Manufacturing method of flexible printed circuit boards

Abstract

To provide a method of forming a fine wiring pattern in a manufacturing of a flexible printed wiring board having an inter-layer conductive path.SOLUTION: A manufacturing method of a flexible printed wiring board 1 of an embodiment, comprises the steps of: preparing a both-layer metal extension lamination board 2 having an insulation base material 30, a metal foil 10, and a metal foil 20; forming a conductive hole H1 in which the metal foil 20 is exposed onto a bottom surface; forming a plating resist pattern 13 having an opening 13a where the conductive hole H1 is exposed onto the metal foil 10; forming a metal plating 40 having a filling part 41 and a projection part 42 to the opening 13a; etching the projection part 42 of the metal plating 40; removing the plating resist pattern 13; forming an etching resist film 14 so as to coat the metal plating 40 on to the metal foil 10; forming an etching resist pattern 14a; and forming a wiring pattern WP1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a flexible printed wiring board and a flexible printed wiring board, and more particularly, to a method for manufacturing a flexible printed wiring board for forming a fine wiring pattern and a flexible printed wiring board in the manufacture of a flexible printed wiring board having interlayer conductive paths. [Background Art]

[0002] A flexible printed wiring board (FPC) is a wiring board in which a wiring pattern is formed from a metal foil such as copper on a thin resin film such as polyimide. Owing to its thinness and flexibility, flexible printed wiring boards are used inside narrow housings and in portions involving mechanical operation.

[0003] Electronic devices, including information communication devices such as smartphones, are pursuing miniaturization and weight reduction. Flexible printed wiring boards meet these objectives and are widely used in electronic devices. Accordingly, further miniaturization and higher density are required for flexible printed wiring boards. Higher density of a substrate means forming more wirings on a substrate with a limited area. To form more wirings, miniaturization and multilayering of wiring patterns are necessary.

[0004] Etching is used for forming wiring patterns. The subtractive method is one of the methods for forming wiring patterns by etching, and is generally used in forming wiring patterns for flexible printed wiring boards. In the subtractive method, etching in the depth direction of the wiring and etching in the width direction of the wiring occur isotropically. For this reason, in order to form a fine wiring pattern, a thinner metal foil used for the wiring is more advantageous.

[0005] On the other hand, for multilayer construction, interlayer conductive paths such as vias are formed in substrates having metal foil on both sides or in multilayer substrates, and wiring in different layers is electrically connected by these interlayer conductive paths. When forming interlayer conductive paths, metal plating is formed in the conductive holes. Patent documents 1 and 2 describe a method called button plating, in which the plating treatment is applied only to specific parts of the substrate, as one embodiment of a method for forming metal plating. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2006-108270 [Patent Document 2] Japanese Patent Application Publication No. 11-195849 [Overview of the project] [Problems that the invention aims to solve]

[0007] An example of a method for manufacturing a flexible printed circuit board having wiring patterns and interlayer conductive paths using the button plating method will be explained with reference to Figures 5A to 5C. Figures 5A to 5C are cross-sectional view steps illustrating a comparative example of a method for manufacturing a flexible printed circuit board.

[0008] First, as shown in Figure 5A(1), a double-sided metal-clad laminate is prepared, comprising a metal foil 100, a metal foil 200, and an insulating substrate 300. Next, as shown in Figure 5A(2), conductive holes H are formed in the double-sided metal-clad laminate. Then, as shown in Figure 5A(3), a plating resist pattern 130 and a plating resist pattern 230 are formed on the upper and lower surfaces of the double-sided metal-clad laminate, respectively. The plating resist pattern 130 has an opening 130a that exposes the conductive holes H. That is, the plating resist pattern 130 does not cover the conductive holes H.

[0009] Next, as shown in Figure 5A(4), a metal plating 400 (button plating) is formed by applying a metal plating treatment to the opening 130a. This forms an interlayer conductive path that electrically connects the metal foil 100 and the metal foil 200. The metal plating 400 has a filling portion 410 and a protruding portion 420. The filling portion 410 is the part of the metal plating 400 that fills the conductive hole H, and is the lower part of the dotted line in Figure 5A(4). The protruding portion 420 is the part of the metal plating 400 located above the filling portion 410, and is the upper part of the dotted line in Figure 5A(4). The protruding portion 420 covers the part of the metal foil 100 that surrounds the conductive hole H (opening edge). The protruding portion 420 is also called a button land.

[0010] Next, as shown in Figure 5B(1), the plating resist patterns 130 and 230 are removed.

[0011] Next, as shown in Figure 5B(2), an etching resist film 140 is formed on the metal foil 100, and an etching resist film 240 is formed on the metal foil 200. The etching resist film 140 is formed to cover the metal plating 400. Since the metal plating 400 has protrusions 420, the etching resist film 140 is formed to embed the protrusions 420. Next, as shown in Figure 5B(3), the etching resist films 140 and 240 are exposed and developed to form etching resist patterns 140a and 240a.

[0012] Next, as shown in Figure 5C(1), etching is performed to remove the metal foil 100 that is not covered by the etching resist pattern 140a and the metal foil 200 that is not covered by the etching resist pattern 240a. After that, as shown in Figure 5C(2), the etching resist patterns 140a and 240a are removed.

