WO2009038950A2

WO2009038950A2 - Flexible circuit board, manufacturing method thereof, and electronic device using the same

Info

Publication number: WO2009038950A2
Application number: PCT/US2008/074798
Authority: WO
Inventors: Yoshiyuki Ohkura; Tatsunori Koyanagi
Original assignee: 3M Innovative Properties Co
Current assignee: 3M Innovative Properties Co
Priority date: 2007-09-20
Filing date: 2008-08-29
Publication date: 2009-03-26
Anticipated expiration: 2010-03-20
Also published as: JP2009076663A; WO2009038950A3

Abstract

An aspect of the invention provides a flexible circuit board, comprising: a substrate having two portions separated by a bendable portion, said substrate having a plurality of thin sections on one surface of said bendable portion, and said thin sections extending in a direction that intersects with the direction of bending, and having a supporting section between said plurality of mutually adjacent thin sections, and the width of said thin section in the bending direction is larger than the width of said supporting section in said bending direction; a circuit comprising a conductive material formed and adhered onto a said substrate; and a cover layer on said first surface covering at least a portion of the surface of said circuit.

Description

FLEXIBLE CIRCUIT BOARD, MANUFACTURING METHOD THEREOF, AND ELECTRONIC DEVICE USING THE SAME

TECHNICAL FIELD

The present invention relates to a flexible circuit board and in further detail relates to a flexible circuit board used in a bent condition for applications such as connecting a liquid crystal display panel of a liquid crystal display device to a printed wiring board, and the like.

BACKGROUND

Flexible circuit boards manufactured by printing a circuit onto a flexible substrate made of a polymer or the like are commonly used for internal wiring of electronic devices such as mobile phones and display devices such as liquid crystal television because of the excellent flexibility characteristics. In order to make these electronic devices smaller and thinner, flexible circuit boards are commonly used in a bent or folded condition in the device.

Flexible circuit boards can easily be bent, but because these circuit boards undergo elastic deformation when in the bent condition, a reactive force is generated when returning from a bent condition to a straight condition. If these reactive forces are strong, assembly productivity when assembling the electronic devices will be reduced. Furthermore, if both ends are connected to a printed circuit board or the like these reactive forces will act in a direction to separate the connecting section, and there is a concern that the long-term reliability of the connecting section will be reduced.

In order to correspond to these issues, the following conventional methods to increase the flexibility of a flexible circuit board have been proposed.

Japanese Unexamined Patent Application H05-335696 describes "a flexible circuit board comprising a circuit conductor and surface coating material wherein at least one of insulative substrate or the surface coating material in the bendable portion is removed to a depth that does not expose the circuit and a width by irradiating excimer laser light or alike in order to form a bending groove."

Japanese Unexamined Patent Application 2006-210771 describes a printed- wiring board "comprises the flexible section made of the conductor thin film for forming a circuit pattern, and the base-laminated section in which a base is laminated on the conductor thin film. The conductor thin film is covered with an insulating material layer made of a single material at least at the flexible section." Furthermore, instead of reducing the thickness of the flexible circuit board in specified areas, an extremely thin substrate is sold. DuPont KAPTON polyimide film type 5OH available from DuPont has a thickness of 12.5 μm.

Incidentally, the method of forming a cover layer on the circuit body that is formed on the flexible substrate in a prescribed pattern can be one of several commonly known methods. Of these methods, using the method of applying a liquid solder resist with a certain viscosity to prescribed sections on the surface of a circuit by silk screen printing is preferable because of the lower material costs and compatibility with in-line production when manufacturing the flexible circuit board. The liquid solder resist that is applied is then hardened by ultraviolet light or the like to form a cover layer.

If one bending groove is formed in the bendable portion, the bending groove must be long in the direction of bending in order to sufficiently improve the flexibility. When a silkscreen mask with the form of the desired pattern is placed on the bendable portion having one bending groove of the flexible circuit board, the substrate in the bendable portion will warp because of the thinness of the substrate and therefore the silkscreen mask will not conform to the surface of the substrate. Therefore, the distance between the silkscreen mask and a substrate surface will not be uniform, and areas where the solder resist layer is thick will occur on the substrate, and furthermore, liquid solder resist could be applied to non-intended regions of the substrate. As a result, the thickness of the cover layer formed by hardening the solder resist layer will not be uniform, and there may be problems in that a cover layer with the desired pattern cannot be formed. This phenomenon becomes more significant as the bending groove lengthens in the direction of bending.

