JPH07312468A - Flexible circuit board - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To provide a flexible wiring board, which has good flexibility, particularly in a multilayer wiring board, and which allows wiring patterns to be wired by thermocompression bonding, at low cost. [Structure] A thermoplastic resin film 2 is fused on one side or both sides of a polyimide resin film 1 by a thermoplasticity of the thermoplastic resin film 2 to form an insulating substrate for circuit formation, and at least the thermoplastic resin film 2 is formed. The wiring pattern 3 is formed on the surface of the wiring pattern by the thermoplasticity of the thermoplastic resin film 2.
Flexible circuit board in which the base material is fused.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible wiring board on which a wiring pattern can be advantageously formed by a transfer method or the like.

[0002]

2. Description of the Related Art A conventional single-layer flexible wiring board is generally a single-layer thermosetting polyimide film used as an insulating substrate for circuit formation, and a copper foil is attached to the surface of the circuit board with an adhesive, and the copper foil is etched. To form the wiring pattern.

That is, the wiring pattern was attached to the surface of the single-layer polyimide film via an adhesive. Epoxy-based or acrylic-based adhesives are often used as the adhesive due to its adhesiveness.

In a two-layer flexible wiring board, wiring patterns are similarly formed on both sides of a polyimide resin film, and these are interconnected by through holes. A method of forming these wiring patterns by printing a conductive paste is also known.

A multilayer wiring board having three or more layers is formed by stacking a plurality of wiring boards having the above structure with an adhesive. In this case, (number of layers-1) sheets of polyimide resin film are required, the polyimide resin film is expensive, and the flexibility of bonding two or more sheets is extremely impaired. Does not exist as a substrate. Further, since the polyimide film is expensive, a substrate having three or more layers is a rigid substrate using glass epoxy resin or the like.

[0006]

Since the surface layer of the conventional wiring board has a wiring pattern formed on the surface of the insulating substrate, this wiring pattern naturally travels from the surface of the insulating substrate and the bonding strength (wiring strength) There is a limit to the pattern holding strength) and there is a risk of peeling and damage due to external force.
In particular, in a wiring pattern formed by printing a conductive paste, the holding strength of the wiring pattern becomes a problem. The effect of the cover coat for protecting the wiring pattern can be considered as a material for obtaining the above holding strength. However, a strong cover coat impairs the original flexibility, and a thin and soft cover coat cannot be expected to improve the holding strength. On the other hand, an adhesive used for forming a wiring pattern by etching copper foil and for a multilayer wiring board has a drawback that it adds rigidity to the wiring board by curing and impairs the flexibility of the polyimide resin forming the insulating board. As mentioned above for substrates with three or more layers,
Since two or more polyimide films are used, the desired flexibility is significantly impaired and, in addition, it is expensive.

[0007]

As a means for solving the above problems, the present invention forms a circuit forming insulating substrate by fusing a thermoplastic resin film to a polyimide resin film as a base material. The surface of is used as a contact surface of the wiring pattern. That is, there is no adhesive layer between layers.

The present invention also provides a flexible circuit board in which a wiring pattern is formed on the surface of the polyimide resin film or the thermoplastic resin film and the wiring pattern is embedded in the surface layer of the thermoplastic resin film. . That is, there is some difficulty in flexibility, but a combination of a polyimide resin film having dimensional accuracy and strength and a thermoplastic resin film that is excellent in flexibility but has problems in dimensional accuracy and strength provides a certain level of hardness and dimensional accuracy. Thus, a circuit board having strength and flexibility is obtained. This flexibility is remarkable in the multilayer wiring board.

As a suitable material for the thermoplastic resin film, thermoplastic polyether ether ketone, thermoplastic polysulfone, or thermoplastic polyimide was used.

Further, the thermoplastic resin film is fused on both sides of the thermosetting polyimide resin film as a base material to form an insulating substrate for circuit formation, and the wiring pattern is formed.

A cover coat made of a thermoplastic resin such as a thermoplastic polyimide resin is applied to the surface of the wiring pattern.

[0012]

In the above flexible wiring board, the base material can be easily and firmly fused by utilizing the thermoplasticization of the thermoplastic resin film in which the wiring pattern is fused on the surface of the polyimide resin film. Further, as a mode of this fusion, a wiring pattern can be formed by embedding a wiring pattern in the surface layer by thermoplasticization of the thermoplastic resin film, so that the adhesion strength is remarkably increased and pattern peeling due to impact or external force is caused. And effectively solve the problem of damage.

Generally, even when the wiring pattern is surface-fused to the thermoplastic film or the same pattern is embedded and fused, a required adhesion strength can be added without impairing the flexibility and the problem of pattern peeling due to bending can be solved. .

