Method of making flexible printed circuit board and flexible printed circuit board

According to an aspect of the present disclosures, a method of making a flexible printed circuit board, which includes a base film having an insulating property, a conductive pattern disposed on either one or both surfaces of the base film, and a cover layer covering a conductive-pattern side of a laminated structure inclusive of the base film and the conductive pattern, includes a superimposing step of superimposing a cover film on the conductive-pattern side of the laminated structure, the cover film having a first resin layer and a second resin layer that is laminated to an inner side of the first resin layer and that softens at a lower temperature than does the first resin layer, and a pressure bonding step of vacuum bagging the laminated structure and the cover film at a temperature higher than a softening temperature of the second resin layer.

TECHNICAL FIELD

The disclosures herein relate to a method of making a flexible printed circuit board and a flexible printed circuit board. The present application claims priority to Japanese patent application No. 2017-228276 filed on Nov. 28, 2017, and the entire contents of the Japanese patent application are hereby incorporated by reference.

BACKGROUND ART

In recent years, to keep up with reduction in the size and weight of electronic devices, electronic components such as planar coils constituting electronic devices have been mounted on flexible printed circuit boards and reduced in size (see, for example, Japanese Patent Application Laid-Open No. 2012-89700).

A planar coil described in the above-noted publication is made by forming a primary copper plating layer on a substrate, removing the substrate, and forming a secondary copper plating layer on the same surface of the primary copper plating layer that has been in contact with the substrate. As a result, the planar coil has an increased aspect ratio compared to a planar coil comprised only of a primary copper plating layer formed on a substrate, and can thus be reduced in size to a certain extent.

Patent Document

Patent Document 1: Japanese Patent Application Publication No. 2012-89700

SUMMARY OF THE INVENTION

According to an aspect of the present disclosures, a method of making a flexible printed circuit board, which includes a base film having an insulating property, a conductive pattern disposed on either one or both surfaces of the base film, and a cover layer covering a conductive-pattern side of a laminated structure inclusive of the base film and the conductive pattern, includes a superimposing step of superimposing a cover film on the conductive-pattern side of the laminated structure, the cover film having a first resin layer and a second resin layer that is laminated to an inner side of the first resin layer and that softens at a lower temperature than does the first resin layer, and a pressure bonding step of vacuum bagging the laminated structure and the cover film at a temperature higher than a softening temperature of the second resin layer.

According to another aspect of the present disclosures, a flexible printed circuit board which includes a base film having an insulating property, a conductive pattern disposed on either one or both surfaces of the base film, and a cover layer covering a conductive-pattern side of a laminated structure inclusive of the base film and the conductive pattern is such that the cover layer includes a first resin layer covering the laminated structure, and a second resin layer laminated to an inner side of the first resin layer and injected onto the conductive-pattern side of the laminated structure, wherein the main polymer of the first resin layer is a polyimide, and the main polymer of the second resin layer is an epoxy resin.

MODE FOR CARRYING OUT THE INVENTION

Problem to be Solved by the Present Disclosures

The planar coil described in the previously-noted publication is covered with an inner resin coating layer, and a resin overcoating layer and an outer insulating layer are laminated in this order on the inner resin coating layer, thereby to produce a flexible printed circuit board.

Electronic components mounted on the conventional flexible printed circuit board as described above are reduced in size. However, what is required is the reduction in the size of an entire flexible printed circuit board. In this regard, there is room for improvement in the thickness of a cover layer covering the electronic components on the conventional flexible printed circuit board.

The present disclosures are made in light of the circumstances described above, and is directed to providing a method of making a flexible printed circuit board and a flexible printed circuit board that allows the thickness of a cover layer covering an electronic component to be decreased.

Advantage of the Present Disclosures

A flexible printed circuit board made by a method of making a flexible printed circuit board according to the present disclosures and a flexible printed circuit board according to the present disclosures allow the thickness of a cover layer covering an electronic component to be decreased, which allows reduction in the size of an entire flexible printed circuit board.

