Manufacturing process of high-density fine-line flexible circuit board by etching first and then copper plating

Abstract

The invention provides a process for manufacturing a high-density fine-line flexible circuit board by etching and then plating copper, which comprises the following steps of: selecting a non-adhesive flexible copper-clad plate of which the copper foil thickness is 6 microns, 9 microns, 12 microns or 18 microns, and processing a positioning hole, an element hole, a via hole and a blind hole through numerical control drilling or laser drilling; a dry film double-sided laminated copper-clad plate with the thickness of 20 [mu] m or 25 [mu] m is adopted, exposure is carried out through a laser direct imaging technology and an automatic alignment laser direct writing device, then developing, etching and film stripping procedures are sequentially completed through a vacuumizing etching machine, a semi-finished product with thin lines on the two sides is obtained, and the line width distance of the thin lines can reach 0.0125 mm or above. By adopting a reverse process of etching first and then plating copper, and by utilizing the characteristic that the original thin copper foil (6-18 microns) is easy to etch, the thin circuit with the line width distance of 0.05 mm or below and even 0.0125 mm or above can be accurately manufactured, and the problem that the thick copper foil is difficult to etch the superfine circuit in the traditional process is solved.

Description

Technical Field

[0001] This invention relates to the field of flexible circuit board technology, specifically to a manufacturing process for a high-density fine-line flexible circuit board that involves etching followed by copper plating. Background Technology

[0002] With the development of multifunctional and miniaturized electronic products, the wiring density of flexible circuit boards is becoming increasingly dense, and the line width spacing is becoming smaller, making circuit fabrication increasingly difficult. Currently, double-sided multilayer flexible circuit boards with line widths below 0.05mm use traditional copper plating followed by circuit fabrication. However, due to the increased thickness of the copper foil caused by copper plating, etching fine lines is extremely difficult, making it virtually impossible to achieve line widths below 0.035mm. Summary of the Invention

[0003] (a) Technical problems to be solved

[0004] To address the shortcomings of existing technologies, this invention provides a process for manufacturing high-density fine-line flexible circuit boards by first etching and then plating copper. The purpose of this process is to overcome the deficiencies of existing technologies and achieve the goal of manufacturing high-density fine-line flexible circuit boards.

[0005] (II) Technical Solution

[0006] To achieve the above objectives, the present invention provides the following technical solution: a manufacturing process for a high-density fine-line flexible circuit board that involves etching followed by copper plating, comprising the following steps:

[0007] Step 1: Select a flexible copper-clad laminate with a copper foil thickness of 6um, 9um, 12um or 18um without adhesive, and process positioning holes, component holes, vias and blind holes by CNC drilling or laser drilling; use a dry film double-sided laminate with a thickness of 20um or 25um, expose it by laser direct imaging technology and automatic alignment laser direct writing equipment, and then complete the development, etching and film removal processes in sequence by a vacuum etching machine to obtain a semi-finished product with fine lines on both sides. The width of the fine lines can reach more than 0.0125mm.

[0008] Step 2: Perform a second dry film application on the above semi-finished products. After exposure and development, the via pads, component via pads and corresponding vias and component holes are exposed, while the remaining parts are protected by dry film.

[0009] Step 3: Immerse the developed semi-finished product in a toner solution for black hole treatment. After drying, cover the surface of the dry film, the surface of the polyimide substrate on the hole wall, and the hole plate with a layer of toner. Then, use a micro-etching solution to micro-etch the semi-finished product, micro-etching the copper foil surface by 0.6-1µm, so that the toner attached to the copper foil surface falls off naturally, leaving the toner on the surface of the dry film and the polyimide substrate to form a conductive layer.

[0010] Step 4: Perform copper plating on the semi-finished product after black hole treatment. Utilize the conductivity of carbon powder to uniformly plate a copper layer of specified thickness onto the dry film surface, hole walls, and hole plate.

[0011] Step 5: Perform a third dry film application on the copper-plated semi-finished product. After exposure and development, protect the vias, component holes and corresponding hole pads, exposing the non-essential protected parts of the copper plating on the second dry film.

[0012] Step 6: Etch away the unnecessary copper layers exposed above, retaining the copper layers in the vias, component holes, and hole pads protected by the dry film;

[0013] Step 7: Use stripping solution to remove the carbon layer and dry film adhering to the surface of the semi-finished product from each step, exposing fine lines, vias, component holes, and copper-plated hole pads and hole walls, realizing hole metallization and completing the electrical connection between the upper and lower layers of circuitry.