Furthermore, if a part of substrate is removed from the bending portion after forming the cover layer, steps of removing material from the bending portion of the substrate must be repeated to the already processed substrate. In this case, ablation by laser light is commonly performed, but this step is performed by scanning a laser light onto the substrate, so only one substrate can be processed at a time and processing costs will be expensive. Furthermore, fragments of the substrate that are removed by ablation will adhere to the board, so a washing process must also be added in order to remove the adhering smear. The bendable portion can also be formed by etching, but this is not preferable because the series of steps of applying photoresist, exposing, developing, and removing must be repeated again. If one bending groove is formed in the bendable portion, the bending groove must be long in the direction of bending in order to sufficiently improve the flexibility. Therefore, if the liquid solder resist which forms the cover layer is applied by silk screen printing to the surface of the substrate after first forming the bendable section in the substrate, problems will occur with the quality of the printed solder resist layer.

If an extremely thin substrate is used to improve the flexibility of the flexible circuit board will not be a problem, the rigidity of the whole flexible circuit board will be lower. Therefore the flexible circuit board may be difficult to handle and productivity during the process of assembling the electronic device may be poor. SUMMARY

An objective of the present invention is to provide a flexible circuit board with improved flexibility in the bendable portion without reducing the handling properties of the flexible circuit board. Another objective of the present invention is to provide a flexible circuit board that can form a covered layer with minimal thickness variation in prescribed sections due to silk screen printing, as well as to a manufacturing method for this flexible circuit board, and to an electronic device which uses this flexible circuit board.

The first aspect of the present invention provides a flexible circuit board, comprising: a substrate made of flexible material comprising a first portion and a second portion separated by a bendable portion, wherein said substrate has a first surface and a second surface and has a plurality of thin sections on the second surface side of said bendable portion in a direction of bending, and said thin sections extending in a direction that intersects with the direction of bending, and has a supporting section between said plurality of mutually adjacent thin sections, such that the width of said thin section in the bending direction is larger than the width of said supporting section in said bending direction; a circuit comprising a conductive material formed and adhered onto said first surface of said substrate; and a cover layer established on said first surface so as to cover at least a portion of the surface of said circuit.

Another aspect of the present invention is to provide a flexible circuit board, comprising: a substrate made of flexible material comprising a first portion and a second portion separated by a bendable portion, wherein said substrate has a first surface and a second surface, and has one thin section on the second surface side of said flexible part, and has one or more supporting sections in said thin sections; a circuit comprising a conductive material formed and adhered onto said first surface of said substrate; and a cover layer established on said first surface so as to cover at least a portion of the surface of said circuit. In the first aspect, the present invention further provides a manufacturing method for a flexible circuit board comprising the following steps: a. a step of preparing a substrate; b. a step of forming a circuit on said substrate; c. a step of removing a portion of said substrate material from the second surface side of the substrate in the bendable portion, thus forming a plurality of thin sections extending from near one side end surface of said substrate in a direction that intersects with the bending direction to near the other side end surface, and a supporting section between the plurality of mutually adjacent thin sections, such that the width of the thin section in the direction of bending is wider than the width of the supporting section in the direction of bending; d. a step of forming a cover layer on said substrate surface in order to cover at least a portion of said circuit.

Furthermore, in another aspect, the present invention also provides a manufacturing method for a flexible circuit board comprising the following steps: a. a step of preparing a substrate; b. a step of forming a circuit on said substrate; c. a step of removing a portion of said substrate material from the second surface side of the substrate which is the bendable portion, and forming one thin section and one or more supporting sections in the thin section. d. a step of forming a cover layer on said substrate surface in order to cover at least a portion of said circuit.

At least one aspect of the present invention provides a flexible circuit board with increased board flexibility without impacting handling properties. Furthermore, at least one aspect of the present invention provides a flexible circuit board that minimizes warp of the substrate when applying a liquid solder resist by silk screen printing and effectively controls a drop in quality of the cover layer obtained. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a top plan view of a flexible circuit board according to the first embodiment of the present invention.

Fig. 2 is a cross-sectional view of a flexible circuit board according to the first embodiment of the present invention.

Fig. 3 is a bottom plan view of a flexible circuit board according to the first embodiment of the present invention.

