JP4074327B2 - Flexible printed circuit board - Google Patents

Description

  The present invention relates to a flexible printed board in which a metal foil layer is formed only on one side of an insulating layer.

In various electronic devices, a flexible printed circuit board (hereinafter sometimes abbreviated as “FPC”) on which a circuit is formed is used. FIG. 5 shows a cross-sectional configuration of a commonly used double-sided FPC.
In FIG. 5A, the FPC 51 has a metal foil layer 54 formed on both sides of an insulating layer 52 via an adhesive layer 53. The FPC 51 is partially provided with through holes 55 for conducting the metal foil layers 54 on both sides.

  FIG. 5B shows a state where the FPC 51 is bent and electronic components are mounted. Drive ICs 56 a and 56 b are mounted on the metal foil layers 54 on both sides, respectively, and these drive ICs 56 a and 56 b are electrically connected by through holes 55.

  An example of the invention made to enhance the convenience of FPC is described in Patent Document 1, Patent Document 2, and Patent Document 3.

  Patent Document 1 describes a single FPC that includes a double-sided FPC part and a single-sided FPC part. The single-sided FPC part is folded and overlapped with another single-sided FPC part. Since the portion constituting the bent portion is a single-sided FPC portion, the device is devised so that the bending performance is excellent.

  In addition, what is described in Patent Document 2 is an etching for forming a conductor pattern on both sides of an FPC by making a hole by machining with a base film of a single-sided FPC provided with an adhesive and pasting a conductor. A terminal pattern is formed by forming a conductive pattern by cutting and cutting at a hole portion. The purpose is to form the terminal section at a high density and to improve processing time and processing cost.

  Patent Document 3 discloses that a circuit pattern is formed by a conductor between a base film and an overcoat film, a pad portion of the circuit pattern is opened in the base film and the overcoat film, and the base film and A flexible printed circuit board in which the window opening portion of the overcoat film is different is described.

JP-A-11-195850 JP-A-4-356996 Japanese Utility Model Publication No. 61-114872

  However, in a conventional FPC, as long as a double-sided FPC is used, a through hole for conducting metal foils on the front and back by forming a metal foil layer on both sides is indispensable. It becomes necessary to perform plating for wall conduction, and the number of processing steps increases.

  In addition, what is described in Patent Document 3 is configured to form a pad portion by etching a metal layer serving as a conductive portion, and to mount an electronic component via a lead in a region where a window is opened. However, with this structure, it is necessary to precisely open windows on both sides of the FPC in a narrow area where the leads are connected, and the positioning accuracy of bonding and window opening is limited. There is a problem that it is difficult to mount electronic components. In the field of FPC, fine pitch is conspicuous due to the demand for high-density mounting. However, in the structure described in Patent Document 3, it is difficult to cope with fine pitch in relation to the alignment accuracy. Furthermore, in recent years, it has been required to be compactly mounted in a limited space such as a mobile phone, and for this purpose, an FPC that can be folded and used is required, but this structure is not suitable for folding and use. Therefore, it cannot meet the demand for compact mounting.

  The present invention has been made to solve the above-described problems, and it is not necessary to form a through hole, and it is possible to ensure the strength at the time of mounting, and it is a fine pitch FPC. Even if it is used by being folded, it has a sufficient function, and therefore an object is to provide a flexible printed board capable of mounting electronic components compactly in a limited space.

  In order to solve the above problems, the flexible printed board of the present invention is a flexible printed board in which a metal foil layer is formed only on one side of an insulating layer, and the surface of the metal foil layer is roughened. In the region where irregularities are formed and the insulating layer is partially removed, a surface flattening process is performed to flatten the unevenness of the surface of the metal foil layer, and the electronic component is directly mounted on the surface on which the surface flattening process has been performed. In an area where the insulating layer is removed, an overcoat layer for reinforcing the metal foil layer is provided on the surface of the metal foil layer opposite to the surface flattened side. It is provided along the metal foil layer.

  An electronic component mounted on a flexible printed circuit board is mounted on a metal foil layer provided only on one side of the insulating layer and is conducted by the metal foil layer, so that a through hole penetrating the insulating layer is formed at all. In addition, the electronic component mounted on the metal foil layer can be electrically conducted. When the through hole is formed, it is necessary to perform plating for the conduction of the inner wall. However, in the present invention, since it is not necessary to form the through hole at all, the processing steps can be greatly simplified. Moreover, since the overcoat layer for reinforcing the metal foil layer is provided in the region from which the insulating layer has been removed, sufficient strength can be ensured when the electronic component is mounted.