Further, the step of fusing the thermoplastic resin film to the polyimide resin film and the step of fusing the wiring pattern or the copper foil for etching to the thermoplastic resin film are completed in one step of thermocompression bonding by a hot pressing method. Therefore, labor saving and cost reduction can be achieved.

For a multi-layer wiring board having three or more layers, by using a substrate in which thermoplastic resin films are fused on both sides of one polyimide resin film as a base material, up to four layers can be easily formed by one polyimide resin film. It can be formed, the flexibility can be enhanced, and the substrate cost can be reduced.

Further, by using thermoplastic polyetheretherketone, thermoplastic polysulfone, or thermoplastic polyimide as the thermoplastic film, the adhesiveness (fusing property) with the polyimide serving as the core material and the wiring pattern, and the embedded wiring form It can be obtained well.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to FIGS.

When a glass epoxy resin plate is used as an insulating substrate for forming a circuit, it has a drawback that it is easily broken. As a substrate material for solving this drawback, a polyimide resin film is mainly used in view of heat resistance, strength and dimensional accuracy. ing.

Since the polyimide resin film is a relatively hard material, good flexibility can be obtained with a single-layer circuit board. However, when two or more layers of boards, that is, two or more polyimide resin films overlap, the flexibility is extremely high. Be damaged by. Therefore, in a circuit board made of a substrate material of two or more layers of polyimide resin film, a slit-like hole is often provided at a bent portion of the film. In addition, a conventional wiring board is prepared by using this polyimide resin film alone as a core material, and laminating copper foil on its surface with an adhesive agent as a board, and forming a wiring pattern by etching the copper foil. ing. Generally, the adhesive is also hard and impairs the flexibility.

Here, a polyimide resin film, preferably a thermosetting polyimide resin film 1 is used as a core material, and a thermoplastic resin film 2 is fused thereto by thermocompression bonding to form an insulating substrate. It served as a substrate for circuit formation.

As the thermoplastic resin film, thermoplastic polyetheretherketone is most suitable, and thermoplastic polysulfone or thermoplastic polyimide is also suitable.

As shown in FIGS. 1 and 2, a thermoplastic resin film 2 made of a polyetheretherketone film is heat-bonded by thermocompression bonding to one or both surfaces of the thermosetting polyimide resin film 1, The wiring pattern 3 is adhered to the outer surface of the thermoplastic resin film 2 (polyether ether ketone film), and the wiring pattern 3 is fused to the base material surface layer portion in an embedded state by utilizing the thermoplasticity of the film 2, or Surface fusion is performed without embedding to form a single-sided wiring board (FIG. 1) or a double-sided wiring board (FIG. 2).

The wiring pattern 3 is thermocompression-bonded to the thermoplastic film 2 as shown in FIGS. 1A and 2A, and the film 2 is plasticized while the pattern 3 is embedded in the base material surface layer to plasticize the film 2. Fusion of the base metal, or Fig. 1B,
As shown in FIG. 2B, the surface of the base material is fused by plasticizing the film 2 on the surface of the base material.

Thus, the flexible wiring board is formed by preparing in advance a board in which the thermoplastic resin film 2 is fused to the polyimide resin film 1 as the base material, and then the wiring pattern 3 is thermocompression bonded. Alternatively, the flexible wiring board can be formed in one step by thermocompression bonding the film 2 to the film 1 and thermocompression bonding the wiring pattern 3.

In FIG. 3, the wiring pattern 3 is formed on one surface or both surfaces of the thermosetting polyimide resin film 1 by a method such as printing, and the thermoplastic resin film 2 is formed on the surface of the wiring pattern 3. Was heat-pressed and the base material was fused.

In this case, the wiring pattern 3 is exposed at the edge of the wiring substrate and used as a terminal, or another wiring pattern is thermocompression-bonded to the outer surface of the thermoplastic resin film 2 by the method described below to form an inner surface. Wiring pattern 3 on the side
It is possible to connect to the outside by short-circuiting with.

Particularly, when Kapton manufactured by Toray DuPont Co., Ltd. was used as the polyimide resin film and polyether ether ketone was used as the thermoplastic resin film, the affinity was the best and the adhesion could be improved.

Attachment Chemical formula 1 shows the chemical structure of PMDA type polyimide, and chemical formula 2 shows the chemical structure of BPDA type polyimide, and the chemical formula 2 corresponds to the above Kapton.

[0029]

[Chemical 1]

[0030]

[Chemical 2] Next, based on FIGS. 4 and 5, together with the manufacturing method of the double-sided wiring board shown in FIG. 2, the board structure in which the double-sided wiring patterns are connected to each other and the manufacturing method thereof will be described.