Fig. 4 is a diagram showing a part of a liquid crystal display device that uses the flexible circuit board according to the first embodiment of the present invention. Fig. 5 is a bottom plan view of a flexible circuit board according to the second embodiment of the present invention.

Fig. 6 is a bottom plan view of a flexible circuit board according to the third embodiment of the present invention.

Fig. 7 is a bottom plan view of a flexible circuit board according to the fourth embodiment of the present invention.

DETAILED DESCRIPTION

Several embodiments of the flexible circuit board of the present invention will be described while referring to the attached drawings.

Fig. 1 is a top plan view of a first embodiment of the flexible circuit board according to the present invention. The flexible circuit board 10 comprises a substrate 12 made of a flexible material, and an electrically conductive circuit 14 formed on the surface of the substrate. Substrate 12 has a film-like from. The flexible circuit board 10 also has a cover layer 16 on a part of substrate 12 and circuit 14 that is formed by hardening a solder resist. Fig. 2 and Fig. 3 are a cross-sectional view and a bottom plain view of the flexible circuit board 10 shown in Fig. 1. Substrate 12 has a bendable portion 20 near the center in the bending direction 18 of the flexible circuit board 10, and also has a first portion 22 at one end in the bending direction 18 and a second portion 24 at the other end. The bendable portion 20 is a section that provides higher flexibility to the substrate because a portion of the material of the substrate 12 has been removed. The bendable portion 20 is not necessarily at the geometric center in the bending direction 18 of the substrate 12, and can also be at a position shifted toward either the first portion side or the second portion side. The first portion 22 and a second portion 24 are sections which can be connected to a printed circuit board or the like which is electrically connected to this flexible circuit board 10. Substrate 12 further comprises a first surface 26 that a circuit 14 is formed on, a second surface 28 which is the back surface thereof, and side end surfaces 30, 31 that are essentially parallel to the bending direction 18 of the flexible circuit board 10 and which intersect with both the first surface 26 and the second surface 28.

The substrate 12 can be a single layer substrate made from a single film, or can be a multilayer film wherein a plurality of films are overlaid. The substrate 12 can be formed from various types of materials that can provide flexibility to the substrate, but normally forming a film-like shape by an arbitrary forming method using an insulative resin material is advantageous. Preferable resin materials for the substrate include polyimide resin, liquid crystal polymer, polyethylene terephthalate, polyethylene naphthalate, and the like. Polyimide resin is especially preferable. The substrate can have various thicknesses depending on the application and construction of the flexible circuit board. As the substrate becomes thinner, rigidity is reduced, handling properties may be compromised and the substrate may be prone to breaking. As the substrate becomes thicker, it will inhibit making the product lighter. Substrate thickness is typically greater than 15μm but less than about 300μm, preferably greater than about 25μm but less than about 125μm. Furthermore, the substrate is normally formed as a long member, and preferably is rolled on a core for storage and transport, and unrolled when used for manufacturing the flexible circuit board. The bendable portion 20 comprises a plurality of thin sections 32 extending in a direction that intersect with the bending direction 18 of the substrate 12, a supporting section 34 positioned between the thin sections 32 and ending at both side ends 30, 31 of the substrate 12, and a second surface 28 of the substrate 12.

The thin section 32 is a thinner section than the thickness of the substrate 12 and comprises a third surface 36 that is essentially a surface parallel to the second surface, and a side wall 38 that joins the third surface and the second surface. In other words, bendable portion 20 comprises a plurality of grooves 39 and the grooves 39 are defined by a bottom surface (third surface 36) and 2 opposing side walls 38. The thin section 32 can be formed by chemically or mechanically removing a portion of the substrate 12 on the second surface 28 side. The sidewalls 38 face each other with the thin section 32 therebetween, and are inclined in a direction that mutually separates as the two side walls 38 move from the third surface 36 toward the second surface 28. A surface which has this type of an incline can easily be formed by forming the thin section 32 using a chemical method such as etching. The side walls 38 can be perpendicular with regards to the second surface 28 of the substrate, but if the side walls 38 are inclined, the change in the rigidity at the interface between the thin section 32 and the supporting section 34 when bending the flexible circuit board 10 will be smoother, and sharp bending of the flexible circuit board 10 at this interface can be suppressed. The incline of the side walls 38 can be any incline, but the angle of the incline α is preferably an obtuse angle in order to prevent sharp bending of the flexible circuit boards 10. The angle of incline α is preferably 100° or higher and 175° or lower. More preferably, the angle of incline α is preferably 135° or higher. Note, the angle of incline α is the angle formed between the third surface 36 of the thin section 32 and the sidewall 38.