[0013] Through the above process, a flexible printed circuit board is manufactured having a wiring pattern WP10 with multiple wires 110, a land 120, a WP20 with multiple wires 210, and a land 220 facing the land 120 with an insulating substrate 300 in between.

[0014] The above-described method for manufacturing flexible printed circuit boards has the following problems when it comes to miniaturizing the wiring pattern.

[0015] As shown in Figure 5B(1), the protrusions 420 of the metal plating 400 protrude from the metal foil 100. In order to prevent the metal plating 400 from being etched in subsequent processes, as shown in Figure 5B(2), it is necessary to embed the protrusions 420 of the metal plating 400 that protrude from the metal foil 100 when forming the etching resist film 140. For this reason, the etching resist film 140 is formed thickly. However, if the etching resist film 140 is made thick, the etching resolution decreases, making it difficult to form fine wiring patterns WP10.

[0016] The present invention is based on the above technical understanding and aims to provide a method for manufacturing a flexible printed circuit board for forming a fine wiring pattern in the manufacture of a flexible printed circuit board having interlayer conductive paths, and a flexible printed circuit board. [Means for solving the problem]

[0017] The inventors of the present invention conducted diligent studies to solve the above problems and, as a result, came up with the technical idea that by etching at least a portion of the protruding parts of the metal plating to reduce the protrusions of the metal plating, it is possible to form a thin etching resist film covering the metal plating, thereby enabling the formation of fine wiring patterns.

[0018] The method for manufacturing a flexible printed circuit board according to the present invention is: A step of preparing a double-sided metal-clad laminate having an insulating substrate having a first main surface and a second main surface opposite to the first main surface, a first metal foil provided on the first main surface, and a second metal foil provided on the second main surface, A step of partially removing the first metal foil and the insulating substrate to form a conductive hole on the bottom surface in which the second metal foil is exposed, a step of forming, on the first metal foil, a plating resist pattern having an opening that exposes the conduction hole; a step of forming metal plating having a filling portion filling the conduction hole and a protruding portion located on the filling portion and covering an opening edge portion of the first metal foil surrounding the conduction hole, by performing a metal plating process on the opening; a step of etching the protruding portion of the metal plating exposed at the opening of the plating resist pattern; a step of removing the plating resist pattern; a step of forming, on the first metal foil, an etching resist film so as to cover the metal plating; a step of exposing and developing the etching resist film to form an etching resist pattern; a step of forming a wiring pattern by etching and removing the first metal foil not covered with the etching resist pattern, and removing the etching resist pattern; comprising the above steps.

[0019] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be performed so as to leave the protruding portion.

[0020] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be performed so as to completely remove the protruding portion.

[0021] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be further performed so as to remove a part of the metal foil of the first metal foil covered by the protruding portion, and / or a part of the filling portion of the metal plating.

[0022] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be performed such that an upper surface of the etched metal plating and an upper surface of the first metal foil constituting the wiring pattern are substantially flush with each other.

[0023] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be performed such that a height difference between an upper surface of the etched metal plating and an upper surface of the first metal foil constituting the wiring pattern is within ±5 µm.

[0024] Further, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating may be performed such that the height difference is within ±2 µm.

[0025] Further, in the method for manufacturing the flexible printed wiring board, the plating resist pattern has a dummy opening, in the step of forming the metal plating, a dummy plating is formed on the first metal foil exposed through the dummy opening, in the step of etching the protruding portion of the metal plating, at least a part of the dummy plating may be removed.

[0026] Further, in the method for manufacturing the flexible printed wiring board, at least one of the respective steps may be performed by a roll-to-roll method.

[0027] Further, in the method for manufacturing the flexible printed wiring board, the first and second metal foils may be rolled copper foils.

[0028] Further, in the method for manufacturing the flexible printed wiring board, The aforementioned metal plating may be electrolytic copper plating.

[0029] Furthermore, in the method for manufacturing the flexible printed circuit board, The step of forming the plating resist pattern having the opening is, A step of forming a plating resist film on the first metal foil that covers the first metal foil and the conductive holes, The process involves exposing and developing the plating resist film to form the opening, Includes, The aforementioned exposure may be performed by a direct drawing method that does not use an exposure mask.

[0030] The flexible printed circuit board according to the present invention is An insulating substrate having a first main surface and a second main surface opposite to the first main surface, Wiring provided on the first main surface, A land provided on the first main surface, The conductive layer provided on the second main surface, The land and the conductive layer are electrically connected, and the upper surface of the metal plating is substantially flush with the upper surface of the wiring. It is characterized by having the following features.

[0031] Furthermore, in the flexible printed circuit board, The height difference between the upper surface of the metal plating and the upper surface of the wiring may be within ±5 μm.

[0032] Furthermore, in the flexible printed circuit board, The height difference may be kept within ±2 μm.

