Flexible circuit board containing transparent cover film and preparation method thereof

Abstract

The present invention discloses a flexible circuit board containing a transparent cover film and a preparation method thereof, comprising a transparent composite PI substrate and a transparent cover film formed on at least one side of the transparent composite PI substrate; the transparent composite PI substrate comprises a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer and a second copper foil layer from top to bottom; the transparent cover film comprises a second UV-resistant transparent adhesive layer and a second UV-resistant transparent polyimide layer; the glass transition temperature of the first UV-resistant transparent polyimide layer and the glass transition temperature of the second UV-resistant transparent polyimide layer are both greater than 320°C. The circuit board of the present invention has extremely high transmittance, extremely low haze, resistance to QUV irradiation, high Tg, high deflection, low rebound force, voltage resistance and low CTE.

Description

Technical Field

[0001] The present invention belongs to the technical field of FPC (flexible circuit board) and its preparation, and particularly relates to a double-sided copper foil substrate, which is mainly used in transparent substrates such as optical instruments, lighting instruments, touch screens, solar panels, flexible mobile phones, AMOLED (active matrix organic light emitting diodes), large-screen TVs, and car displays. Background Art

[0002] Compared with traditional display technology, organic light-emitting diode (OLED) display technology has a series of advantages such as ultra-light, ultra-thin, wide viewing angle, high definition, low temperature resistance, and good shock resistance. Therefore, organic light-emitting display technology has made great progress in the past 20 years, and various high-performance and long-life organic light-emitting materials and devices have been continuously introduced. With the further development of organic light-emitting display technology, active-driven OLED displays with richer information, soft OLED displays, and OLED light sources that can be used for lighting will create a more colorful world for us in the near future. OLED has a very broad application prospect. In the display field, OLED can not only be used in mobile phones, MP3 / MP4, digital cameras, GPS, PDA, 5G communication terminals, wall-mounted TVs, desktop and laptop computers, home appliances, industrial instruments and other civilian products. OLED is an ideal display with a wider application prospect. At the same time, OLED is the only display technology among all current display technologies that can produce large-size, high-brightness, high-resolution soft screens, and the thickness of the device is only two layers of plastic sheets. By then, "dreamlike displays" such as curtain-type TVs and rollable electronic newspapers will gradually become a reality. In the field of lighting, OLED can not only be used as general indoor and outdoor lighting, backlight source, decorative lighting and other fields, but can even be used to prepare artistic flexible luminous wallpaper, monochrome or color luminous windows, wearable luminous warning signs and other products.

[0003] For this purpose, there are two main solutions for transparent design in flexible circuit board substrates on the market. One is a transparent PI substrate with adhesive using a transparent PI film and a transparent adhesive that is resistant to yellowing. One is a highly transparent substrate using a highly transparent PET / PEN film and a transparent adhesive that is resistant to yellowing. The other is a transparent composite substrate (the so-called composite substrate is an asymmetric double-sided copper-clad substrate, consisting of a first copper foil layer, an insulating layer, an adhesive layer, and a second copper foil layer). However, both have some disadvantages.

[0004] At present, transparent PI films on the market have the following disadvantages:

[0005] 1. The traditional PI film preparation method is cast film or blown film method, followed by uniaxial or biaxial stretching, which has high requirements on equipment and needs to undergo high temperature 500-800℃ dehydration closed-loop treatment. The process is complex and the cost is extremely high.

[0006] 2. It is difficult to produce ultra-thin films of 5-15um, and optical properties such as transmittance and haze are affected;

[0007] 3. Or without anti-UV absorber, it will turn yellow and become brittle under long-term ultraviolet light.

[0008] Currently, transparent PET / PEN films on the market have the following disadvantages:

[0009] 1. Transparent PET / PEN film is matched with transparent glue to make transparent products. However, due to the melting point of PET / PEN at 225-260℃, the heat resistance is poor and it cannot withstand high temperature (288℃) FPC processes such as SMT reflow soldering. There are concerns about its application in flexible substrates.

