Reinforcement plate, flexible printed circuit board and glue dispensing method

Abstract

The present invention relates to a reinforcing plate, a flexible printed circuit board, and a method for dispensing glue. The reinforcing plate includes a reinforcing substrate, on which at least one slit is etched running through the thickness direction of the reinforcing substrate, and also includes a strippable coating. At least one slit on the lower surface of the reinforcing substrate is filled with the strippable coating, and the strippable coating filled in at least one slit is flush with the upper surface of the reinforcing substrate. In this way, when dispensing glue on the device on the upper surface of the reinforcing plate, since the slit on the reinforcing plate has been filled with the strippable coating, the liquid glue will not flow into the slit, but will flow directly between the device and the reinforcing plate. After the liquid glue solidifies, the strippable coating on the reinforcing plate can be directly peeled off, which ensures that there are slits on the reinforcing plate to reduce internal stress, and these slits will not affect the dispensing glue between the device and the reinforcing plate, thereby improving the flatness and ensuring a good dispensing effect.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible printed circuit boards, and in particular to a reinforcing plate, a flexible printed circuit board and a glue dispensing method. Background Art

[0002] Flexible printed circuits (FPCs), also known as flexible printed circuits, are made from a polyester film or polyimide substrate, with circuits etched onto copper foil. They offer high reliability and excellent flexibility. They offer advantages such as small size, light weight, high flexibility, and excellent electrical performance, meeting the demands for high density, miniaturization, lightweighting, thinness, and high reliability.

[0003] Because flexible printed circuit boards (FPCBs) have a certain degree of flexibility, when using them to manufacture related electrical devices, it is necessary to increase the strength of the FPCBs by using reinforcing plates. The FPCBs can be bonded to the surface of the reinforcing plates with conductive adhesive to achieve a certain degree of mechanical stability. In cases where there are many devices that require reinforcement, a larger reinforcing plate is required. However, the larger the area of ​​the reinforcing plate, the stress generated within the material when subjected to external forces will cause the surface flatness of the reinforcing plate to deteriorate. To address the issue of reinforcing plate flatness, the reinforcing plate is usually divided into multiple pieces and pressed together, with gaps between adjacent pieces to reduce the stress within the reinforcing plate. However, due to the gaps in the reinforcing plate, when dispensing glue between the reinforcing plate and the device, the glue easily flows into these gaps, reducing the amount of glue flowing between the reinforcing plate and the device, making it difficult for the reinforcing plate and the device to adhere, resulting in poor dispensing effect. Summary of the Invention

[0004] The embodiments of the present invention provide a reinforcement plate, a flexible printed circuit board, and a glue dispensing method, which are used to solve the technical problem of poor glue dispensing effect between the reinforcement plate and the device.

[0005] In a first aspect, the present invention provides a reinforcing plate for use in a flexible printed circuit board, comprising:

[0006] A reinforcing substrate, wherein at least one slit is etched on the reinforcing substrate and runs through the thickness direction of the reinforcing substrate;

[0007] The strippable coating is coated on the lower surface of the reinforcing substrate, the at least one gap is filled with the strippable coating in the strippable coating, and the strippable coating filled in the at least one gap is flush with the upper surface of the reinforcing substrate.

[0008] Optionally, the reinforcing substrate is any one of a steel plate, a polyimide plate, a glass fiber cloth plate, and an aluminum foil plate.

[0009] Optionally, the strippable coating in the strippable coating is a strippable glue or a strippable resin.

[0010] Optionally, four corners of the reinforcement substrate are all provided with rounded chamfers.

[0011] Optionally, the reinforcing substrate is a rectangular plate structure with a length of 36 mm, a width of 26 mm, and a thickness of 0.15 mm.

[0012] Optionally, the at least one gap on the reinforcing substrate divides the reinforcing substrate into M areas, where M is the number of devices to be bonded.

[0013] Optionally, the device to be bonded includes a 4-in-1 fingerprint recognition chip, and the 4-in-1 fingerprint recognition chip includes 4 fingerprint recognition sub-chips. Three gaps are set on the reinforcement substrate, and the reinforcement substrate is divided into 4 areas corresponding to the 4 fingerprint recognition sub-chips one by one. Each area is used to paste the corresponding fingerprint recognition sub-chip in the 4-in-1 fingerprint recognition chip.

