Method for manufacturing flexible circuit board and flexible circuit board

Abstract

The present application relates to the technical field of flexible circuit board manufacturing, and particularly relates to a flexible circuit board manufacturing method and a flexible circuit board. A wet film layer is formed to cover the surface of a selective copper plating layer and extend to cover the sidewall of the selective copper plating layer until the surface of the FPC substrate, completely covering the top surface and the sidewall of the selective copper plating layer, eliminating the limitation of the high step on the dry film layer fitting, and making the dry film selection only need to be based on the fine line analysis requirement, using thin dry film to adapt to fine lines throughout the process, solving the problem of insufficient analysis accuracy of thick dry film and step coverage failure of thin dry film. The wet film layer is pre-baked to a semi-cured state, has enough strength in the subsequent dry film fitting, and ensures the integrity of the covering structure. After development, the boundary of the wet film layer covers the sidewall of the selective copper plating layer, and the boundary of the wet film layer and the dry film layer is flush in the vertical direction, preventing the etching liquid from penetrating along the edge of the selective copper plating layer and avoiding the edge of the selective copper plating layer from being etched, achieving the consideration of high step plating and fine line accurate analysis, and greatly improving the product yield.

Description

Technical Field

[0001] This invention relates to the field of flexible circuit board manufacturing technology, and in particular to a method for manufacturing a flexible circuit board and a flexible circuit board. Background Technology

[0002] With the rapid development of electronic products towards thinner, lighter, and more integrated designs, the wiring density of flexible printed circuit boards (FPCs) continues to increase, leading to ever-higher requirements for line width and spacing. FPC production commonly employs a combination of plated board plating and selective plating. This process ensures the conductivity of the copper within the vias while avoiding excessive copper layer thickness caused by full-board plating, thus providing a foundation for subsequent fine-line etching.

[0003] However, when the thickness of the copper plating layer reaches 25μm or more, a steep step will form between the copper-plated area and the unplated base copper area. To cover this step, a dry film with a thickness of ≥30μm is required. Due to its large thickness, it can provide sufficient volume of plastic flow material during bonding, which can fill the corner area at the root of the step to form a smooth transition. However, the thick dry film causes blurred imaging edges due to increased light scattering, and the unpolymerized parts are difficult to remove completely during development. During etching, the sidewalls of the resist layer are steep, and the etching solution cannot uniformly enter the gap between the lines, making it impossible to resolve fine-pitch lines with a spacing of less than 60μm. If a ≤25μm thin dry film is selected, although it can be adapted to fine line resolution, the material volume is insufficient to fill the corners at the root of high steps. Furthermore, it is rapidly stretched and thinned or even cracked at the junction of the step sidewall and top surface. At the same time, stress concentration is prone to occur at the corners of the steps, and air bubbles are easily trapped in the corner areas, leading to dry film cracking, poor adhesion, and inability to completely cover copper-plated steps larger than 25μm. Ultimately, this results in the edge of the copper-plated layer being etched during etching, open circuits or short circuits in the lines, and a significant decrease in product yield. This has long restricted the mass production capacity and process stability of high-density fine FPC products.