[0010] 2. It is difficult to produce ultra-thin films of 5-15um, and optical properties such as transmittance and haze are affected, and the thickness is mostly above 25um;

[0011] 3. Due to the molecular structure, the overall CTE of PET / PEN materials is relatively large, which affects the downstream line design, especially the fine line design;

[0012] 4. If there is no anti-UV absorber, it will turn yellow and become brittle under long-term ultraviolet light.

[0013] For example, Taiwan Patent No. I560056B and China Patent No. 104109489B disclose a covering film using PET, PEN, PBT with a transparent resin and a hard coating layer, and Taiwan Patent No. 201629725A discloses a conductive substrate for a touch panel using PET, PEN, and PEI resin with a penetration rate greater than 90%.

[0014] Currently, transparent composite substrates on the market have the following disadvantages:

[0015] 1. The flatness and stiffness of the PI layer after etching are poor;

[0016] 2. The roughness of the copper foil surface is required to be high (due to the light transmittance requirement of the transparent layer, the surface roughness may need to be controlled within a specific low range, Rz < 2.0). Low profile copper foil with low roughness needs to be selected for production, and the cost is limited. Otherwise, it will affect the optical properties of the material such as transmittance and haze. Summary of the invention

[0017] The main technical problem solved by the present invention is to provide a flexible circuit board containing a transparent cover film and a preparation method thereof, wherein the circuit board has extremely high transmittance, extremely low haze, resistance to QUV irradiation, high Tg, high deflection, low rebound force, voltage resistance and low CTE, and the thickness design is more flexible, wherein the transparent layer thickness of the transparent composite PI substrate without the copper foil layer can be as low as 15μm. Compared with the prior art, the transparent PI film substrate with glue has lower cost and can be made into ultra-thin substrates, which is beneficial to light transmittance; compared with the prior art, the highly transparent PET / PEN film substrate has high heat resistance and low CTE, can cooperate with the FPC high temperature process, make fine circuit design and can make ultra-thin substrates, which is beneficial to light transmittance.

[0018] In order to solve the above technical problems, a technical solution adopted by the present invention is: a flexible circuit board containing a transparent cover film, comprising a transparent composite PI substrate and a transparent cover film formed on at least one side of the transparent composite PI substrate;

[0019] The transparent composite PI substrate includes, from top to bottom, a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer, and a second copper foil layer;

[0020] The transparent cover film includes a second UV resistant transparent adhesive layer and a second UV resistant transparent polyimide layer;

[0021] The light transmittance of the first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer is greater than 88% and the haze is less than 1%, and the L value of the L*a*b color system measured by a UV-visible light meter is 92-99, the a value is -2.0-2.0, and the b value is -1.0-1.0;

[0022] The glass transition temperature of the first UV resistant transparent polyimide layer and the glass transition temperature of the second UV resistant transparent polyimide layer are both greater than 320° C., the light transmittance of the first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer is greater than 88% and the haze is less than 1%, the L value of the L*a*b color system measured by an ultraviolet visible light brightness meter is 92-99, the a value is -2.0-2.0 and the b value is -1.0-1.0, the surface hardness is greater than 2H, and the CTE is less than 30ppm;

[0023] The thickness of the first UV resistant transparent polyimide layer and the thickness of the second UV resistant transparent polyimide layer are both 3-20 μm; the thickness of the first UV resistant transparent adhesive layer and the thickness of the second UV resistant transparent adhesive layer are both 5-50 μm.

[0024] Furthermore, the transparent covering film is formed on the upper surface of the transparent composite PI substrate and the lower surface of the transparent composite PI substrate, respectively, and the flexible circuit board includes, from top to bottom, a second UV-resistant transparent polyimide layer, a second UV-resistant transparent adhesive layer, a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer, a second copper foil layer, a second UV-resistant transparent adhesive layer, and a second UV-resistant transparent polyimide layer.

