A method for manufacturing a flexible wiring board

Abstract

The application provides a preparation method of a flexible circuit board, which comprises the following steps: S10: drilling: drilling holes on the flexible circuit board according to a preset design; S20: first copper deposition: forming a thin copper layer on the hole wall by chemical copper deposition; S30: first copper electroplating: electroplating copper on the flexible circuit board after the first copper deposition to form a thick copper layer on the board surface and the hole wall; S40: resin hole filling: filling resin in the through hole or the blind hole and solidifying the resin; S50: ceramic grinding: alternately grinding two board surfaces of the flexible circuit board after the step S40; S60: second copper deposition: forming a thin copper layer on the board surface and the resin surface of the filled hole after the step S50; and S70: second copper electroplating: fully electroplating the flexible circuit board after the step S60 to make the copper thickness of the board surface reach a requirement.

Description

Technical Field

[0001] This invention belongs to the technical field of flexible circuit boards, and specifically relates to a method for preparing a flexible circuit board. Background Technology

[0002] As electronic products trend towards lighter, thinner, and smaller designs, circuit board wiring space is becoming increasingly dense. To improve wiring space, the VIPPO (Via-in-Pad Plated Over) process is gaining popularity. The VIPPO process enables the creation of high-end HDI multilayer boards and the placement of high-speed / high-frequency circuits; or it facilitates miniaturized and thinner designs, making it suitable for highly integrated products such as mobile phones, wearable devices, and drones. It is currently a mainstream solution in high-tech fields such as 5G, AI, and automotive electronics.

[0003] However, the VIPPO process is currently only suitable for the production of rigid circuit boards and cannot be used for flexible circuit boards. This is because the material of flexible circuit boards is relatively soft, and during the ceramic grinding process, unevenness of the board surface can easily occur, leading to abnormal phenomena such as the exposure of the substrate during grinding. In addition, the stress of the ceramic mechanical brush can cause irregular local deformation of the flexible circuit board, making the prepared flexible circuit board unable to meet the design requirements.

[0004] Therefore, it is necessary to develop a method for fabricating flexible circuit boards to enable the application of VIPPO technology in flexible circuit boards. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a reliable method for preparing a flexible circuit board with good product quality.

[0006] This invention provides a method for preparing a flexible circuit board, which includes the following steps:

[0007] S10: Drilling: Drilling holes in the flexible circuit board according to the preset design;

[0008] S20: One-time copper plating: A thin copper layer is formed on the hole wall using chemical copper plating;

[0009] S30: First-time copper plating: After the first-time copper plating, the flexible circuit board is electroplated with copper to form a thick copper layer on the board surface and hole walls;

[0010] S40: Resin plugging: Resin is used to fill through holes or blind holes and then cured.

[0011] S50: Ceramic grinding plate: The two surfaces of the flexible circuit board after step S40 are alternately ground;

[0012] S60: Secondary copper plating: A thin copper layer is formed on the board surface and the surface of the plugging resin after the polishing in step S50.

[0013] S70: Secondary copper plating: The flexible circuit board after step S60 is electroplated throughout to ensure that the copper thickness on the board meets the requirements.

[0014] Preferably, the S50 ceramic grinding plate step includes the following steps:

[0015] The flexible circuit board is fixedly connected to the rigid fixture, and the first surface of the flexible circuit board is polished on one side.

[0016] Separate the flexible circuit board and the rigid fixture. After flipping the flexible circuit board over, fix the first board surface, which has been polished, to the rigid fixture.

[0017] The second surface of the flexible circuit board is polished on one side only.

[0018] Separate the flexible circuit board from the rigid fixture, and complete the grinding of both surfaces of the flexible circuit board.

[0019] Preferably, when fixing the flexible circuit board to the rigid fixture, solder paste is used to fix the flexible circuit board and the rigid fixture.