[0033] Furthermore, in the flexible printed circuit board, Multiple metal platings are provided, The upper surfaces of the plurality of metal platings may be substantially flush with the upper surface of the wiring provided on the first main surface. [Effects of the Invention]

[0034] According to the manufacturing method of a flexible printed circuit board according to the present invention, before removing the plating resist pattern, the protruding parts of the metal plating are etched to remove at least a portion of the protruding parts. This eliminates the need to embed thick protruding parts with an etching resist film. Therefore, a thinner etching resist film can be used. As a result, fine wiring patterns can be formed in the manufacturing of a flexible printed circuit board having interlayer conductive paths. [Brief explanation of the drawing]

[0035] [Figure 1] This is a flowchart showing a method for manufacturing a flexible printed circuit board according to an embodiment. [Figure 2A] This is a cross-sectional view illustrating a process for manufacturing a flexible printed circuit board according to the first embodiment. [Figure 2B] Figure 2A is a cross-sectional view illustrating the process for manufacturing a flexible printed circuit board according to the first embodiment. [Figure 2C] Figure 2B is a cross-sectional view illustrating the process for manufacturing a flexible printed circuit board according to the first embodiment. [Figure 3A] This is a cross-sectional view illustrating a process for manufacturing a flexible printed circuit board according to a second embodiment. [Figure 3B] This is a cross-sectional view illustrating a process for manufacturing a flexible printed circuit board according to a second embodiment, following Figure 3A. [Figure 4] This is a plan view of a flexible printed circuit board for illustrating an example of a dummy opening according to the third embodiment. [Figure 5A] This is a cross-sectional view illustrating a process for manufacturing a flexible printed circuit board according to a comparative example. [Figure 5B] Figure 5A is a cross-sectional view illustrating the process for manufacturing a flexible printed circuit board according to a comparative example. [Figure 5C]Figure 5B is a cross-sectional view illustrating the process for manufacturing a flexible printed circuit board according to a comparative example. [Modes for carrying out the invention]

[0036] As mentioned above, the inventors of the present invention conceived of the technical idea of ​​etching at least a portion of the protrusions of the metal plating before forming an etching resist film in order to form a fine wiring pattern on a flexible printed circuit board having interlayer conductive paths. Generally, when etching the protrusions of the metal plating, there is an error in the degree to which the protrusions are etched. Considering this, the state after etching will be one of the following: One is when the amount of etching is small and the protrusions are not completely removed. The other is when the amount of etching is large and the protrusions are completely removed.

[0037] The first and second embodiments described below correspond to the respective cases described above. Specifically, the first embodiment is for cases where the amount of etching is small and the protrusions are not completely removed, while the second embodiment is for cases where the amount of etching is large and the protrusions are completely removed.

[0038] Embodiments of the present invention will be described below with reference to the drawings. In each figure, components having equivalent functions are denoted by the same reference numerals. The drawings are schematic and mainly show characteristic parts of each embodiment, and the relationship between thickness and planar dimensions, the ratio of the thickness of each layer, etc., may differ from reality.

[0039] (First embodiment) An example of a method for manufacturing a flexible printed circuit board according to the first embodiment will be described with reference to Figures 1 and 2A to 2C. Figure 1 is a flowchart illustrating the method for manufacturing a flexible printed circuit board according to this embodiment. Figures 2A to 2C are cross-sectional views illustrating the process for manufacturing a flexible printed circuit board according to this embodiment.

[0040] As shown in Figure 2A(1), a double-sided metal-clad laminate 2 is prepared (step S1). The double-sided metal-clad laminate 2 has a metal foil 10, a metal foil 20, and an insulating base material 30. More specifically, the double-sided metal-clad laminate 2 has an insulating base material 30 having an upper surface (first main surface) and a lower surface (second main surface opposite the first main surface), a metal foil 10 provided on the first main surface, and a metal foil 20 provided on the second main surface.

[0041] The metal foils 10 and 20 are, for example, copper foil (electrolytic copper foil) with a thickness of 9 μm. However, metal foils 10 and 20 with a thickness of 5 to 72 μm may also be used. Furthermore, the material of metal foils 10 and 20 may be a metal other than copper (for example, silver, aluminum, etc.). When copper foil is used as metal foil 10 and 20, rolled copper foil may be used. Using rolled copper foil makes it possible to provide a highly flexible printed circuit board. Also, half-etched metal foils 10 and 20 may be used.

[0042] The insulating substrate 30 is, for example, made of polyimide with a thickness of 25 μm. However, the insulating substrate 30 may also be made of materials with a thickness of 6 to 100 μm. Furthermore, the material of the insulating substrate 30 may be, for example, fluorine-based materials such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) and PTFE (polytetrafluoroethylene), polyimide-based materials such as MPI (modified polyimide) and PI (polyimide), PEEK (polyether ether ketone), PET (polyethylene terephthalate), or PEN (polyethylene naphthalate).

[0043] Next, as shown in Figure 2A(2), a conductive hole H1 is formed by partially removing the metal foil 10 and the insulating substrate 30 (step S2). The conductive hole H1 is for forming an interlayer conductive path. The metal foil 20 is exposed at the bottom of the conductive hole H1. In other words, the conductive hole H1 is a bottomed hole. By making it a bottomed hole, it is not necessary to suppress etching of unnecessary parts during etching in step S5, which will be described later, and the etching process can be made easier.