[0025] Furthermore, the first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer are both transparent polyimide layers containing an anti-UV absorber.

[0026] Furthermore, the first UV resistant transparent adhesive layer and the second UV resistant transparent adhesive layer are both acrylic adhesive layers or polyester adhesive layers.

[0027] Furthermore, the first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer are both polyester adhesive layers, and the polyester adhesive layers include an organic component B, an amine low-temperature curing agent and an anti-UV absorber; the weight percentage of each component of the polyester adhesive layer is: the organic component B is 90-95%, the amine low-temperature curing agent is 4-8% and the anti-UV absorber is 0.01-2%.

[0028] Furthermore, the organic component B is at least one of methyl acrylate, ethyl acrylate, polyphenylmethyl acrylate and polymethacrylate.

[0029] Further, the anti-UV absorber is at least one of benzophenone, 2-hydroxy-4-methoxybenzophenone, [2-hydroxy-4-(octyloxy)phenyl]phenyl ketone, 2-hydroxy-4-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, benzophenone-12, benzophenone-12, 2-hydroxy-4-n-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, ultraviolet absorber BP-12, benzophenone-12, ultraviolet absorber 531 and ultraviolet absorber UV-531.

[0030] The further technical solution adopted by the present invention to solve the technical problem is:

[0031] The preparation method of the transparent composite PI substrate comprises the following steps:

[0032] S1, mixing the raw materials of the first UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the first UV-resistant transparent adhesive layer according to a ratio, and setting them aside;

[0033] S2, coating the precursor of the first UV-resistant transparent polyimide layer prepared in step S1 on the first copper foil layer, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a transparent composite PI semi-finished product;

[0034] A first UV-resistant transparent adhesive layer is coated on the surface of the first UV-resistant transparent polyimide layer of the transparent composite PI semi-finished product, and a second copper foil layer is pressed and baked, and the transparent composite PI substrate is formed after winding.

[0035] The parameters of the baking process are: temperature 60-90°C, fan speed 750-800rpm; pressure during pressing 0.8-1.5kgf / cm 2 ;

[0036] S3, curing: the transparent composite PI substrate prepared in step S2 is cured at a low temperature of 45-55° C. for 1.5-2.5 h to obtain a finished transparent composite PI substrate.

[0037] Furthermore, the method for preparing the transparent covering film comprises the following steps:

[0038] S1, mixing the raw materials of the second UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the second UV-resistant transparent adhesive layer according to a ratio, and setting them aside;

[0039] S2, coating the precursor of the second UV-resistant transparent polyimide layer prepared in step S1 on the release film, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a semi-finished transparent covering film;

[0040] A second UV-resistant transparent adhesive layer is coated on the surface of the second UV-resistant transparent polyimide layer of the semi-finished transparent covering film, and a release paper is attached and baked, and the transparent covering film is formed after winding.

[0041] The parameters of the baking process are: temperature is 50°C, and the fan speed is 750-800rpm.

[0042] Furthermore, the method for preparing the flexible circuit board containing the transparent cover film comprises the following steps:

[0043] S1, etching the circuit on the transparent composite PI substrate for standby use;

[0044] S2. A transparent cover film is respectively applied to the upper and lower surfaces of the transparent composite PI substrate, and the transparent FPC product is prepared after pressing and ripening.

[0045] The beneficial effects of the present invention include at least the following points:

[0046] 1. The highly transparent composite PI substrate and its matching transparent cover film provided by the present invention are composed of a UV-resistant transparent polyimide layer and a UV-resistant transparent adhesive layer, and have extremely high transmittance, extremely low haze, QUV irradiation resistance, high Tg and high deflection, low rebound force and other characteristics, wherein the light transmittance is ≥88% and the haze is ≤1%, so the transparency is high, the mechanical strength is sufficient, and the thickness design is flexible, which is particularly suitable for the production of thin substrates, reaching 15-50μm, and different thicknesses can be designed according to the requirements of different electronic products;