[0004] To address the aforementioned issues, the industry has attempted to adjust the selective plating process sequence (i.e., etching fine lines first and then selective plating). However, due to the thin and soft nature of the FPC substrate, this process causes substrate wrinkles and defects. Furthermore, areas requiring thickening through selective plating after etching cannot guarantee the flow and stability of the plating current. Therefore, these attempts have failed to effectively resolve the contradiction between high-step selective plating and fine-line resolution. While existing technologies utilize a combination of wet and dry films, these are limited to filling recessed steps or pits on the surface of multilayer board substrates. Specifically, the wet film is filled into the pits to restore a smooth surface before the dry film is applied. However, for raised copper steps formed by localized selective plating in FPCs, the liquid wet film flows and collapses on the top and sidewalls of the raised areas due to gravity and surface tension, making it difficult to stably cover the edges of the raised areas. Moreover, wet films are generally considered insufficient in precision for the fabrication of fine lines on the outer layers of FPCs and are unsuitable as an outer resist layer. Thus, existing technologies have consistently failed to overcome the contradiction between high-step coverage and fine-line resolution. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for manufacturing a flexible circuit board and a flexible circuit board, which can simultaneously ensure reliable coverage of the copper plating step and accurate resolution of fine lines in non-copper plating areas when a high step is formed between the selectively plated copper layer and the base copper layer, and avoid the edge of the selectively plated copper layer being eroded.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for manufacturing a flexible circuit board includes the following steps: S1. Provide an FPC substrate and form a base copper layer on the surface of the FPC substrate; S2. A selective copper plating layer is formed, wherein the selective copper plating layer is disposed on the surface of the base copper layer; S3. A wet film layer is formed, which covers the surface of the selected copper plating layer and extends to cover the sidewalls of the selected copper plating layer until the surface of the FPC substrate. S4. Perform a pre-baking operation on the wet film layer to make the wet film layer semi-cured; S5. A dry film layer is formed, wherein the dry film layer covers the surface of the wet film layer and the surface of the base copper layer respectively; S6. Expose and develop the dry film layer to form a patterned dry film resist layer. After exposure and development, the dry film layer above the selected copper plating layer and the wet film layer are retained simultaneously. The boundary of the wet film layer covers the sidewall of the selected copper plating layer, and the boundaries of the wet film layer and the dry film layer are vertically aligned. S7. Using the dry film resist layer as a mask, perform an etching operation on the base copper layer to remove the base copper layer that is not covered by the dry film resist layer. S8. Remove the dry film layer and the wet film layer in sequence to obtain the finished flexible circuit board.

[0007] Another technical solution adopted in this invention is: A flexible circuit board is prepared by the above-described method for manufacturing flexible circuit boards.

[0008] The beneficial effects of this invention are as follows: By forming a wet film layer that covers the surface of the selectively plated copper layer and extends to cover its sidewalls up to the surface of the FPC substrate, the wet film layer completely encapsulates the top surface and sidewalls of the selectively plated copper layer. Before significant flow occurs, the wet film layer is pre-baked to a semi-cured state, rapidly shaping its coverage and overcoming the problem of liquid wet film flowing and collapsing on raised surfaces due to gravity and surface tension. Simultaneously, this semi-cured state ensures sufficient structural strength for the subsequent dry film layer lamination process, preventing deformation due to pressing and ensuring the integrity of the encapsulation structure. After the wet film layer completely covers the top surface and sidewalls of the selectively plated copper layer, the limitations imposed by high steps on dry film layer lamination are eliminated. This allows the selection of the dry film layer to be based solely on the resolution requirements of fine lines in non-selectively plated areas, enabling the use of thin dry films to adapt to fine lines throughout the process. This resolves the contradiction between insufficient resolution of thick dry films and the failure of thin dry films to cover steps. After exposure and development, the dry film layer and wet film layer above the selective copper plating layer are retained simultaneously. The boundary of the wet film layer covers the sidewall of the selective copper plating layer, and the boundaries of the wet film layer and the dry film layer are vertically aligned, forming a reliable double-layer protection for the selective copper plating layer. This effectively prevents the etching solution from penetrating along the edge of the selective copper plating layer and avoids the edge of the selective copper plating layer being etched. It achieves a balance between high-step selective plating and precise resolution of fine lines, greatly improving product yield. Attached Figure Description

[0009] Figure 1 This is a flowchart illustrating the steps of the flexible circuit board manufacturing method of the present invention. Figure 2 This is a cross-sectional structural diagram of step S1 of the flexible circuit board manufacturing method of the present invention; Figure 3 This is a cross-sectional structural diagram of step S2 in the method for manufacturing a flexible circuit board according to the present invention. Figure 4 This is a cross-sectional structural diagram of step S3 in the method for manufacturing a flexible circuit board according to the present invention. Figure 5 This is a cross-sectional structural diagram of step S5 in the method for manufacturing a flexible circuit board according to the present invention. Figure 6 This is a cross-sectional structural diagram of step S6 in the method for manufacturing a flexible circuit board according to the present invention. Figure 7 This is a cross-sectional structural diagram of step S7 in the method for manufacturing a flexible circuit board according to the present invention. Figure 8This is a cross-sectional structural diagram of step S8 in the method for manufacturing a flexible circuit board according to the present invention. Figure 9 This is a cross-sectional structural diagram of step S3 in the method for manufacturing a flexible circuit board according to the present invention. Label Explanation: 1. FPC substrate; 2. Base copper layer; 3. Optional copper plating layer; 4. Wet film layer; 41. Wet film oil overflow area; 5. Dry film layer. Detailed Implementation