Description of Embodiments of the Present Disclosures

According to an aspect of the present disclosures, a method of making a flexible printed circuit board, which includes a base film having an insulating property, a conductive pattern disposed on either one or both surfaces of the base film, and a cover layer covering a conductive-pattern side of a laminated structure inclusive of the base film and the conductive pattern, includes a superimposing step of superimposing a cover film on the conductive-pattern side of the laminated structure, the cover film having a first resin layer and a second resin layer that is laminated to an inner side of the first resin layer and that softens at a lower temperature than does the first resin layer, and a pressure bonding step of vacuum bagging the laminated structure and the cover film at a temperature higher than a softening temperature of the second resin layer.

The method of making a flexible printed circuit board uses a two-layer cover film that has a first resin layer and a second resin layer that is laminated to an inner side of the first resin layer. In the method of making a flexible printed circuit board, the cover film is vacuum bagged in the pressure bonding step at a temperature higher than the softening temperature of the second resin layer wherein the softening temperature of the second resin layer is lower than that of the first resin layer. As a result, the second resin layer is selectively softened to cover the conductive pattern laminated to the base film. The first resin layer laminated to the outer side of the second resin layer seals the second resin layer, which provides a cover layer. Use of the method of making a flexible printed circuit board ensures that the conductive pattern is more reliably coated even when the thickness of the cover layer is reduced, compared with the case in which the first resin layer and the second resin layer are separately laminated. This serves to reduce the size of an entire flexible printed circuit board.

The main polymer of the first resin layer may be a polyimide, and the main polymer of the second resin layer may be an epoxy resin. Use of an epoxy resin as the main polymer of the second resin layer allows the conductive pattern to be reliably coated with the second resin layer due to its fluidity obtained at the time of softening. Further, use of a polyimide as the main polymer of the first resin layer allows the second resin layer to be selectively softened, so that the second resin layer can be securely sealed with the first resin layer.

The average circuit pitch of the conductive pattern may be greater than or equal to 5 μm and less than or equal to 20 μm. Setting the average circuit pitch of a conductive pattern within the range described above allows the density of the conductive pattern to be increased, thereby providing a reliable coating with the second resin layer while maintaining the high density of mounted electronic components.

The conductive pattern may form a planar coil element. The planar coil element is designed to have a relatively thick conductive pattern, so that decreasing the thickness of the cover layer covering the planar coil greatly contributes to reducing the size of an entire flexible printed circuit board.

According to another aspect of the present disclosures, a flexible printed circuit board which includes a base film having an insulating property, a conductive pattern disposed on either one or both surfaces of the base film, and a cover layer covering a conductive-pattern side of a laminated structure inclusive of the base film and the conductive pattern is such that the cover layer includes a first resin layer covering the laminated structure, and a second resin layer laminated to an inner side of the first resin layer and injected onto the conductive-pattern side of the laminated structure, wherein the main polymer of the first resin layer is a polyimide, and the main polymer of the second resin layer is an epoxy resin.

Such a flexible printed circuit board uses an epoxy resin as the main polymer of the second resin layer so as to allow the conductive pattern to be reliably coated with the second resin layer due to its fluidity obtained at the time of softening. Further, the flexible printed circuit board uses a polyimide as the main polymer of the first resin layer so as to allow the second resin layer to be selectively softened, which results in the second resin layer being securely sealed with the first resin layer. As a result, the flexible printed circuit board allows the conductive pattern to be more reliably coated even when the thickness of the cover layer is reduced, which serves to reduce the size of an entire flexible printed circuit board.

Details of Embodiments of the Present Disclosures

In the following, embodiments of a method of making a flexible printed circuit board and a flexible printed circuit board according to the present disclosures will be described with reference to the drawings.

The method of making a flexible printed circuit board includes, as illustrated inFIG.1, a laminated-structure forming step S1of laminating a conductive pattern to a base film, a superimposing step S2of superimposing a cover film on a conductive pattern side of the laminated structure, and a pressure bonding step S3of vacuum bagging the laminated structure and the cover film.