[0020] Preferably, the step of fixing the flexible circuit board to the rigid fixture includes the following steps:

[0021] Silkscreen solder paste is applied to the surface of the rigid fixture.

[0022] The flexible circuit board is attached to a rigid fixture printed with solder paste;

[0023] A flat cover plate is pressed onto the flexible circuit board, and the solder paste is melted and solidified by the reflow line to fix the flexible circuit board to the rigid fixture.

[0024] Remove the flat cover plate.

[0025] Preferably, the solder paste is a low-temperature solder paste, and the melting temperature of the solder paste is 135℃-145℃.

[0026] Preferably, the step of separating the flexible circuit board and the rigid fixture includes the following steps:

[0027] Place the rigid fixture and flexible circuit board on the heating platform and heat them to the solder paste melting temperature.

[0028] Separate rigid fixtures and flexible circuit boards;

[0029] Use solder paste stripping solution to remove solder paste from flexible circuit boards and rigid fixtures.

[0030] Preferably, the size of the rigid fixture is larger than the size of the flexible circuit board; when the flexible circuit board is placed in the middle of the rigid fixture, the distance between the four edges of the flexible circuit board and the corresponding four edges of the rigid fixture is greater than 10mm.

[0031] Preferably, the rigid fixture is a copper-clad laminate.

[0032] Preferably, after the ceramic grinding plate treatment in step S50, the overall expansion / contraction rate of the flexible circuit board is less than 0.02%.

[0033] Compared with existing technologies, the flexible circuit board fabrication method of this solution has at least the following advantages:

[0034] 1. The flexible circuit board preparation method provided in this solution uses a single-sided grinding method to alternately grind both sides of the flexible circuit board during the ceramic grinding process. Specifically, the flexible circuit board is first fixedly connected to a rigid fixture, and then the first side of the flexible circuit board is ground. After grinding, the flexible circuit board and the rigid fixture are separated, the flexible circuit board is flipped over, and then fixedly connected to the rigid fixture again. Then the second side of the flexible circuit board is ground. With the assistance of the rigid fixture and the alternating single-sided grinding method, the flexible circuit board will not have unevenness or deformation problems during the ceramic grinding process, resulting in a high yield of the flexible circuit board. Attached Figure Description

[0035] The above and other objects, features, and advantages of the invention will become clearer through a more detailed description of the preferred embodiments illustrated in the accompanying drawings. The same reference numerals denote the same parts throughout the drawings, and the drawings are not intentionally drawn to scale with actual dimensions; the focus is on illustrating the gist of the invention.

[0036] Figure 1 A schematic flowchart illustrating the method for fabricating a flexible circuit board according to an embodiment of the present invention;

[0037] Figure 2 A schematic diagram of the module for a method of fabricating a flexible circuit board provided in an embodiment of the present invention;

[0038] Figure 3 A schematic diagram showing the connection between the flexible circuit board and the rigid fixture in the ceramic grinding step of the method for preparing a flexible circuit board according to an embodiment of the present invention.

[0039] Figure 4 This is a schematic diagram of the modules for the steps of Specific Embodiment 1. Detailed Implementation

[0040] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and implement it. However, the embodiments are not intended to limit the present invention. In this embodiment, it should be understood that the terms "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0041] It should be noted that when one element is considered to be "connected" to another element, it can be directly connected to and integrated with the other element, or there may be an intervening element present. The terms "mounted," "one end," "the other end," and similar expressions used in this invention are for illustrative purposes only.

[0042] Please refer to Figures 1-2 This invention provides a method for preparing a flexible circuit board, which includes the following steps:

[0043] S10: Drilling: Drilling holes in the flexible circuit board according to a preset design. Understandably, before drilling, a drilling design file is created based on requirements and imported into the drilling equipment to achieve precise drilling of the flexible circuit board. The drilling process is used to form physical channels for vias on the flexible circuit board, serving as the basis for subsequent interlayer electrical connections.