[0044] In step S2, the metal foil 10 and insulating substrate 30 can be removed using, for example, the conformal mask method. When using the conformal mask method, first, the metal foil 10 is partially removed by etching or the like. Next, the insulating substrate 30 exposed in the area where the metal foil 10 has been removed is removed by irradiating it with laser light. This forms a conductive hole H1 on the bottom surface in which the metal foil 20 is exposed. The type of laser that can be used is, for example, a UV-YAG laser or a carbon dioxide laser. A UV-YAG laser is preferred for forming high-density wiring. In addition, other methods besides the conformal mask method (for example, the window method or the direct drilling method) may be used to form the conductive hole H1.

[0045] The diameter of the conductive hole H1 is, for example, 50 μm at the opening (i.e., the portion where the metal foil 10 has been removed). Alternatively, by adjusting the laser irradiation conditions, the conductive hole H1 may be formed such that its inner diameter decreases as it deepens from the opening, as shown in Figure 2A(2). This facilitates the formation of the metal plating 40, described later, in the conductive hole H1.

[0046] After forming the conductive holes H1, the metal foil 20 exposed on the bottom surface may be cleaned (for example, by desmearing). Then, the conductive holes H1 may be treated to make them conductive (for example, by electroless plating).

[0047] Next, as shown in Figure 2A(3), a plating resist pattern 13 is formed on the metal foil 10 and a plating resist pattern 23 is formed on the metal foil 20 (step S3). The plating resist pattern 13 has an opening 13a in which the conductive holes H1 are exposed.

[0048] An example of the process for forming the plating resist patterns 13 and 23 will be explained in more detail. First, a plating resist film is formed on the metal foil 10 to cover the metal foil 10 and the conductive holes H1. Next, a plating resist film is formed on the metal foil 20 to cover the metal foil 20. Then, the plating resist film is exposed and developed to form the opening 13a. The plating resist patterns 13 and 23 are formed by these steps.

[0049] For exposure of the plating resist film, methods such as direct writing (direct writing exposure), proximity exposure, and projection exposure are used. Preferably, a direct writing method that does not use an exposure mask (for example, laser direct writing or electron beam direct writing) is used. By using the direct writing method, even if the insulating substrate 30 or the like expands or contracts in a process prior to the exposure process, an exposure pattern corresponding to that expansion or contraction can be drawn. Therefore, misalignment between the conductive hole H1 and the opening 13a can be suppressed. As a result, for example, the diameter of the opening 13a can be reduced.

[0050] Next, as shown in Figure 2A(4), a metal plating 40 is formed by applying a metal plating treatment to the opening 13a (step S4). This forms an interlayer conductive path that electrically connects the metal foil 10 and the metal foil 20. In this embodiment, since the conductive hole H1 was a bottomed hole, the interlayer conductive path becomes a bottomed via. In addition, since the metal plating 40 will be etched in the subsequent steps, it is preferable that the metal plating 40 be a filled via.

[0051] The metal plating 40 has a filling portion 41 and a protruding portion 42. The filling portion 41 is the part of the metal plating 40 that fills the conductive hole H1, and is the lower part of the dotted line in Figure 2A(4). The protruding portion 42 is the part of the metal plating 40 located above the filling portion 41, and is the upper part of the dotted line in Figure 2A(4). The protruding portion 42 covers the part of the metal foil 10 that surrounds the conductive hole H1 (opening edge). The protruding portion 42 is also called a button land. The metal plating 40 is formed, for example, so that the thickness of the protruding portion 42 is 13 μm.

[0052] In this embodiment, the metal plating 40 is electroplated. More specifically, in step S4, an electroplating treatment is performed to form the metal plating 40. By using electroplating, the plating treatment time can be shortened compared to using electroless plating, and manufacturing efficiency can be improved.

[0053] Furthermore, the metal plating 40 may use the same metal as the metal foil 10. Using the same metal ensures consistent physical, electrical, and chemical properties, simplifying handling. In particular, if the metal foil 10 is copper foil, the metal plating 40 may be electrolytic copper plating. By forming electrolytic copper plating, the metal of the metal plating 40 can be the same metal (copper) as the rolled copper foil preferred in the case of flexible printed circuit boards.

[0054] Next, as shown in Figure 2B(1), the protrusions 42 of the metal plating 40 are etched (step S5). More specifically, at least a portion of the protrusions 42 of the metal plating 40 exposed in the opening 13a of the plating resist pattern 13 is etched. For etching, for example, copper chloride is used.

[0055] In this embodiment, the etching process for the protrusions 42 of the metal plating 40 is performed in such a way that the protrusions 42 remain. As a result, a portion of the protrusions 42 is removed, resulting in a less protruding protrusion 42a. For example, if the thickness of the protrusion 42 is 13 μm, 10 μm is removed by etching. In this case, the thickness of the protrusion 42a after etching will be approximately 3 μm.

[0056] Next, as shown in Figure 2B(2), the plating resist patterns 13 and 23 are removed (step S6).

[0057] Next, as shown in Figure 2B(3), an etching resist film 14 is formed on the metal foil 10 and an etching resist film 24 is formed on the metal foil 20 (step S7). The etching resist film 14 is formed to cover the metal plating 40. In this embodiment, since the metal plating 40 has protrusions 42a, the etching resist film 14 is formed to embed the protrusions 42a.

[0058] In this embodiment, since the thickness of the protrusion 42a is smaller than the thickness of the protrusion 42, a thinner etching resist film 14 (for example, one with a thickness of 7 to 15 μm) can be formed.