As shown in FIG. 4A, a metal plate 4 and a stainless steel plate are used as a suitable material, a Teflon (fluorine resin) coat is applied to the surface, and a wiring pattern 3a is applied to the surface of the Teflon coat by a method such as printing. Further, the bumps 5 made of a conductive material are provided at required positions on the surface of the wiring pattern 3a.
Is prepared by printing or the like to prepare the transfer plate 6a.

On the other hand, a transfer plate 6b is prepared by printing a wiring pattern 3b on the surface of a Teflon-coated metal plate 4, for example, a stainless plate.

Further, the thermosetting polyimide resin film 1 is provided with through holes 7 at positions corresponding to the bumps 5, the thermoplastic resin films 2a and 2b are superposed on both surfaces of the film 1, and the films 2a and 2b are further laminated. Transfer plate 6 on the outer surface of
A and 6b are superposed, and the superposed body is set as one set and installed in a heat press machine. After a required heating time, melting of the film 2 is promoted, and the press machine is operated at a required timing to perform thermocompression bonding, After crimping, the metal plate 4 is removed.

As a result, as shown in FIG. 4B, the wiring patterns 3a and 3b are embedded in the surface layer of the molten layer while the thermoplastic films 2a and 2b are fused by thermoplasticization, and the bumps 5 are formed on the film 2a. , 2b, and further strongly pressed against the surface of the wiring pattern 3b through the through hole 7 of the film 1, the tip is crushed and connected to the surface of the pattern 3b. Thus, the two-layer wiring board in which the patterns 3a and 3b are short-circuited via the bumps 5 is formed.

In the case of the double-sided wiring board shown in FIG. 2, the bumps 5 are not used and thermocompression bonding is performed. Further, as shown in FIG. 4C, the wiring pattern 3a of the flexible wiring board,
The surface of 3b is covered with a cover coat 8 made of a thermoplastic resin. Thermoplastic polyimide is suitable as the thermoplastic resin for the cover coat.

The cover coat 8 is formed by fusing a thermoplastic polyimide film to the surfaces of the wiring patterns 3a and 3b by thermocompression bonding, or as a suitable example, a liquid material (varnish) obtained by dissolving thermoplastic polyimide in a solvent is used as the wiring pattern 3a. ，
The surface of 3b is applied by printing or the like, and then taken in a heating furnace and heat-treated to be brought into close contact with the pattern surface.

Conventionally, a thermosetting polyimide film is adhered as the cover coat 8 through an adhesive, and the present invention eliminates the sticking by the adhesive.

The cover coat 8 is applied so as to expose the end of the wiring pattern 3a, and nickel-gold plating 9 is applied to the exposed end, which is used as a terminal for external connection.

FIG. 6 exemplifies the manufacturing method and structure of the four-layer wiring board.

As shown in FIG. 6A, the wiring pattern 3 is formed on both surfaces of the thermosetting resin film 1 as a core material by printing or the like.
c, 3d are applied, and both wiring patterns 3c, 3d are provided with through holes 7
Interconnected by a metal filled in. In addition, a pair of transfer plates 6c and 6d having wiring patterns 3a and bumps 5 having the same structure as the transfer plate 6a of FIG. 4 are prepared, and as described above, a set of the transfer plates 6c and 6d are stacked and taken into a heat press machine. The thermoplastic resin films 2a and 2b are melted and thermocompression bonded.

After thermocompression bonding, the metal plates 4 of the transfer plates 6c and 6d are removed to obtain the flexible wiring board shown in FIG. 6B.

Thus, the wiring pattern 3a transferred from the transfer plate 6c and the wiring pattern 3c are connected via the bump 5, and the wiring pattern 3 transferred from the transfer plate 6d.
A four-layer wiring board in which a and the wiring pattern 3d are connected via the bumps 5 is obtained.

Next, as shown in FIG. 6C, a cover coat 8 of thermoplastic polyimide resin similar to that of FIG. 4C is applied.

FIG. 6 shows the transfer plates 6c, 6d, and 3
It also suggests the case of manufacturing a two-layer wiring board with a combination of wiring patterns c and 3d.

In the above embodiment, the wiring pattern formation by printing the conductor paste has been described. However, as shown in FIG. 7, the wiring board can be manufactured in the same manner when the wiring pattern is formed by etching the copper foil.

That is, as shown in FIG. 7A, a copper foil 10b having bumps 5 formed at predetermined positions on the surface and a copper foil 10a having no bumps 5 are prepared, and a polyimide resin film ( A thermoplastic resin film (polyether ether ketone 2a, 2a, 2) is formed on both sides of the Kapton.
The superposed body in which b is arranged is set as one set and arranged in a hot press machine, and after a required heating time, the press machine is operated at a required timing to perform thermocompression bonding.