[0047] Second, the raw materials in the UV-resistant transparent polyimide layer and the UV-resistant transparent adhesive layer of the present invention are reasonably proportioned, and both contain UV-resistant absorbers, which can enhance their yellowing resistance under irradiation. At the same time, the precursor of the UV-resistant transparent polyimide layer does not require high temperature (200-800°C) dehydration and ring closure, and can be cured for 3-10 minutes at low temperature (100-150°C) to dry on the surface, which has low requirements on equipment, is easy to produce, and has a high yield rate;

[0048] 3. In the preparation method of the transparent composite PI substrate and the transparent cover film of the present invention, the UV-resistant transparent polyimide layer and the UV-resistant transparent adhesive layer are formed by coating, so the thickness of each layer is controllable, the thickness range is wider, and there is no need to adopt the traditional casting method or film blowing method, and then perform uniaxial or biaxial stretching to form a film, nor is there a need for high-temperature dehydration closed-loop treatment under high temperature conditions of 500-800°C. The preparation method of the present invention has higher controllability and operability, and lower requirements on equipment;

[0049] Fourth, the transparent FPC of the present invention has the characteristics of high temperature resistance, high Tg, high flexibility, low rebound force, low CTE, voltage resistance, high surface hardness, etc. Due to its extremely low rebound force, the rebound force can be ≤5gf when combined with an ultra-thin cover film. Therefore, it can be bent multiple times without bending and has high transparency. It is particularly suitable for use in high-performance AMOLED display screens with soft and hard combined boards. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 is a schematic structural diagram of a flexible circuit board according to an embodiment of the present invention;

[0051] Figure 2 It is a schematic diagram of the structure of a transparent substrate in the prior art;

[0052] Figure 3 It is a structural schematic diagram of a transparent composite substrate in the prior art;

[0053] The parts in the accompanying drawings are marked as follows:

[0054] Flexible circuit board 100

[0055] A first copper foil layer 101, a first UV resistant transparent polyimide layer 102, a first UV resistant transparent adhesive layer 103, a second copper foil layer 104, a second UV resistant transparent polyimide layer 105, and a second UV resistant transparent adhesive layer 106;

[0056] Transparent substrate 200

[0057] A first copper foil layer 201, a first transparent adhesive layer 202, a transparent PEN / PET / PI layer 203, a second transparent adhesive layer 204 and a second copper foil layer 205;

[0058] Transparent composite substrate 300

[0059] a first copper foil layer 301 , a transparent PI layer 302 , a transparent adhesive layer 303 , and a second copper foil layer 304 . DETAILED DESCRIPTION

[0060] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making a clearer and more definite definition of the protection scope of the present invention.

[0061] Embodiment: A flexible circuit board 100 containing a transparent cover film comprises a transparent composite PI substrate and a transparent cover film formed on at least one side of the transparent composite PI substrate;

[0062] The transparent composite PI substrate includes, from top to bottom, a first copper foil layer 101, a first UV-resistant transparent polyimide layer 102, a first UV-resistant transparent adhesive layer 103, and a second copper foil layer 104;

[0063] The transparent cover film includes a second UV resistant transparent adhesive layer 106 and a second UV resistant transparent polyimide layer 105;

[0064] The light transmittance of the first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer is greater than 88% and the haze is less than 1%, and the L value of the L*a*b color system measured by a UV-visible light meter is 92-99, the a value is -2.0-2.0, and the b value is -1.0-1.0;

[0065] The glass transition temperature of the first UV resistant transparent polyimide layer and the glass transition temperature of the second UV resistant transparent polyimide layer are both greater than 320° C., the light transmittance of the first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer is greater than 88% and the haze is less than 1%, the L value of the L*a*b color system measured by an ultraviolet visible light brightness meter is 92-99, the a value is -2.0-2.0 and the b value is -1.0-1.0, the surface hardness is greater than 2H, and the CTE is less than 30ppm;

[0066] The thickness of the first UV resistant transparent polyimide layer and the thickness of the second UV resistant transparent polyimide layer are both 3-20 μm; the thickness of the first UV resistant transparent adhesive layer and the thickness of the second UV resistant transparent adhesive layer are both 5-50 μm.