[0059] Furthermore, since the etching resist film 24 is not provided on the lower surface side of the insulating substrate 30, a thinner film can be formed. Note that if there are multiple conductive holes H1, the protrusions 42a may also be present on the lower surface side (second main surface side) of the insulating substrate 30. Even in this case, as described above, since the thickness of the protrusions 42a is smaller than the thickness of the protrusions 42, a thinner etching resist film 24 can be formed.

[0060] Next, as shown in Figure 2B(4), the etching resist film 14 is exposed and developed to form an etching resist pattern 14a. The etching resist film 24 is also exposed and developed to form an etching resist pattern 24a (step S8).

[0061] In this embodiment, because the etching resist films 14 and 24 are thin, the resolution of exposure and development is increased, and etching resist patterns 14a and 24a with fine patterns can be formed. In addition, the thinness of the etching resist patterns 14a and 24a makes it easier for the etching solution to reach the metal foil. As a result, the etching resolution is increased, and finer wiring patterns can be formed.

[0062] Next, as shown in Figure 2C(1), the metal foil 10 that is not covered by the etching resist pattern 14a is removed by etching. Also, the metal foil 20 that is not covered by the etching resist pattern 24a is removed by etching (step S9).

[0063] Next, as shown in Figure 2C(2), the etching resist patterns 14a and 24a are removed (step S10). As a result, a wiring pattern WP1 including multiple wirings 11 and lands 12 are formed on the upper surface (first main surface) of the insulating substrate 30. On the lower surface (second main surface) of the insulating substrate 30, a wiring pattern WP2 including multiple wirings 21 and lands 22 are formed.

[0064] In this embodiment, since the etching resist patterns 14a and 24a have fine patterns and are thin, the etching resolution is increased, and fine wiring patterns WP1 and WP2 can be formed.

[0065] By following the steps S1 to S10 described above, the flexible printed circuit board 1 according to this embodiment can be manufactured.

[0066] As described above, in this embodiment, before removing the plating resist patterns 13 and 23, the protrusions 42 of the metal plating 40 are etched to remove a portion of the protrusions 42, thereby forming less protruding protrusions 42a. This eliminates the need to embed the thick protrusions 42 with the etching resist film 14. Therefore, a thinner etching resist film 14 can be used to embed the protrusions 42a. Similarly, a thinner etching resist film 24 can be used.

[0067] By thinning the etching resist films 14 and 24, the resolution of exposure and development is increased, enabling the formation of high-resolution etching resist patterns 14a and 24a. Because the etching resist patterns 14a and 24a have high resolution and are thin, the etching resolution is increased, enabling the formation of fine wiring patterns WP1 and WP2. In other words, fine wiring patterns WP1 and WP2 can be formed in the manufacturing of a flexible printed circuit board 1 having interlayer conductive paths.

[0068] Furthermore, in the process of etching the protrusions 42 of the metal plating 40, the plating resist pattern 13 is used as an etching mask. This shortens the process. In addition, because misalignment between the plating resist pattern 13 and the metal plating 40 can be suppressed, the protrusions 42 can be etched more reliably.

[0069] Furthermore, by using rolled copper foil as the metal foils 10 and 20, and by not forming the metal plating 40 on the bent portion of the flexible printed circuit board, it is possible to manufacture a flexible printed circuit board with high flexibility.

[0070] (Second embodiment) Next, a method for manufacturing a flexible printed circuit board according to the second embodiment will be described with reference to Figures 1, 2A, 3A, and 3B. One of the differences between this embodiment and the first embodiment is the process of etching the protruding portion 42 of the metal plating 40. Hereafter, this embodiment will be described focusing on the differences from the first embodiment, and descriptions of similar parts will be omitted.

[0071] As shown in Figure 2A(1), a double-sided metal-clad laminate 2 is prepared (step S1). In this embodiment, the double-sided metal-clad laminate 2 is made of, for example, rolled copper foil with a thickness of 12 μm for the metal foils 10 and 20, and polyimide with a thickness of 12.5 μm for the insulating substrate 30. When copper foil is used as the metal foils 10 and 20, electrolytic copper foil may also be used.

[0072] Next, as shown in Figure 2A(2), a conductive hole H1 is formed by partially removing the metal foil 10 and the insulating substrate 30 (step S2). The diameter of the conductive hole H1 is, for example, 35 μm at the opening (i.e., the portion where the metal foil 10 has been removed).

[0073] Steps S3 and S4 are the same as in the first embodiment (see Figures 2A(3), (4)).

[0074] Next, as shown in Figure 3A(1), the protruding portions 42 of the metal plating 40 are etched (step S5). In this embodiment, unlike the first embodiment, the protruding portions 42 of the metal plating 40 are completely removed in this step. Specifically, the etching is performed in an over-etching manner.

[0075] If over-etching is to occur, the protrusions 42 may be completely removed, and further, a portion of the metal foil 10 and / or a portion of the filling portion 41 may be removed. More specifically, step S5 may be to completely remove the protrusions 42 of the metal plating 40, and further, a portion of the metal foil covering the protrusions 42 of the metal foil 10 and / or a portion of the filling portion 41 of the metal plating 40 may be removed. This thins the metal foil 10 and / or the filling portion 41. The degree of over-etching should be such that the electrical connection between the metal foil 10 and the metal plating 40 is not lost.