[0010] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0011] Please refer to Figure 1 A method for manufacturing a flexible circuit board includes the following steps: S1. Provide an FPC substrate 1, and form a base copper layer 2 on the surface of the FPC substrate 1; S2. A selective copper plating layer 3 is formed, wherein the selective copper plating layer 3 is disposed on the surface of the base copper layer 2; S3. A wet film layer 4 is formed, which covers the surface of the copper plating layer 3 and extends to cover the sidewalls of the copper plating layer 3 until the surface of the FPC substrate 1. S4. Perform a pre-baking operation on the wet film layer 4 to make the wet film layer 4 semi-cured; S5. A dry film layer 5 is formed, which covers the surface of the wet film layer 4 and the surface of the base copper layer 2 respectively. S6. Exposure and development are performed on the dry film layer 5 to form a patterned dry film resist layer. After exposure and development, the dry film layer 5 above the copper plating layer 3 and the wet film layer 4 are retained simultaneously. The boundary of the wet film layer 4 covers the sidewall of the copper plating layer 3, and the boundary of the wet film layer 4 and the dry film layer 5 are vertically aligned. S7. Using the dry film resist layer as a mask, perform an etching operation on the base copper layer 2 to remove the base copper layer 2 that is not covered by the dry film resist layer. S8. Remove the dry film layer 5 and the wet film layer 4 in sequence to obtain the finished flexible circuit board.

[0012] As can be seen from the above description, the beneficial effects of the present invention are as follows: By forming a wet film layer 4, which covers the surface of the selectively plated copper layer 3 and extends to cover the sidewalls of the selectively plated copper layer 3 up to the surface of the FPC substrate 1, the wet film layer 4 completely covers the top surface and sidewalls of the selectively plated copper layer 3. Before significant flow occurs, the wet film layer 4 is pre-baked to a semi-cured state, quickly shaping its coverage and overcoming the problem of liquid wet film flowing and collapsing on raised surfaces due to gravity and surface tension. Simultaneously, this semi-cured state ensures sufficient structural strength for the wet film layer 4 during the subsequent lamination of the dry film layer 5, preventing deformation due to pressing and ensuring the integrity of the coverage structure. After the wet film layer 4 completely covers the top surface and sidewalls of the selectively plated copper layer 3, the limitation of high steps on the lamination of the dry film layer 5 is eliminated. This allows the selection of the dry film layer 5 to be based solely on the resolution requirements of fine lines in non-selectively plated areas, enabling the use of thin dry films to adapt to fine lines throughout the process. This resolves the contradiction between insufficient resolution of thick dry films and the failure of thin dry films to cover steps. After exposure and development, the dry film layer 5 and wet film layer 5 above the selectively plated copper layer 3 are retained simultaneously. The boundary of the wet film layer 4 covers the sidewall of the selectively plated copper layer 3, and the boundaries of the wet film layer 4 and the dry film layer 5 are vertically aligned, forming a reliable double-layer protection for the selectively plated copper layer 3. This effectively prevents the etching solution from penetrating along the edge of the selectively plated copper layer 3 and avoids the edge of the selectively plated copper layer 3 being etched. This achieves a balance between high-step selective plating and precise resolution of fine lines, significantly improving product yield.

[0013] For further details, please refer to Figure 9 In step S3, the wet film layer 4 covers the surface of the copper plating layer 3 and extends to cover the sidewalls of the copper plating layer 3 until the width of the FPC substrate 1 is 0.3mm~0.5mm.

[0014] As can be seen from the above description, an outward expansion of not less than 0.3mm ensures that the wet film layer 4 has sufficient coverage width over the edge of the selectively plated copper layer 3, forming an effective anti-etching barrier; an outward expansion of not more than 0.5mm avoids the wet film layer 4 from excessively occupying the circuit layout space in the non-selective plating area, ensuring the feasibility of high-density circuit design. This range achieves the best balance between protection effect and circuit density.

[0015] Furthermore, the boundary shape of the selected copper plating layer 3 is wavy.

[0016] As described above, the wavy boundary facilitates the discharge of air bubbles along the trough channels during the printing of the wet film layer 4 and the lamination of the dry film layer 5, significantly reducing the probability of air bubble residue in the step area and improving the consistency and density of the dry film layer 5. Simultaneously, the wavy boundary increases the contact area and adhesion between the edge of the selectively plated copper layer 3 and the wet film layer 4, enhancing the coverage of the edge of the selectively plated copper layer 3 by the wet film layer 4 and further improving the reliability of protection for the edge of the selectively plated copper layer.