[0044] S20: First-stage copper plating: A thin copper layer is formed on the hole wall using chemical copper plating. Understandably, a first-stage copper plating process is performed on the flexible circuit board after drilling. Chemical copper plating allows the copper layer to uniformly cover the hole wall. For example, palladium colloid can be adsorbed onto the insulating hole wall. Under palladium catalysis, copper ions are reduced to copper, thus covering the hole wall. In this embodiment, the thickness of the thin copper layer is 0.2-0.5 μm. First-stage copper plating makes the insulating hole wall conductive and provides a conductive seed layer for subsequent copper electroplating processes.

[0045] S30: Single-stage copper plating: This involves electroplating copper onto the flexible circuit board after the initial copper plating, forming a thick copper layer on the board surface and hole walls. Essentially, performing a single-stage copper plating process on the entire flexible circuit board after the initial copper plating process thickens the thin copper layer formed in the initial copper plating process into a thick copper layer with a thickness of 5-10 μm. This single-stage copper plating thickens the copper layer, enhancing conductivity and providing a sufficiently strong substrate for subsequent resin plugging, preventing the copper layer on the hole walls from thinning during the plugging process.

[0046] S40: Resin plugging: Resin is used to fill through holes or blind holes and then cured. It is understood that resin plugging can fill through holes, which can prevent chemical residues from remaining in the through holes during subsequent electroplating, leading to corrosion and other problems. On the other hand, it can improve the reliability, heat resistance and pressure resistance of flexible circuit boards.

[0047] S50: Ceramic Grinding Plate: The two surfaces of the flexible circuit board processed in step S40 are alternately ground. It can be understood that the ceramic grinding plate uses a brush roller containing ceramic abrasive to grind the surface of the flexible circuit board. It is used to grind the resin bumps that protrude after the resin plugs the holes to smooth them out, and to grind away the oxide layer, impurities and glue residue left in the previous process. On the other hand, it can also slightly roughen the surface of the board, providing a clean, rough and strong bonding surface for the subsequent secondary copper plating.

[0048] S60: Secondary copper plating: A thin copper layer is formed on the board surface and the via-filling resin surface after grinding in step S50. It is understandable that after the ceramic grinding process is completed, part of the copper layer on the board surface and the bumps in the resin area will be ground off, making the resin area completely non-conductive. Therefore, secondary copper plating is required on the flexible circuit board to make the via-filling resin surface and the entire surface of the flexible circuit board conductive again.

[0049] S70: Secondary copper plating: The flexible circuit board processed in step S60 is subjected to full-board electroplating to ensure the copper thickness on the board surface meets the requirements. Understandably, secondary copper plating thickens the copper layer on the surface of the flexible circuit board, achieving the design requirements such as 1 oz or 2 oz; and ultimately forms a stable and reliable conductive layer, ensuring circuit continuity, high current carrying capacity, thermal shock resistance, and high reliability.

[0050] The above steps can complete the processing and preparation of flexible circuit boards, solving problems such as exposed substrate and abnormal dimensional expansion and contraction after the ceramic grinding process, thus improving product quality and yield.

[0051] In this embodiment, the S50 ceramic grinding plate step includes the following steps:

[0052] The flexible circuit board is fixedly connected to the rigid fixture, and the first surface of the flexible circuit board is polished on one side.

[0053] Separate the flexible circuit board and the rigid fixture. After flipping the flexible circuit board over, fix the first board surface, which has been polished, to the rigid fixture.

[0054] The second surface of the flexible circuit board is polished on one side only.

[0055] Separate the flexible circuit board from the rigid fixture, and complete the grinding of both surfaces of the flexible circuit board.