[0067] The transparent covering film is formed on the upper surface of the transparent composite PI substrate and the lower surface of the transparent composite PI substrate respectively, and the flexible circuit board includes, from top to bottom, a second UV-resistant transparent polyimide layer, a second UV-resistant transparent adhesive layer, a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer, a second copper foil layer, a second UV-resistant transparent adhesive layer and a second UV-resistant transparent polyimide layer.

[0068] The first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer are both transparent polyimide layers containing an anti-UV absorber.

[0069] The first UV resistant transparent adhesive layer and the second UV resistant transparent adhesive layer are both acrylic adhesive layers or polyester adhesive layers.

[0070] The first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer are both polyester adhesive layers, and the polyester adhesive layers include an organic component B, an amine low-temperature curing agent and an anti-UV absorber; the weight percentage of each component of the polyester adhesive layer is: the organic component B is 90-95%, the amine low-temperature curing agent is 4-8% and the anti-UV absorber is 0.01-2%.

[0071] The amine low-temperature curing agent is an epoxide addition polyamine, a Michael addition polyamine, a fatty amine curing agent, an alicyclic amine curing agent, a polyamide curing agent or a polyamine polyamide curing agent.

[0072] The organic component B is at least one of methyl acrylate, ethyl acrylate, polyphenylmethyl acrylate and polymethacrylate.

[0073] The anti-UV absorber is at least one of phenyl ketone, 2-hydroxy-4-methoxybenzophenone, [2-hydroxy-4-(octyloxy)phenyl]phenyl ketone, 2-hydroxy-4-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, benzophenone-12, benzophenone-12, 2-hydroxy-4-n-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, ultraviolet absorber BP-12, benzophenone-12, ultraviolet absorber 531 and ultraviolet absorber UV-531.

[0074] The preparation method of the transparent composite PI substrate comprises the following steps:

[0075] S1, mixing the raw materials of the first UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the first UV-resistant transparent adhesive layer according to a ratio, and setting them aside;

[0076] S2, coating the precursor of the first UV-resistant transparent polyimide layer prepared in step S1 on the first copper foil layer, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a transparent composite PI semi-finished product;

[0077] A first UV-resistant transparent adhesive layer is coated on the surface of the first UV-resistant transparent polyimide layer of the transparent composite PI semi-finished product, and a second copper foil layer is pressed and baked, and the transparent composite PI substrate is formed after winding.

[0078] The parameters of the baking process are: temperature 60-90°C, fan speed 750-800rpm; pressure during pressing 0.8-1.5kgf / cm 2 ;

[0079] S3, curing: the transparent composite PI substrate prepared in step S2 is cured at a low temperature of 45-55° C. for 1.5-2.5 h to obtain a finished transparent composite PI substrate.

[0080] The method for preparing the transparent covering film comprises the following steps:

[0081] S1, mixing the raw materials of the second UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the second UV-resistant transparent adhesive layer according to a ratio, and setting them aside;

[0082] S2, coating the precursor of the second UV-resistant transparent polyimide layer prepared in step S1 on the release film, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a semi-finished transparent covering film;

[0083] A second UV-resistant transparent adhesive layer is coated on the surface of the second UV-resistant transparent polyimide layer of the semi-finished transparent covering film, and a release paper is attached and baked, and the transparent covering film is formed after winding.

[0084] The parameters of the baking process are: temperature is 50°C, and the fan speed is 750-800rpm.

[0085] The method for preparing the flexible circuit board containing the transparent cover film comprises the following steps:

[0086] S1, etching the circuit on the transparent composite PI substrate for standby use;

[0087] S2. A transparent cover film is respectively applied to the upper and lower surfaces of the transparent composite PI substrate, and the transparent FPC product is prepared after pressing and ripening.

