Manufacturing method of flexible printed circuit board and flexible printed circuit board

Abstract

This invention provides a method for forming fine circuit patterns in the manufacture of a flexible printed circuit board with interlayer conductive paths. The method for manufacturing the flexible printed circuit board 1 in the embodiment includes the following steps: preparing a double-sided metal-coated laminate 2 having an insulating substrate 30, a metal foil 10, and a metal foil 20; forming a through-hole H1 with the bottom surface exposing the metal foil 20; forming an anti-plating pattern 13 with an opening 13a exposing the through-hole H1 on the metal foil 10; forming a metal plating layer 40 with a filling portion 41 and a protrusion 42 in the opening 13a; etching the protrusion 42 of the metal plating layer 40; removing the anti-plating pattern 13; forming an etch-resistant film 14 on the metal foil 10 to cover the metal plating layer 40; forming the etch-resistant pattern 14a; and forming a circuit pattern WP1.

Description

Method for manufacturing flexible printed circuit board and flexible printed circuit board The present invention relates to a flexible printed circuit board and a method for manufacturing the same, and more particularly to a method for manufacturing a flexible printed circuit board for forming a fine circuit pattern in the manufacture of a flexible printed circuit board having an interlayer conductive path and the flexible printed circuit board. Flexible printed circuit boards (FPCs) are circuit boards with a circuit pattern formed using metal foils such as copper on a resin film such as polyimide. Because FPCs are thin and flexible, they can be used in narrow enclosures or in areas involving mechanical movement. Electronic devices, starting with information and communication devices like smartphones, are constantly striving for miniaturization and weight reduction. Flexible printed circuit boards (FPCs) meet this goal and are commonly used in electronic devices. Consequently, FPCs are in high demand for further miniaturization and density. Densification of substrates means forming more circuits within a limited substrate area. To achieve this, finer and more multilayered circuit patterns are necessary. Etching is required to create circuit patterns. Subtractive etching is one method of creating circuit patterns, commonly used for forming circuit patterns on flexible printed circuit boards. In subtractive etching, etching occurs simultaneously and in equal amounts across the width of the circuit. Therefore, to create fine circuit patterns, thinner metal foil is preferred. On the other hand, for multilayering, interlayer conductive paths, such as vias, are formed on a substrate with metal foil on both sides or on a multilayer substrate, electrically connecting circuits on different layers through the interlayer conductive paths. When forming the interlayer conductive paths, a metal plating layer is formed on the vias. Patent Documents 1 and 2 describe a method called button plating, which involves performing an electroplating process on only specific portions of a substrate, as one example of a method for forming the metal plating layer. [Prior Art Document] [Patent Document] [Patent Document 1] JP 2006-108270 A [Patent Document 2] JPH 11-195849 A [Problem to be Solved by the Invention] A method for manufacturing a flexible printed wiring board having a circuit pattern and interlayer conductive paths using a button plating method will be described with reference to Figures 5A to 5C. Figures 5A to 5C are cross-sectional views of the process of manufacturing a flexible printed wiring board as a comparative example. First, as shown in FIG5A(1), a double-sided metal-clad laminate having metal foil 100, metal foil 200, and an insulating substrate 300 is prepared. Next, as shown in FIG5A(2), a via hole H is formed in the double-sided metal-clad laminate. Next, as shown in FIG5A(3), a resist pattern 130 and a resist pattern 230 are formed on the upper and lower surfaces of the double-sided metal-clad laminate, respectively. The resist pattern 130 has an opening 130a that exposes the via hole H. In other words, the resist pattern 130 does not cover the via hole H. Next, as shown in FIG5A (4), a metal plating layer 400 (button plating) is formed by applying a metal plating process to the opening 130a. Thus, an interlayer conductive path electrically connecting the metal foil 100 and the metal foil 200 is formed. The metal plating layer 400 has a filling portion 410 and a protrusion 420. The filling portion 410 is the portion of the metal plating layer 400 that fills the via H, and is the portion below the dotted line in FIG5A (4). The protrusion 420 is the portion of the metal plating layer 400 that is located above the filling portion 410, and is the portion above the dotted line in FIG5A (4). The protrusion 420 is the portion (opening edge) that covers the metal foil 100 surrounding the via H. The protrusion 420 is also called a button land. Next, as shown in FIG5B (1), the plating resist patterns 130 and 230 are removed. Next, as shown in FIG5B (2), an anti-etching film 140 is formed on the metal foil 100, and an anti-etching film 240 is formed on the metal foil 200. The anti-etching film 140 is formed so as to cover the metal plating layer 400. Since the metal plating layer 400 has a protrusion 420, the anti-etching film 140 is formed so as to cover the protrusion 420. Next, as shown in FIG5B (3), the anti-etching films 140 and 240 are exposed and developed to form anti-etching patterns 140a and 240a. Next, as shown in FIG5C (1), etching is performed to remove the metal foil 100 not covered by the anti-etching pattern 140a and the metal foil 200 not covered by the anti-etching pattern 240a. Thereafter, as shown in FIG5C (2), the anti-etching patterns 140a and 240a are removed. Through the above steps, a flexible printed wiring board including the wiring pattern WP10 having a plurality of wirings 110 , the pads 120 , the WP20 having a plurality of wirings 210 , and the pads 220 facing the pads 120 with the insulating substrate 300 interposed therebetween is manufactured. The above-mentioned method for manufacturing a flexible printed wiring board has the following problems when making