[0056] Understandably, in step S50, when grinding the ceramic slab, fixing the flexible circuit board to the rigid fixture prevents deformation and unevenness caused by the brush rollers. For example, the flexible circuit board is laid flat on the rigid fixture and fixedly connected to it to prevent loosening or displacement during grinding. After the flexible circuit board is fixed, the first surface facing upwards is ground. After grinding the first surface, the connection between the flexible circuit board and the rigid fixture is separated, and both are cleaned. The ground first surface is then fixed downwards to the rigid fixture, with the second surface facing upwards, and the second surface is ground. After grinding the second surface, the flexible circuit board and rigid fixture are separated again and cleaned to complete the ceramic grinding process for the flexible circuit board. By using a single-sided alternating grinding method in conjunction with the assistance of the rigid fixture, the flexible circuit board is prevented from deforming during ceramic grinding, reducing its expansion and contraction rate and keeping it within acceptable limits.

[0057] Please refer to Figure 3 In this embodiment, when the flexible circuit board 1 is fixedly connected to the rigid fixture 3, solder paste is used to fix the flexible circuit board 1 and the rigid fixture 3. It is understood that solder paste has good fluidity after being heated and melted. When the flexible circuit board 1 is connected to the rigid fixture 3 with solder paste and the solder paste is heated, the molten solder paste can automatically fill the tiny gaps between the flexible circuit board 1 and the rigid fixture 3, forming a uniform, continuous and strong solder paste layer 2, which tightly adheres the flexible circuit board 1 to the rigid fixture 3. After the solder paste solidifies, the flexible circuit board 1 forms a flat rigid plane, which can ensure uniform grinding during polishing, making the flexible circuit board 1 uniform in thickness, with good through-hole flatness, no height difference and no deformation. Furthermore, solder paste has the advantages of easy unloading, easy cleaning, and no damage to the board. In this embodiment, the surface of the flexible circuit board 1 needs to be polished alternately. Therefore, the flexible circuit board 1 needs to be fixedly connected to the rigid fixture 3 and unloaded twice. Unloading can be completed by heating the solder paste. It will not corrode the flexible circuit board. After cleaning, there will be no residue or damage to the circuit and substrate.

[0058] In a preferred embodiment, the solder paste is a low-temperature solder paste with a melting temperature of 135°C-145°C. Using low-temperature solder paste reduces processing difficulty. The heating temperature is not high during the connection and removal of the flexible circuit board from the rigid fixture, further ensuring production safety and reducing the impact of heating temperature on the flexible circuit board, thus preventing damage.

[0059] In a preferred embodiment, the step of fixing the flexible circuit board to the rigid fixture includes the following steps:

[0060] Silkscreen solder paste is applied to the surface of the rigid fixture.

[0061] The flexible circuit board is attached to a rigid fixture printed with solder paste;

[0062] A flat cover plate is pressed onto the flexible circuit board, and the solder paste is melted and solidified by the reflow line to fix the flexible circuit board to the rigid fixture.

[0063] Remove the flat cover plate.

[0064] Understandably, screen printing allows solder paste to be evenly spread on a rigid fixture. When the flexible circuit board is attached to the rigid fixture and the solder paste is heated, the uniformity of the solder paste between the flexible circuit board and the rigid fixture can be ensured, thus improving the flatness of the flexible circuit board. The flat cover plate can further ensure the flatness of the flexible circuit board after it is fixedly connected, preventing problems such as warping and wrinkling, and laying the foundation for subsequent uniform ceramic polishing.

[0065] In a preferred embodiment, the step of separating the flexible circuit board and the rigid fixture includes the following steps:

[0066] Place the rigid fixture and flexible circuit board on the heating platform and heat them to the solder paste melting temperature.

[0067] Separate rigid fixtures and flexible circuit boards;

[0068] Use solder paste stripping solution to remove solder paste from flexible circuit boards and rigid fixtures.

[0069] Understandably, thanks to the easy removal and cleaning properties of solder paste, flexible circuit boards can be easily separated from rigid fixtures. After cleaning with solder paste removal solution, it can be ensured that there is no solder paste residue on the flexible circuit board and the flexible circuit board will not be damaged.