[0014] In a second aspect, the present invention provides a flexible printed circuit board, comprising a flexible substrate and the reinforcing plate described in the first aspect, wherein the flexible substrate is provided with a hollow region corresponding to the reinforcing plate, the area of ​​the hollow region being larger than the area of ​​the device to be bonded, the at least one slit being located in the hollow region, and the upper surfaces of the flexible substrate and the reinforcing plate being bonded together by conductive adhesive.

[0015] In a third aspect, the present invention provides a dispensing method, applied to the flexible printed circuit board described in the second aspect, comprising:

[0016] Placing the device to be bonded on the reinforcing plate in the hollow area, wherein the device to be bonded is placed in the corresponding area divided by the at least one slit;

[0017] Filling the gap closest to the edge of the chip to be bonded with liquid glue, so that the liquid glue flows between the device to be bonded and the reinforcing plate;

[0018] After the liquid glue is cured, the peelable coating on the lower surface of the reinforcing plate is peeled off.

[0019] In a fourth aspect, the present invention provides a method for manufacturing a reinforcing plate, comprising:

[0020] Etching at least one slit on the reinforcing substrate, wherein each of the at least one slit runs through the thickness direction of the reinforcing substrate;

[0021] A peelable coating is applied on the lower surface of the reinforcing substrate to form a peelable coating layer, and the peelable coating filled in the at least one gap is flush with the upper surface of the reinforcing substrate.

[0022] The above one or more technical solutions in the embodiments of the present application have at least one or more of the following technical effects:

[0023] In a technical solution of an embodiment of the present invention, a reinforcing plate is provided, comprising a reinforcing substrate having at least one slit etched therein extending through the thickness of the reinforcing substrate, and a strippable coating, wherein at least one slit on the lower surface of the reinforcing substrate is filled with the strippable coating, and the strippable coating filled in the at least one slit is flush with the upper surface of the reinforcing substrate. Thus, using the reinforcing plate of the embodiment of the present invention, when dispensing glue to a device on the upper surface of the reinforcing plate, since the slit in the reinforcing plate is already filled with the strippable coating, liquid glue will not flow into the slit, but will flow directly between the device and the reinforcing plate. After the liquid glue solidifies, the strippable coating on the reinforcing plate can be directly peeled off. This ensures that the slits on the reinforcing plate exist to reduce internal stress, while these slits do not affect the dispensing of glue between the device and the reinforcing plate, thereby improving flatness and ensuring a good dispensing effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the cross-sectional structure of a reinforcement plate in the first embodiment of the present invention;

[0025] Figure 2 A schematic top view of a reinforcement plate in the first embodiment of the present invention;

[0026] Figure 3 Schematic diagram of a hollowed-out area of ​​a flexible substrate in a flexible printed circuit board in a second embodiment of the present invention;

[0027] Figure 4 A schematic diagram of a chip placed on a reinforcing plate in a third embodiment of the present invention;

[0028] Figure 5 FIG. 1 is a schematic diagram of a flexible printed circuit board after the strippable coating is stripped from the reinforcing plate in the fourth embodiment of the present invention. DETAILED DESCRIPTION

[0029] The embodiments of the present invention provide a reinforcement plate, a flexible printed circuit board, and a dispensing method, which are used to provide a TOF reinforcement plate with automatic zoom, so as to achieve clear images within a variable depth of field range.

[0030] In order to solve the above technical problems, the technical solution provided by the present invention is as follows:

[0031] In the technical solution of an embodiment of the present invention, a reinforcing plate is provided for use in a flexible printed circuit board, comprising: a reinforcing substrate having at least one slit etched thereon extending through the thickness of the reinforcing substrate; and a strippable coating applied to the lower surface of the reinforcing substrate, wherein the at least one slit is filled with the strippable coating, and the strippable coating filled in the at least one slit is flush with the upper surface of the reinforcing substrate. When gluing components on the upper surface of the reinforcing plate, since the slits in the reinforcing plate are already filled with the strippable coating, liquid glue will not flow into the slits, but will flow directly between the components and the reinforcing plate. After the liquid glue solidifies, the strippable coating on the reinforcing plate can be directly peeled off. This ensures that the reinforcing plate has slits to reduce internal stress, while these slits do not affect the gluing between the components and the reinforcing plate, thereby improving flatness and ensuring a good gluing effect.