the circuit pattern finer. As shown in FIG5B (1), the protrusion 420 of the metal plating layer 400 protrudes from the metal foil 100. To prevent the metal plating layer 400 from being etched in a subsequent step, as shown in FIG5B (2), when forming the anti-etching film 140, it is necessary to cover the protrusion 420 of the metal plating layer 400 protruding from the metal foil 100. Therefore, the anti-etching film 140 is formed to be relatively thick. However, when the anti-etching film 140 is thickened, the etching resolution decreases, making it difficult to form a fine circuit pattern WP10. The present invention is based on the recognition of the above-mentioned technology, and aims to provide a method for manufacturing a flexible printed wiring board and a flexible printed wiring board for forming a fine circuit pattern in the manufacture of a flexible printed wiring board having interlayer conductive paths. [Means for Solving the Problem] The inventors have conducted earnest research to solve the above-mentioned problem, and as a result, they have obtained a technical idea of ​​reducing the protrusion of the metal plating by etching at least a part of the protrusion of the metal plating, so that the anti-etching film covering the metal plating can be formed to be thinner, thereby enabling the formation of a fine circuit pattern. A method for manufacturing a flexible printed wiring board according to the present invention is characterized by comprising the following steps: preparing a double-sided metal-clad laminate comprising an insulating substrate having a first main surface and a second main surface opposite the first main surface, a first metal foil disposed on the first main surface, and a second metal foil disposed on the second main surface; partially removing the first metal foil and the insulating substrate to form a via hole whose bottom surface exposes the second metal foil; forming a resist pattern having an opening exposing the via hole on the first metal foil; electroplating the opening to form a metal plating layer having a filling portion filling the via hole and a protruding portion located above the filling portion and covering an edge of the via hole in the first metal foil; etching the protruding portion of the metal plating layer exposed at the opening of the resist pattern; removing the resist pattern; forming an etching resist film on the first metal foil so as to cover the metal plating layer; exposing and developing the etching resist film to form an etching resist pattern; and removing the portion of the first metal foil not covered by the etching resist pattern by etching, and then removing the etching resist pattern to form a circuit pattern. Furthermore, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating layer may be performed in such a manner that the protruding portion remains. Furthermore, in the method for manufacturing the flexible printed circuit board, the step of etching the protruding portion of the metal plating layer can be performed in a manner such that the protruding portion is completely removed. Furthermore, in the method for manufacturing the flexible printed circuit board, the step of etching the protruding portion of the metal plating layer may be performed by further removing a portion of the first metal foil covered by the protruding portion and / or a portion of the filling portion of the metal plating layer. Furthermore, in the method for manufacturing the flexible printed circuit board, the step of etching the protruding portion of the metal plating layer can be performed so that the upper surface of the etched metal plating layer and the upper surface of the first metal foil constituting the circuit pattern are substantially in the same plane. Furthermore, in the method for manufacturing the flexible printed circuit board, the step of etching the protruding portion of the metal plating layer can be performed so that the height difference between the upper surface of the etched metal plating layer and the upper surface of the first metal foil constituting the circuit pattern is within ±5 μm. Furthermore, in the method for manufacturing the flexible printed wiring board, the step of etching the protruding portion of the metal plating layer can be performed so that the height difference is within ±2 μm. In addition, in the manufacturing method of the flexible printed circuit board, the resist pattern may have a dummy opening; in the step of forming the metal plating layer, a dummy plating layer may be formed on the first metal foil exposed to the dummy opening; and in the step of etching the protruding portion of the metal plating layer, at least a portion of the dummy plating layer may be removed. Furthermore, in the method for manufacturing the flexible printed wiring board, at least one step may be performed in a roll-to-roll manner. Furthermore, in the method for manufacturing the flexible printed circuit board, the first and second metal foils may be rolled copper foils. Furthermore, in the method for manufacturing the flexible printed wiring board, the metal plating layer may be an electrolytic copper plating layer. Furthermore, in the method for manufacturing the flexible printed circuit board, the step of forming the resist pattern having the opening may include: forming a resist film on the first metal foil, covering the first metal foil and the via hole; and exposing and developing the resist film to form the opening, wherein the exposure may be performed by a direct writing method without using an exposure mask. The flexible printed circuit board described in the present invention is characterized by comprising: an insulating substrate having a first main surface and a second main surface opposite the first main surface; a circuit disposed on the first main surface; a solder pad disposed on the first main surface; a conductive layer disposed on the second main surface; and a metal plating layer electrically connected to the solder pad and the conductive layer, with its upper surface being substantially coplanar with the upper surface of the circuit. Furthermore, in the flexible printed wiring board, a height difference between an upper surface of the metal plating layer and an upper surface of the circuit may be within ±5 μm. Furthermore, in