line width is 10 μm, and the grid spacing is 100 μm.

[0030] A preparation method of a multilayer high-shielding electromagnetic shielding film for a flexible printed circuit board comprises the following steps: Step S1: Corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: Roughen the conductive metal foil, and then form a conductive layer by making a grid-like structure through a photolithography process; Step S3: Thermocompression bond the lower surface of the conductive layer and the first adhesive layer; then coat a second adhesive layer on the other side of the conductive layer; Step S4: Melt and extrude the functional polymer at 235°C, then successively perform casting, film casting, and biaxial stretching to obtain a polyester film. Then, corona-treat the polyester film and thermocompression bond it with the second adhesive layer to form a substrate layer; Step S5: Cure to obtain a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards; the curing treatment is specifically: harden at 60°C for 30 minutes, and then cure at room temperature for more than 20 hours. Example 2

[0031] A multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards, which sequentially includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer from top to bottom; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 17 parts of amino-terminated hyperbranched polyimide, 1.5 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 3.5 parts of melamine, 23 parts of amino-terminated hyperbranched polyamide, 6 parts of 1,3,5-triglycidyl-S-triazine trione, 43 parts of solvent, and 6 parts of conductive filler.

[0032] The thickness of the insulating layer is 15 μm; the thickness of the first adhesive layer is 10 μm; the thickness of the conductive layer is 5 μm; the thickness of the second adhesive layer is 10 μm; the thickness of the substrate layer is 20 μm; the insulating layer is made of polyethylene terephthalate; the polyethylene terephthalate is SKC-PET-V7610 polyester film.

[0033] The preparation method of the functional polymer comprises the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, and a catalyst into a high-boiling-point solvent, stirring at 115°C for 1.5 hours to obtain a reaction solution, then adding the reaction solution into a high-pressure reaction kettle, closing the feeding port after the feeding is completed, purging the air in the reaction kettle with an inert gas, then stirring and reacting at 135°C under normal pressure for 3.5 hours, further reducing the pressure to 40 Pa, raising the temperature to 255°C, maintaining the temperature and pressure and stirring and reacting for 12 hours, cooling to room temperature after the reaction is completed, precipitating in water, washing the precipitated polymer with ethanol 4 times, and finally drying to constant weight at 87°C in a vacuum dryer to obtain the functional polymer; the molar ratio of the biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, the catalyst, and the high-boiling-point solvent is 1:0.7:0.3:0.9:11; the high-boiling-point solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate according to a mass ratio of 1.2:0.9:1; the inert gas is helium.

[0034] The amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B; the amino-terminated hyperbranched polyamide is amino-terminated hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.; the solvent is ethyl acetate; the conductive filler is copper powder; the particle size of the copper powder is 1400 mesh; the conductive layer is made of a conductive metal, and the conductive metal is copper; it is a network structure, the grid line width is 20 μm, and the grid spacing is 200 μm.

[0035] A preparation method of the multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board comprises the following steps: Step S1: corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: roughen the conductive metal foil, and then form a grid-like structure through a lithography process to form a conductive layer; Step S3: thermally press and laminate the lower surface of the conductive layer and the first adhesive layer; and then coat a second adhesive layer on the other side of the conductive layer; Step S4: melt-extrude the functional polymer at 240°C, then successively pass through casting, film casting, and biaxial stretching to obtain a polyester film, then corona-treat the polyester film, and then thermally press and laminate it with the second adhesive layer to form a substrate layer; Step S5: Aging treatment to obtain a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards; the aging treatment specifically is: hardening at 65°C for 35 minutes, and then aging at room temperature for more than 20 hours. Example 3

[0036] A multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards, which sequentially includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer from top to bottom; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 20 parts of amino-terminated hyperbranched polyimide, 2 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 4 parts of melamine, 25 parts of amino-terminated hyperbranched polyamide, 6.5 parts of 1,3,5-triglycidyl-S-triazine trione, 45 parts of solvent, and 6.5 parts of conductive filler.

