A cover opening and windowing method for rigid-flex printed circuit board

Abstract

The application discloses a kind of rigid-flex combination board uncovering windowing method, comprising the following steps: making the inner layer circuit of soft board core plate, soft board core plate includes soft board area and soft-hard combination area;In soft board area, cover film and protective tape are sequentially aligned and attached;Soft board core plate and copper foil are compressed into first sub-board with PP, and before stacking, the periphery corresponding to the soft board area on PP is cut through by laser, to form a cutting seam;Make circuit on first sub-board, and remove copper layer on first sub-board corresponding to soft board area;Then, each layer of circuit layer is made layer by layer, and the copper foil corresponding to the soft board area in each layer is etched off;Cutting is carried out by laser, and the waste material part and protective tape on the soft board area are removed by uncovering;Surface treatment and forming are carried out again to obtain rigid-flex combination board.The method solves the problem that rigid-flex combination circuit board cannot use mechanical depth control uncovering and laser cutting uncovering due to the lack of hard board core plate in laminated structure and the design of windowing pads in flexible area.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing technology, specifically to a method for opening and removing the cover of a rigid-flex board. Background Technology

[0002] Rigid-flex PCBs, also known as flexible PCBs, are composed of rigid and flexible boards selectively laminated together. Their compact structure uses metallized vias to form conductive connections. Rigid-flex PCBs combine the regularity and toughness of conventional rigid boards with the flexibility and suppleness of flexible boards, allowing for movement, bending, folding, and twisting, and enabling three-dimensional wiring. This significantly saves installation space for electronic components, aligning with the trend towards "lighter, thinner, shorter, and smaller" electronic products. In the past decade or so, they have been widely used in numerous fields such as military, aerospace, automotive electronics, medical electronics, and consumer electronics, becoming one of the fastest-growing types of PCB products and representing one of the main future development directions for PCBs.

[0003] Opening the window is a core process in rigid-flex PCB manufacturing. There are several methods for opening windows, among which the most widely used are mechanical depth control and laser windowing. Mechanical depth control involves pre-fabricating a blind groove on the rigid plate adjacent to the flexible plate. After pressing, the final form uses mechanical depth control milling to align with the blind groove, removing the rigid plate at the window location to expose the flexible plate. Laser cutting utilizes the high-precision depth control cutting capabilities of a UV laser cutter to remove the rigid plate at the window location, exposing the flexible plate.

[0004] Conventional mechanical depth control and laser cutting methods have limitations. They are only suitable for rigid-flex boards with ordinary rigid core boards and flexible core boards laminated together. They are not suitable for high-density interconnect rigid-flex boards with flexible core boards and copper foil laminated layer by layer, boards without rigid core boards, or boards with windowed pads in the flexible area. Without a rigid core board, the PP and copper foil are too thin to perform depth control milling of blind slots, and mechanical depth control processes cannot be used for uncovering. Laser cutting requires precise adjustment of the appropriate laser energy according to the window thickness to ensure that the rigid layer adjacent to the flexible board is cut through without damaging it. For flexible layers, dummy copper pads are generally designed in adjacent rigid layers. After the window is opened, the dummy copper pads are etched away using an etching method. For areas with pads in the window, etching is not possible, so laser cutting cannot be used simply. In addition, when the flexible layer is laminated with the copper foil, to avoid corrosion of the flexible area's circuits and pads due to copper foil wear during pretreatment and the entry of chemicals during the etching process, PP cannot be opened. However, not opening windows in PP also requires avoiding adhesive flow and powder falling onto the flexible layer, which would affect quality and appearance. This is also a technical challenge. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a method for opening windows on rigid-flex boards, which solves the problem that high-density interconnect rigid-flex boards cannot be opened using mechanical depth control or laser cutting due to the absence of a rigid core board in the laminated structure and the presence of windowed pads in the flexible area.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for opening a window on a rigid-flexible bonded plate, comprising the following steps:

[0007] S1. Fabricating the inner layer circuitry of a flexible circuit board core; the flexible circuit board core includes a flexible area and a rigid-flex area;

[0008] S2. Align and attach the cover film to the flexible board area on the flexible board core board;

[0009] S3. Apply protective tape to the covering film;

[0010] S4. Press the flexible core board and copper foil together with PP to form the first sub-board. Before stacking, laser cut through the perimeter of the area corresponding to the flexible board on the PP to form a slit, and fill the slit with PP glue during the pressing process.