[0017] Furthermore, in step S3, the wet film layer 4 covers the surface of the selectively plated copper layer 3 and extends to cover the sidewalls of the selectively plated copper layer 3 until the width of the FPC substrate 1 surface is measured with reference to the position of the wave crest.

[0018] As can be seen from the above description, since the wavy boundary has peaks and troughs, measuring with the peak position as the reference can ensure that the edge of each copper plating layer 3 on the wavy boundary is covered with an expansion that is not less than the minimum requirement, thus ensuring the comprehensiveness and consistency of the protection; at the same time, it avoids excessive expansion at the trough due to using the trough as the reference, which would occupy unnecessary line space, and maximizes the line layout area while ensuring the protection effect.

[0019] Furthermore, the difference between the thickness of the selected copper layer 3 and the thickness of the base copper layer 2 is greater than or equal to 25 μm.

[0020] As can be seen from the above description, when the thickness difference reaches more than 25μm, a steep step is formed between the copper plating area and the surrounding base copper area. Under this condition, the traditional single dry film process will face the problems of insufficient resolution of thick dry film and failure of thin dry film to cover the step. However, the present invention pre-covers the edge of the high step by wet film layer 4, so that the subsequent bonding and selection of dry film layer 5 are completely free from the constraints of step height. Under this harsh condition, high-quality circuit fabrication is achieved, which fully demonstrates the technical value of the present invention.

[0021] Furthermore, in step S4, the baking temperature of the pre-baking operation is 65℃~85℃, and the baking time of the pre-baking operation is 15min~30min.

[0022] As can be seen from the above description, under these conditions, the solvent in the wet film layer 4 evaporates moderately, and the resin undergoes partial cross-linking, so that the wet film layer 4 reaches an ideal semi-cured state where it does not transfer when lightly touched with a finger and does not fray when the polyester film is torn. It has sufficient structural strength to resist the extrusion deformation when the dry film layer 5 is subsequently applied, and maintains a certain degree of adhesive activity to ensure good bonding with the dry film layer 5. At the same time, it avoids over-curing, which would make it difficult to remove the film with organic solvents later.

[0023] Furthermore, in step S3, the wet film layer 4 is formed of a liquid photoresist, the liquid photoresist has a rotational viscosity at 25°C greater than or equal to 8000 cps, and the liquid photoresist has a solid content greater than or equal to 55 wt%.

[0024] As described above, high viscosity ensures that the wet film layer 4 remains stably positioned on the top surface and edge sidewalls of the selectively plated copper layer 3 after printing, preventing it from flowing or collapsing due to gravity or surface tension, thus guaranteeing the integrity of the coating structure. High solids content ensures that the wet film layer 4 has sufficient film thickness and density after pre-baking, forming an effective etching barrier. This specific parameter of the liquid photoresist is highly compatible with the wet film coating process of this invention, forming the material basis for achieving reliable edge protection of the selectively plated copper layer 3.

[0025] Furthermore, in step S5, the dry film layer 5 is laminated using a vacuum laminator, and the lamination process conditions are as follows: The bonding pressure is 0.4MPa~0.5MPa, the bonding speed is 0.5m / min, and the pressure roller temperature is 90℃~100℃.

[0026] As described above, the synergistic effect of these parameters enables high-quality bonding of the dry film layer 5 onto the undulating surface with the copper-plated protrusions 3. A bonding pressure ≥0.4MPa ensures that the dry film layer 5 is fully pressed against the FPC substrate 1 and the wet film layer 4 surface without any air bubbles. A bonding pressure ≤0.5MPa avoids deformation of the encapsulation structure caused by excessive pressure and strong compression of the semi-cured wet film layer 4. A pressure roller temperature of 90℃~100℃ allows the dry film layer 5 to obtain suitable fluidity to fill the small undulations, while avoiding excessive temperature that could lead to over-curing of the wet film layer 4. A bonding speed of 0.5m / min ensures that the dry film layer 5 has sufficient pressing time on each surface, ensuring uniform bonding quality.

[0027] Furthermore, in step S8, a two-step stripping method is used to remove the film layer, specifically as follows: First, use an alkaline stripping solution to remove the dry film layer 5, and then use an organic solvent to remove the wet film layer 4.