[0014] (III) Beneficial Effects

[0015] This invention provides a process for manufacturing high-density, fine-line flexible circuit boards by first etching and then plating copper. It offers the following advantages:

[0016] 1. Breakthrough in fine line fabrication bottleneck: By adopting a reverse process of "etching first and then copper plating", the original thin copper foil (6-18um) is easy to etch, which can accurately produce fine lines with line widths of less than 0.05mm or even more than 0.0125mm, solving the problem that thick copper foil is difficult to etch ultra-fine lines in traditional processes.

[0017] 2. Ensure the reliability of hole metallization: A conductive layer is formed through black hole treatment, which, together with the subsequent copper plating process, forms a stable copper layer on the hole wall and hole plate, ensuring the electrical connection effect of the upper and lower layer circuits, and taking into account the needs of fine line fabrication and hole metallization.

[0018] 3. Strong process feasibility: Through three steps of dry film application, exposure and development, orderly isolation and protection of circuit fabrication and hole metallization are achieved. Each process uses existing mature equipment and materials, without the need for additional special processes, which facilitates industrial promotion. Attached Figure Description

[0019] Figure 1 This is a cross-sectional view of the flexible copper-clad laminate after the first etching of the drilled holes, which is part of the high-density fine-line flexible circuit board manufacturing process proposed in this invention, involving etching followed by copper plating.

[0020] Figure 2 This is a cross-sectional view of a flexible circuit board after the first dry film development following the etching and copper plating process proposed in this invention.

[0021] Figure 3This is a cross-sectional view of the flexible circuit board after black holes, based on the high-density fine-line flexible circuit board manufacturing process of etching followed by copper plating proposed in this invention.

[0022] Figure 4 This is a cross-sectional view of the flexible circuit board after copper plating of the black holes, which is part of the high-density fine-line flexible circuit board manufacturing process of etching followed by copper plating proposed in this invention.

[0023] Figure 5 The cross-sectional view of the flexible circuit board after copper plating, dry film lamination, and development is provided in the present invention, which is a high-density fine-line flexible circuit board manufacturing process of etching and copper plating.

[0024] Figure 6 This is a cross-sectional view of a flexible circuit board after etching an unprotected copper layer, based on the high-density fine-line flexible circuit board manufacturing process of etching followed by copper plating proposed in this invention.

[0025] Figure 7 This is a cross-sectional view of the flexible circuit board after removing the protective dry film, which is part of the high-density fine-line flexible circuit board manufacturing process proposed in this invention, involving etching followed by copper plating. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Example:

[0028] like Figure 1-7 As shown, this embodiment of the invention provides a manufacturing process for a high-density fine-line flexible circuit board that involves etching followed by copper plating, comprising the following steps:

[0029] Step 1: Select a thin copper foil (e.g., 6µm, 9µm, 12µm, 18µm) adhesive-free flexible copper clad laminate and perform CNC drilling or laser drilling (including positioning holes, component holes, vias, blind vias, etc.). Apply 25µm or 20µm dry film with good resolution and good via coverage to both sides of the copper clad laminate. Then, expose the laminate using laser direct imaging technology and a laser direct writing device with automatic alignment function. Develop, etch, and remove the film using a vacuum etching machine to obtain a semi-finished flexible circuit board with fine lines on both sides (6µm thin copper can produce fine lines with a line pitch of 0.0125mm or more). Figure 1As shown in the figure, 0 is the polyimide substrate, 2 is the via or small hole, 3 is the component hole or large hole, 4 and 6 are fine lines, 5 is the spacing of the fine lines, A1 and A2, B1 and B2 are the upper and lower layers of via or small hole lines, and A3 and A4, B3 and B4 are the upper and lower layers of component holes or large holes lines.

[0030] Step Two: Apply dry film, expose and develop to reveal the upper layer circuitry A1, A2, B1, B2 via pads and via 2, and the lower layer circuitry component via pads A3, A4, B3, B4 and component via 3. Other areas are protected by the dry film. Figure 2 As shown;

[0031] Step 3: The developed semi-finished product undergoes black hole treatment. This process involves immersing the entire sheet in a toner solution and drying it. During this process, the dry film, the substrate (0 polyimide) inside the hole walls, and the hole pads are all coated with a layer of toner. Since copper itself is conductive, toner is unnecessary, and the toner remaining on the copper surface would create a carbon medium between the copper and the original copper after subsequent copper plating, affecting product quality. Therefore, the toner on the copper surface must be removed. After black hole treatment, a micro-etching solution is used to micro-etch away 0.6-1µm of copper. The toner on the copper surface naturally falls off. However, the polyimide and dry film are organic materials, which cannot be micro-etched by the solution, and the attached toner will not fall off due to their conductivity. Therefore, after black hole treatment, there is a layer of toner (E) on the dry film, and two layers of toner (E12 and E34) on the polyimide inside the hole walls. While A1 and A2, and A3 and A4 remain the original copper layers, this results in the dry film outer layer, the perforated disks, and the polyimide outer layer of the perforation walls all being conductive, providing a conductive layer for subsequent copper plating. Figure 3 As shown;

[0032] Step 4: The semi-finished product after black hole plating is copper plated. Since the outer layer of the dry film, the hole pads, and the polyimide outer layer of the hole walls are all conductive, these areas are uniformly plated with a copper layer of the required thickness. Figure 4 The numbers F, AB12, AB34, F12, and F34 are shown.