[0076] Figure 3A(1) shows a case where the protrusion 42 is completely removed, and a portion of the metal foil 10 and a portion of the filler 41 are also removed. As a result, the metal foil 10 and the filler 41 are thinned. In the example shown in Figure 3A(1), for example, if the thickness of the protrusion 42 is 13 μm, the etching amount is set to 15 μm.

[0077] Step S5 may also be performed in such a way that it just removes the protruding portion 42 of the metal plating 40.

[0078] Next, as shown in Figure 3A(2), the plating resist patterns 13 and 23 are removed (step S6).

[0079] Next, as shown in Figure 3A(3), an etching resist film 14 is formed on the metal foil 10, and an etching resist film 24 is formed on the metal foil 20 (step S7). The etching resist film 14 is formed to cover the metal plating 40.

[0080] In this embodiment, since the protrusions 42 of the metal plating 40 are completely removed, the etching resist film 14 does not need to fill the protrusions 42. Therefore, a thin etching resist film 14 (for example, one with a thickness of 7 to 15 μm) can be used. Furthermore, since it is not necessary to fill the protrusions 42, an even thinner etching resist film 14 may be formed compared to the first embodiment.

[0081] Steps S8 to S10 are the same as in the first embodiment (see Figures 3A(4), 3B(1),(2)).

[0082] After going through the above steps S1 to S10, a flexible printed circuit board 1A according to the second embodiment is manufactured as shown in Figure 3B(2).

[0083] As described above, in this embodiment, the protrusions 42 of the metal plating 40 are completely removed before removing the plating resist patterns 13 and 23. As a result, the etching resist film 14 does not need to fill the protrusions 42, and a thinner etching resist film 14 can be used. Also, as in the first embodiment, a thinner etching resist film 24 can be used. By making the etching resist films 14 and 24 thinner, the resolution of exposure and development is increased, and etching resist patterns 14a and 24a with higher resolution can be formed. Because the etching resist patterns 14a and 24a have higher resolution, the etching resolution is increased, and finer wiring patterns WP1 and WP2 can be formed.

[0084] (Summary of the first and second embodiments) According to the first embodiment, in the etching process of the protrusions 42 of the metal plating 40, the process is carried out in such a way that a portion of the protrusions 42 is left intact. As a result, a portion of the protrusions 42 is removed, resulting in protrusions 42a with less protrusion, so that thin etching resist films 14 and 24 can be used. Consequently, fine wiring patterns can be formed in the manufacture of flexible printed circuit boards having interlayer conductive paths.

[0085] On the other hand, according to the second embodiment, in the etching process of the protrusions 42 of the metal plating 40, the protrusions 42 are completely removed. This makes it possible to obtain the same effects as in the first embodiment.

[0086] The etching process for the protrusions 42 of the metal plating 40 may be performed such that the upper surface of the metal plating 40 after etching and the upper surface of the metal foil 10 constituting the wiring pattern WP1 (i.e., the upper surface of the wiring 11) are substantially the same plane. For example, etching may be performed with the aim of precisely removing the protrusions 42. As described above, errors occur in the degree of etching, so the state after etching will be the same as in the first or second embodiment. In either case, a thin etching resist film can be used, and a fine wiring pattern can be formed.

[0087] Furthermore, the etching process for the protrusions 42 of the metal plating 40 may be performed such that the height difference between the upper surface of the etched metal plating 40 and the upper surface of the metal foil 10 constituting the wiring pattern WP1 is within ±5 μm. This allows for the use of a thinner etching resist film and the formation of a finer wiring pattern.

[0088] Furthermore, the etching process of the protrusions 42 of the metal plating 40 may be performed such that the height difference is within ±2 μm. This allows for the use of an even thinner etching resist film and the formation of even finer wiring patterns.

[0089] (Third embodiment) Next, a method for manufacturing a flexible printed circuit board according to the third embodiment will be described with reference to Figures 1 and 4. One of the differences between this embodiment and the first and second embodiments is the presence of a dummy opening. Hereafter, this embodiment will be described focusing on the differences from the first and second embodiments, and similar parts will be omitted.

[0090] In this embodiment, a dummy opening is formed in the plating resist pattern 13 in step S3. The process of forming the plating resist patterns 13 and 23 according to this embodiment (step S3) will be described in more detail below.

[0091] First, a plating resist film is formed on the metal foil 10, covering the metal foil 10 and the conductive holes H1. Also, a plating resist film is formed on the metal foil 20, covering the metal foil 20.

[0092] Next, the plating resist film is exposed and developed to form the openings 13a. In this embodiment, as shown in Figure 4(1), multiple conductive holes H1 are provided. Therefore, multiple openings 13a are formed in the plating resist film.

[0093] As shown in Figure 4(1), each conductive hole H1 is generally arranged according to the shape of the wiring, and therefore is not evenly distributed in a plan view. As a result, there are areas where the conductive holes H1 are sparse (upper part of the figure) and areas where they are dense (lower part of the figure).