[0011] S5. Fabricate the circuit on the first daughterboard and remove the copper layer in the corresponding flexible board area on the first daughterboard.

[0012] S6. Copper foil is laminated layer by layer on both surfaces of the first sub-board to accumulate the required number of circuit layers to form the production board. After each copper foil lamination, the circuit is fabricated and the copper layer in the corresponding flexible board area is removed.

[0013] S7. Fabricate a solder mask layer on the production board;

[0014] S8. Then, the laser is used to cut the production board at the position corresponding to the cut, the cover is removed to remove the waste material on the corresponding soft board area, and the protective tape on the soft board area is removed.

[0015] S9. Finally, the production board is surface treated and shaped to obtain a rigid-flexible composite board.

[0016] Furthermore, in step S2, the covering film is 0.5 mm larger on one side than the flexible plate area.

[0017] Furthermore, steps S2 and S3 include the following steps:

[0018] S21. The cover film and the flexible core board are completely bonded together by a rapid pressing method;

[0019] S22. Bake the flexible core board to cure the covering film.

[0020] Furthermore, in step S21, the parameters for rapid pressing are: temperature 180℃, pressure 100KG, and pressing time 1-2min.

[0021] Furthermore, in step S4, the width of the slit is 0.025 mm.

[0022] Furthermore, when the required number of circuit layers on the production board is eight, step S6 includes:

[0023] S61. Press the first sub-board and copper foil together with PP to form the second sub-board;

[0024] S62. Fabricate circuitry on the second daughterboard and remove the copper layer from the corresponding flexible board area on the second daughterboard.

[0025] S63. Press the second sub-board and copper foil together with PP to form a production board;

[0026] S64. Fabricate circuits on the production board and remove the copper layer in the corresponding flexible circuit board area on the production board.

[0027] Furthermore, in step S61, before the laminated plates are pressed together, a slit is formed on the periphery of the area corresponding to the flexible plate on the PP by laser cutting, and the slit is filled by PP adhesive during the pressing process.

[0028] Furthermore, when the required number of circuit layers on the production board is ten or more, repeat steps S63 and S64 until the required number of circuit layers is reached.

[0029] Furthermore, in step S8, the laser is used to cut inward to the kerf.

[0030] Furthermore, the following steps are included between steps S8 and S9:

[0031] S81. Remove the film from the production board.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] Before lamination, a protective tape is applied to the cover film of the flexible circuit board core. The PP to be laminated is not opened. Laser cutting is used to cut through the PP adjacent to the flexible circuit board core, forming a separation slit to facilitate later removal. However, the cut-through and separated part is not removed and is laminated together. During the lamination process, the PP glue flows to fill the slit, avoiding the problem of chemical injection into the flexible circuit area and corroding the circuit and pads in later processes. It also solves the problem of PP glue overflow and powder falling into the flexible circuit area. After the flexible circuit board core and copper foil are laminated, the copper foil in the corresponding flexible circuit area is etched away, leaving only PP, completing the first lamination structure. Then, each circuit layer is made layer by layer, and the copper foil in the corresponding flexible circuit area in each layer is etched away. Since the copper foil in each layer in the corresponding flexible circuit area has been removed during the circuit etching, only the PP layer remains. Therefore, only low-energy laser cutting is needed to burn through to the PP slit to achieve the cover removal, which greatly reduces the difficulty of laser cutting and eliminates the need to design a false copper pad buffer to successfully produce a rigid-flex board.

[0034] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and will become apparent from the description or may be learned by practice of the invention. Detailed Implementation

[0035] To better understand the technical content of the present invention, the present invention will be further introduced and described below in conjunction with specific embodiments. It should be noted that if there are descriptions such as "first" and "second" in the text, they are used to distinguish different components, plates, etc., and do not represent the order of priority, nor do they limit "first" and "second" to different types.