In the following, a flexible printed circuit board made by a method of making a flexible printed circuit board will be described. The flexible printed circuit board illustrated inFIG.3includes a base film1having an insulating property, conductive patterns2laminated to both surfaces of the base film1, and cover layers4covering the conductive patterns2of the laminated structure3including the base film and the conductive patterns2. The conductive patterns2laminated to both surfaces of the base film1are connected to each other through a through-hole5.

The base film1has an insulating property, and is flexible. Examples of the main component of the base film1include synthetic resins such as a polyimide, polyethylene terephthalate, a fluorine resin, and a liquid crystal polymer. Among these, polyimide is preferable due to its excellent insulating property, flexibility, and heat resistance. The term “main component” refers to a component accounting for the highest content, and may refer to a component with a content of 50 wt % or more, for example.

The lower limit of the average thickness of the base film1is preferably 5 μm, more preferably 10 μm, and still more preferably 15 μm. The upper limit of the average thickness of the base film1is preferably 150 μm, more preferably 100 μm, and still more preferably 50 μm. Use of the average thickness of the base film1that is less than the lower limit may create a risk that the insulating property and mechanical strength are insufficient. Conversely, use of the average thickness of the base film1that exceeds the upper limit may create a risk of disregarding the demand for reduction in the size of a flexible printed circuit board. The term “average thickness” refers to the distance between an average line of the front surface boundary and an average line of the back surface boundary at a cross-section of the object taken along the thickness direction thereof within the range in which measurements are made. The term “average line” refers to an imaginary line which is drawn along the boundary such that the total area of peaks formed between the boundary and the imaginary line (i.e., the total area above the imaginary line) and the total area of troughs (i.e., the total area below the imaginary line) are equal to each other.

The conductive patterns2constitute structures such as electronic components, electrical interconnect structures, the ground, shields, and the like. In the flexible printed circuit board ofFIG.2, the conductive patterns2form planar coil elements. The design of a planar coil element uses a relatively thick conductive pattern2, so that decreasing the thickness of the cover layers4covering the planar coils greatly contributes to reducing the size of an entire flexible printed circuit board.

The conductive pattern2is not limited to a particular material as long as the material has electrical conductivity. Examples include metals such as copper, aluminum, and nickel. In general, copper is used due to its relatively low price and high conductivity. The conductive patterns2may also be plated on the surface thereof.

The lower limit of the average thickness of the conductive pattern2is preferably 5 μm, more preferably 10 μm, and still more preferably 20 μm. The upper limit of the average thickness of the conductive pattern2is preferably 80 μm and more preferably 60 μm. Use of the average thickness of the conductive pattern2that is less than the lower limit may create a risk that the conductivity of the conductive pattern2is insufficient. Conversely, use of the average thickness of the conductive pattern2that exceeds the upper limit results in the flexible printed circuit board being needlessly thick, which may create a risk of disregarding the demand for reduction in the thickness of a flexible printed circuit board.

The average width of the conductive patterns2is determined as appropriate according to the structure of an electronic component, an electrical interconnect structure, the ground, shields, and the like. The lower limit of the average width of the conductive patterns2is preferably 2 μm and more preferably 5 μm. Further, the upper limit of the average width of the conductive patterns2is preferably 15 μm and more preferably 10 μm. Use of the average width of the conductive patterns2that is less than the lower limit may create a risk that the conductivity of the conductive patterns2is insufficient. Use of the average width of the conductive patterns2that exceeds the upper limit causes the density of the conductive patterns to be decreased, which may create a risk that the high-density mounting of electronic components or the like becomes difficult.