[0088] The first copper foil layer and the second copper foil layer are both rolled copper foil layers or electrolytic copper foil layers; the roughness of the first copper foil layer and the roughness of the second copper foil layer are both <2.0; the thickness of the first copper foil layer and the thickness of the second copper foil layer are 2-35 μm respectively.

[0089] The embodiments of the present invention are compared with the transparent substrate and the transparent composite substrate of the prior art for related performance (in the comparative example, sample 1 is a transparent composite substrate, sample 2 is a PEN film, sample 3 is a PET film, and sample 4 is a transparent PI film). Figure 2 As shown, the transparent substrate 200 includes, from top to bottom, a first copper foil layer 201, a first transparent adhesive layer 202, a transparent PEN / PET / PI layer 203, a second transparent adhesive layer 204 and a second copper foil layer 205; Figure 3 As shown, the transparent composite substrate 300 includes a first copper foil layer 301, a transparent PI layer 302, a transparent adhesive layer 303 and a second copper foil layer 304 from top to bottom. The test results are shown in Table 1:

[0090] Table 1

[0091]

[0092]

[0093] From the experimental data of the embodiments (samples 1 to 3) and the comparative example (sample 1) in Table 1, it can be seen that the flexible circuit board of the present invention is compared with the transparent composite substrate of the prior art, although the anti-UV transparent adhesive layer and the anti-UV transparent polyimide layer are added to the upper and lower surfaces of the transparent composite substrate respectively, and the total thickness is relatively high, but the embodiment has high transmittance before and after QUV irradiation, low haze, low CTE, and low rebound force;

[0094] From the experimental data of the embodiments (sample 1-sample 3) and the comparative examples (sample 2-sample 4) in Table 1, it can be seen that compared with the transparent substrate in the prior art, the flexible circuit board of the present invention has a UV-resistant transparent adhesive layer and a UV-resistant transparent polyimide layer added to the upper and lower surfaces of the transparent composite substrate respectively, but the total thickness is not much different, and the embodiment has high transmittance before and after QUV irradiation, low haze, low CTE, high Tg, low rebound force, and high solder resistance.

[0095] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A flexible circuit board containing a transparent cover film, characterized in that: It comprises a transparent composite PI substrate and a transparent covering film formed on at least one side of the transparent composite PI substrate; The transparent composite PI substrate includes, from top to bottom, a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer, and a second copper foil layer; The transparent cover film includes a second UV resistant transparent adhesive layer and a second UV resistant transparent polyimide layer; The light transmittance of the first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer is greater than 88% and the haze is less than 1%, and the L value of the L*a*b color system measured by a UV-visible light meter is 92-99, the a value is -2.0-2.0, and the b value is -1.0-1.0; The glass transition temperature of the first UV resistant transparent polyimide layer and the glass transition temperature of the second UV resistant transparent polyimide layer are both greater than 320° C., the light transmittance of the first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer is greater than 88% and the haze is less than 1%, the L value of the L*a*b color system measured by an ultraviolet visible light brightness meter is 92-99, the a value is -2.0-2.0 and the b value is -1.0-1.0, the surface hardness is greater than 2H, and the CTE is less than 30ppm; The thickness of the first UV resistant transparent polyimide layer and the thickness of the second UV resistant transparent polyimide layer are both 3-20 μm; the thickness of the first UV resistant transparent adhesive layer and the thickness of the second UV resistant transparent adhesive layer are both 5-50 μm.

2. The flexible circuit board with a transparent cover film according to claim 1, characterized in that: The transparent covering film is formed on the upper surface of the transparent composite PI substrate and the lower surface of the transparent composite PI substrate, respectively, and the flexible circuit board includes, from top to bottom, a second UV-resistant transparent polyimide layer, a second UV-resistant transparent adhesive layer, a first copper foil layer, a first UV-resistant transparent polyimide layer, a first UV-resistant transparent adhesive layer, a second copper foil layer, a second UV-resistant transparent adhesive layer, and a second UV-resistant transparent polyimide layer.