[0028] As described above, due to the different chemical compositions and solubility characteristics of the dry film layer 5 and the wet film layer 4, the alkaline stripping solution has a high solubility for the dry film layer 5 but a poor solubility for the wet film layer 4, while the organic solvent has good solubility for the wet film layer 4. The two-step stripping method first uses an alkaline stripping solution to quickly remove the dry film layer 5, and then uses an organic solvent to thoroughly remove the wet film layer 4. This method ensures both stripping efficiency and complete removal of both film layers, avoiding poor circuit appearance or subsequent process defects caused by film residue.

[0029] Please refer to Figures 1 to 8 A flexible circuit board is prepared by the above-described method for manufacturing flexible circuit boards.

[0030] As can be seen from the above description, the beneficial effects of the present invention are as follows: The flexible circuit board has a covering portion on the outer edge of the selectively plated copper layer 3. The wet film layer 4 covers the surface of the selectively plated copper layer 3 and extends to cover the sidewalls of the selectively plated copper layer 3 until it reaches the surface of the FPC substrate 1, horizontally extending outward by 0.3mm~0.5mm. The height of the top surface of the covering portion continuously decreases from the top edge of the selectively plated copper layer 3 towards the surface of the FPC substrate 1. This smooths the steep steps at the edge of the selectively plated copper layer 3 into a gradual transition shape, eliminates stress concentration points, improves the mechanical reliability of the product, and effectively protects the edge of the selectively plated copper layer 3 from the etchant during the etching process, ensuring edge flatness and circuit integrity. It can be widely used in electronic products that require high-density, high-reliability flexible circuit boards.

[0031] The flexible printed circuit board involved in this invention is an FPC product manufactured using a plate plating and selective plating process. This type of FPC includes an FPC substrate, a base copper layer formed on the surface of the FPC substrate, and a selectively plated copper layer covering the surface of the base copper layer. The selectively plated copper layer generally corresponds to vias, connection pads, or local circuit areas that require high current transmission, while the base copper layer is used to form fine circuits in non-selectively plated areas.

[0032] When the selected copper plating layer is relatively thin (e.g., less than 25μm), the height difference between the selected copper plating layer and the base copper layer is small, and conventional dry film processes can meet the processing needs of both the selected plating area and the fine line area. However, when the selected copper plating layer thickness reaches more than 25μm and the target line spacing requirement in the non-selective plating area is less than 60μm, existing dry film processes are caught in a dilemma: thick dry films can cover high steps but cannot resolve fine lines, while thin dry films can resolve fine lines but cannot completely cover high steps. This contradiction is particularly prominent in high-end products such as foldable screen phone hinge FPCs, high-density display module connection FPCs, and wearable device motherboard FPCs, which has long restricted the mass production capability of high-density fine FPC products.

[0033] To at least address the aforementioned problems, embodiments of the present invention provide a method for manufacturing a flexible circuit board and a flexible circuit board in which a wet film layer is pre-formed on the surface of the selectively plated copper layer to cover its top surface and edge sidewalls, and then pre-baked and shaped. A thin dry film, the shape of which is determined entirely based on the requirements for fine line resolution, is then laminated onto the wet film. This ensures reliable coverage of the selectively plated copper steps while achieving accurate resolution of fine lines in non-selectively plated areas, preventing the edges of the selectively plated copper layer from being etched during the etching process. Specific implementation methods are described below: One embodiment of the present invention is as follows: This embodiment is suitable for FPC products with a copper plating layer 3 thickness of 30μm (the copper plating layer 3 is located in the selected plating area) and a line spacing of 50μm in the non-selected plating area. The specific process steps are as follows: S1, please refer to 1 and Figure 2An FPC substrate 1 is provided, and a base copper layer 2 is formed on the surface of the FPC substrate 1. Specifically, the FPC substrate 1 is plated to form a 12μm thick base copper layer 2.

[0034] S2, please refer to 1 and Figure 3 A selectively plated copper layer 3 is formed on the surface of the base copper layer 2. Specifically, a 30 μm thick selectively plated copper layer 3 is formed through local selective plating, and the film is stripped after selective plating for later use. The difference between the thickness of the selectively plated copper layer 3 and the thickness of the base copper layer 2 is 18 μm.