  In addition, since the surface of the metal foil layer is roughened to form irregularities, even if it is compatible with fine pitch, the adhesion with the insulating layer is strong and the insulating layer is partially removed. In this area, the surface flattening process is performed to flatten the unevenness of the surface of the metal foil layer, and the surface that has been subjected to the surface flattening process is wide enough to directly mount electronic components. An electronic component can be directly mounted thereon, and electrical connection with the electronic component can be maintained in a good state. Therefore, unlike the one described in Patent Document 3, there is no limitation on the processing accuracy in the manufacturing process when mounting electronic components.

  In the present invention, the insulating layer and the overcoat layer are formed using polyimide or a liquid crystal polymer.

  When the insulating layer and the overcoat layer are formed of polyimide, a function of reinforcing the metal foil layer can be stably obtained for a wide range of uses. In addition, when the insulating layer and the overcoat layer are formed of liquid crystal polymer, the pattern can be formed stably and with high precision in applications that require precise pattern formation, and in harsh environments such as temperature and chemicals. For applications that need to be used below, good heat resistance and chemical resistance can be obtained. Further, when used in a high frequency environment, excellent electrical characteristics can be obtained even in a high frequency environment. In addition, the water absorption is low, and high dimensional stability and excellent electrical characteristics can be maintained even in a high humidity environment.

  In the present invention, the thickness of the metal foil layer is 9 μm or more and 35 μm or less.

  In order to correspond to the fine pitch type in which the pitch between adjacent conductive parts is 50 μm or less, the thickness of the metal foil layer is reduced from the interlocking between the size of the pitch and the thickness of the metal foil layer due to the processing limit in the etching process. It is necessary to suppress it to 35 μm or less. On the other hand, if the thickness of the metal foil layer is less than 9 μm, the metal foil layer cannot sufficiently exhibit the function as a conductor. Thus, by setting the thickness of the metal foil layer to 9 μm or more and 35 μm or less, it is possible to cope with fine pitch while ensuring the function as the conductor of the metal foil layer, and more metal than the conventional one. Since the foil layer is thin, it is possible to mount electronic components stably even when folded and used, and even when mounted in a limited space like a mobile phone, it can be mounted compactly. It is possible.

  In the present invention, the surface flattening treatment is characterized in that wet etching and chemical polishing are combined, or wet etching, plasma etching, and chemical polishing are combined.

  Wet etching or plasma etching can efficiently remove the residue present on the irregularities on the surface of the metal foil layer, and the metal foil to the level where electronic components can be mounted in a well-connected state by subsequent chemical polishing. The layer surface can be planarized.

  In the present invention, a back sheet is provided on the surface of the metal foil layer opposite to the surface subjected to the surface flattening process in a region other than the region where the overcoat layer is provided and where the insulating layer is removed. Thus, a connector part to which the joining terminal can be attached and detached is formed.

  In order to function as a connector part, it is necessary to have enough mechanical strength to withstand the attachment and detachment of the joining terminal, but a back sheet is provided on the surface of the metal foil layer opposite to the surface subjected to the surface flattening treatment. Therefore, the mechanical strength required for attaching / detaching the joining terminal can be ensured. Therefore, it is possible to make a part of the flexible printed circuit board function as a connector part to be joined to the joining terminal, there is no need to use a connector as a separate part, the number of parts can be reduced, and the joining terminal in a narrow space It is also effective for attaching and detaching.

  In this invention, the metal plating is given to the surface by which the surface flat process of the said metal foil layer was made | formed.

  Since the surface flattening treatment of the metal foil layer is performed, when metal plating is performed on this surface, the plated metal layer is formed flat on the surface of the metal foil layer. Therefore, the electrical connection when an electronic component is mounted on this surface is good. Moreover, in the connector part, the electrical connection with the joining terminal is maintained well.

  According to the present invention, it is possible to realize a flexible printed circuit board that does not require the formation of a through hole and that can ensure strength during mounting.

Below, this invention is demonstrated based on the embodiment.
FIG. 1 shows a cross-sectional configuration of a flexible printed circuit board according to an embodiment of the present invention.
In the FPC 1 shown in FIG. 1A, a metal foil layer 4 is formed on only one side of the insulating layer 2 via an adhesive layer 3. The metal foil layer 4 is preferably formed of copper or a copper alloy having good conductivity. The surface of the metal foil layer 4 that is in contact with the adhesive layer 3 is roughened to form irregularities. . This roughening process will be described in detail later.