The supporting section 34 is a section of substrate 12 that is thicker than the thin section 32. An upper end surface 40 of the supporting section 34 forms a plane that is essentially the same as the second surface 28 of the substrate 12. This means that a supporting section 34 separates 2 adjacently located grooves 39. Upper end surface 40 contacts the surface where the substrate 12 is placed when liquid solder resist is applied by a printing method as described later and suppresses warping of the first surface 26 of the substrate 12 during the process. Thereby, the gap that occurs between the silkscreen mask and the bendable portion of the substrate 12 can be essentially uniform. Normally, the upper end surface 40 can be formed using the second surface 28 of the substrate 12, but the upper end surface 40 can also be formed to be lower than the second surface 28.

The cross-sectional shape of the groove 39 formed by one thin section 32, and mutually adjacent side walls 38 on either side of the thin section 32 is not restricted to a trapezoid as shown in Fig. 2, and can be formed with a rectangular or V-shape, or can have a U-shape with the thickness of the thin section continuously changing. Depending on the cross-sectional shape of the groove 39, the third surface 36 of the thin section may essentially be nonexistent such as the case of a V-shaped cross-section. In the present embodiment, the thin section 32 extends to both side end surfaces 30, 31 of the substrate 12, but it is also acceptable for the thin section to not extend to both side end surfaces 30, 31. In other words, the thin section 32 can be formed to end near both side end surfaces 30, 31 of the substrate 12. Note, near both end surfaces 30, 31 refers to locations some limited distance from the side end surface 30, 31, and that are separated by no more than 1/3 of the width of the substrate 12 (shortest distance between both side end surfaces 30, 31 in a direction that intersects with the direction of bending).

The width W of the supporting section 34 in the bending direction 18 and the thickness and width X of the thin section 32 in the bending direction 18 are appropriately set to match the conditions where the flexible circuit board 10 will be used. The flexibility of the flexible circuit board 10 will increase if the thin section 32 is thinner, and normally the thickness is 25 μm or less, and more preferably 15 μm or less. On the other hand, the thin section 32 has a thickness of 5 μm or higher, and more preferably 10 μm or higher. If the thin section is thinner than this, there will be concerns of breaking the board during handling. Furthermore, if the thin section 32 is formed by etching, the thickness of the thin section 32 is preferably less than half the thickness of the substrate 12. This is because if the substrate 12 has through holes or the like, the through holes can also be formed at the same time as forming the thin section 32 by etching from both sides of the substrate 12.

Width X of thin section 32 varies with intended usage. Control of the warp of the first surface 26 of the substrate 12 will be insufficient if the width X is too large. Flexibility of substrate 12 will not be improved enough if the width X is too narrow. Due to this reason, width X is typically about 2mm or less and about 0. lmm or greater, preferably about 0.9mm or less and 0.4mm or greater.

Furthermore, in order to effectively improve the flexibility, the width X of the thin section is typically set to be wider than the width W of the supporting section. Note, the width X of the thin section is the distance between the points (lines) where the side walls 38 that face each other on either side of one thin section 32 intersect with the top end surface 40 and the second surface 28. Similarly, the width W of the supporting member is defined as the distance between the points (lines) where the upper end surface 40 of one supporting section intersects with the most adjacent sidewall 38 and the supporting section 34.

The width X of the thin section 32 and the width W of the supporting section 34 can be uniform or non-uniform across the entire the length of the bendable portion 20. If these widths are uniform, the flexible circuit board 10 will be able to be bent with a uniformly curved radius along the entire length of the bendable portion 20. On the other hand, if there is a desire to locally change the bending radius, the bending radius can be adjusted by setting these widths to be non-uniform. The thin section 32 can be formed by removing the substrate material using various types of physical and chemical methods known by one skilled in the art. These methods include laser ablation using direct or indirect exposure, or etching by using a solvent to selectively dissolve a region that has been exposed by a photoresist layer formed on the substrate 12. Of these methods, the thin section 32 of the present invention is preferably formed by etching. Forming the thin section 32 by etching is efficient because other configurations in the substrate 12 besides the thin section 32 can also be simultaneously formed. Furthermore, unlike ablation, smearing will not occur so there is no need to add an additional smear removing process. On the other hand, if the side walls 38 of the thin section 32 are set to be inclined, the inclined side walls 38 can easily be formed by the etching dissolving properties.