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0037] Example

[0038] This embodiment provides a method for manufacturing a ten-layer rigid-flexible composite panel, which includes a process of opening a window by removing the cover, and includes the following processing steps in sequence:

[0039] (1) Cutting: Cut flexible core board, copper foil and PP according to the panel size of 520mm×620mm. The thickness of flexible core board is 0.075mm. The copper layer thickness on both surfaces of flexible core board is 0.5oz. The thickness of copper foil is 0.5oz. The flexible core board includes a flexible board area and a flexible-rigid bonding area.

[0040] (2) Inner layer circuit fabrication (negative film process): Inner layer pattern transfer, photosensitive film is coated using a vertical coating machine, and the film thickness is controlled at 8μm. The inner layer circuit is exposed on the flexible core board using a fully automatic exposure machine with 5-6 exposure rulers (21 exposure rulers); Inner layer etching, the inner layer circuit is etched on the exposed and developed flexible core board, and the inner layer line width is measured to be 3mil. The inner layer circuit includes the pads located in the flexible board area; Inner layer AOI, and then the inner layer circuit is inspected for defects such as open circuits, short circuits, line gaps, and line pinholes. Defective products are scrapped, and defect-free products are sent to the next process.

[0041] (3) Browning: A brown oxide layer is generated on the copper layer surface of the flexible board core through a chemical reaction, which increases the roughness of the copper surface and enhances the bonding force with the cover film during lamination.

[0042] (4) Applying cover film: The cover film is aligned and applied to the flexible board area on the flexible board core board, and the cover film is 0.5mm larger on each side than the flexible board area. Of course, the cover film has a window at the corresponding pad position to expose the pad; the cover film is made of polyimide (PI material).

[0043] Before lamination, according to the shape of the flexible board area, a window is made on the cover film of the same size as the flexible board core board to correspond to the rigid-flex bonding area.

[0044] (5) Rapid pressing: The adhesive layer (epoxy resin) of the cover film is completely bonded to the flexible board area of ​​the flexible board core board in a short time through high temperature and high pressure, so as to protect the flexible board area. The parameters for rapid pressing are: temperature 180℃, pressure 100KG, and pressing time 1-2min.

[0045] (6) Baking plate: The soft core board is baked at 150℃ for 1 hour to completely cure the covering film.

[0046] (7) Apply protective tape: Apply protective tape to the area corresponding to the flexible board on the cover film. The size of the protective tape is the same as the size of the flexible board area and the tape is aligned and overlapped, so that the protective tape can be removed after the cover is removed.

[0047] (8) Laser cutting: The perimeter of the PP corresponding to the soft board area is cut through by laser cutting to form a 0.025mm slit, so that the waste material part that needs to be uncovered later is separated from the PP.

[0048] (9) Browning: A brown oxide layer is generated on the copper layer surface of the flexible board core through a chemical reaction, which increases the roughness of the copper surface and enhances the bonding force with PP during lamination.

[0049] (10) First lamination: After the flexible circuit board core and copper foil are pre-stacked together with PP (the specific board arrangement order from top to bottom is copper foil, PP, flexible circuit board core, PP, copper foil), the first sub-board is formed after lamination. The PP is the PP sheet with slits mentioned above. Since the slit size formed by laser cutting is relatively small, the slit can be filled by PP glue flow during the lamination process. Thus, after lamination, the entire flexible circuit board area is protected by PP. In conjunction with the protective tape, it can avoid the problem of chemical solution entering the flexible circuit board area and corroding the circuit and pads in subsequent processes. At the same time, it solves the problem of PP glue overflow and powder falling into the flexible circuit board area. Of course, in order to reduce the glue overflow problem, non-glue PP can be used for the first lamination.

[0050] (11) Drilling: Based on the drilling data, drill holes on the first sub-board using mechanical drilling.