[0094] Because the conductive holes H1 are not evenly spaced, the amount of metal plating 40 deposited in the opening 13a in step S4 differs for each conductive hole H1. For example, in areas where the conductive holes H1 are sparse, the metal plating 40 is formed thicker (the top surface is higher). On the other hand, in areas where the conductive holes H1 are densely packed, the metal plating 40 is formed thinner (the top surface is lower). Thus, the height of the top surface of the metal plating 40 for each conductive hole H1 is not uniform. Therefore, the height of the top surface of the metal plating 40 after etching the protruding portion 42 of the metal plating 40 in step S5 is also not uniform.

[0095] Therefore, in this embodiment, as shown in Figure 4(2), dummy openings 13b are formed by exposing and developing the plating resist film. This makes it possible to uniformly deposit the amount of metal plating in each conductive hole H1 and opening 13a. Thus, the plating resist pattern 13 has dummy openings 13b. The formation order of each opening 13a and each dummy opening 13b is arbitrary and may be simultaneous. As shown in Figure 4(2), the metal foil 10 is exposed in the dummy opening 13b. The dummy opening may also be formed on the second main surface side of the insulating substrate 30. In this case, the metal foil 20 is exposed in the dummy opening. Furthermore, dummy openings may be formed on both the first main surface side and the second main surface side of the insulating substrate 30.

[0096] In the step of forming the metal plating 40 (step S4), a dummy plating is formed on the metal foil 10 exposed in the dummy opening 13b. As described above, the amount of metal plating deposited in each conductive hole H1 and opening 13a can be made uniform. That is, the height of the upper surface of the metal plating 40 in each opening 13a can be made uniform.

[0097] As shown in Figure 4(2), the position and shape of the dummy opening 13b are arbitrary. This is because at least a portion of the dummy plating formed inside the dummy opening 13b will be removed in a later step.

[0098] More specifically, in the step of etching the protrusions 42 of the metal plating 40 (step S5), at least a portion of the dummy plating is removed. If the amount of etched dummy plating is small, some of the dummy plating remains. On the other hand, if the amount of etched dummy plating is large, it is completely removed.

[0099] In step S5, even if the dummy plating is not completely removed and some remains on the metal foil, this is not a problem as long as it does not affect the formation of the etching resist films 14 and 24. For example, it is sufficient if the upper surface of the dummy plating and the upper surface of the metal foil 10 (and / or metal foil 20) are substantially the same plane. Furthermore, any remaining dummy plating formed on metal foils other than the metal foil that will become the wiring 11 (wiring 21) will be completely removed in the etching process (step S9) in which the metal foils 10 and 20 that are not covered with the etching resist patterns 14a and 24a are removed by etching.

[0100] According to the manufacturing method of the flexible printed circuit board of this embodiment, even when forming multiple conductive holes H1, the same effects as in the first and second embodiments can be obtained. That is, even when forming multiple conductive holes H1, the upper surfaces of the multiple metal platings 40 and the upper surface of the metal foil 10 (i.e., the upper surface of the wiring 11) can be made substantially the same plane. As a result, a thin etching resist film can be used, and a fine wiring pattern can be formed.

[0101] In the first to third embodiments, the opening of the conductive hole H1 is located on the first main surface of the insulating substrate 30. However, the embodiment is not limited to this, and the main surface of the insulating substrate 30 on which the opening of the conductive hole H1 is located (i.e., the main surface of the insulating substrate 30 opposite to the bottom surface of the conductive hole H1) may be either the first main surface or the second main surface. Furthermore, multiple conductive holes H1 may be formed, and the openings of at least two of these conductive holes H1 may be located on different main surfaces of the first and second main surfaces.

[0102] Furthermore, while the first to third embodiments used a double-sided metal-clad laminate having two layers of metal foil, a multilayer flexible printed circuit board having three or more layers of metal foil may be used as the starting material. In this case, the outermost layer of the multilayer flexible printed circuit board, which is the target for forming the wiring pattern and interlayer conductive paths, is considered the first main surface of the double-sided metal-clad laminate in this embodiment, and the method according to this embodiment is applied.

[0103] Furthermore, in the manufacturing process of flexible printed circuit boards according to the first to third steps, at least one of the steps (steps S1 to S10) may be carried out using a roll-to-roll method (continuous transport). This can improve the manufacturing efficiency of flexible printed circuit boards. Alternatively, all steps may be carried out using a roll-to-roll method.

[0104] (Flexible printed circuit board) As shown in Figures 2C(2) and 3B(2), the flexible printed circuit board manufactured by the method described in the first and second embodiments comprises an insulating substrate 30, wiring 11, lands 12, lands 22 (conductive layer), and metal plating 40.

[0105] The insulating substrate 30 has an upper surface (first main surface) and a lower surface (second main surface opposite the first main surface).

[0106] The wiring 11 is provided on the first main surface of the insulating substrate 30. The land 12 is provided on the first main surface of the insulating substrate 30 and is connected to the wiring 11. Note that both the wiring 11 and the land 12 may be provided in multiples.

[0107] Land 22 is provided on the second main surface of the insulating substrate 30 and faces land 12 across the insulating substrate 30. Alternatively, a ground layer may be provided in the position of land 22. Furthermore, one or more wires 21 may be provided on the second main surface of the insulating substrate 30.