The lower limit of the average circuit pitch of the conductive patterns2is preferably 5 μm and more preferably 7 μm. The upper limit of the average circuit pitch of the conductive patterns2is preferably 20 μm, more preferably 15 μm, and still more preferably 10 μm. Use of the average circuit pitch of the conductive patterns2that is less than the lower limit may create a risk that the conductive patterns2cannot be easily covered with a second resin layer7of the cover layers4, which will be described later. Use of the average circuit pitch of the conductive patterns2that exceeds the upper limit causes the density of the conductive patterns to be decreased, which may create a risk that the high-density mounting of electronic components or the like becomes difficult. The term “the average circuit pitch of a conductive pattern” refers to the distance between the centers of conductive pattern lines next to each other when straight conductive pattern lines are arranged at the highest density.

The lower limit of the aspect ratio of the conductive patterns2(i.e., the ratio of the average thickness to the average width of the conductive patterns2) is preferably 1.5 and more preferably 2. The upper limit of the aspect ratio of the conductive patterns2is preferably 5 and more preferably 4. Use of the average aspect ratio of the conductive patterns2that is less than the lower limit makes it necessary for the average width to be increased in order to reduce the resistance of the conductive patterns2, which may create a risk that the high-density mounting of electronic components or the like becomes difficult. Conversely, use of the average aspect ratio of the conductive patterns2that exceeds the upper limit may create a risk that the conductive patterns2cannot be easily covered with the second resin layer7of the cover layers4.

The through-hole5provides conduction between the conductive patterns2laminated to the respective surfaces of the base film1. Specifically, the through-hole5passes through the base film1and the conductive patterns2laminated to the respective surfaces of the base film1to provide electrical connection between the conductive patterns2laminated to the respective surfaces. A penetrating hole5amay be formed in the laminated structure3made by laminating the base film1and the conductive patterns2, and plating5bor the like may be applied to the penetrating hole5a, so that the through-hole is formed. The above-described plating5bis not limited to a particular plating, and may be either electroplating or electroless plating. Nonetheless, electroplating is more preferred. Examples of the above-described plating types include copper plating, gold plating, nickel plating, plating of an alloy thereof, and the like. In particular, copper plating or copper alloy plating is preferable from the viewpoint of satisfactory electrical conductivity and cost reduction. Further, the through-hole5may alternately be formed by injecting a silver paste, a copper paste, or the like into the penetrating hole5aand then causing the paste to be hardened by heat.

The average diameter of the through-hole5is selected as appropriate in consideration of workability, conductivity, and the like, and may be greater than or equal to 20 μm and less than or equal to 2000 Provision of the through-hole5in the flexible printed circuit board allows electrical connection to be easily and reliably made between the conductive patterns2laminated to both surfaces of the base film1, thereby facilitating an increase in density.

The cover layer4includes a first resin layer covering the laminated structure3and a second resin layer7laminated to the inner side of the first resin layer6.

The main polymer of the first resin layer6may be a polyimide, polyethylene terephthalate, a fluorine resin, or the like. In particular, a polyimide having a high softening temperature is preferable.

The lower limit of the average thickness of the first resin layer6is preferably 1 μm and more preferably 3 μm. Further, the upper limit of the average thickness of the first resin layer6is preferably 8 μm and more preferably 6 μm. Use of the average thickness of the first resin layer6that is less than the lower limit may create a risk that the strength of the first resin layer6is insufficient so as to fail to provide a sufficient coating for the second resin layer7. Conversely, use of the average thickness of the first resin layer6that exceeds the upper limit results in the flexible printed circuit board being needlessly thick, which may create a risk of disregarding the demand for reduction in the thickness of a flexible printed circuit board.

The lower limit of the softening temperature of the first resin layer6is preferably 150° C., more preferably 200° C., and still more preferably 300° C. Use of the softening temperature of the first resin layer6that is less than the lower limit may create a risk that the first resin layer6softens together with the second resin layer7in the pressure bonding step S3, which will be described later, at the time of making the flexible printed circuit board, which results in a failure to provide a sufficient coating for the second resin layer7. Although the upper limit of the softening temperature of the first resin layer is not limited to a particular value, the upper limit may be 600° C., for example.

The second resin layer7, which is laminated to the inner side of the first resin layer6, is injected onto the conductive pattern2of the laminated structure3.