[0032] The technical solution of the present invention is described in detail below through the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present application and the specific features in the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. Unless there is a conflict, the embodiments of the present application and the technical features in the embodiments can be combined with each other.

[0033] The term "and / or" in this document simply describes a relationship between related objects, indicating that three possible relationships exist. For example, "A and / or B" can mean: A exists alone, A and B exist simultaneously, or B exists alone. Additionally, the character " / " in this document generally indicates that the related objects are in an "or" relationship.

[0034] The first embodiment of the present invention provides a reinforcing plate for use in a flexible printed circuit board, comprising: a reinforcing substrate having at least one slit etched therein extending through the thickness of the reinforcing substrate; and a strippable coating applied to the lower surface of the reinforcing substrate, the at least one slit being filled with the strippable coating, the strippable coating in the at least one slit being flush with the upper surface of the reinforcing substrate. The at least one slit in the reinforcing substrate divides the reinforcing substrate into M regions, where M is the number of components to be bonded.

[0035] Specifically, in this embodiment, please refer to Figure 1 and Figure 2 , Figure 1 is a cross-sectional schematic diagram of a reinforcement plate in the first embodiment of the present invention, Figure 2 is a top view of a reinforcement plate in the first embodiment of the present invention, Figure 1 and Figure 23 slits are provided on the reinforcing substrate 10, dividing the reinforcing substrate 10 into 4 areas for attaching the devices to be reinforced. Specifically, the 3 slits can be etched on the reinforcing substrate by chemical etching.

[0036] Furthermore, a strippable coating 20 is coated on the lower surface of the reinforcing substrate 10 . The strippable coating 20 fills the three gaps on the reinforcing substrate 10 and is flush with the upper surface of the reinforcing substrate 10 .

[0037] In this way, the device is placed in the four corresponding areas. When glue is dispensed on the upper surface of the reinforcing substrate 10, the dispensed liquid glue flows between the device and the reinforcing substrate 10. After curing, the device and the reinforcing substrate 10 are firmly bonded together. The peelable coating on the reinforcing substrate can be directly peeled off. In this way, gaps are ensured on the reinforcing substrate to reduce internal stress, and these gaps do not affect the glue dispensing between the device and the reinforcing substrate, thereby improving flatness and ensuring a good glue dispensing effect.

[0038] Furthermore, in this embodiment, Figure 1 and Figure 2 The corresponding device to be bonded includes a 4-in-1 fingerprint recognition chip, which includes four fingerprint recognition sub-chips. Three slits are provided on the reinforcing substrate, dividing it into four areas corresponding to the four fingerprint recognition sub-chips. Each area is used to bond a corresponding fingerprint recognition sub-chip in the 4-in-1 fingerprint recognition chip. Based on the size of the 4-in-1 fingerprint recognition chip, the reinforcing substrate 10 is a rectangular plate-shaped structure with a length of 36 mm, a width of 26 mm, and a thickness of 0.15 mm. In this embodiment, a reinforcing substrate 10 with a thickness of 0.15 mm is selected to improve the strength of the FPC board. The thickness of the reinforcing substrate 10 has a crucial impact on the performance of the FPC board. On the one hand, a reinforcing substrate that is too thin will not achieve the ideal reinforcement effect. On the other hand, a reinforcing substrate that is too thick will limit the application of the FPC board in electronic devices. Selecting a reinforcing substrate with a thickness of 0.15 mm ensures a good reinforcement effect while not significantly affecting the volume of the FPC board, which is conducive to its widespread application in electronic devices.

[0039] Furthermore, in this embodiment, the reinforcing substrate is any one of a steel plate, a polyimide plate, a glass fiber cloth plate, and an aluminum foil plate.