[0035] S3, please refer to 1 and Figure 4 A wet film layer 4 is formed, which covers the surface of the selected copper plating layer 3 and extends to cover the sidewalls of the selected copper plating layer 3 until it reaches the surface of the FPC substrate 1. Specifically, a liquid photoresist (rotational viscosity ≥8000cps at 25°C, solid content ≥55wt%) is selected, and the wet film layer 4 is applied to the surface of the selected copper plating layer 3 by printing. The wet film layer 4 extends from the top surface of the selected copper plating layer 3 through the sidewalls to the outer edge of the FPC substrate 1 surface, horizontally extending outward by 0.4mm (i.e., Figure 9 (represented by the letter W in the text). When the wet film layer 4 is formed, a wet film oil overflow area 41 will be generated. This wet film oil overflow area 41 refers to the non-target area that the wet film spreads to due to abnormal flow during printing or subsequent processes.

[0036] S4. Perform a pre-baking operation on the wet film layer 4 to make the wet film layer 4 semi-cured. Specifically, place the FPC substrate 1 covered with the wet film layer 4 at 75°C for 20 minutes to bake, so that the wet film layer 4 reaches a semi-cured state where it does not shift when lightly touched with a finger and does not fray when the polyester film is torn.

[0037] S5, please refer to 1 and Figure 5 A dry film layer 5 is formed, which covers both the surface of the wet film layer 4 and the surface of the base copper layer 2. Specifically, a 25μm thick dry film is selected, and the entire board is laminated using a vacuum laminator. The process conditions are: lamination pressure of 0.45MPa, lamination speed of 0.5m / min, and pressure roller temperature of 95℃.

[0038] S6, please refer to 1 and Figure 6 The dry film layer 5 is exposed and developed to form a patterned dry film resist layer (i.e., the dry film layer 5 with the developed circuit pattern). After exposure and development, the dry film layer 5 above the selected copper plating layer 3 and the wet film layer 4 are retained simultaneously. The boundary of the wet film layer 4 covers the sidewall of the selected copper plating layer 3, and the boundary of the wet film layer 4 and the dry film layer 5 are vertically aligned. Specifically, ultraviolet light is used for exposure, and a weakly alkaline developer is used for development to form a preset circuit pattern. The spacing between adjacent lines in the circuit pattern is 50 μm.

[0039] S7, Please refer to 1 and Figure 7 Using the dry film resist layer as a mask, the base copper layer 2 is etched to remove the base copper layer 2 that is not covered by the dry film resist layer. Specifically, an etching solution is used for etching.

[0040] S8, Please refer to 1 and Figure 8 The dry film layer 5 and the wet film layer 4 are removed sequentially to obtain the finished FPC circuit board. Specifically, the dry film layer 5 is first removed using an alkaline stripping solution, and then the wet film layer 4 is removed using an organic solvent. After cleaning and drying, the finished flexible circuit board is obtained.

[0041] The finished product test results of this embodiment show that the 50μm fine lines are complete without open circuits or short circuits, the edges of the selected copper plating layer 3 are smooth without etching, and the product yield rate is significantly improved.

[0042] One embodiment of the present invention is as follows: This embodiment is suitable for flexible circuit board products with high-thickness selective plating and ultra-fine lines. The thickness of the selectively plated copper layer 3 is 50μm, and the line spacing in the non-selectively plated areas is 40μm. The process of this embodiment is basically the same as that of the first embodiment, except that: In step S1, please refer to Figure 2 The FPC substrate 1 is formed with a 12μm thick base copper layer 2 through a plate plating process.

[0043] In step S2, please refer to Figure 3 A 50μm thick selectively plated copper layer 3 is formed by selective plating in certain areas. The difference between the thickness of the selectively plated copper layer 3 and the thickness of the base copper layer 2 is 38μm (≥25μm).

[0044] In step S3, please refer to Figure 4 The wet film layer 4 covers the surface of the copper plating layer 3 and extends to cover the sidewalls of the copper plating layer 3 until it reaches the surface of the FPC substrate 1, horizontally extending outward by 0.5 mm (i.e., Figure 9 (represented by the letter W in the text), the wet film layer 4 has a printing thickness of 25μm.

[0045] In step S4, the baking temperature for the pre-baking operation is 85℃ and the baking time is 25min.

[0046] In step S5, based on the requirement of a fine line spacing of 40μm, a thin dry film with a thickness of 20μm is selected and the whole board is laminated using a vacuum laminator. The process conditions are: lamination pressure of 0.45MPa, lamination speed of 0.5m / min, and pressure roller temperature of 95℃.