the flexible printed wiring board, the height difference may be within ±2 μm. Furthermore, in the flexible printed circuit board, a plurality of metal plating layers may be provided, and the upper surfaces of the plurality of metal plating layers and the upper surface of the circuit provided on the first main surface may be substantially in the same plane. Effects of the Invention: According to the method for manufacturing a flexible printed wiring board of the present invention, the metal-plated protrusion is etched to remove at least a portion of the protrusion before removing the plating resist pattern. This eliminates the need to mask the thick protrusion with an etch resist film. Consequently, a thinner etch resist film can be used. Consequently, when manufacturing a flexible printed wiring board having interlayer conductive paths, a fine circuit pattern can be formed. As described above, the inventors developed the technical concept of etching at least a portion of the protrusion of the metal plating layer before forming the etching-resistant film in order to form a fine circuit pattern on a flexible printed wiring board having interlayer conductive paths. Typically, when etching the protrusion of the metal plating layer, variations in the extent of etching occur. Considering this, the post-etching state can be either of the following: A small amount of etching may not completely remove the protrusion. Alternatively, a large amount of etching may completely remove the protrusion. The first and second embodiments described below correspond to the above-mentioned individual cases. That is, the first embodiment is when the etching amount is small and the protrusion is not completely removed, and the second embodiment is when the etching amount is large and the protrusion is completely removed. Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Throughout the figures, elements with equivalent functions are designated by the same reference numerals. Furthermore, the figures are schematic, primarily illustrating the key features of each embodiment. The relationship between thickness and planar dimensions, or the thickness ratios of various layers, may differ from actual values. (First Embodiment) Referring to FIG. 1 and FIG. 2A to FIG. 2C , an example of a method for manufacturing a flexible printed wiring board according to a first embodiment will be described. FIG. 1 is a flow chart illustrating the method for manufacturing a flexible printed wiring board according to this embodiment. FIG. 2A to FIG. 2C are cross-sectional views illustrating the steps of the method for manufacturing a flexible printed wiring board according to this embodiment. As shown in FIG2A(1), a double-sided metal-clad laminate 2 is prepared (step S1). This double-sided metal-clad laminate 2 includes a metal foil 10, a metal foil 20, and an insulating substrate 30. More specifically, the double-sided metal-clad laminate 2 includes an insulating substrate 30 having an upper surface (a first main surface) and a lower surface (a second main surface opposite to the first main surface), the metal foil 10 provided on the first main surface, and the metal foil 20 provided on the second main surface. Metal foils 10 and 20 are, for example, copper foil (electrolytic copper foil) with a thickness of 9 μm. Alternatively, the thickness of metal foils 10 and 20 may range from 5 to 72 μm. Furthermore, metal foils 10 and 20 may be made of metals other than copper (e.g., silver, aluminum, etc.). When copper foil is used for metal foils 10 and 20, rolled copper foil may also be used. Using rolled copper foil allows for the provision of highly flexible flexible printed circuit boards. Furthermore, metal foils 10 and 20 may also be half-etched. The insulating substrate 30 is made of, for example, polyimide with a thickness of 25 μm. Alternatively, the insulating substrate 30 may have a thickness of 6 to 100 μm. Furthermore, the insulating substrate 30 may be made of fluorinated materials such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) and PTFE (polytetrafluoroethylene), polyimide materials such as MPI (modified polyimide) and PI (polyimide), PEEK (polyetheretherketone), PET (polyethylene terephthalate), or PIN (polyethylene naphthalate). Next, as shown in FIG2A (2), the metal foil 10 and the insulating substrate 30 are partially removed to form a via hole H1 (step S2). The via hole H1 is used to form an interlayer conductive path. The bottom surface of the via hole H1 exposes the metal foil 20. In other words, the via hole H1 is a bottomed hole. By forming a bottomed hole, it is not necessary to suppress etching of unnecessary portions during etching in step S5, which can make the etching process easier. In step S2, for example, the metal foil 10 and the insulating substrate 30 can be removed using a conformal mask method. When the conformal mask method is used, the metal foil 10 is first partially removed by etching or the like. Next, the portion of the metal foil 10 from which the metal foil 10 has been removed is irradiated with laser light to remove the insulating substrate 30 exposed in that portion. Thus, a via H1 is formed in which the metal foil 20 is exposed on the bottom surface. As a type of laser, for example, a UV-YAG laser or a carbonic acid laser can be used. For the formation of high-density circuits, a UV-YAG laser is preferred. In addition, for the formation of the via H1, other methods other than the conformal mask method (for example, a large window method or a direct drilling method) can also be used. The diameter of the via hole H1 at the opening (i.e., the portion where the metal foil 10 is removed) is, for example, 50 μm. Alternatively, by adjusting the laser light irradiation conditions, as shown in FIG2A (2), the via hole H1 can be formed so that its inner diameter decreases as the depth increases from the opening. This facilitates the formation of the metal plating layer 40 described later in the via hole H1. After forming the via hole H1 , the metal foil 20 exposed on the bottom surface may be cleaned (eg, desmeared). Thereafter, the via hole H1 may be made conductive (eg, electroless plating). Next, as shown in FIG2A (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 that exposes the via hole H1. An example