  The FPC 1 is partially treated so that the insulating layer 2 and the adhesive layer 3 are removed, and the surface of the metal foil layer 4 on the side where the insulating layer 2 and the adhesive layer 3 are removed becomes flat. In the region A from which the insulating layer 2 and the adhesive layer 3 are removed, an overcoat layer 5 for reinforcing the metal foil layer 4 is formed on the surface of the metal foil layer 4 opposite to the surface subjected to the surface flattening process. , Provided over the entire region A. The region A from which the insulating layer 2 and the adhesive layer 3 are removed can be provided at the end portion of the FPC 1, but is not limited to the end portion and can be provided at an arbitrary position in the middle of the FPC 1. The surface flattening process of the metal foil layer 4 will be described in detail later.

  FIG. 1B shows a state where the FPC 1 is bent and an electronic component is mounted. A drive IC 6 a as an example of an electronic component is mounted on the first metal foil surface 4 a of the metal foil layer 4, and the drive IC 6 a is electrically connected by the metal foil layer 4. In the region A from which the insulating layer 2 and the adhesive layer 3 have been removed, the driving IC 6b is mounted on the second metal foil surface 4b, which is the surface of the metal foil layer 4 that has been subjected to the surface flattening process. The metal foil layer 4 is electrically connected. As described above, in the present invention, the area A from which the insulating layer 2 and the adhesive layer 3 are removed is an area in which an electronic component such as the driving IC 6b can be directly mounted without using a connecting member such as a lead. have.

In this region, an overcoat layer 5 for reinforcing the metal foil layer 4 is provided on the surface of the metal foil layer 4 on the side opposite to the surface subjected to the surface flattening process. The driving IC 6 b has a strength that is retained by the overcoat layer 5. By mounting in this way, it is not necessary to form a through hole, the number of processing steps can be reduced, and sufficient strength can be ensured when an electronic component is mounted.
In addition, the metal foil layer 4 may be subjected to precious metal plating such as gold, solder / nickel plating, etc. in the region where the drive ICs 6a and 6b are mounted, and a cover coat is applied to the other non-mounting regions. Also good. Even when such processing is performed, since the metal foil layer 4 is formed only on one surface of the insulating layer 2, the number of processing steps can be reduced.

The thickness of the metal foil layer 4 is preferably in the range of 2 μm to 70 μm, and more preferably in the range of 9 μm to 35 μm. The single-sided FPC has the property that it is easier to bend than the double-sided FPC. However, in the present invention, the thickness of the metal foil layer 4 is set in the range of 9 μm to 35 μm to make it easier to bend than the conventional one. Secured. In order to enable mounting in a narrow space, it is indispensable to bend and use the FPC, and the present invention meets the purpose of use in such a situation.
Here, the reason why the upper limit of the thickness of the metal foil layer 4 as a more preferable range is set to 35 μm is to cope with the fine pitch. In recent years, in the field of FPC, fine pitch pitches with the pitch between adjacent conductive parts being 50 μm or less have become mainstream, but the size of the pitch and the thickness of the metal foil layer 4 have processing limitations in the etching process. Therefore, they are linked to each other. Therefore, in order to reduce the pitch in this way, it is necessary to suppress the thickness of the metal foil layer 4 to 35 μm or less. If this thickness is exceeded, it becomes difficult to cope with the fine pitch. Moreover, since the metal foil layer 4 cannot fully exhibit the function as a conductor when the thickness of the metal foil layer 4 is less than 9 μm, the lower limit is set to 9 μm.

  The thickness of the adhesive layer 3 is preferably in the range of 1 μm to 30 μm. The insulating layer 2 can be formed using polyimide or liquid crystal polymer. When polyimide is used, the thickness is preferably 7.5 μm to 225 μm, and when using liquid crystal polymer, the thickness is 12.5 μm to It is good to set it as 225 micrometers. In any case, the range of 12.5 μm to 50 μm is more preferable. As an example, the metal foil layer 4 can be formed with a thickness of 18 μm, the adhesive layer 3 with a thickness of 15 μm, and the insulating layer 2 formed of polyimide or a liquid crystal polymer with a thickness of 25 μm. By forming each layer in this way, a flexible printed circuit board with extremely excellent bending characteristics can be produced, and an FPC with extremely excellent bending characteristics that can be bent at 180 ° with a curvature radius of 200 μm is realized. it can.