[0047] In step S6, after exposure and development, a preset circuit pattern is precisely formed. The spacing between adjacent lines in the circuit pattern is 40μm. The dry film layer 5 above the copper plating layer 3 and the wet film layer 4 are retained simultaneously with flush boundaries.

[0048] The remaining process steps are the same as those in the first embodiment.

[0049] This embodiment can effectively smooth out 50μm ultra-high selective plating steps, the dry film layer 5 is uniformly bonded without defects, the 40μm ultra-fine circuit has high integrity, and the selective copper plating layer 3 has excellent edge protection effect.

[0050] One embodiment of the present invention is as follows: The process in this embodiment is basically the same as that in the first embodiment, except that: The boundary shape of the copper plating layer 3 is selected to be wavy.

[0051] In step S3, please refer to Figure 4 The wet film layer 4 extends from the surface of the selectively plated copper layer 3 through the sidewall of the selectively plated copper layer 3 to the surface of the FPC substrate 1 outside the edge of the selectively plated copper layer 3. The horizontal outward expansion width is measured with the position of the wave crest as a reference, and the horizontal outward expansion is 0.4 mm (i.e. Figure 9 The letter W is used to represent it.

[0052] The wavy boundary facilitates the discharge of air bubbles along the trough channel during the printing of the wet film layer 4 and the lamination of the dry film layer 5, reduces the probability of air bubble residue in the step area, and improves the consistency and density of the dry film layer 5.

[0053] The remaining process steps are the same as those in the first embodiment.

[0054] One embodiment of the present invention is as follows: The process in this embodiment is basically the same as that in the first embodiment, except that: The thickness of the selectively plated copper layer 3 is 45 μm, the thickness of the base copper layer 2 is 10 μm, and the difference between the thickness of the selectively plated copper layer 3 and the thickness of the base copper layer 2 is 35 μm (≥25 μm); the line spacing in the non-selectively plated area is 55 μm.

[0055] In step S3, the wet film layer 4 covers the surface of the selected copper plating layer 3 and extends to cover the sidewalls of the selected copper plating layer 3 until it reaches the surface of the FPC substrate 1, horizontally expanding outward by 0.35mm (i.e., Figure 9 The letter W is used to represent it.

[0056] In step S4, the baking temperature for the pre-baking operation is 65℃ and the baking time is 30 minutes.

[0057] In step S5, a 25μm thin dry film is selected, with a bonding pressure of 0.4MPa, a bonding speed of 0.5m / min, and a pressure roller temperature of 90℃.

[0058] In step S6, after exposure and development, a 55μm circuit pattern is formed. The dry film layer 5 above the copper plating layer 3 and the wet film layer 4 are retained simultaneously with flush boundaries.

[0059] The remaining process steps are consistent with those in the first embodiment. This embodiment also achieves effective coverage of high-selectivity plating steps and precise forming of fine lines.

[0060] One embodiment of the present invention is as follows: The process in this embodiment is basically the same as that in the first embodiment, except that: The boundary shape of the copper plating layer 3 is wavy, with a vertical distance of 0.25 mm from the crest to the trough and a wavelength of 0.5 mm.

[0061] In step S3, please refer to Figure 4 The wet film layer 4 covers the surface of the copper plating layer 3 and extends to cover the sidewalls of the copper plating layer 3 until it reaches the surface of the FPC substrate 1. The horizontal outward expansion width is measured with reference to the position of the wave crest, and the horizontal outward expansion is 0.45mm (i.e., Figure 9 The letter W is used to represent it.

[0062] In step S4, the baking temperature for the pre-baking operation is 80℃ and the baking time is 18min.

[0063] In step S5, the process conditions are: bonding pressure of 0.5 MPa, bonding speed of 0.5 m / min, and pressure roller temperature of 100℃.

[0064] The remaining process steps are the same as those in the first embodiment.

[0065] The wavy boundary, combined with optimized pre-baking and bonding parameters, further improves the consistency of dry film layer 5 bonding and product batch stability.

[0066] Please refer to Figures 1 to 9 As shown, one embodiment of the present invention is as follows: A flexible circuit board is prepared by the manufacturing method of the flexible circuit board in the above five embodiments.