[0070] In this embodiment, the size of the rigid fixture is larger than the size of the flexible circuit board. When the flexible circuit board is placed in the middle of the rigid fixture, the distance between the four edges of the flexible circuit board and the corresponding four edges of the rigid fixture is greater than 10mm. It is understood that when the flexible circuit board is centered on the surface of the rigid fixture, the platform of the rigid fixture must be larger than the flexible circuit board, and a distance of at least 10mm must be maintained on all four sides. This ensures that when the flexible circuit board and the rigid fixture are fixedly connected, especially when the flat cover plate is pressed onto the flexible circuit board and solder paste is cured, even if the flexible circuit board shifts, it will remain completely attached to the surface of the rigid fixture and will not bulge out from the four sides of the rigid fixture.

[0071] In this embodiment, the rigid fixture is a copper-clad laminate.

[0072] In this embodiment, after the ceramic grinding process in step S50, the overall expansion / contraction rate of the flexible circuit board is less than 0.02%. It is understood that the expansion / contraction rate of the flexible circuit board is measured by a two-dimensional measuring device, which measures the change in the overall board size. The expansion / contraction rate is calculated as: expansion / contraction rate = (size after ceramic grinding - size before ceramic grinding) / size before ceramic grinding. Under existing conventional techniques, after ceramic grinding, the expansion / contraction rate of the flexible circuit board will be greater than 0.010, and the change in expansion / contraction rate is irregular, varying due to issues such as flatness and warping of the flexible circuit board during each ceramic grinding process. Furthermore, due to the unevenness of the flexible circuit board surface, localized areas may expose the substrate during grinding, resulting in scrap. The method described in this solution ensures the flatness of the flexible circuit board during ceramic grinding, resulting in an expansion / contraction rate of less than 0.02% before and after the ceramic grinding process, demonstrating significant effectiveness.

[0073] To further illustrate the ceramic grinding plate step in this solution in detail, a specific embodiment is provided:

[0074] Example 1

[0075] Please refer to Figure 4 The specific implementation steps for ceramic grinding plates of flexible circuit boards:

[0076] 1) A 2.0mm thick FR4 copper-clad laminate is used as a rigid fixture for the auxiliary flexible circuit board ceramic grinding plate, and its external dimensions are 10mm larger than the four sides of the flexible circuit board.

[0077] 2) Silkscreen low-temperature solder paste (melting temperature 140℃) onto the surface of the FR4 copper-clad laminate mentioned above.

[0078] 3) Lay the flexible circuit board onto the FR4 copper-clad board printed with solder paste;

[0079] 4) A flat aluminum cover plate is pressed onto the flexible circuit board, and the solder paste is melted and cured by the reflow line to fix the flexible circuit board to the FR4 copper-clad board.

[0080] 5) Remove the flat aluminum cover plate;

[0081] 6) The first surface of the flexible circuit board is polished using a single-sided ceramic grinding plate.

[0082] 7) After polishing, place the FR4 copper-clad laminate and the flexible circuit board together on the heating platform and heat to 140°C. The solder paste will remelt due to the heat, and the flexible circuit board and the FR4 copper-clad laminate will be separated.

[0083] 8) Use solder paste removal solution to remove solder paste from flexible circuit boards and FR4 copper-clad laminates;

[0084] 9) Screen print low-temperature solder paste (melting temperature 140℃) on the surface of the FR4 copper-clad board again.

[0085] 10) Attach the polished first surface of the flexible circuit board to the FR4 copper-clad board printed with solder paste;

[0086] 11) A flat aluminum cover plate is pressed onto the flexible circuit board, and the solder paste is melted and cured by the reflow line to fix the flexible circuit board to the FR4 copper-clad board.

[0087] 12) Remove the flat aluminum cover plate;

[0088] 13) The second surface of the flexible circuit board is polished using a single-sided ceramic grinding plate.