3. The flexible circuit board with a transparent cover film according to claim 1, characterized in that: The first UV resistant transparent polyimide layer and the second UV resistant transparent polyimide layer are both transparent polyimide layers containing an anti-UV absorber.

4. The flexible circuit board with a transparent cover film according to claim 1, characterized in that: The first UV resistant transparent adhesive layer and the second UV resistant transparent adhesive layer are both acrylic adhesive layers or polyester adhesive layers.

5. The flexible circuit board with a transparent cover film according to claim 4, characterized in that: The first UV-resistant transparent adhesive layer and the second UV-resistant transparent adhesive layer are both polyester adhesive layers, and the polyester adhesive layers include an organic component B, an amine low-temperature curing agent and an anti-UV absorber; the weight percentage of each component of the polyester adhesive layer is: the organic component B is 90-95%, the amine low-temperature curing agent is 4-8% and the anti-UV absorber is 0.01-2%.

6. The flexible circuit board with a transparent cover film according to claim 5, characterized in that: The organic component B is at least one of methyl acrylate, ethyl acrylate, polyphenylmethyl acrylate and polymethacrylate.

7. The flexible circuit board with a transparent cover film according to claim 3 or 5, characterized in that: The anti-UV absorber is at least one of phenyl ketone, 2-hydroxy-4-methoxybenzophenone, [2-hydroxy-4-(octyloxy)phenyl]phenyl ketone, 2-hydroxy-4-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, benzophenone-12, benzophenone-12, 2-hydroxy-4-n-octyloxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, ultraviolet absorber BP-12, benzophenone-12, ultraviolet absorber 531 and ultraviolet absorber UV-531.

8. The flexible circuit board with a transparent cover film according to claim 1, characterized in that: The preparation method of the transparent composite PI substrate comprises the following steps: S1, mixing the raw materials of the first UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the first UV-resistant transparent adhesive layer according to a ratio, and setting them aside; S2, coating the precursor of the first UV-resistant transparent polyimide layer prepared in step S1 on the first copper foil layer, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a transparent composite PI semi-finished product; A first UV-resistant transparent adhesive layer is coated on the surface of the first UV-resistant transparent polyimide layer of the transparent composite PI semi-finished product, and a second copper foil layer is pressed and baked, and the transparent composite PI substrate is formed after winding. The parameters of the baking process are: temperature 60-90°C, fan speed 750-800rpm; pressure during pressing 0.8-1.5kgf / cm 2 ; S3, curing: the transparent composite PI substrate prepared in step S2 is cured at a low temperature of 45-55° C. for 1.5-2.5 h to obtain a finished transparent composite PI substrate.

9. The flexible circuit board with a transparent cover film according to claim 8, characterized in that: The method for preparing the transparent covering film comprises the following steps: S1, mixing the raw materials of the second UV-resistant transparent polyimide layer according to a ratio, and also mixing the raw materials of the second UV-resistant transparent adhesive layer according to a ratio, and setting them aside; S2, coating the precursor of the second UV-resistant transparent polyimide layer prepared in step S1 on the release film, and curing at a low temperature of 100-150° C. for 3-10 minutes to form a semi-finished transparent covering film; A second UV-resistant transparent adhesive layer is coated on the surface of the second UV-resistant transparent polyimide layer of the semi-finished transparent covering film, and a release paper is attached and baked, and the transparent covering film is formed after winding. The parameters of the baking process are: temperature is 50°C, and the fan speed is 750-800rpm.

10. A method for preparing a flexible circuit board containing a transparent cover film according to claim 9, characterized in that: The following steps are involved: S1, etching the circuit on the transparent composite PI substrate for standby use; S2. A transparent cover film is respectively applied to the upper and lower surfaces of the transparent composite PI substrate, and the transparent FPC product is prepared after pressing and ripening.

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