[0067] In summary, the present invention provides a method for manufacturing a flexible circuit board and a flexible circuit board in which a wet film layer is formed. This wet film layer covers the surface of a selectively plated copper layer and extends to cover the sidewalls of the selectively plated copper layer up to the surface of the FPC substrate. The wet film layer completely covers the top surface and sidewalls of the selectively plated copper layer. Before significant flow occurs, the wet film layer is pre-baked to a semi-cured state, rapidly shaping the covering form and overcoming the problem of liquid wet film flowing and collapsing on raised surfaces due to gravity and surface tension. Simultaneously, this semi-cured state ensures sufficient structural strength for the subsequent dry film layer lamination process, preventing deformation due to pressing and ensuring the integrity of the covering structure. After the wet film layer completely covers the top surface and sidewalls of the selectively plated copper layer, the limitation of high steps on dry film layer lamination is eliminated. This allows the selection of the dry film layer to be based solely on the resolution requirements of fine lines in non-selectively plated areas, enabling the use of thin dry films to adapt to fine lines throughout the process. This resolves the contradiction between insufficient resolution of thick dry films and the failure of thin dry films to cover steps. After exposure and development, the dry film layer and wet film layer above the selective copper plating layer are retained simultaneously. The boundary of the wet film layer covers the sidewall of the selective copper plating layer, and the boundaries of the wet film layer and the dry film layer are vertically aligned, forming a reliable double-layer protection for the selective copper plating layer. This effectively prevents the etching solution from penetrating along the edge of the selective copper plating layer and avoids the edge of the selective copper plating layer being etched. It achieves a balance between high-step selective plating and precise resolution of fine lines, greatly improving product yield.

[0068] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for manufacturing a flexible circuit board, characterized in that, Includes the following steps: S1. Provide an FPC substrate and form a base copper layer on the surface of the FPC substrate; S2. A selective copper plating layer is formed, wherein the selective copper plating layer is disposed on the surface of the base copper layer; S3. A wet film layer is formed, which covers the surface of the selected copper plating layer and extends to cover the sidewalls of the selected copper plating layer until the surface of the FPC substrate. S4. Perform a pre-baking operation on the wet film layer to make the wet film layer semi-cured; S5. A dry film layer is formed, wherein the dry film layer covers the surface of the wet film layer and the surface of the base copper layer respectively; S6. Expose and develop the dry film layer to form a patterned dry film resist layer. After exposure and development, the dry film layer above the selected copper plating layer and the wet film layer are retained simultaneously. The boundary of the wet film layer covers the sidewall of the selected copper plating layer, and the boundaries of the wet film layer and the dry film layer are vertically aligned. S7. Using the dry film resist layer as a mask, perform an etching operation on the base copper layer to remove the base copper layer that is not covered by the dry film resist layer. S8. Remove the dry film layer and the wet film layer in sequence to obtain the finished flexible circuit board.

2. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, In step S3, the wet film layer covers the surface of the selectively plated copper layer and extends to cover the sidewalls of the selectively plated copper layer until the width of the FPC substrate surface is 0.3mm~0.5mm.

3. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, The boundary shape of the selected copper plating layer is wavy.

4. The method for manufacturing a flexible circuit board according to claim 3, characterized in that, In step S3, the wet film layer covers the surface of the selectively plated copper layer and extends to cover the sidewalls of the selectively plated copper layer until the width of the FPC substrate surface is measured with reference to the position of the wave crest.

5. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, The difference between the thickness of the selected copper layer and the thickness of the base copper layer is greater than or equal to 25 μm.

6. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, In step S4, the baking temperature for the pre-baking operation is 65℃~85℃, and the baking time for the pre-baking operation is 15min~30min.

7. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, In step S3, the wet film layer is formed by a liquid photoresist, the liquid photoresist has a rotational viscosity at 25°C greater than or equal to 8000 cps, and the liquid photoresist has a solid content greater than or equal to 55 wt%.

8. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, In step S5, the dry film layer is laminated using a vacuum laminator. The lamination process conditions are as follows: The bonding pressure is 0.4MPa~0.5MPa, the bonding speed is 0.5m / min, and the pressure roller temperature is 90℃~100℃.

9. The method for manufacturing a flexible circuit board according to claim 1, characterized in that, In step S8, a two-step stripping method is used to remove the film layer, specifically as follows: First, use an alkaline stripping solution to remove the dry film layer, and then use an organic solvent to remove the wet film layer.

10. A flexible circuit board, characterized in that, It is prepared by the method of manufacturing a flexible circuit board according to any one of claims 1 to 9.

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