of the steps for forming plating resist patterns 13 and 23 will be described in more detail below. First, a plating resist film is formed on metal foil 10, covering metal foil 10 and via H1. Furthermore, a plating resist film is formed on metal foil 20, covering metal foil 20. Next, the plating resist film is exposed and developed to form openings 13a. Through these steps, plating resist patterns 13 and 23 are formed. Furthermore, exposure of the resist can be performed using direct writing methods (direct writing exposure), proximity exposure, projection exposure, and the like. Direct writing methods that do not use an exposure mask (e.g., laser direct writing or electron beam direct writing) are preferred. Using direct writing methods allows for an exposure pattern to be created that corresponds to the expansion or contraction of the insulating substrate 30 or the like, even if the insulating substrate 30 expands or contracts during the steps preceding the exposure step. This can minimize positional deviation between the via H1 and the opening 13a. This can, for example, reduce the diameter of the opening 13a. Next, as shown in FIG2A (4), a metal plating layer 40 is formed by applying a metal electroplating process to the opening 13a (step S4). In this way, an interlayer conductive path is formed that electrically connects the metal foil 10 and the metal foil 20. In this embodiment, since the via H1 is a bottomed hole, the interlayer conductive path is a bottomed through-hole. In addition, since the metal plating layer 40 will be etched in a subsequent step, the metal plating layer 40 is preferably a filled via. The metal plating layer 40 has a filling portion 41 and a protrusion 42. The filling portion 41 is the portion of the metal plating layer 40 that fills the via H1, that is, the portion below the dotted line in FIG2A(4). The protrusion 42 is the portion of the metal plating layer 40 that is located above the filling portion 41, that is, the portion above the dotted line in FIG2A(4). The protrusion 42 covers the portion of the metal foil 10 surrounding the via H1 (opening edge). The protrusion 42 is also called a button pad. The metal plating layer 40 is formed so that the thickness of the protrusion 42 is, for example, 13 μm. In this embodiment, the metal plating layer 40 is an electrolytic plating layer. More specifically, in step S4, an electrolytic plating process is performed to form the metal plating layer 40. By using electrolytic plating, the plating process time can be shortened compared to the case of using chemical plating, and the manufacturing efficiency can be improved. Furthermore, the metal plating layer 40 can be made of the same metal as the metal foil 10. Using the same metal allows for consistent physical, electrical, and chemical properties, making handling easier. In particular, when the metal foil 10 is copper foil, the metal plating layer 40 can also be electrolytic copper plating. By forming an electrolytic copper plating layer, the metal of the metal plating layer 40 can be the same metal (copper) as the rolled copper foil preferred for flexible printed wiring boards. Next, as shown in FIG2B (1), the protruding portion 42 of the metal plating layer 40 is etched (step S5). More specifically, at least a portion of the protruding portion 42 of the metal plating layer 40 exposed in the opening 13a of the resist pattern 13 is etched. Here, etching is performed using, for example, copper chloride. In this embodiment, the step of etching the protrusion 42 of the metal plating layer 40 is performed so that the protrusion remains. This removes a portion of the protrusion 42, leaving 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 is approximately 3 μm. Next, as shown in FIG. 2B ( 2 ), the plating resist patterns 13 , 23 are removed (step S6 ). Next, as shown in FIG2B (3), an etching resistant film 14 is formed on the metal foil 10, and an etching resistant film 24 is formed on the metal foil 20 (step S7). The etching resistant film 14 is formed so as to cover the metal plating layer 40. In this embodiment, since the metal plating layer 40 has a protrusion 42a, the etching resistant film 14 is formed so as to cover the protrusion 42a. In this embodiment, since the thickness of the protrusion 42 a is smaller than the thickness of the protrusion 42 , a thinner etching resistant film 14 (eg, a film with a thickness of 7 to 15 μm) can be formed. Furthermore, since the metal plating layer 40 is not provided on the lower surface side of the insulating substrate 30, a thinner etching resistant film 24 can be formed. Furthermore, when a plurality of via holes H1 are present, the protrusion 42a may 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 protrusion 42a is smaller than the thickness of the protrusion 42, a thinner etching resistant film 24 can be formed. Next, as shown in FIG2B (4), the anti-etching film 14 is exposed and developed to form an anti-etching pattern 14a. In addition, the anti-etching film 24 is also exposed and developed to form an anti-etching pattern 24a (step S8). In this embodiment, the thinner etch-resistant films 14 and 24 improve the resolution of exposure and development, enabling the formation of finer etch-resistant patterns 14a and 24a. Furthermore, the thinner etch-resistant patterns 14a and 24a facilitate the etchant's access to the metal foil. This improves etching resolution and enables the formation of finer circuit patterns. Next, as shown in FIG2C (1), the metal foil 10 not covered by the anti-etching pattern 14a is removed by etching. Furthermore, the metal foil 20 not covered by the anti-etching pattern 24a is also removed by etching (step S9). Next, as shown in FIG2C (2), the etching resist patterns 14a and 24a are removed (step S10). Thus, a wiring pattern WP1 including a plurality of wirings 11 and pads 12 are formed on the upper surface (first main surface) of the insulating substrate 30. Furthermore, a wiring pattern WP2 including a plurality of wirings 21 and pads 22 are also formed on the lower surface (second main surface) of the insulating substrate 30. In this embodiment, since the etching resist patterns 14 a and 24 a have fine patterns and are thin, etching resolution is improved, and fine wiring patterns WP1 and WP2 can be formed. Through