[0051] (12) Copper plating: A thin layer of copper is deposited on the hole wall through a chemical reaction to provide a basis for the subsequent full-board electroplating. The backlight test is level 10, and the copper plating thickness in the hole is 0.5μm.

[0052] (13) Full board electroplating: Based on the mechanism of electrochemical reaction, a layer of copper is electroplated on the basis of copper plating to ensure that the copper thickness of the hole meets the product requirements. The electroplating parameters are set according to the completed copper thickness of the hole.

[0053] (14) Inner layer circuit fabrication (negative film process): Inner layer pattern transfer, photosensitive film is coated using a vertical coating machine, and the film thickness is controlled at 8μm. A fully automatic exposure machine is used, and the inner layer circuit is exposed on the first daughter board with 5-6 exposure rulers (21 exposure rulers); Inner layer etching, the inner layer circuit is etched on the first daughter board after exposure and development. The inner layer line width is measured to be 3mil. During etching, the copper layer in the corresponding flexible board area on the first daughter board is removed at the same time; Inner layer AOI, and then the inner layer circuit is inspected for defects such as open circuits, short circuits, line gaps, and line pinholes. Defective products are scrapped, and defect-free products are sent to the next process.

[0054] (15) Second pressing: First, the first sub-board is browned, and then the first sub-board and copper foil are pre-stacked together with PP (the specific board arrangement order from top to bottom is copper foil, PP, first sub-board, PP, copper foil). After pressing, the second sub-board is formed. The PP is also the PP sheet with slits mentioned above. During the pressing process, the slits can be filled by PP glue flow. The two inner layers of PP adjacent to the flexible core board are cut through to form slits. This is mainly because the PP and copper foil are too thin, and the precision requirements for the later laser depth control cutting are high. The later laser cutting only needs to cut through to the slit to form a butt joint before the cover can be removed. Forming slits on the two inner layers of PP can increase the overall thickness of the slits, thereby reducing the precision requirements for laser depth control cutting and avoiding burning the inner protective tape and cover film.

[0055] (16) Laser drilling: Based on the drilling data, use laser drilling to drill blind holes on the second sub-board.

[0056] (17) Copper plating: A thin layer of copper is deposited on the hole wall through a chemical reaction to provide a basis for the subsequent full-board electroplating. The backlight test is level 10, and the copper plating thickness in the hole is 0.5μm.

[0057] (18) Full board filling electroplating: Based on the mechanism of electrochemical reaction, a layer of copper is electroplated on the basis of copper plating to ensure that the copper thickness of the hole meets the product requirements. The electroplating parameters are set according to the completed copper thickness.

[0058] (19) Inner layer circuit fabrication (negative film process): Inner layer pattern transfer, photosensitive film is coated using a vertical coating machine, and the film thickness is controlled at 8μm. A fully automatic exposure machine is used, and the inner layer circuit is exposed on the second daughter board with 5-6 exposure rulers (21 exposure rulers); Inner layer etching, the inner layer circuit is etched on the second daughter board after exposure and development. The inner layer line width is measured as 3mil. During etching, the copper layer in the corresponding flexible board area on the second daughter board is removed at the same time; Inner layer AOI, and then the inner layer circuit is inspected for defects such as open circuits, short circuits, line gaps, and line pinholes. Defective products are scrapped, and defect-free products are sent to the next process.

[0059] (20) Third pressing: First, the second sub-board is browned, and then the second sub-board and copper foil are pre-stacked together with PP (the specific board arrangement order from top to bottom is copper foil, PP, second sub-board, PP, copper foil), and then pressed to form the third sub-board. The PP is a PP sheet without slits.

[0060] (21) Laser drilling: Based on the drilling data, use laser drilling to drill blind holes on the third sub-board.

[0061] (22) Copper plating: A thin layer of copper is deposited on the hole wall through a chemical reaction to provide a basis for the subsequent full-board electroplating. The backlight test is level 10, and the copper plating thickness in the hole is 0.5μm.

[0062] (23) Full board filling electroplating: Based on the mechanism of electrochemical reaction, a layer of copper is electroplated on the basis of copper plating to ensure that the copper thickness of the hole meets the product requirements. The electroplating parameters are set according to the completed copper thickness.