The main polymer of the second resin layer7may be an epoxy resin, an acrylic resin, a butyral resin, or the like. In particular, an epoxy resin having a relatively low softening temperature is preferable.

The second resin layer7covers the conductive pattern2. The lower limit of the average thickness (i.e., thickness D inFIG.3) of the second resin layer7between the top surface of the conductive pattern2and the first resin layer6is preferably 0.2 μm and more preferably 0.5 μm. Further, the upper limit of the average thickness D of the second resin layer7is preferably 1.5 μm and more preferably 1 μm. Use of the average thickness D of the second resin layer7that is less than the lower limit may create a risk that the adhesion of the first resin layer6is insufficient so as to fail to provide a sufficient protection by the cover layer4for the conductive pattern2. Conversely, use of the average thickness D of the second resin layer7that exceeds the upper limit results in the flexible printed circuit board being needlessly thick, which may create a risk of disregarding the demand for reduction in the thickness of a flexible printed circuit board.

In the portion where the conductive pattern2is not laminated, the second resin layer7fills the gaps between the lines of the conductive pattern2to cover the base film1. As illustrated inFIG.4, the average thickness of such a filling portion of the second resin layer7may be less than the average thickness of the conductive pattern2. However, the filling portion preferably has a thickness providing the same height as the portion covering the conductive pattern as illustrated inFIG.3. Making the average thickness of the filling portion of the second resin layer7the same thickness as the portion covering the conductive pattern2allows the first resin layer to be implemented as a flat plate shape. This arrangement enhances the effect of the cover layer4to provide protection for the conductive pattern2, and, at the same time, prevents tension and stress from being applied by the cover layer4to electronic components or the like formed by the conductive pattern2on the base film1.

In the case in which the average thickness of the filling portion of the second resin layer7is less than the average thickness of the conductive pattern2as illustrated inFIG.4, the ratio of the average thickness of the filling portion of the second resin layer7to the average thickness of the conductive pattern2has a lower limit that is preferably 50%, more preferably 75%, and still more preferably 90%. When the ratio of the average thickness of the filling portion of the second resin layer7is less than the lower limit, there may be a risk of insufficient protection for the conductive pattern2due to the surface irregularities of the cover layer4.

The lower limit of the softening temperature of the second resin layer7is preferably 50° C. and more preferably 70° C. The upper limit of the softening temperature of the second resin layer7is preferably 150° C. and more preferably 120° C. Use of the softening temperature of the second resin layer7that is lower than the lower limit may create a risk of deformation of the cover layer4due to heat generated by electronic components during operations. Conversely, use of the softening temperature of the second resin layer7that exceeds the upper limit may create a risk that the second resin layer7does not sufficiently softens in the pressure bonding step S3, which will be described later, at the time of making a flexible printed circuit board, thereby causing the second resin layer7to fail to fill the gaps sufficiently between the lines of the conductive pattern2.

The second resin layer7softens at a lower temperature than does the first resin layer6. The lower limit of a difference in softening temperature between the first resin layer6and the second resin layer7is preferably 50° C., more preferably 100° C., and still more preferably 300° C. Use of the difference in softening temperature that is less than the lower limit may create a risk that the first resin layer6softens together with the second resin layer in the pressure bonding step S3, which will be described later, at the time of making a flexible printed circuit board, which results in a failure to provide a sufficient coating for the second resin layer7. The upper limit of the softening temperature is not limited to a particular value, and may be 400° C., for example.

Reducing the thickness of the cover layer4allows the thickness of the flexible printed circuit board to be reduced. The upper limit of the average thickness of the flexible printed circuit board is preferably 160 μm and more preferably 150 μm. Use of the average thickness of the flexible printed circuit board that exceeds the upper limit may create a risk of disregarding the demand for reduction in the thickness of the flexible printed circuit board. The lower limit of the average thickness of the flexible printed circuit board is not limited to a particular value, and may be 50 μm, for example, as determined in view of a resistance value or the like required for the conductive pattern2.

[Method of Making Flexible Printed Circuit Board]

In the following, the steps of the method of making the flexible printed circuit board will be described in detail.