The circuit 14 is a conductive circuit formed and adhered onto the substrate 12 in an arbitrary pattern in order for the flexible circuit board 10 to achieve the desired electrical function. A plurality of circuits 14 are formed so as to conduct electricity between the first portion 22 and the second portion 24 on the first surface 26 of the substrate 12. The circuit 14 can also be formed on the substrate 12 with an adhesive layer therebetween. The circuit 14 can also have a pattern onto which an electronic component such as an IC or the like is placed. The circuit 14 can also be formed on the second surface 28 of both the first portion 22 and the second portion 24 of the substrate 12. In this case, through holes between the layers known as via holes can be formed in the substrate 12 in order to conduct electricity between the circuit 14 on the first surface 26 and the circuit 14 on the second surface 28.

The circuit 14 can be made from any conductive material, but copper or aluminum and the like are preferably used. Furthermore, in order to increase the reliability of the electrical connections to the printed circuit board or the like, a metallic film comprising another metal such as gold or tin can be formed on the surface of the circuit 14 which acts as an electrical connection with other circuit boards or the like.

The circuit can have a variety of thicknesses depending on the construction of the flexible circuit board and the construction of this circuit, but normally the thickness is less than approximately 50 μm, preferably in a range between 5 and 40 μm, and more preferably in a range between 10 and 30 μm.

In order to prevent mutual shorting between the circuits 14, a cover layer 16 is formed on the surface of the substrate 12 except in both side sections which form the contacts between the circuit 14 and other boards or the like. The cover layer 16 can be formed to cover the entire surface of the substrate 12, or can be formed to cover only an arbitrary region that excludes the electrical contacts with other printed circuit boards and IC chips and the like, as with the present embodiment. With the flexible circuit board of the present invention, the cover layer 16 is formed by applying a liquid solder resist to the substrate by silk screen printing, and then hardening the solder resist. The liquid solder resist can be any type of currently existing light hardening or thermal hardening the liquid solder resist. The silk screen printing method is a method where a silkscreen mask is placed over an object (substrate) so that the solder resist will only be applied in the desired regions, and then liquid solder resist is transferred onto the surface of the object (substrate) through the silkscreen mask. Silkscreen printing has the advantages that the cover layer can easily be selectively formed in only the desired locations at a low material cost as compared to a method of laminating a solder resist film. In order to uniformly apply liquid solder resist only to the desired regions using silkscreen printing, the silkscreen mask must be adhered to the object of printing. If the silkscreen mask is not locally adhered to the object for printing, the application thickness of the solder resist in that section will increase, and in some situations there is a possibility that the solder resist will be applied to unintended regions. Therefore, a reduction in the quality of the silk screen printing can effectively be prevented by establishing a supporting section 34 in the bendable portion 20 in order to suppress the warp of the thin section 32.

The liquid solder resist that is printed on the substrate is then hardened by heat or light in order to form the cover layer 16. The thickness of the cover layer 16 is arbitrarily set according to the conditions for using the flexible circuit board and the properties of the liquid solder resist that is used, but is normally greater than 5 μm and less than 40 μm. In order to more effectively demonstrate the effects of the present invention, the bending flexibility of the cover layer 16 is preferably greater than the bending flexibility of the substrate 12. This can be adjusted by reducing the modulus of elasticity of the material which forms the cover layer 16 or by reducing the thickness of the cover layer 16 and/or reducing the area of application. This is because regardless of the method used, the loss of flexibility of the flexible circuit board can be minimized without providing a bendable portion similar to the present invention in the cover layer 16. If necessary, a reinforcement plate can also be used with the flexible circuit board 10 of the present invention. This reinforcing plate is normally attached on the surface of the substrate 12 that the circuit 14 is not formed, corresponding to the pattern of the circuit 14. The reinforcing plate is attached using an empty region where the circuit 14 or other functional element is not formed, and therefore can be formed either continuously or with an arbitrary non-continuous pattern. To give an example, the pattern of the reinforcing plate can be rectangular, L-shaped, or round. Any of these patterns can easily be formed using standard technology such as die punching a sheet-like reinforcing plate. A connector region or the like can be suggested as an example of a location where a reinforcing plate is placed. Fig. 4 is a diagram schematically showing an example of a portion of a liquid crystal display device 70 that uses the flexible circuit board according to the present invention. A back light 74 is provided on the back surface of a liquid crystal panel 72, and a drive circuit board 76 which drives the liquid crystal panel is provided behind the back light. A flexible circuit board 10 is used in order to electrically connect the drive circuit board 76 and the liquid crystal panel 72. The flexible circuit board 10 also has a circuit 14' on the second surface side, and this circuit 14' and the circuit 14 on the first side are electrically connected by via holes which are not shown in the drawings. The flexible circuit board 10 is electrically and mechanically connected to the liquid crystal panel 72, and on the other end to the drive circuit board 76. In Fig. 4, a shape where the second surface 28 of the flexible circuit board 10 faces inward is shown, but the second surface 28 can also be used in a condition facing outward.