[0089] 14) After polishing, place the FR4 copper-clad laminate and the flexible circuit board together on the heating platform and heat to 140°C. The solder paste will remelt due to the heat, and the flexible circuit board and the FR4 copper-clad laminate will be separated.

[0090] 15) Use solder paste stripping solution to remove solder paste from flexible circuit boards and FR4 copper-clad laminates;

[0091] 16) After the above steps, the ceramic grinding plate for the flexible circuit board is made.

[0092] In this specification, unless otherwise expressly specified and limited, "above" or "below" the second feature can mean that the first and second features are in direct contact, or that the first and second features are in indirect contact through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0093] In the description of this specification, the references to terms such as "preferred embodiment," "another embodiment," "other embodiment," or "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0094] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for fabricating a flexible circuit board, characterized in that, Includes the following steps: S10: Drilling: Drilling holes in the flexible circuit board according to the preset design; S20: One-time copper plating: A thin copper layer is formed on the hole wall using chemical copper plating; S30: First-time copper plating: After the first-time copper plating, the flexible circuit board is electroplated with copper to form a thick copper layer on the board surface and hole walls; S40: Resin plugging: Resin is used to fill through holes or blind holes and then cured. S50: Ceramic grinding plate: The two surfaces of the flexible circuit board after step S40 are alternately ground; S60: Secondary copper plating: A thin copper layer is formed on the board surface and the surface of the plugging resin after the polishing in step S50. S70: Secondary copper plating: The flexible circuit board after step S60 is electroplated throughout to ensure that the copper thickness on the board meets the requirements.

2. The method for preparing a flexible circuit board as described in claim 1, characterized in that, The S50 ceramic grinding process includes the following steps: The flexible circuit board is fixedly connected to the rigid fixture, and the first surface of the flexible circuit board is polished on one side. Separate the flexible circuit board and the rigid fixture. After flipping the flexible circuit board over, fix the first board surface, which has been polished, to the rigid fixture. The second surface of the flexible circuit board is polished on one side only. Separate the flexible circuit board from the rigid fixture, and complete the grinding of both surfaces of the flexible circuit board.

3. The method for preparing a flexible circuit board as described in claim 2, characterized in that, When fixing the flexible circuit board to the rigid fixture, solder paste is used to fix the flexible circuit board and the rigid fixture.

4. The method for preparing a flexible circuit board as described in claim 3, characterized in that, The step of fixing the flexible circuit board to the rigid fixture includes the following steps: Silkscreen solder paste is applied to the surface of the rigid fixture. The flexible circuit board is attached to a rigid fixture printed with solder paste; A flat cover plate is pressed onto the flexible circuit board, and the solder paste is melted and solidified by the reflow line to fix the flexible circuit board to the rigid fixture. Remove the flat cover plate.

5. The method for preparing a flexible circuit board as described in claim 3, characterized in that, The solder paste is a low-temperature solder paste, and the melting temperature of the solder paste is 135℃-145℃.

6. The method for preparing a flexible circuit board as described in claim 2, characterized in that, The steps for separating the flexible circuit board from the rigid fixture include the following: Place the rigid fixture and flexible circuit board on the heating platform and heat them to the solder paste melting temperature. Separate rigid fixtures and flexible circuit boards; Use solder paste stripping solution to remove solder paste from flexible circuit boards and rigid fixtures.

7. The method for preparing a flexible circuit board as described in claim 2, characterized in that, The size of the rigid fixture is larger than that of the flexible circuit board; when the flexible circuit board is placed in the middle of the rigid fixture, the distance between the four edges of the flexible circuit board and the corresponding four edges of the rigid fixture is greater than 10mm.

8. The method for preparing a flexible circuit board as described in claim 2, characterized in that, The rigid fixture is a copper-clad laminate.

9. The method for preparing a flexible circuit board as described in claim 1, characterized in that, After the ceramic grinding process in step S50, the overall expansion and contraction rate of the flexible circuit board is less than 0.02%.

Images