the above steps S1 to S10 , the flexible printed circuit board 1 of this embodiment can be manufactured. As described above, in this embodiment, the protrusion 42 of the metal plating layer 40 is etched to remove a portion of the protrusion 42 before removing the plating resist patterns 13 and 23, thereby forming a less protruding protrusion 42a. Therefore, it is not necessary to cover the thick protrusion 42 with the etching resist film 14. Therefore, a thinner etching resist film 14 can be used to cover the protrusion 42a. Alternatively, a thinner etching resist film 24 can be used. As the thickness of the etching resist films 14 and 24 decreases, the resolution of exposure and development increases, enabling the formation of high-resolution etching resist patterns 14a and 24a. The high resolution and thin thickness of the etching resist patterns 14a and 24a improves etching resolution, enabling the formation of fine wiring patterns WP1 and WP2. This allows the formation of fine wiring patterns WP1 and WP2 during the manufacture of the flexible printed wiring board 1 having interlayer conductive paths. Furthermore, in the step of etching the protrusion 42 of the metal plating layer 40, the resist pattern 13 is directly used as an etching mask. Therefore, this step can be shortened. Furthermore, since positional deviation between the resist pattern 13 and the metal plating layer 40 can be suppressed, the protrusion 42 can be etched more accurately. Furthermore, by using rolled copper foil as the metal foils 10 and 20 and not forming the metal plating layer 40 on the bent portion of the flexible printed wiring board 40 , a flexible printed wiring board having high flexibility can be manufactured. Second Embodiment: A method for manufacturing a flexible printed wiring board according to a second embodiment will be described below with reference to Figures 1, 2A, 3A, and 3B. One difference between this embodiment and the first embodiment is the step of etching the protrusion 42 of the metal plating layer 40. The following description will focus on the differences between this embodiment and the first embodiment, and descriptions of the common features will be omitted. As shown in FIG2A (1), a double-sided metal-clad laminate 2 is prepared (step S1). In the double-sided metal-clad laminate 2 of this embodiment, for example, the metal foils 10 and 20 are rolled copper foils with a thickness of 12 μm, and the insulating substrate 30 is made of polyimide with a thickness of 12.5 μm. Alternatively, when copper foil is used as the metal foils 10 and 20, electrolytic copper foil may be used. Next, as shown in FIG2A (2), the metal foil 10 and the insulating substrate 30 are partially removed to form a via hole H1 (step S2). The diameter of the via hole H1 at the opening (ie, the portion where the metal foil 10 is removed) is, for example, 35 μm. Step S3 and step S4 are the same as those in the first embodiment (see FIG. 2A ( 3 ) and ( 4 )). Next, as shown in FIG3A (1), the protruding portion 42 of the metal plating layer 40 is etched (step S5). In this embodiment, unlike the first embodiment, the protruding portion 42 of the metal plating layer 40 is completely removed in this step. Specifically, this is done by over-etching. When overetching is performed, the protrusion 42 is completely removed, and a portion of the metal foil 10 and / or a portion of the filling portion 41 may also be removed. More specifically, step S5 completely removes the protrusion 42 of the metal plating layer 40, and may also remove a portion of the metal foil 10 covered by the protrusion 42 and / or a portion of the filling portion 41 of the metal plating layer 40. As a result, the metal foil 10 and / or the filling portion 41 become thinner. The extent of overetching should be sufficient to avoid disrupting the electrical connection between the metal foil 10 and the metal plating layer 40. FIG3A(1) shows a case where the protrusion 42 is completely removed, and further, a portion of the metal foil 10 and a portion of the filling portion 41 are removed. As a result, the metal foil 10 and the filling portion 41 become thinner. In the example shown in FIG3A(1), for example, when the thickness of the protrusion 42 is 13 μm, the etching amount is set to 15 μm. In addition, step S5 may also be performed in a manner of just removing the protruding portion 42 of the metal plating layer 40 . Next, as shown in FIG. 3A ( 2 ), the plating resist patterns 13 and 23 are removed (step S6 ). Next, as shown in FIG3A ( 3 ), an etching resistant film 14 is formed on the metal foil 10 , and an etching resistant film 24 is formed on the metal foil 20 (step S7 ). The etching resistant film 14 is formed so as to cover the metal plating layer 40 . In this embodiment, since the protrusion 42 of the metal plating layer 40 is completely removed, the anti-etching film 14 does not need to cover the protrusion 42. Therefore, a thinner anti-etching film 14 (for example, a film with a thickness of 7 to 15 μm) can be used. In addition, since it is not necessary to cover the protrusion 42, the anti-etching film 14 can be formed thinner than in the first embodiment. Steps S8 to S10 are the same as those in the first embodiment (see Figures 3A (4) and 3B (1), (2)). After the above steps S1 to S10 , as shown in FIG3B ( 2 ), the flexible printed circuit board 1A of the second embodiment is manufactured. As described above, in this embodiment, the protrusion 42 of the metal plating layer 40 is completely removed before removing the plating resist patterns 13 and 23. Therefore, since the anti-etching film 14 does not need to cover the protrusion 42, a thinner film can be used for the anti-etching film 14. In addition, as in the first embodiment, a thinner film can be used for the anti-etching film 24. As the anti-etching films 14 and 24 become thinner, the resolution of exposure and development increases, and higher-resolution anti-etching patterns 14a and 24a can be formed. Due to the higher resolution of the anti-etching patterns 14a and 24a, the etching resolution is improved, and finer line patterns WP1 and WP2 can be formed. (Summary of the First and Second Embodiments) According to the first embodiment, the step of etching the protrusions 42 of the metal plating layer 40 