Fig. 5 shows a bottom plan view of a flexible circuit board 10a according to the second embodiment of the present invention. In this embodiment, the bendable portion 20a has a plurality of groove-shaped thin sections 32a, and a linking section (supporting section) 46a links each of the supporting sections 34a as well as between the supporting sections 34a and the first portion 22a and the second portion 24a. In other words, thin section 32 (groove 39) of example 1 is divided into two thin sections 32a by a linking section (supporting section) 46a.

With this construction, the linking section helps to suppresses sharp bending of the substrate 12a at the interface between the supporting sections 34a and the thin sections 32a during bending. Furthermore, this linking section 46a acts in a similar manner to the supporting section 34a and helps to suppresses warp of the first surface 26a during printing of the cover layer.

In Fig. 5, a plurality of linking sections 46a are formed in a straight line in the direction of bending, but each of the linking sections 46a can also be in different positions in the width direction of the substrate 12a. Furthermore, a plurality of linking sections 46a can be formed in parallel.

With the embodiment of Fig. 5, the width of each of the linking sections 46a is essentially uniform, but varying the width is also possible. For example, it is possible for the width to be the narrowest at approximately the center in the bending direction 18a of the linking section 46a and for the width to increase towards the supporting section 34a. Fig. 6 and Fig. 7 show several examples of other embodiments of the present invention.

Fig. 6 is a bottom plan view of a flexible circuit board 10b according to the third embodiment of the present invention. In this embodiment, a plurality of supporting section islands 34b are arranged in a thin section 32b in two rows in the bending direction 18b of a flexible circuit board 10b. In other words, flexible circuit board 10b of this embodiment comprises bendable portion 20b comprising one recess (thin section 32b) and plurality of posts (supporting section islands 34b). Islands refers to a condition where the outer circumference 41b on the upper side surface 40b of one supporting section 34b is entirely in contact with one side wall 38b of the thin section 32b. The supporting sections 34b are arranged in the bending direction 18b such that one row of supporting sections 34b is located at a position between two mutually adjacent supporting sections 34b in an adjacent row. In other words, the supporting sections 34b of two adjacent rows will be in mutually different positions with regards to the bending direction 18b. In this embodiment, the area of the thin section can be further increased compared to the first and second embodiment, so the flexibility of the flexible circuit board can be enhanced. Furthermore, the supporting sections 34b are in mutually different positions, so the flexibility of the bendable portion will be more uniform in the direction of bending compared to the case where the supporting sections 34b are arranged in a single row or compared to the case where two rows of supporting sections are aligned with regards to the position in the bending direction.

Fig. 7 shows a bottom surface diagram of a flexible circuit board 10c according to the fourth embodiment of the present invention. The bendable portion 20c has one thin section 32c, and a plurality of supporting sections 34c are arranged in the thin section 32c. The supporting sections 34c are in mutually different positions with regards to the bending direction 18c than the adjacent row of supporting sections 34c, similar to the embodiment shown in Fig. 6. With this embodiment, the supporting sections 34c extend from one side edge 30c or 3 Ic of the substrate 12c toward the center in the width direction of the substrate

12c, and the leading edge thereof is near the center in the width direction of the substrate. With the flexible circuit board of this embodiment as well, the flexibility of the bendable portion is further improved, and the bending rigidity of the bendable portion is more uniform in the bending direction, and in particular, a sharp bend in the flexible circuit board 10 can effectively be prevented even for the case of a small bending radius.

The third and fourth embodiments can also have a linking section that links between supporting sections and/or between the supporting sections and the first portion or the second portion. Furthermore, combinations of two or more of these embodiments can be used. Of course the supporting sections can be arranged in a configuration where the positions are not mutually different, or arranged in a single row in the bending direction.