[0063] (24) Inner layer circuit fabrication (negative film process): Inner layer pattern transfer, photosensitive film is coated using a vertical coating machine, and the film thickness is controlled at 8μm. A fully automatic exposure machine is used to complete the exposure of the inner layer circuit on the third daughter board with 5-6 exposure rulers (21 exposure rulers); Inner layer etching, the inner layer circuit is etched on the third daughter board after exposure and development. The inner layer line width is measured as 3mil. During etching, the copper layer in the corresponding flexible board area on the third daughter board is removed at the same time; Inner layer AOI, and then the inner layer circuit is inspected for defects such as open circuits, short circuits, line gaps, and line pinholes. Defective products are scrapped, and defect-free products are sent to the next process.

[0064] (25) Fourth pressing: First, the third sub-board is browned, and then the third sub-board and copper foil are pre-stacked together with PP (the specific board arrangement order from top to bottom is copper foil, PP, third sub-board, PP, copper foil), and then pressed to form a production board. The PP is a PP sheet without slits.

[0065] (26) Drilling: Drill holes on the production board using laser drilling and mechanical drilling methods, based on the drilling data.

[0066] (27) Copper plating: A thin layer of copper is deposited on the hole wall through a chemical reaction to provide a basis for the subsequent full-board electroplating. The backlight test is level 10, and the copper plating thickness in the hole is 0.5μm.

[0067] (28) Full-board hole-filling electroplating: Based on the mechanism of electrochemical reaction, a layer of copper is electroplated on the basis of copper plating.

[0068] (29) Full-board electroplating: Based on the mechanism of electrochemical reaction, a layer of copper is electroplated on the basis of copper plating to ensure that the copper thickness of the hole meets the product requirements. The electroplating parameters are set according to the completed copper thickness of the hole.

[0069] (30) Outer layer circuit fabrication (negative film process): outer layer pattern transfer, photosensitive film is coated using a vertical coating machine, the film thickness of the photosensitive film is controlled at 8μm, and the outer layer circuit is exposed on the production board using a fully automatic exposure machine with 5-6 exposure rulers (21 exposure rulers); outer layer vacuum etching, etching the outer layer circuit onto the exposed and developed production board, and removing the copper layer in the corresponding flexible board area on the production board during etching; outer layer AOI, and then checking for defects such as open and short circuits, line gaps, and line pinholes in the outer layer circuit. Defective products are scrapped, and defect-free products are sent to the next process.

[0070] (31) Solder resist and screen printing: After screen printing solder resist ink on the surface of the production board, it is then subjected to pre-curing, exposure, development and heat curing treatment in sequence to cure the solder resist ink into a solder resist layer; specifically, the solder resist ink on the TOP side and the characters on the TOP side are added with "UL mark", thereby coating a layer on the lines and substrates that do not need to be soldered to prevent bridging between lines during soldering, provide a permanent electrical environment and chemical corrosion resistance, and at the same time beautify the appearance.

[0071] (32) Laser cutting: The laser cuts at the corresponding cut position on the production board, that is, cuts inward to the cut and connects with the cut. The PP waste part on the corresponding flexible board area is removed, and the protective tape on the flexible board area is removed to expose the flexible board area. Since the copper foil of each layer of the corresponding flexible board area has been etched away when each layer of circuit is made, only the PP layer is left. Therefore, low-energy laser cutting can be used to burn through to the PP cut to achieve the removal of the cover, which greatly reduces the difficulty of laser cutting. The rigid-flex board can be successfully made without designing a false layer copper pad buffer.

[0072] (33) Film removal: Remove the film from the production board to remove residual adhesive from the pads in the flexible board area.

[0073] (34) Laser cutting: Cut the required shape in the flexible board area according to existing technology and design requirements.

[0074] (35) Surface treatment (immersion nickel and gold): The copper surface of the solder pads at the solder mask opening position is uniformly deposited with a nickel layer and a gold layer of a certain required thickness through chemical principle. The thickness of the nickel layer is 3-5μm; the thickness of the gold layer is 0.05-0.1μm.