[0037] The thickness of the insulating layer is 15μm; the thickness of the first adhesive layer is 10μm; the thickness of the conductive layer is 5μm; the thickness of the second adhesive layer is 10μm; the thickness of the substrate layer is 20μm; the insulating layer is made of polyethylene naphthalate; the polyethylene naphthalate is Teonex® TN8050SC polyethylene naphthalate film.

[0038] The preparation method of the functional polymer includes the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, and a catalyst into a high-boiling solvent, stirring at 118°C for 2 hours to obtain a reaction solution, then adding the reaction solution into a high-pressure reaction kettle, closing the feeding port after feeding, purging the air in the reaction kettle with an inert gas, then stirring and reacting at 140°C under normal pressure for 4 hours, then reducing the pressure to 60Pa, raising the temperature to 260°C, maintaining the temperature and pressure and stirring and reacting for 13 hours, cooling to room temperature after the reaction, precipitating in water, then washing the precipitated polymer with ethanol 5 times, and finally drying in a vacuum dryer at 90°C to constant weight to obtain the functional polymer; the molar ratio of biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, the catalyst, and the high-boiling solvent is 1:0.7:0.3:1:12; the high-boiling solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate in a mass ratio of 1.5:1:1; the inert gas is neon.

[0039] The amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B; the amino-terminated hyperbranched polyamide is amino-terminated hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.; the solvent is methyl ethyl ketone; the conductive filler is copper powder; the particle size of the copper powder is 1500 mesh; the conductive layer is made of conductive metal, and the conductive metal is nickel; it is a network structure, the grid line width is 30 μm, and the grid spacing is 300 μm.

[0040] A preparation method of a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board includes the following steps: Step S1: Corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: Roughen the conductive metal foil, and then form a conductive layer by making a grid-like structure through a photolithography process; Step S3: Thermocompression bond the lower surface of the conductive layer and the first adhesive layer; then coat a second adhesive layer on the other side of the conductive layer; Step S4: Melt and extrude the functional polymer at 250 °C, then successively pass through casting, sheet casting, and biaxial stretching to obtain a polyester film, then corona-treat the polyester film, and then thermocompression bond it with the second adhesive layer to form a substrate layer; Step S5: Carry out a curing treatment to obtain a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board; the curing treatment is specifically: harden at 70 °C for 40 minutes, and then cure at room temperature for more than 20 hours. Example 4

[0041] A multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board includes, from top to bottom, an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 23 parts of amino-terminated hyperbranched polyimide, 2.5 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 4.5 parts of melamine, 28 parts of amino-terminated hyperbranched polyamide, 7.5 parts of 1,3,5-triglycidyl-S-triazine trione, 48 parts of solvent, and 7.5 parts of conductive filler.

[0042] The thickness of the insulating layer is 15 μm; the thickness of the first adhesive layer is 10 μm; the thickness of the conductive layer is 5 μm; the thickness of the second adhesive layer is 10 μm; the thickness of the substrate layer is 20 μm; the insulating layer is made of polyimide; the polyimide is DuPont KAPTON® polyimide film.

[0043] The preparation method of the functional polymer includes the following steps: Add biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, and a catalyst into a high-boiling solvent, stir at 122 °C for 2.5 hours to obtain a reaction solution, then add this reaction solution into a high-pressure reaction kettle, close the feeding port after feeding, displace the air in the reaction kettle with an inert gas, then stir and react at 145 °C under normal pressure for 4.5 hours, then reduce the pressure to 90 Pa, raise the temperature to 265 °C, keep the temperature and pressure constant and stir and react for 15 hours. After the reaction is completed, cool to room temperature, precipitate in water, then wash the precipitated polymer with ethanol 6 times, and finally dry it to constant weight at 93 °C in a vacuum dryer to obtain the functional polymer; the molar ratio of biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalic alcohol, the catalyst, and the high-boiling solvent is 1:0.7:0.3:1.1:14; the high-boiling solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate in a mass ratio of 1.8:1.1:1; the inert gas is argon.