Next, a preferred example of a manufacturing method for the flexible circuit board of the present invention will be described using the flexible circuit board 10 of the first embodiment as an example.

First, a substrate 12 made of an arbitrary material was prepared. In order to efficiently form the flexible circuit board 10 of the present invention, continuous processing using a long substrate 12 is preferable for production efficiency.

Next, one surface of the substrate 12 was metalized by sputtering. The use of a substrate that has already been metalized on the substrate surface is also possible, in which case the metalizing step can be omitted. Furthermore, a copper (Cu) layer which will become the circuit 14 was deposited on the entire surface of the first surface 26 of the substrate 12 by sulfuric acid copper plating. The thickness of the copper was between approximately 1 and 7 μm.

Next, photoresist was applied to entirely cover both surfaces of the substrate 12. A liquid resist or a dry film resist can be used as the photoresist. After applying the photoresist, exposure was performed through a photomask, and then developing was performed. As a result of exposing and developing, photoresist was removed in the regions corresponding to the locations where the circuit 14 is to be formed on the first surface 26 of the substrate 12, and the photoresist was left to remain in all other regions. Furthermore, photoresist was removed in the regions corresponding to the thin sections 32 of the bendable portion 20 on the second surface 28 of the substrate 12, and the photoresist was left to remain in all other regions. On the first surface 26 of the substrate 12, additional copper was deposited by copper plating on the surface (copper plated surface) that was exposed by the photoresist. The copper plating can be performed at the same conditions as the aforementioned copper plating. A copper layer with a thickness of between approximately 8 and 50 μm was formed in the regions where the circuit 14 is to be formed by this additional plating process. Note, the base copper plating surface was remaining on the entire surface of the substrate 12 at this stage, so the circuit 14 was not electrically independent.

Continuing, the thin section 32 was formed. This section can be formed by dissolving and removing the surface of the substrate that has been exposed from the photoresist using for example a strong alkaline etching agent. As a result of this etching, a thin section 32 with inclined side walls 38 can be formed. Laser ablation for example may be used in place of etching when forming the thin section 32.

After forming the thin section 32 as described above, the photoresist was dissolved and removed from both surfaces of the substrate 12. After this photoresist removing process, a portion of the copper layer was entirely removed by etching from the first surface 26 of the substrate 12, thereby making each of the circuits 14 electrically independent.

Next, liquid solder resist was applied by a silk screen printing in arbitrary regions of the first surface 26 of the substrate 12. The liquid solder resist can be a liquid with a viscosity suitable for the printing method used. Furthermore, light hardening or thermal hardening liquid solder resist can be appropriately selected. After applying liquid solder resist to arbitrary locations on the substrate 12 by silk screen printing, the liquid solder resist was hardened to form the cover layer 16.

Finally, a metal film can be formed by plating in the areas of the circuit 14 not covered by the cover layer in order to increase the contact with the external board. The metal layer can be formed by gold (Au) plating or tin (Sn) plating. EXAMPLES

Example 1

A flexible circuit board corresponding to the first embodiment was manufactured by the following procedures. One surface of a long polyimide substrate (product name "KAPTON 10OE", product of du Pont) with a width of 11 mm and a thickness of 25 μm was sputtered to form a seed layer of Cr/Ni and a layer of Cu on the seed layer. Next, a copper layer with a thickness between 3 and 5 μm was deposited by sulfuric acid copper plating on the entire surface.

Photoreactive photoresist was then applied to the entire surface on both sides of the substrate. The photoresist on both surfaces was exposed to ultraviolet light through a photomask and then developed.

The substrate was then immersed in a plating tank, and copper was deposited by sulfuric acid copper plating to approximately 15 μm on the copper layer surface that was exposed from the photomask. Continuing, the substrate was then immersed in a strong alkaline solution and the exposed side of the polyimide substrate was etched in order to form a groove-shaped thin section in the bendable portion extending from one side end surface of the substrate to the other side end surface.

After dissolving and removing the photoresist from both surfaces of the substrate, the metal section was removed from the entire surface by etching in order to obtain mutually independent circuits. Wiring with a line/space of approximately 75 μm/75 μm and a thickness of approximately 15 μm was formed in four lines in the bending direction of the substrate.