  The overcoat layer 5 is formed along the metal foil layer 4 via an adhesive, and can be formed of polyimide or a liquid crystal polymer. The thickness of the adhesive is 2 μm to 70 μm. When polyimide is used, the thickness of the overcoat layer 5 is preferably 7.5 μm to 225 μm. When the liquid crystal polymer is used, the thickness of the overcoat layer 5 is 12. It is good to set it as 5 micrometers-225 micrometers. As an example, the thickness of the adhesive can be 25 μm, and the thickness of the overcoat layer 5 formed of polyimide or liquid crystal polymer can be 25 μm.

The method of forming the FPC 1 having the structure shown in FIG. 1 differs depending on whether the FPC 1 is manufactured based on a single layer material of metal foil or resin, or based on a composite material in which the metal foil and resin are laminated. .
When producing based on the composite material, a circuit is formed by copper etching on the metal foil layer 4 and a laminated resin such as polyimide or liquid crystal polymer is processed to form a metal foil in a predetermined region. Make it bare. Alternatively, this process is reversed to expose the metal foil in a predetermined area, and then a circuit is formed on the metal foil layer 4 by copper etching. Next, plating is performed to protect the surface of the metal foil layer 4 as necessary, and the overcoat layer 5 is formed in a region where the metal foil is exposed.

  In the case of manufacturing based on a single layer material, a pattern is formed on a resin material such as polyimide or liquid crystal polymer so that a predetermined region is an opening. A metal foil is bonded to this so that the metal foil in a predetermined region is exposed, and then a circuit is formed on the metal foil layer 4 by copper etching. Next, plating is performed to protect the surface of the metal foil layer 4 as necessary, and the overcoat layer 5 is formed in a region where the metal foil is exposed.

  In the above description, it is assumed that the insulating layer 2 and the metal foil layer 4 are bonded via the adhesive layer 3, but the insulating layer 2 and the metal foil layer 4 are not bonded via the adhesive layer 3. It can also be directly attached by means such as thermocompression bonding. Moreover, FPC can also be comprised using the copper clad laminated material which does not have the contact bonding layer 3 as a composite material. Similarly, when the overcoat layer 5 is formed, polyimide or a liquid crystal polymer can be directly attached to the metal foil layer 4 without using an adhesive.

A method of removing the insulating layer 2 and the adhesive layer 3 to expose the metal foil layer 4 or a method of removing the insulating layer 2 and exposing the metal foil layer 4 when the adhesive layer 3 is not provided. Several methods are available.
First, when the adhesive layer 3 is not provided and the insulating layer 2 is a thermoplastic polyimide, wet etching can be used. According to this method, fine processing with high position accuracy is possible by photolithography, the processing time is short, and the apparatus can be configured at low cost.
Second, plasma etching can be used. According to this method, fine processing with high position accuracy is possible by photolithography, but the processing time is long, the device configuration is likely to be expensive, and the cost is difficult to reduce.

Third, laser etching can be used. According to this method, fine processing is possible, but the positional accuracy is inferior and carbides are generated, so that desmear treatment is required.
Fourth, when an FPC is manufactured based on a single layer material, it can be machined by a mold or a router. However, since this method has poor positional accuracy and is not suitable for fine processing, it can be used for FPC of rough patterns.

  Also, several methods can be used as a method for forming the overcoat layer 5, for example, a method of thermocompression bonding an insulating resin film of polyimide or liquid crystal polymer by applying heat and pressure, such as a thermocompression press, Examples thereof include a method of attaching an insulating resin film of polyimide or liquid crystal polymer via an adhesive, or a method of applying an insulating resin varnish made of polyimide or liquid crystal polymer.

  In the FPC of the present invention, the insulating layer 2 and the adhesive layer 3 are removed by the above-described method, and an electronic component is mounted in a region where the metal foil layer 4 is exposed, but the processing accuracy required depending on the application Is different. Therefore, these processing methods can be selected according to the processing accuracy.

Next, the process of planarizing the metal foil layer 4 after removing the insulating layer 2 and the adhesive layer 3 will be specifically described.
In the fine pitch FPC, when the metal foil layer 4 is bonded to the insulating layer 2 via the adhesive layer 3, in order to increase the contact area between the metal foil layer 4 and the adhesive layer 3, In order to increase the contact area between the metal foil layer 4 and the insulating layer 2 when the insulating layer 2 and the metal foil layer 4 are directly attached by means such as thermocompression bonding without using the layer 3, the metal foil Layer 4 is roughened. This roughening treatment is performed by forming irregularities on the surface of the metal foil layer 4, and usually irregularities having a size of 0.5 μm to 2 μm are often formed, but in this example, it corresponds to a fine pitch. In order to strengthen the adhesive strength for this purpose, irregularities with dimensions of 2 μm to 10 μm are formed.