[0044] The amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B; the amino-terminated hyperbranched polyamide is amino-terminated hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.; the solvent is toluene; the conductive filler is copper powder; the particle size of the copper powder is 1700 mesh; the conductive layer is made of a conductive metal, and the conductive metal is copper; it is a network structure, the grid line width is 40 μm, and the grid spacing is 400 μm.

[0045] A preparation method of a multi-layer high-shielding electromagnetic shielding film for a flexible printed circuit board includes the following steps: Step S1: Corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: Roughen the conductive metal foil, and then form a grid-like structure through a lithography process to form a conductive layer; Step S3: Thermocompression bond the lower surface of the conductive layer and the first adhesive layer; then coat a second adhesive layer on the other side of the conductive layer; Step S4: Melt and extrude the functional polymer at 255°C, then successively pass through casting, film casting, and biaxial stretching to obtain a polyester film. Then, corona treatment is performed on the polyester film, and then it is thermocompression bonded with the second adhesive layer to form a substrate layer; Step S5: Aging treatment to obtain a multilayer high-shielding electromagnetic shielding film for flexible printed circuit boards; the aging treatment is specifically: hardening at 75°C for 45 minutes, and then aging at room temperature for more than 20 hours. Example 5

[0046] A multilayer high-shielding electromagnetic shielding film for flexible printed circuit boards, which successively includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer from top to bottom; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 25 parts of amino-terminated hyperbranched polyimide, 3 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 5 parts of melamine, 30 parts of amino-terminated hyperbranched polyamide, 8 parts of 1,3,5-triglycidyl-S-triazine trione, 50 parts of solvent, and 8 parts of conductive filler.

[0047] The thickness of the insulating layer is 15μm; the thickness of the first adhesive layer is 10μm; the thickness of the conductive layer is 5μm; the thickness of the second adhesive layer is 10μm; the thickness of the substrate layer is 20μm; the insulating layer is polyethylene terephthalate; the polyethylene terephthalate is SKC-PET-V7610 polyester film.

[0048] The preparation method of the functional polymer comprises the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, and a catalyst into a high-boiling solvent, stirring at 125 °C for 3 hours to obtain a reaction solution, then adding the reaction solution into a high-pressure reaction kettle, closing the feeding port after the feeding is completed, purging the air in the reaction kettle with an inert gas, then stirring and reacting at 150 °C under normal pressure for 5 hours, then reducing the pressure to 100 Pa, raising the temperature to 270 °C, maintaining the temperature and pressure and stirring and reacting for 16 hours, cooling to room temperature after the reaction is completed, precipitating in water, then washing the precipitated polymer with ethanol 6 times, and finally drying to constant weight at 95 °C in a vacuum dryer to obtain the functional polymer; the molar ratio of the biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decane dimethanol, 2,3,5,6-tetrafluoroterephthalyl alcohol, the catalyst, and the high-boiling solvent is 1:0.7:0.3:1.2:15; the high-boiling solvent is dimethyl sulfoxide; the catalyst is a mixture formed by mixing antimony glycolate, p-toluenesulfonic acid, and tetrabutyl titanate according to a mass ratio of 2:1.2:1; the inert gas is nitrogen.

[0049] The amino-terminated hyperbranched polyimide is prepared by the method of Example 1 of the Chinese invention patent with the authorized publication number CN115558284B; the terminal amino hyperbranched polyamide is terminal amino hyperbranched polyamide Hyper N103, provided by Wuhan Hyperbranched Resin Technology Co., Ltd.; the solvent is toluene; the conductive filler is copper powder; the particle size of the copper powder is 1800 mesh; the conductive layer is made of a conductive metal, and the conductive metal is copper; it is a network structure, the grid line width is 50 μm, and the grid spacing is 500 μm.