Thermal hardening liquid solder resist (UPICOAT, product of Ube Industries, Ltd.) was printed onto the copper wiring and then hardened for one hour at 150° to obtain a cover layer with a thickness between 15 and 18 μm. Ni and Au plating was performed on the regions of the circuit that were exposed from the cover layer. The substrate was then cut off to form flexible circuit boards with a length of 130 mm and a width of 11 mm. The dimensions of each part of the bendable portion of the flexible circuit board obtained are as shown in Table 1. Liquid Solder Resist Printing Variability Evaluation

The application variability of the liquid solder resist for the flexible circuit board obtained was evaluated by observing the cover layer under an stereomicroscope at a zoom of 10 times. Samples with significant application variation were evaluated as "fail", and samples with no application variation or insignificant variation were evaluated as "pass".

The evaluation results are shown in Table 1.

Measurement of Bending Strength

The bending strength of the flexible circuit board was measured as the loop stiffness. A flexible circuit board with a width of 11 mm and the length of 130 mm manufactured as described above was bent in a loop with a loop length of 60 mm and secured to the test device. At this time, the bendable portion was adjusted to be near the center of the loop. The loop was pressed and flattened at a speed of 3.5 mm/s until the loop height was 10 mm, and then the loop stiffness was measured. A Toyo Seiki Seisaku-Sho loop stiffness tester D was used for measuring. The measurement results are shown in Table 1. Examples 2 through 4

Flexible circuit boards similar to embodiment 1 were manufactured, but the various dimensions of the photomask were changed to obtain a board with the dimensions in the bendable portion as shown in Table 1

Comparative Examples 1 A flexible circuit board was manufactured similar to embodiment 1 , except that a thin section was not formed in the bendable portion of the substrate.

Comparative examples 2 and 3

A flexible circuit board was manufactured similar to embodiment 1 , except that only one thin section with the dimensions shown in Table 1 was formed. TABLE 1

NA: No corresponding shape or dimension.

Claims

1. A flexible circuit board, comprising: a substrate made of flexible material comprising a first portion and a second portion separated by a bendable portion, wherein said substrate has a first surface and a second surface and has a plurality of thin sections on the second surface side of said bendable portion, and said thin sections extending in a direction that intersects with the direction of bending, and has a supporting section between adjacent thin sections, such that the width of said thin section in the bending direction is larger than the width of said supporting section in said bending direction; a circuit comprising a conductive material on said first surface of said substrate; and a cover layer established on said first surface so as to cover at least a portion of the surface of said circuit.

2. The flexible circuit board according to Claim 1 , wherein said thin sections extend to at least one side end surface of said substrate.

3. A flexible circuit board, comprising: a substrate made of flexible material comprising a first portion and a second portion separated by a bendable portion, wherein said substrate has a first surface and a second surface, and has one thin section on the second surface side of said bendable portion, and has one or more supporting sections in said thin sections; a circuit comprising a conductive material formed and adhered onto said first surface of said substrate; and a cover layer established on said first surface so as to cover at least a portion of the surface of said circuit.

4. The flexible circuit board according to Claim 3, wherein said supporting section has a supporting surface and the entire outer circumference of said supporting surface is in contact with said thin section.

5. The flexible circuit board according to Claim 1 , wherein said cover layer is made by hardening a liquid solder resist applied on said substrate by silk screen printing.

6. The flexible circuit board according to Claim 5, wherein said cover layer is formed after forming said thin section.

7. The flexible circuit board according to Claim 1, wherein in said bendable portion, the area occupied by said thin section is wider than the area occupied by said supporting section.

8. An electronic device comprising a flexible circuit board according to Claim 1.

9. A manufacturing method for a flexible circuit board comprising: preparing a substrate; forming a circuit on said substrate; selectively removing substrate material from the second surface side of the substrate to form a bendable portion having a plurality of thin sections extending from near one side end surface of said substrate in a direction that intersects with the bending direction of the bendable portion to near the other side end surface and having a supporting section between adjacent thin sections, such that the width of the thin sections in the direction of bending is wider than the width of the supporting section in the direction of bending; and forming a cover layer on said substrate surface in order to cover at least a portion of said circuit.

10. A manufacturing method for a flexible circuit board comprising the following: preparing a substrate; forming a circuit on said substrate; selectively removing said substrate material from the second surface side of said substrate to form a bendable portion having one thin section and one or more supporting sections in the thin section; and forming a cover layer on said substrate surface in order to cover at least a portion of said circuit.