[0033] Step 5: Apply dry film again for exposure and development to protect via pads AB12 and F12, as well as component via pads AB34 and F34. Develop away the unexposed dry film, revealing the copper layer G plated on the first dry film that does not require protection, and the exposed but not developed via pad protective dry film G12 and component via pad protective dry film G34. Figure 5 As shown;

[0034] Step Six: Etch away the exposed copper layer G, leaving the protective dry film G12 and G34 that were not developed. The via pads AB12 and F12, as well as the component via pads AB34 and F34, are protected. Figure 6 As shown;

[0035] Step 7: Use stripping solution to remove the carbon layer E, the dry film C from step 2, and the protective dry films G12 and G34, exposing the original circuit lines 4 and 6 with spacing 5; the vias or pinholes between A1 and A2, B1 and B2; the component holes or large holes between A3 and A4, B3 and B4; and the newly obtained copper-plated vias F12 and AB12, and component holes F34 and AB34. This achieves hole metallization and connects the upper and lower layer circuits. Figure 7 As shown, a double-sided flexible circuit board with fine lines and electrical connections between the upper and lower layers was obtained. A multi-layer flexible circuit board with fine lines can also be obtained using the same method.

[0036] Working principle: After drilling holes in the double-sided flexible copper-clad laminate, dry film is first applied for exposure, development, and etching of the circuitry. Since the entire board is not copper-plated, the original copper foil is very thin, making it ideal for etching fine lines, especially those smaller than 0.05mm. Then, dry film is applied for exposure and development to expose the via pads and component via pads, followed by black hole plating and copper plating. Dry film is applied again for exposure and development to protect the vias and component vias, exposing a second layer of copper on the unprotected portion of the dry film. This copper layer is then etched away, and the film is stripped to expose the via pads and inner holes. A layer of copper is then plated onto the via pads and hole walls to achieve the desired hole metallization. This etching-then-copper-plating (hole metallization) process is the opposite of the traditional copper-plating (hole metallization)-then-etching process, making it ideal for etching fine lines.

[0037] Although this process is relatively complex, requiring three dry film exposure and development steps, first etching fine lines with original thin copper, and then protecting the lines and holes with metallization, this method can produce double-sided multilayer flexible circuit boards with fine lines.

[0038] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for manufacturing a high-density fine-line flexible circuit board by etching followed by copper plating, comprising the following steps: Step 1: Select a flexible copper-clad laminate with a copper foil thickness of 6um, 9um, 12um or 18um without adhesive, and process positioning holes, component holes, vias and blind holes by CNC drilling or laser drilling; use a dry film double-sided laminate with a thickness of 20um or 25um, expose it by laser direct imaging technology and automatic alignment laser direct writing equipment, and then complete the development, etching and film removal processes in sequence by a vacuum etching machine to obtain a semi-finished product with fine lines on both sides. The width of the fine lines can reach more than 0.0125mm. Step 2: Perform a second dry film application on the above semi-finished products. After exposure and development, the via pads, component via pads and corresponding vias and component holes are exposed, while the remaining parts are protected by dry film. Step 3: Immerse the developed semi-finished product in a toner solution for black hole treatment. After drying, cover the surface of the dry film, the surface of the polyimide substrate on the hole wall, and the hole plate with a layer of toner. Then, use a micro-etching solution to micro-etch the semi-finished product, micro-etching the copper foil surface by 0.6-1µm, so that the toner attached to the copper foil surface falls off naturally, leaving the toner on the surface of the dry film and the polyimide substrate to form a conductive layer. Step 4: Perform copper plating on the semi-finished product after black hole treatment. Utilize the conductivity of carbon powder to uniformly plate a copper layer of specified thickness onto the dry film surface, hole walls, and hole plate. Step 5: Perform a third dry film application on the copper-plated semi-finished product. After exposure and development, protect the vias, component holes and corresponding hole pads, exposing the non-essential protected parts of the copper plating on the second dry film. Step 6: Etch away the unnecessary copper layers exposed above, retaining the copper layers in the vias, component holes, and hole pads protected by the dry film; Step 7: Use stripping solution to remove the carbon layer and dry film adhering to the surface of the semi-finished product from each step, exposing fine lines, vias, component holes, and copper-plated hole pads and hole walls, realizing hole metallization and completing the electrical connection between the upper and lower layers of circuitry.

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