is performed so that a portion of the protrusions 42 remains. Therefore, since a portion of the protrusions 42 is removed, resulting in less protruding protrusions 42a, thinner etching resists 14 and 24 can be used. Consequently, when manufacturing a flexible printed wiring board having interlayer conductive paths, finer circuit patterns can be formed. On the other hand, according to the second embodiment, in the step of etching the protruding portion 42 of the metal plating layer 40, the protruding portion 42 is completely removed. Therefore, the same effect as in the first embodiment can be obtained. Furthermore, during the step of etching the protrusions 42 of the metal plating layer 40, the etching can be performed so that the upper surface of the metal plating layer 40 and the upper surface of the metal foil 10 (i.e., the upper surface of the wiring 11) forming the wiring pattern WP1 are substantially flush with each other. For example, etching can be performed with the goal of just removing the protrusions 42. As described above, due to variations in the etching degree, the state after etching will be similar to that of the first or second embodiment. In either case, a thinner etching resist film can be used, enabling the formation of a fine wiring pattern. Furthermore, the step of etching the protrusions 42 of the metal plating layer 40 can be performed so that the height difference between the upper surface of the metal plating layer 40 after etching and the upper surface of the metal foil 10 constituting the wiring pattern WP1 is within ±5 μm. Therefore, a thinner etching resist film can be used, and a finer wiring pattern can be formed. Furthermore, the step of etching the protrusions 42 of the metal plating layer 40 can also keep the height difference within ±2 μm. Therefore, a thinner etching resist film can be used, and a finer circuit pattern can be formed. (Third Embodiment) A method for manufacturing a flexible printed wiring board according to a third embodiment will be described below with reference to Figures 1 and 4 . One difference between this embodiment and the first and second embodiments is the presence of dummy openings. The following description will focus primarily on the differences between this embodiment and the first and second embodiments, and descriptions of the common features will be omitted. In this embodiment, in step S3, a dummy opening is formed in the plating resist pattern 13. The step of forming the plating resist patterns 13, 23 according to this embodiment (step S3) will be described in more detail below. First, a plating resist film is formed on the metal foil 10 so as to cover the metal foil 10 and the via hole H1 . Furthermore, a plating resist film is also formed on the metal foil 20 so as to cover the metal foil 20 . Next, the resist film is exposed and developed to form openings 13a. In this embodiment, as shown in FIG4 (1), a plurality of via holes H1 are provided. Therefore, a plurality of openings are formed in the resist film 13a. As shown in Figure 4 (1), since each via H1 is generally arranged according to the shape of the circuit, it is not uniformly arranged in a plan view. Therefore, there are areas where the vias H1 are sparse (upper side in the figure) and areas where they are dense (lower side in the figure). Because the vias H1 are unevenly distributed, the amount of metal plating 40 deposited in the openings 13a in step S4 varies across vias H1. For example, in areas where the vias H1 are sparsely populated, the metal plating 40 is thicker (with a higher top surface). On the other hand, in areas where the vias H1 are densely populated, the metal plating 40 is thinner (with a lower top surface). Consequently, the top surface height of the metal plating 40 varies across vias H1. Consequently, after etching the protrusions 42 of the metal plating 40 in step S5, the top surface height of the metal plating 40 also varies. Therefore, in this embodiment, as shown in FIG4 (2), the dummy opening 13b is formed by exposure and development of the resist coating. Therefore, the amount of metal plating deposited in each via H1 and the opening 13a can be made consistent with each other. Therefore, the resist pattern 13 has a dummy opening 13b. In addition, the order of forming each opening 13a and each dummy opening 13b can be arbitrary or performed simultaneously. As shown in FIG4 (2), the metal foil 10 is exposed in the dummy opening 13b. In addition, the dummy opening can 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. In addition, the dummy opening can also be formed on both the first main surface side and the second main surface side of the insulating substrate 30. In the step of forming metal plating layer 40 (step S4), a dummy plating layer is formed on the metal foil 10 exposed in dummy opening 13b. As described above, the amount of metal plating deposited in each via H1 and opening 13a can be made uniform. In other words, the top surface height of metal plating layer 40 can be made uniform in each opening 13a. In addition, as shown in FIG4 (2), the position and shape of the dummy opening 13b are arbitrary. This is because at least a portion of the dummy plating layer formed in the dummy opening 13b will be removed in a subsequent step. More specifically, during the step of etching the protruding portion 42 of the metal plating layer 40 (step S5), at least a portion of the dummy plating layer is removed. If the amount of dummy plating etched is small, a portion of the dummy plating layer may remain. On the other hand, if the amount of dummy plating etched is large, it may be completely removed. Furthermore, even if the dummy plating layer is not completely removed in step S5 and some remains on the metal foil, this is not a problem as long as the extent does not affect the formation of the etching resist films 14 and 24. For example, the upper surface of the dummy plating layer and the upper surface of the metal foil 10 (and / or metal foil 20) can be substantially flush. Furthermore, any remaining dummy plating formed on the metal foil other than the metal foil that will form the wiring 11 (wiring 21) will be completely removed during the step (step S9) of removing the portions of the metal foil 10 and 20 not covered by the etching resist patterns 