In the laminated-structure forming step S1, the conductive patterns2are laminated to the base film1The through-hole5is further formed according to need. The specific procedure is as follows.

A conductive layer is formed on both surfaces of the base film1.

The conductor layer may be formed by bonding a conductor foil through an adhesive or by a deposition method known in the art, for example. Examples of the conductor include copper, silver, gold, nickel, and the like. The adhesive is not limited to any particular adhesive as long as the adhesive is capable bonding the conductor to the base film1, and various adhesives known in the art may be used. Examples of the deposition method include vapor deposition, plating, and the like. The conductor layer is preferably formed by bonding a copper foil to the base film1through a polyimide adhesive.

The conductive layers are then patterned to form the conductive patterns2.

Patterning of the conductor layer may be performed by a method known in the art, such as photoetching. Photoetching is performed by forming a resist film with a predetermined pattern on one surface of the conductive layer, by subsequently treating the conductive layer exposed from the resist film with an etchant, and by removing the resist film.

In the case of forming the through-hole5, the penetrating hole5aextending through the base film and the conductive patterns2laminated to both surfaces of the base film1is formed after the conductive patterns2are formed. Then, the plating5bis applied to the circumferential wall of the penetrating hole5a.

The through-hole may be formed by forming the penetrating hole5aas described above, followed by injecting, and then causing to be hardened by heat, silver paste, copper paste, or the like in the penetrating hole5a.

In the superimposing step S2, the cover film8including the first resin layer6and the second resin layer7laminated to the inner side of the first resin layer6is superimposed on the conductive pattern2of the laminated structure3formed in the laminated-structure forming step S1, as illustrated inFIG.5.

Specifically, the exterior surface of the second resin layer7of the cover film8is placed in contact with the surfaces of the conductive pattern2and the through-hole5of the laminated structure3. Since the conductive pattern2is formed on both surfaces of the base film1, two cover films8are used such that the cover films8are superimposed on the respective surfaces.

The first resin layers6of the cover films8become the first resin layers6of the flexible printed circuit board that has previously been described. The first resin layers6of the cover films8are the same as or similar to the first resin layers of the flexible printed circuit board that has previously been described, and a description thereof will be omitted.

The second resin layers7of the cover films8are softened in the pressure bonding step S3, which will be described later, to cover the conductive patterns2as well as to fill the gaps between the lines of the conductive patterns2, thereby becoming the second resin layers7of the flexible printed circuit board that has previously been described.

The average thickness of the second resin layer7of each cover film8is determined by an amount of resin required both to cover the conductive pattern and to fill the gaps between the lines of the conductive pattern2. The lower limit of the average thickness of the second resin layer7of the cover film8is preferably 20 μm and more preferably 30 μm. The upper limit of the average thickness of the second resin layer7of the cover film8is preferably 50 μm and more preferably 40 μm. When the average thickness of the second resin layer7of the cover film8is less than the lower limit, there may be a risk that the cover layer4formed after the pressure bonding step S3have large surface irregularities, resulting in an insufficient protection for the conductive pattern2. Conversely, use of the average thickness of the second resin layer7of the cover film8that exceeds the upper limit results in the produced flexible printed circuit board being needlessly thick, which may create a risk of disregarding the demand for reduction in the thickness of a flexible printed circuit board.

The second resin layers7of the cover films8are the same as or similar to the second resin layers of the flexible printed circuit board that has previously been described, except for their average thicknesses. A further description thereof will thus be omitted.

In the pressure bonding step S3, the laminated structure3and the cover film8are vacuum bagged at a temperature higher than the softening temperature of the second resin layer7.

The lower limit of the temperature for vacuum bagging is preferably 60° C. and more preferably 70° C. The upper limit of the temperature for vacuum bagging is preferably 100° C. and more preferably 90° C. Use of a vacuum bagging temperature that is less than the lower limit may create a risk that the second resin layer7does not sufficiently softens, thereby causing the second resin layer7to fail to fill the gaps sufficiently between the lines of the conductive pattern2. Use of a vacuum bagging temperature that exceeds the upper limit may create a risk of deterioration in the characteristics of electronic components and the like mounted on the base film1.