[0050] A preparation method of the multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards comprises the following steps: Step S1: corona-treat the insulating layer, and then coat a first adhesive on its lower surface to form a first adhesive layer; Step S2: roughen the conductive metal foil, and then form a conductive layer by making a grid-like structure through a lithography process; Step S3: thermally press and laminate the lower surface of the conductive layer and the first adhesive layer; then coat a second adhesive layer on the other side of the conductive layer; Step S4: melt and extrude the functional polymer at 260 °C, then obtain a polyester film through casting, film casting, and biaxial stretching in sequence, then corona-treat the polyester film, and then thermally press and laminate it with the second adhesive layer to form a substrate layer; Step S5: carry out a curing treatment to obtain a multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards; the curing treatment is specifically: hardening at 80 °C for 50 minutes, and then curing at room temperature for more than 20 hours.

[0051] Comparative Example 1 A multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards is basically the same as that in Example 1, except that SKC-PET-V7610 polyester film is used instead of the functional polymer.

[0052] Comparative Example 2 A multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards is basically the same as that in Example 1, except that 4,4'-diaminodiphenylsulfone difluoride is not added.

[0053] To further illustrate the beneficial technical effects of the multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards involved in each embodiment of the present invention, relevant performance tests were carried out on the multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards involved in Examples 1-5 and Comparative Examples 1-2. The test results are shown in Table 1, and the test methods are as follows: (1) Electromagnetic shielding effectiveness: Tested according to the standard of GB / T30142-2013; (2) Tensile strength: Tested according to the standard of GB / T1040.3-2006, specimen type 1B; test speed 50mm / min; (3) Dynamic bending resistance: Evaluated by the U-shaped dynamic bending test. By fixing the folding radius at 0.5mm, the film is repeatedly folded and extended, and the number of folds when the crease appears is observed. If there is no crease after 250,000 bends, the bending resistance performance passes, otherwise it fails.

[0054] (4) High temperature and high humidity resistance: After aging at 85°C / 85%RH for 1000h and then returning to room temperature environment, the electromagnetic shielding effectiveness test is carried out again, and the electromagnetic shielding effectiveness retention rate is statistically calculated. The larger the value, the better the high temperature and high humidity resistance performance.

[0055] As can be seen from Table 1, the multi-layer high-shielding electromagnetic shielding film for flexible printed circuit boards disclosed in the embodiments of the present invention has more excellent electromagnetic shielding effect, better tensile performance, bending resistance performance and high temperature and high humidity resistance performance than the products of the comparative examples; the use of the functional polymer and 4,4'-diaminodiphenylsulfone difluoride is beneficial to improving the above performances.

[0056] Table 1 Project Electromagnetic shielding effectiveness Tensile strength Dynamic bending resistance High temperature and high humidity resistance Unit dB MPa — % Example 1 88 282 Passed by 97.73 Example 2 90 285 Passed by 98.33 Example 3 93 290 Passed by 99.03 Example 4 94 293 Passed by 99.36 Example 5 96 295 Passed by 99.69 Comparative Example 1 87 246 Failed 93.10 Comparative Example 2 85 278 Failed 95.29 The foregoing has shown and described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and what is described in the above embodiments and the specification is only the principle of the present invention. Without departing from the spirit and scope of the present invention, various changes and improvements will occur to the present invention, and all these changes and improvements fall within the scope of the present invention claimed. The scope of protection required by the present invention is defined by the appended claims and their equivalents.

Claims

1. A multi-layer high-shielding electromagnetic shielding film for a flexible circuit board, characterized in that: From top to bottom, it includes an insulating layer, a first adhesive layer, a conductive layer, a second adhesive layer, and a substrate layer; the substrate layer is made of a functional polymer; the functional polymer includes structural units introduced by the following monomers: biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decanedimethanol, and 2,3,5,6-tetrafluorophenylene glycol; the first adhesive and the second adhesive are independently made by uniformly mixing the following components: 15-25 parts of amino-terminated hyperbranched polyimide, 1-3 parts of 3,3'-diamino-4,4'-difluorodiphenyl sulfone, 3-5 parts of melamine, 20-30 parts of amino-terminated hyperbranched polyamide, 5-8 parts of 1,3,5-triglycidyl-S-triazinetrione, 40-50 parts of solvent, and 5-8 parts of conductive filler.

2. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 1, characterized in that: The thickness of the insulating layer is 5-25 μm; the thickness of the first adhesive layer is 5-15 μm; the thickness of the conductive layer is 1-10 μm; the thickness of the second adhesive layer is 5-15 μm; and the thickness of the substrate layer is 10-30 μm.

3. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 1, characterized in that: The insulating layer is made of any one of polyimide, polyethylene terephthalate and polyethylene naphthalate.

4. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 3, characterized in that: The polyimide is DuPont KAPTON® polyimide film; the polyethylene terephthalate is SKC-PET-V7610 polyester film; and the polyethylene naphthalate is Teonex® TN8050SC polyethylene naphthalate film.

5. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 1, characterized in that: The preparation method of the functional polymer comprises the following steps: adding biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decanedimethanol, 2,3,5,6-tetrafluorophenylene glycol and a catalyst into a high boiling point solvent, stirring at 110-125°C for 1-3 hours to obtain a reaction solution, then adding the reaction solution into a high pressure reactor, closing the feed port after the addition, replacing the air in the reactor with an inert gas, then stirring and reacting at 130-150°C under normal pressure for 3-5 hours, then reducing the pressure to 20-100 Pa, heating to 250-270°C, stirring and reacting at the same temperature and pressure for 10-16 hours, cooling to room temperature after the reaction, precipitating in water, washing the precipitated polymer with ethanol for 3-6 times, and finally drying in a vacuum dryer at 85-95°C to a constant weight to obtain the functional polymer.

6. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 5, characterized in that: The molar ratio of the biphenyl dicarboxylic acid, 4,8-tricyclo[5.2.1.O2,7]decanedimethanol, 2,3,5,6-tetrafluorophenylenedimethanol, catalyst and high boiling point solvent is 1:0.7:0.3:(0.8-1.2):(9-15).

7. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 5, characterized in that: The high boiling point solvent is dimethyl sulfoxide; the catalyst is a mixture of antimony glycol, p-toluenesulfonic acid, and tetrabutyl titanate in a mass ratio of (1-2):(0.8-1.2):1; and the inert gas is any one of nitrogen, helium, neon, and argon.

8. The multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 1, characterized in that: The amino-terminated hyperbranched polyamide is amino-terminated hyperbranched polyamide Hyper N103; the solvent is any one of toluene, ethyl acetate, and butanone; the conductive filler is any one of copper powder, carbon nanotubes, and graphene; the particle size of the copper powder is 1300-1800 mesh; the conductive layer is made of a conductive metal, and the conductive metal is copper, aluminum, nickel, or an alloy thereof; It is a network structure with a grid line width of 10-50μm and a grid spacing of 100-500μm.

9. A method for preparing a multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to any one of claims 1 to 8, characterized in that: The steps include: Step S1, performing corona treatment on the insulating layer, and then coating the first adhesive on the lower surface thereof to form a first adhesive layer; Step S2, roughening the conductive metal foil, and then forming a grid structure through a photolithography process to form a conductive layer; Step S3, hot-pressing the conductive layer and the lower surface of the first adhesive layer; and then coating the second adhesive layer on the other side of the conductive layer; Step S4, melt-extrude the functional polymer at 235-260° C., and then sequentially perform casting, sheet casting, and biaxial stretching to obtain a polyester film, then perform corona treatment on the polyester film, and then perform hot pressing bonding with the second adhesive layer to form a substrate layer; Step S5, aging treatment to obtain a multi-layer high-shielding electromagnetic shielding film for a flexible circuit board.

10. The method for preparing a multi-layer high-shielding electromagnetic shielding film for a flexible circuit board according to claim 9, characterized in that: The aging treatment specifically includes: hardening at 60° C.-80° C. for 30-50 minutes, and then aging at room temperature for more than 20 hours.