[0040] In the aforementioned embodiment, if the device to be bonded includes a 4-in-1 fingerprint recognition chip, a stainless steel plate can be used as the reinforcing substrate 10. Stainless steel plates are resistant to deformation, have high strength, and can provide both electrical conductivity and sensing. Specifically, 304 or 301 stainless steel plates can be used as the reinforcing substrate 10. Furthermore, nickel-plated steel sheets can also be used as the reinforcing substrate 10. Nickel has strong corrosion resistance, which helps protect the steel sheet. Nickel also has strong plasticity and good bonding strength with the substrate. Therefore, the use of nickel-plated steel sheets can effectively extend the service life of the reinforcing substrate 10 and improve the reliability of the FPC board.

[0041] Furthermore, if the heat dissipation of the device to be bonded is large, the reinforcing substrate 10 can be made of a polyimide board PI made of polyimide material, which has high temperature resistance.

[0042] If the FPC corresponding to the reinforcing plate in this embodiment is used in products requiring high-performance electronic insulation, the reinforcing substrate 10 can be made of FR-4 fiberglass cloth. FR-4 fiberglass cloth has high mechanical and dielectric properties, stable electrical insulation performance, and good heat resistance and moisture resistance.

[0043] If the FPC corresponding to the reinforcing plate in this embodiment is used in electronic equipment with high heat dissipation requirements, the reinforcing substrate 10 can be an aluminum foil reinforcing plate.

[0044] During the specific implementation process, the reinforcing substrate 10 can also be selected from polytetrafluoroethylene plates, polycarbonate plates, polyester reinforcing plates, etc. according to different application scenarios. The specific selection of the reinforcing substrate 10 can be set according to actual needs, and this embodiment does not limit it.

[0045] Specifically, in this embodiment, the strippable coating in the strippable coating 20 mainly has the function of filling, filling the gaps in the reinforcing substrate 10, and being flush with the upper surface of the reinforcing substrate 10, so as to facilitate the subsequent dispensing operation between the device to be bonded and the reinforcing substrate 10. In addition, the strippable coating also has a protective function. The film-forming substance of the strippable coating is also a polymer material. The coating solidifies to form a continuous and dense strippable coating attached to the protected object, isolating the invasion of external media such as air, water vapor, salt spray, and microorganisms, and reducing chemical corrosion to achieve an anti-corrosion effect. In addition, the coating has a certain thickness and elasticity, which can alleviate external force collisions and human scratches to achieve a protective effect. Furthermore, the strippable coating also has a decontamination function. During the film formation process, the strippable coating absorbs pollutants on the protected surface through surface adsorption, causing them to transfer from the material surface to the film and become part of the film. Since the polymer film-forming substance itself has a certain viscosity, it relies on adhesion to stick to other residual substances on the material surface. The result of the two forces is that the pollutants are enriched on the membrane. As long as the strippable coating is removed, the pollutants on the protected material can be removed.

[0046] Furthermore, in this embodiment, the strippable coating in the strippable coating is a strippable glue or a strippable resin.

[0047] Specifically, in the present embodiment, the strippable coating is a strippable glue or a strippable resin. Specifically, the strippable glue has strong water resistance, oil resistance, corrosion resistance and insulation. After being sprayed on the lower surface of the reinforcing substrate 10, a strippable film is formed. Through the effective isolation of the film on the object, the film is anti-dirt, anti-rust, acid and alkali, anti-scratch and can be peeled off and renewed. The strippable resin is specifically a water-based strippable coating, which is a coating in which a water-based strippable resin is dispersed in water, that is, the solvent of the coating is water. The advantages are environmental protection and safety. It uses cheap water instead of expensive and toxic organic solvents, which will not pollute the environment, save production costs, and improve construction safety. In the specific implementation process, the strippable coating is not limited to the above two types and can be selected according to actual needs. Here, this embodiment does not make any restrictions.

[0048] Furthermore, in this embodiment, the four corners of the reinforcing substrate 10 are all provided with rounded chamfers. Specifically, in this embodiment, the four corners of the reinforcing substrate 10 are provided with rounded chamfers, which solves the problem that the thin reinforcing substrate 10 and the sharp corners can easily puncture the flexible substrate of the FPC. It also effectively prevents workers from scratching their fingers during the installation of the reinforcing substrate 10, ensuring the safety of the workers.