[0108] The metal plating 40 electrically connects the land 12 and the conductive layer. The upper surface of the metal plating 40 is substantially flush with the upper surface of the wiring 11.

[0109] Furthermore, the height difference between the upper surface of the metal plating 40 and the upper surface of the wiring 11 may be within ±5 μm, or within ±2 μm.

[0110] Multiple metal platings 40 may be provided. In this case, the upper surfaces of the multiple metal platings 40 may be substantially the same as the upper surface of the wiring 11. Furthermore, the upper surfaces of at least two of the multiple metal platings 40 may be located on different main surfaces of the first and second main surfaces. In addition, if the wiring 21 is provided on the second main surface, the upper surface of the metal plating 40 whose upper surface is on the second main surface side may be substantially the same as the upper surface of the wiring 21.

[0111] Based on the above description, those skilled in the art may conceive of additional effects and various modifications of the present invention, but the embodiments of the present invention are not limited to the individual embodiments described above. Components from different embodiments may be combined as appropriate. Various additions, modifications, and partial deletions are possible without departing from the conceptual idea and spirit of the present invention derived from the claims and their equivalents. [Explanation of Symbols]

[0112] 1.1A Flexible Printed Circuit Board 2. Double-sided metal-clad laminate 10, 20, 100, 200 metal foils 11,21,110,210 wiring 12,22,120,220 Rand 13,23,130,230 Plating Resist Pattern 13a,130a opening 13b Dummy opening 14, 24, 140, 240 Etching resist film 14a, 24a, 140a, 240a etching resist patterns 30,300 Insulating substrate 40,400 Metal plating 41,410 Filling section 42,42a,420 Projection H1,H Conduction hole WP1, WP2, WP10, WP20 Wiring Pattern

Claims

1. A step of preparing a double-sided metal-clad laminate having an insulating substrate having a first main surface and a second main surface opposite to the first main surface, a first metal foil provided on the first main surface, and a second metal foil provided on the second main surface, A step of partially removing the first metal foil and the insulating substrate to form a conductive hole on the bottom surface in which the second metal foil is exposed, A step of forming a plating resist pattern having openings in which the conductive holes are exposed on the first metal foil, A step of forming a metal plating by applying a metal plating treatment to the opening, the metal plating having a filling portion that fills the conductive hole and a protruding portion that is located above the filling portion and covers the opening edge portion of the first metal foil that surrounds the conductive hole, A step of etching the protruding portion of the metal plating exposed in the opening of the plating resist pattern, A step of removing the aforementioned plating resist pattern, A step of forming an etching resist film on the first metal foil so as to cover the metal plating, The process involves exposing and developing the etching resist film to form an etching resist pattern, A step of forming a wiring pattern by etching away the first metal foil that is not covered with the etching resist pattern and removing the etching resist pattern, A method for manufacturing a flexible printed circuit board, comprising the features described above.

2. The method for manufacturing a flexible printed circuit board according to claim 1, wherein the step of etching the protruding portion of the metal plating is performed in such a way that the protruding portion remains.

3. The method for manufacturing a flexible printed circuit board according to claim 1, wherein the step of etching the protruding portion of the metal plating is performed so as to completely remove the protruding portion.

4. The method for manufacturing a flexible printed circuit board according to claim 3, wherein the step of etching the protruding portion of the metal plating is performed in such a way as to further remove a portion of the metal foil covering the protruding portion of the first metal foil, and / or a portion of the filling portion of the metal plating.

5. The method for manufacturing a flexible printed circuit board according to any one of claims 1 to 4, wherein the step of etching the protruding portion of the metal plating is performed such that the upper surface of the metal plating after etching and the upper surface of the first metal foil constituting the wiring pattern are substantially the same plane.

6. The method for manufacturing a flexible printed circuit board according to claim 5, wherein the step of etching the protruding portion of the metal plating is performed such that the height difference between the upper surface of the metal plating after etching and the upper surface of the first metal foil constituting the wiring pattern is within ±5 μm.

7. The method for manufacturing a flexible printed circuit board according to claim 6, wherein the step of etching the protruding portion of the metal plating is performed such that the height difference is within ±2 μm.

8. The aforementioned plating resist pattern has dummy openings, In the process of forming the metal plating, a dummy plating is formed on the first metal foil exposed in the dummy opening. A method for manufacturing a flexible printed circuit board according to any one of claims 1 to 4, wherein at least a portion of the dummy plating is removed in the step of etching the protruding portion of the metal plating.

9. A method for manufacturing a flexible printed circuit board, wherein at least one step is performed in a roll-to-roll manner, according to the manufacturing method described in any one of claims 1 to 4.

10. The method for manufacturing a flexible printed circuit board according to any one of claims 1 to 4, wherein the first and second metal foils are rolled copper foils.

11. The method for manufacturing a flexible printed circuit board according to any one of claims 1 to 4, wherein the metal plating is electrolytic copper plating.

12. The step of forming the plating resist pattern having the opening is, A step of forming a plating resist film on the first metal foil that covers the first metal foil and the conductive holes, The process involves exposing and developing the plating resist film to form the opening, Includes, The method for manufacturing a flexible printed circuit board according to any one of claims 1 to 4, wherein the exposure is performed by a direct drawing method that does not use an exposure mask.

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