14a and 24a by etching. According to the method for manufacturing a flexible printed wiring board of this embodiment, even when a plurality of via holes H1 are formed, the same effects as those of the first and second embodiments can be achieved. Specifically, even when a plurality of via holes H1 are formed, the upper surfaces of the plurality of metal plating layers 40 and the upper surface of the metal foil 10 (i.e., the upper surface of the traces 11) can be substantially coplanar. Consequently, a thin etching resist film can be used, and a fine trace pattern can be formed. Furthermore, in the first through third embodiments, the opening of the via H1 is located on the first main surface of the insulating substrate 30. However, this is not limiting. The main surface of the insulating substrate 30 where the via H1 is located (i.e., the main surface of the insulating substrate 30 that is opposite the bottom surface of the via H1) can be either the first or second main surface. Furthermore, a plurality of vias H1 can be formed, with at least two of the vias H1 openings located on different main surfaces than the first and second main surfaces. Furthermore, while a double-sided metal-clad laminate with two layers of metal foil was used in the first through third embodiments, a multilayer flexible printed wiring board with three or more layers of metal foil can also be used as the starting material. In this case, the outermost layer of the multilayer flexible printed wiring board, on which the wiring pattern and interlayer conductive paths are formed, will be considered the first major surface of the double-sided metal-clad laminate in these embodiments, and the methods of these embodiments will be applied accordingly. Furthermore, in the flexible printed circuit board manufacturing process according to the first to third embodiments, at least one of the steps (steps S1 to S10) can be performed in a roll-to-roll manner (continuous conveyance). This improves the efficiency of flexible printed circuit board manufacturing. Furthermore, all steps can be performed in a roll-to-roll manner. (Flexible Printed Wiring Board) As shown in Figures 2C (2) and 3B (2), the flexible printed wiring board manufactured by the method described in the first and second embodiments includes an insulating substrate 30, a wiring 11, a pad 12, a pad 22 (conductive layer) and a metal plating layer 40. The insulating base material 30 has an upper surface (a first main surface) and a lower surface (a second main surface opposite to the first main surface). The wiring 11 is provided on the first main surface of the insulating substrate 30. The pad 12 is provided on the first main surface of the insulating substrate 30 and is connected to the wiring 11. In addition, a plurality of wirings 11 and pads 12 may be provided. The pads 22 are provided on the second main surface of the insulating substrate 30 and face the pads 12 across the insulating substrate 30. Alternatively, a ground layer may be provided in place of the pads 22. Furthermore, one or more lines 21 may be provided on the second main surface of the insulating substrate 30. The metal plating layer 40 electrically connects the pad 12 and the conductive layer. The upper surface of the metal plating layer 40 and the upper surface of the circuit 11 are substantially in the same plane. In addition, the height difference between the upper surface of the metal plating layer 40 and the upper surface of the circuit 11 may be within ±5 μm, or the height difference may be within ±2 μm. Alternatively, a plurality of metal plating layers 40 may be provided. In this case, the upper surfaces of the plurality of metal plating layers 40s may be substantially coplanar with the upper surface of the wiring 11. Furthermore, the upper surfaces of at least two of the plurality of metal plating layers 40 may be located on different sides of the first and second main surfaces. Furthermore, when the wiring 21 is provided on the second main surface, the upper surface of the wiring 21 may be substantially coplanar with the upper surface of the metal plating layer 40 located on the second main surface. Based on the above description, those skilled in the art can envision additional effects and various variations of the present invention, but the various aspects of the present invention are not limited to the above embodiments. The constituent elements in different embodiments may also be appropriately combined. Various additions, modifications, and partial deletions may be made without departing from the concept, idea, and gist of the present invention, without departing from the scope of the claims and their equivalents. 1, 1A: Flexible printed circuit board 2: Double-sided metal-clad laminate 10, 20, 100, 200: Metal foil 11, 21, 110, 210: Circuit 12, 22, 120, 220: Pad 13, 23, 130, 230: Resist pattern 13a, 130a: Opening 13b: Dummy opening 14, 24, 140, 240: Etching resistant film 14a, 24a, 140a, 240a: Etching resistant pattern 30, 300: Insulating substrate 40, 400: Metal plating layer 41, 410: Filling portion 42, 42a, 420: Protrusion H1, H: Via holes WP1, WP2, WP10, WP20: Circuit pattern S1-S10: Steps FIG1 is a flowchart illustrating a method for manufacturing a flexible printed wiring board according to an embodiment. FIG2A is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to the first embodiment. FIG2B is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to the first embodiment, continuing from FIG2A . FIG2C is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to the first embodiment, continuing from FIG2B . FIG3A is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to a second embodiment. FIG3B is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to the second embodiment, continuing from FIG3A . FIG4 is a plan view of a flexible printed wiring board illustrating an example of a dummy opening according to a third embodiment. FIG5A is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to a comparative example. FIG5B is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to a comparative example, continuing from FIG5A . FIG5C is a cross-sectional view illustrating a process of manufacturing a flexible printed wiring board according to a comparative example, continuing from FIG5B . S1~S10: Steps