[0049] Based on the same inventive concept as in the above embodiment, the second embodiment of the present invention provides a flexible printed circuit board, comprising: a flexible substrate and the reinforcing plate described in the above first embodiment, please refer to Figure 3 The flexible substrate is provided with a hollow area corresponding to the reinforcing plate, the area of ​​the hollow area is larger than the area of ​​the device to be bonded, and at least one gap is located in the hollow area. The flexible substrate and the upper surface of the reinforcing plate are bonded together by conductive adhesive. Please refer to Figure 4 The chip to be bonded includes a 4-in-1 fingerprint recognition chip, and the four fingerprint recognition sub-chips are placed in the corresponding areas respectively, and the chips are separated by gaps.

[0050] Furthermore, in this embodiment, the flexible substrate material is a polyimide copper-clad laminate. This flexible substrate material utilizes high-purity, structurally uniform rolled copper foil, with its crystal orientation parallel to the circuit alignment, enhancing the product's toughness. Prior to circuit fabrication, the polyimide copper-clad laminate undergoes a high-temperature heat treatment, further enhancing the copper foil's flexural properties and eliminating shrinkage and stress caused by moisture absorption in the polyimide, effectively improving the flexible substrate's ability to flex.

[0051] Furthermore, in this embodiment, the area of ​​the reinforcing plate is larger than that of the flexible substrate. The flexible substrate and the upper surface of the reinforcing plate are bonded together using conductive adhesive. This increases the bonding stress between the reinforcing plate and the flexible substrate, significantly reducing the risk of the reinforcing plate falling off the flexible substrate during operation. The specific structure of the reinforcing plate has been described in detail in the first embodiment above; please refer to the first embodiment for details and will not be repeated in this embodiment.

[0052] The flexible printed circuit board in this embodiment increases the strength of the flexible printed circuit board by providing the reinforcing plate, while ensuring the flatness of the reinforcing surface, and does not affect the subsequent gluing between the reinforcing plate and the device to be bonded, ensuring a good gluing effect.

[0053] Based on the same inventive concept as the above-mentioned embodiment, the third embodiment of the present invention provides a method for manufacturing a reinforcement plate, comprising:

[0054] Etching at least one slit on the reinforcing substrate, wherein each of the at least one slit runs through the thickness direction of the reinforcing substrate;

[0055] A peelable coating is applied on the lower surface of the reinforcing substrate to form a peelable coating layer, and the peelable coating filled in the at least one gap is flush with the upper surface of the reinforcing substrate.

[0056] Specifically, in this embodiment, chemical etching is used to etch a predetermined number of slits into the reinforcing substrate, such as the three slits in the aforementioned embodiment. These three slits divide the reinforcing substrate into four areas, which are used to attach the components to be reinforced. The slits can be rectangular strips or other shapes. In specific implementations, the position, shape, and number of the slits etched into the reinforcing substrate can be customized according to actual needs and are not limited by this embodiment.

[0057] Furthermore, a peelable coating is applied to the lower surface of the reinforcing substrate. The peelable coating 20 completely fills the gaps in the reinforcing substrate and is flush with the upper surface of the reinforcing substrate. The peelable coating used in the specific implementation has been described in detail in the first embodiment above and will not be repeated here.

[0058] In this way, the device is placed in the corresponding area of ​​the upper surface of the reinforcing substrate. When glue is dispensed on the upper surface of the reinforcing substrate, the dispensed liquid glue flows between the device and the reinforcing substrate. After curing, the device and the reinforcing substrate are firmly bonded together. The peelable coating on the reinforcing plate can be directly peeled off. In this way, it is ensured that there are gaps on the reinforcing plate to reduce internal stress, and these gaps will not affect the glue dispensing between the device and the reinforcing plate, thereby improving the flatness and ensuring a good glue dispensing effect.

[0059] Based on the same inventive concept as the aforementioned embodiment, the fourth embodiment of the present invention provides a dispensing method applied to the flexible printed circuit board in the aforementioned second embodiment, comprising:

[0060] Placing the device to be bonded on the reinforcing plate in the hollow area, wherein the device to be bonded is placed in the corresponding area divided by the at least one gap;

[0061] Filling the gap closest to the edge of the chip to be bonded with liquid glue, so that the liquid glue flows between the device to be bonded and the reinforcing plate;

[0062] After the liquid glue is cured, the peelable coating on the lower surface of the reinforcing plate is peeled off.