The lower limit of the vacuum bagging pressure is preferably 0.1 MPa and more preferably 0.2 MPa. The upper limit of the vacuum bagging pressure is preferably 0.5 MPa and more preferably 0.4 MPa. Use of vacuum bagging pressure that is less than the lower limit may create a risk of excessive manufacturing costs. Conversely, use of vacuum bagging pressure that exceeds the upper limit may create a risk that the second resin layer7fails to fill the gaps sufficiently between the lines of the conductive pattern2.

The lower limit of the duration of vacuum bagging is preferably 10 seconds and more preferably 15 seconds. The upper limit of the duration of vacuum bagging is preferably 30 seconds and more preferably 25 seconds. Use of the duration of vacuum bagging that is less than the lower limit may create a risk that the second resin layer7fails to fill the gaps sufficiently between the lines of the conductive pattern2. Conversely, use of the duration of vacuum bagging that exceeds the upper limit may create a risk of reduced manufacturing efficiency.

It may be noted that the cover film8may preferably be made of a photo-curable resin and exposed to ultraviolet light after vacuum bagging. Curing through exposure to ultraviolet light as described above may prevent the cover layer4from deforming due to generated heat during the operation of electronic components.

Advantage

The method of making a flexible printed circuit board uses the two-layer cover film8that has the first resin layer6and the second resin layer7that is laminated to the inner side of the first resin layer6. In the method of making a flexible printed circuit board, the cover film8is vacuum bagged in the pressure bonding step S2at a temperature higher than the softening temperature of the second resin layer7wherein the softening temperature of the second resin layer7is lower than that of the first resin layer6. As a result, the second resin layer7is selectively softened to cover the conductive pattern2laminated to the base film1. The first resin layer6laminated to the outer side of the second resin layer7seals the second resin layer7, which provides the cover layer4. Accordingly, use of this method of making a flexible printed circuit board ensures that the conductive pattern2is more reliably coated even when the thickness of the cover layer4is reduced, compared with the case in which the first resin layer6and the second resin layer7are separately laminated. This serves to reduce the size of an entire flexible printed circuit board.

Further, such a flexible printed circuit board uses an epoxy resin as the main polymer of the second resin layer7so as to allow the conductive pattern2to be reliably coated with the second resin layer due to its fluidity obtained at the time of softening. Moreover, the flexible printed circuit board uses a polyimide as the main polymer of the first resin layer6so as to allow the second resin layer7to be selectively softened, which results in the second resin layer7being securely sealed with the first resin layer6. As a result, the flexible printed circuit board allows the conductive pattern2to be more reliably coated even when the thickness of the cover layer4is reduced, which serves to reduce the size of an entire flexible printed circuit board.

Other Embodiments

The embodiments disclosed herein should be regarded as examples only and as non-limiting in all aspects. The scope of the present disclosures is defined by the claims without being limited to the configurations of the disclosed embodiments, and is intended to include all modifications within the spirit and equivalents of the scope of the claims.

Although the embodiments have been described with respect to a case in which the conductive pattern is laminated on both surfaces of the base film, the conductive pattern may be laminated only on one surface of the base film. In this case, only one sheet of the cover film may be used, and the cover film may be superimposed only on the conductive-pattern side of the laminated structure, followed by vacuum bagging.

Although the embodiments have been described with respect to a case in which the conductive pattern is made of metal, the conductive pattern may alternatively be implemented in other configurations. Such other configurations of the conductive pattern may include a configuration which has a core body formed by a subtractive or semi-additive process, with a thickening layer laminated by plating to the exterior surface of the core body. Use of such a configuration allows the circuit intervals to be shortened to increase the density of a conductive pattern, which further reduces the size of a flexible printed circuit board.

DESCRIPTION OF REFERENCE SYMBOLS