[0075] (36) Electrical test: Test the electrical conductivity of the finished board. The test method used for this board is: flying probe test.

[0076] (37) Molding: According to existing technology and design requirements, the outer shape is shaped with a tolerance of + / -0.05mm; a rigid-flexible bond plate is made.

[0077] (38) FQC: In accordance with the customer's acceptance standards and our company's inspection standards, inspect the appearance of the rigid-flexible plate, and repair any defects in a timely manner to ensure excellent quality control for the customer.

[0078] (39) FQA: Re-test the appearance, hole copper thickness, dielectric layer thickness, green solder mask thickness, inner copper thickness, etc. of the rigid-flex PCB to see if they meet the customer's requirements.

[0079] (40) Packaging: The rigid-flex PCB is sealed and packaged according to the packaging method and quantity required by the customer, and desiccant and humidity card are placed in it before shipment.

[0080] In the above, the PP used for the second to fourth pressing is preferably a flowable PP.

[0081] The technical solutions provided by the embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the embodiments of the present invention. The descriptions of the embodiments above are only for helping to understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the embodiments of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A cover opening and windowing method of a rigid-flex board, characterized by, The method comprises the following steps: S1, manufacturing inner layer circuit of soft core board, the soft core board comprises soft area and soft and hard combination area; S2, aligning and adhering cover film on soft area of soft core board; S3, adhering protective tape on cover film; S4, pressing soft core board and copper foil into first sub-board by PP, cutting through the periphery of the soft area on the PP by laser before stacking, forming a cutting seam, and filling the cutting seam with PP flow adhesive during pressing; S5, manufacturing circuit on the first sub-board, and removing the copper layer corresponding to the soft area on the first sub-board; S61, pressing the first sub-board and the copper foil into a second sub-board by PP; cutting through the periphery of the soft area on the PP by laser before stacking, forming a cutting seam, and filling the cutting seam with PP flow adhesive during pressing; S62, manufacturing circuit on the second sub-board, and removing the copper layer corresponding to the soft area on the second sub-board; S63, pressing the second sub-board and the copper foil into a production board by PP; S64, manufacturing circuit on the production board, and removing the copper layer corresponding to the soft area on the production board; When the number of layers of the production board required to be manufactured is ten or more than ten, repeating steps S63 and S64 until the required number of layers of circuit; the PP used in the third pressing and subsequent pressings is a PP sheet without cutting seam; S7, manufacturing solder resist layer on the production board; S8, then cutting on the production board at the position corresponding to the cutting seam by laser, uncovering and removing the waste part on the soft area, and then removing the protective tape on the soft area; S9, finally, surface treatment and molding are performed on the production board to obtain a rigid-flexible combination board.

2. The cover opening and windowing method of a rigid-flex board according to claim 1, wherein In step S2, the cover film is 0.5mm larger than the soft area on one side.

3. The cover opening and windowing method of a rigid-flex board according to claim 1, wherein, Between steps S2 and S3, the following steps are further included: S21, completely bonding the cover film and the soft core board by rapid pressing; S22, baking the soft core board to cure the cover film.

4. The cover opening and windowing method of the rigid-flex printed circuit board according to claim 3, wherein, In step S21, the parameters of rapid pressing are: temperature 180℃, pressure 100KG, and pressing time 1-2min.

5. The cover opening and windowing method of the rigid-flex board according to claim 1, wherein, In step S4, the width of the cutting seam is 0.025mm.

6. The cover opening and windowing method of a rigid-flex board according to claim 1, wherein In step S61, cutting through the periphery of the soft area on the PP by laser before stacking and pressing, forming a cutting seam, and filling the cutting seam with PP flow adhesive during pressing.

7. The cover opening and windowing method of the rigid-flex printed circuit board according to any one of claims 1 to 6, wherein In step S8, cutting inward to the cutting seam by laser.

8. The cover opening and windowing method of a rigid-flex board according to claim 1, wherein, Between steps S8 and S9, the following steps are further included: S81, removing the film on the production board.