Here, the case where the metal foil layer 4 is bonded to the insulating layer 2 via the adhesive layer 3 will be described, but the insulating layer 2 and the metal foil layer 4 are not bonded via the adhesive layer 3 by means such as thermocompression bonding. Even if it is directly pasted, the same flattening process is performed.
In FIG. 2, the removal process of the insulating layer 2 and the contact bonding layer 3, and the planarization process of the metal foil layer 4 are shown.
A photo-developing resist 10 is laminated on the surface of the insulating layer 2 (FIG. 2A). The resist 10 is alkali resistant. A pattern mask is brought into close contact with the resist 10 and exposed, and the resist 10 is developed (FIG. 2B). The insulating layer 2 and the adhesive layer 3 in the region where the resist 10 has been removed are subjected to the first liquid wet etching. It is removed by treatment (FIG. 2 (c)), which uses a KOH-based mixture.

  By this treatment, the metal foil layer 4 is exposed, but the surface of the metal foil layer 4 is left with the unevenness 11 of about 2 μm to 10 μm as it is due to the roughening treatment, and remains on the unevenness 11 without being removed. A part of the adhesive layer 3 is present. Therefore, a part of the remaining adhesive layer 3 is completely removed by plasma etching or second liquid wet etching (FIG. 2D). When performing the plasma etching process, a reactive gas is supplied and discharged, and the excited molecules are applied to the surface of the metal foil layer 4 to remove a part of the adhesive layer 3 remaining on the irregularities 11. In the case of performing the second liquid wet etching process, the same components as the liquid used in the first liquid wet etching process are used while changing the blending ratio and taking about 5 seconds.

Thereafter, the resist 10 is peeled and removed, and the unevenness 11 is planarized by chemical polishing (FIG. 2E). In chemical polishing, ammonium persulfate added with sulfuric acid is used as a polishing liquid, the temperature is set in a range of 20 ° C. to 40 ° C. (usually 30 ° C.), and dipping is performed for 30 seconds to 60 seconds. Thereby, the dimension of the unevenness | corrugation 11 after chemical polishing becomes 0.1 micrometer-0.5 micrometer.
In addition, when the insulating layer 2 and the metal foil layer 4 are directly attached by means such as thermocompression bonding without using the adhesive layer 3, the metal foil layer 4 having the irregularities 11 and the insulating layer 2 are directly bonded. The planarization of the metal foil layer 4 described above is performed in the same manner.
The fact that the quality of the plating process is determined based on the presence or absence of the above-described planarization process will be described with reference to FIG.

  FIG. 3A shows the state of plating when the flattening process is not performed, and the plating metal (for example, gold) 12 is attached so as to be caught by the irregularities 11 on the surface of the metal foil layer 4, and thus this state. When electronic parts are mounted, contact failure occurs. On the other hand, when the flattening process is performed, the plating metal 12 adheres to the surface of the metal foil layer 4 to form a flat surface as shown in FIG. . Therefore, the electrical connection when an electronic component is mounted on this surface is improved, and the reliability is increased.

  As described above, the FPC according to the present invention is formed by forming the metal foil layer 4 for conducting electronic parts only on one side of the insulating layer 2, so that it is possible to save work and save resources such as copper. It is. In addition, since the strength at the time of mounting can be sufficiently secured, it is particularly useful for mounting on a small electronic device that is used with movement, such as a mobile phone.

Next, an embodiment in which the FPC of the present invention is provided with a portion that functions as a connector will be described with reference to FIG.
In the connector part 20 shown to Fig.4 (a), in the area | region where the insulating layer 2 provided in the metal foil layer 4 via the contact bonding layer 3 was removed, on the surface of the metal foil layer 4 by which the surface flattening process was made | formed The back sheet 22 is provided via the adhesive layer 21 on the surface of the metal foil layer 4 opposite to the surface on which the plating metal 12 is adhered and the surface is flattened. The connector portion 20 is provided with a back sheet 22 having a strength higher than that of the overcoat layer 5 described above because the mechanical strength needs to be ensured also by the attachment / detachment of the joining terminal described below. The sheet 22 can be formed of, for example, a glass epoxy resin that is configured by hardening a glass cloth with an epoxy resin.