[0063] Specifically, in this embodiment, when dispensing glue, the device to be bonded is placed on the reinforcing plate in the hollow area of ​​the flexible printed circuit board. Figure 4 The chip to be bonded includes a 4-in-1 fingerprint recognition chip, and the four fingerprint recognition sub-chips are placed in the corresponding areas respectively, and the chips are separated by gaps.

[0064] Furthermore, a peelable coating is applied to the lower surface of the reinforcing substrate. The peelable coating fills the gaps on the reinforcing substrate and is flush with the upper surface of the reinforcing substrate. In this way, the device is placed in the corresponding area on the upper surface of the reinforcing substrate. When the glue is dispensed on the upper surface of the reinforcing substrate, the dispensed liquid glue flows between the device and the reinforcing substrate. After curing, the device and the reinforcing substrate are firmly bonded together, and the peelable coating on the reinforcing substrate can be directly peeled off. Please refer to Figure 5 , showing a schematic diagram after the peelable coating is peeled off. In this way, it can be ensured that there are gaps on the reinforcement plate to reduce internal stress, and these gaps will not affect the dispensing between the device and the reinforcement plate, ensuring a good dispensing effect while improving flatness.

[0065] Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0066] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A reinforcing plate, applied to a flexible printed circuit board, characterized in that: include: A reinforcing substrate, wherein at least one slit is etched on the reinforcing substrate and runs through the thickness direction of the reinforcing substrate; The strippable coating is coated on the lower surface of the reinforcing substrate, the at least one gap is filled with the strippable coating, and the strippable coating filled in the at least one gap is flush with the upper surface of the reinforcing substrate.

2. The reinforcing plate according to claim 1, wherein: The reinforcing substrate is any one of a steel plate, a polyimide plate, a glass fiber cloth plate, and an aluminum foil plate.

3. The reinforcing plate according to claim 1, wherein: The strippable coating in the strippable coating is a strippable glue or a strippable resin.

4. The reinforcing plate according to claim 1, wherein: The four corners of the reinforcement base plate are all provided with rounded chamfers.

5. The reinforcing plate according to claim 1, wherein: The reinforcing substrate is a rectangular plate structure with a length of 36 mm, a width of 26 mm, and a thickness of 0.15 mm.

6. The reinforcing plate according to claim 5, wherein: The at least one slit on the reinforcing substrate divides the reinforcing substrate into M areas, where M is the number of components to be bonded.

7. The reinforcing plate according to claim 6, wherein: The device to be bonded includes a 4-in-1 fingerprint recognition chip, which includes 4 fingerprint recognition sub-chips. Three gaps are set on the reinforcement substrate, dividing the reinforcement substrate into 4 areas corresponding to the 4 fingerprint recognition sub-chips one by one, and each area is used to paste the corresponding fingerprint recognition sub-chip in the 4-in-1 fingerprint recognition chip.

8. A flexible printed circuit board, characterized in that: The invention comprises a flexible substrate and the reinforcing plate according to any one of claims 1 to 7, wherein the flexible substrate is provided with a hollow region corresponding to the reinforcing plate, the area of ​​the hollow region is larger than the area of ​​the device to be bonded, the at least one slit is located in the hollow region, and the upper surface of the flexible substrate and the reinforcing plate are bonded together by conductive adhesive.

9. A dispensing method, characterized in that: The flexible printed circuit board according to claim 8, comprising: Placing the device to be bonded on the reinforcing plate in the hollow area, wherein the device to be bonded is placed in the corresponding area divided by the at least one gap; Filling the gap closest to the edge of the chip to be bonded with liquid glue, so that the liquid glue flows between the device to be bonded and the reinforcing plate; After the liquid glue is cured, the peelable coating on the lower surface of the reinforcing plate is peeled off.

10. A method for manufacturing a reinforcing plate, characterized in that: include: Etching at least one slit on the reinforcing substrate, wherein each of the at least one slit runs through the thickness direction of the reinforcing substrate; A peelable coating is applied on the lower surface of the reinforcing substrate to form a peelable coating layer, and the peelable coating filled in the at least one gap is flush with the upper surface of the reinforcing substrate.

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