Claims

1. A method for manufacturing a flexible printed circuit board, comprising the following steps: preparing a double-sided metal-clad laminate, comprising an insulating substrate having a first main surface and a second main surface opposite to the first main surface, a first metal foil disposed on the first main surface, and a second metal foil disposed on the second main surface; partially removing the first metal foil and the insulating substrate to form a via with the bottom surface exposed to the second metal foil; forming a resist pattern on the first metal foil having an opening exposing the via; forming a metal plating layer by performing a metal electroplating process on the opening, having a filling portion filling the via and a protrusion located on the filling portion and covering the opening edge of the via in the first metal foil; directly using the resist pattern as an etching mask to etch the protrusion of the metal plating layer exposed at the opening of the resist pattern; removing the resist pattern; and forming an etch-resistant film on the first metal foil in such a way as to cover the metal plating layer. The resist film is exposed and developed to form an resist pattern; and the first metal foil not covered by the resist pattern is removed by etching, and the resist pattern is then removed to form a circuit pattern.

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

3. The method for manufacturing a flexible printed circuit board as claimed in claim 1, wherein the step of etching the protrusion of the metal plating is performed in such a manner as to completely remove the protrusion.

4. The method for manufacturing a flexible printed circuit board as claimed in claim 3, wherein the step of etching the protrusion of the metal plating is performed by further removing a portion of the first metal foil covered by the protrusion and / or a portion of the filling portion of the metal plating.

5. A method for manufacturing a flexible printed circuit board as described in any one of claims 1 to 4, wherein the step of etching the protrusion of the metal plating is performed such that the upper surface of the etched metal plating is substantially on the same plane as the upper surface of the first metal foil constituting the circuit pattern.

6. The method for manufacturing a flexible printed circuit board as claimed in claim 5, wherein the step of etching the protrusion of the metal plating is performed such that the height difference between the upper surface of the etched metal plating and the upper surface of the first metal foil constituting the circuit pattern is within ±5 μm.

7. The method for manufacturing a flexible printed circuit board as claimed in claim 6, wherein the step of etching the protrusion of the metal plating is performed in such a way that the height difference is within ±2 μm.

8. A method for manufacturing a flexible printed circuit board as claimed in any one of claims 1 to 4, wherein the resist pattern has a dummy opening; in the step of forming the metal plating layer, a dummy plating layer is formed on the first metal foil exposed to the dummy opening; and in the step of etching the protrusion of the metal plating layer, at least a portion of the dummy plating layer is removed.

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

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

11. A method for manufacturing a flexible printed circuit board as described in any one of claims 1 to 4, wherein the metal plating is an electrolytic copper plating.

12. A method for manufacturing a flexible printed circuit board as claimed in any one of claims 1 to 4, wherein the step of forming the resist pattern having the opening comprises: forming a resist film covering the first metal foil and the via on the first metal foil; and exposing and developing the resist film to form the opening, wherein the exposure is performed by direct writing without using an exposure mask.

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