As a result, as shown in FIG. 7B, the copper foil 10
a and 10b are fused to the outer surfaces of the thermoplastic films 2a and 2b by the base material fusion by the fusion of the films, and further the thermoplastic resin films 2a and 2b are fused on both sides of the polyimide resin film and the bumps 5 are formed. The films 2a and 2b are pierced through the through holes 7 provided in the polyimide resin film 1 and strongly pressed against the surface of the copper foil 10a.
A laminated body in which 10b are connected is formed.

Next, as shown in FIG. 7C, the copper foil 10 is
Etching processing is performed on a and 10b to form a required wiring pattern. As a result, the wiring patterns 3a and 3b are attached to the thermoplastic resin films 2a and 2b.
Due to the thermoplasticity of b, a double-sided wiring board having the surface fused is formed.

[0049]

According to the present invention, a thermoplastic resin film is fused on the surface of a polyimide resin film to form an insulating substrate, and this thermoplastic resin film is used as a means for adhering a wiring pattern.

Since this insulating substrate is fused by utilizing the thermoplasticity of the polyimide resin film and the thermoplastic resin film, a strong substrate having a high adhesion strength and exhibiting good flexibility can be formed.

That is, a wiring board having one or two layers does not have an adhesive layer, and a more flexible board can be obtained by using a flexible cover coat. Similar to the one-layer wiring board, the wiring board having three to four layers has no adhesive layer and has only one polyimide resin film, and thus has sufficient flexibility. Furthermore, 5 obtained by stacking these
Even with a wiring board of up to 8 layers, since the number of polyimide resin films is two, it is possible to obtain a flexible multilayer wiring board that has never been seen.

When the thermoplastic resin film is polyetheretherketone, it exhibits higher adhesion to the polyimide resin film. The same effect was confirmed for polysulfone film and thermoplastic polyimide film.

Further, the insulating substrate can fuse the wiring pattern by utilizing the thermoplasticity of the thermoplastic resin film, and the thermoplasticization can easily embed the wiring pattern in the surface layer of the film. In addition to being able to adhere extremely strongly, the pattern does not project on the surface, so damage and peeling due to external force can be effectively prevented.

Further, the insulating substrate can form a single-sided, double-sided, multi-layered wiring board very easily in one step by utilizing the thermoplasticity of the thermoplastic resin film by using the hot pressing method, thereby achieving labor saving and cost reduction.

[Brief description of drawings]

1A and 1B are cross-sectional views of a flexible wiring board (a board shown in a single-sided wiring state) showing an embodiment of the present invention.

2A and 2B are cross-sectional views of a flexible wiring board (a board shown in a double-sided wiring state) showing an embodiment of the present invention.

FIG. 3 is a sectional view of a flexible wiring board showing another example of the present invention.

4A, 4B and 4C are cross-sectional views showing a method of manufacturing the double-sided wiring board according to the present invention and a structural example in order of steps.

FIG. 5 is a perspective view showing an outline (main part) of the double-sided wiring board by cutting out.

6A, 6B, 6C, 6D, 6E, 6F, 6G, and 6G are cross-sectional views showing, step by step, a manufacturing method and a structural example of a multilayer wiring board (four layers are taken as an example) according to this invention.

7A, 7B, 7C and 7D are cross-sectional views showing steps of another manufacturing method and a structure example of the multilayer wiring board using the present invention, step by step.

Embedded image A chemical formula showing the chemical structure of PMDA-type polyimide.

[Chemical formula 2] Chemical structure of BPDA type polyimide (Kapton)
A chemical formula that indicates.

[Explanation of symbols]

 1 thermosetting polyimide resin film 2 thermoplastic resin film 3 wiring pattern

Claims (4)

[Claims] 1. A thermoplastic resin film is fused on one or both sides of a polyimide resin film by a thermoplastic material of the thermoplastic resin film to form a circuit-forming insulating substrate.
A flexible circuit board having a base material fused to at least the surface of the thermoplastic resin film by the thermoplasticity of the thermoplastic resin film. 2. The flexible circuit board according to claim 1, wherein the polyimide resin film is Kapton manufactured by Toray DuPont Co., Ltd. 3. The flexible circuit board according to claim 1, wherein the thermoplastic resin film is thermoplastic polyetheretherketone, thermoplastic polysulfone, or thermoplastic polyimide. 4. A thermoplastic resin film is fused on a polyimide resin film by a thermoplastic material of the thermoplastic resin film to form an insulating substrate for forming a circuit, and a polyimide resin film or a thermoplastic resin film is provided on the surface thereof. A flexible circuit board in which a wiring pattern is formed, the wiring pattern is embedded in a surface layer of the thermoplastic resin film, and the base material is fused by the thermoplasticity of the thermoplastic resin film.