FIG. 4B shows a state in which the joining terminal 30 is electrically connected to the connector portion 20 provided in a part of the FPC.
When joining the joining terminal 30 to the connector portion 20 of the FPC, as shown in FIG. 4B, the joining terminal 30 is brought closer to the insulating layer 2 of the FPC along the direction of the arrow. If the junction terminal 30 has a spring contact 31 rich in elasticity at its tip, the spring contact 31 gets over the insulating layer 2 by pressing with its elasticity, and adheres to the region where the insulating layer 2 has been removed. The contact terminal 30 is electrically joined to the connector portion 20 by contacting the plated metal 12. Since the plated metal 12 adheres flatly to the metal foil layer 4, the electrical connection with the junction terminal 30 is maintained well. Moreover, the area | region from which the insulating layer 2 was removed should just ensure the area required in order to join with the junction terminal 30, and once the junction terminal 30 is joined, the spring contact 31 removes the insulation layer 2 Held in the specified area.

When it is desired to release the bonding, when the bonding terminal 30 is moved in the direction opposite to the above direction, the spring contact 31 returns to the original state even after overcoming the insulating layer 2 again.
Such a connector part 20 can be provided at an arbitrary position as required in addition to being provided at the end of the FPC.
By providing the connector portion 20 described above on the FPC, it is possible to function as a connector portion that can be easily attached and detached without using a connector as a separate component, so that the number of components can be reduced. In particular, in the case where the space for mounting is extremely limited, such as a mobile phone, a new space is often required by using separate components, which may hinder high-density mounting. However, the present invention is advantageous in that it can be mounted in a narrow space. In recent years, the miniaturization of connectors has become remarkable, and the work for mounting such a small connector requires precision, and the assembly process becomes complicated, so the FPC itself also has a connector function. It is extremely useful.

  The present invention does not require the formation of a through hole and can be used as a flexible printed circuit board that can ensure strength during mounting, and is labor-saving for mounting a liquid crystal display device such as a mobile phone. It can be used as a flexible printed circuit board that can be reduced in size and resources.

It is a figure showing the section composition of the flexible printed circuit board concerning the embodiment of the present invention. It is a figure which shows the removal process of an insulating layer and an adhesion layer, and the planarization process of a metal foil layer. It is a figure explaining that the quality of a plating process is determined by the presence or absence of a planarization process. It is a figure explaining embodiment which gives the function as a connector. It is a figure which shows the cross-sectional structure of the double-sided flexible printed circuit board used normally.

Explanation of symbols

1 FPC
2 Insulating layer 3 Adhesive layer 4 Metal foil layer 4a First metal foil surface 4b Second metal foil surface 5 Overcoat layer 6a, 6b Driving IC
DESCRIPTION OF SYMBOLS 10 Resist 11 Concavity and convexity 12 Plating metal 20 Connector part 21 Adhesive layer 22 Back sheet 30 Joining terminal 31 Spring contact A Area where the insulating layer 2 and the adhesive layer 3 are removed

Claims (6)

  A flexible printed circuit board in which a metal foil layer is formed only on one side of an insulating layer, the surface of the metal foil layer is roughened to form irregularities, and the insulating layer is partially removed In the region, a surface flattening process is performed to flatten the unevenness of the surface of the metal foil layer, and the surface on which the surface flattening process has been performed has a width that allows electronic components to be directly mounted, and the insulating layer is removed. A flexible print, characterized in that an overcoat layer for reinforcing the metal foil layer is provided along the metal foil layer on the surface of the metal foil layer opposite to the side subjected to surface flattening in the region. substrate.   The flexible printed circuit board according to claim 1, wherein the insulating layer and the overcoat layer are formed using polyimide or a liquid crystal polymer.   The flexible printed circuit board according to claim 1 or 2, wherein the thickness of the metal foil layer is 9 µm or more and 35 µm or less.   4. The flexible print according to claim 1, wherein the surface flattening treatment is performed by combining wet etching and chemical polishing, or by combining wet etching, plasma etching, and chemical polishing. 5. substrate.   A back sheet is provided on the surface of the metal foil layer opposite to the surface subjected to the surface flattening process in a region other than the region where the overcoat layer is provided, and where the insulating layer is removed. The flexible printed circuit board according to claim 1, wherein a detachable connector portion is formed.   The flexible printed circuit board according to any one of claims 1 to 5, wherein a metal plating is applied to a surface of the metal foil layer that has been subjected to a surface flattening process.

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