JP2004079731A - Flexible circuit board - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a both-sided flexible circuit board that is used while the board is repetitively bent in one direction and can prevent the disconnection of a circuit wiring pattern caused by compressive strains resulting from the bending. <P>SOLUTION: This flexible circuit board has a first circuit wiring pattern 12 on the first surface of a flexible insulating base material and a second circuit wiring pattern 13 on the second surface of the base material and is provided with surface protective layers 16 and 19, each composed of a flexible resin film and an adhesive layer, on the the first and second circuit wiring patterns. The first circuit wiring pattern 12 is positioned so that the pattern 12 may come to the outside in the bending direction when the circuit board is bent with respect to the center line of the board in the thickness direction and the second circuit wiring pattern 13 is positioned so that the the pattern 13 may come to the inside in the bending direction. At the same time, the circuit wiring patterns 12 and 13 and surface protective layers 16 and 19 are constituted so that the position of the inside side face of the second circuit wiring pattern 13 in the bending direction may approach the center of the circuit board in the thickness direction. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a flexible circuit board used for wiring of a movable portion in an electronic device, and more particularly to a flexible circuit board having a circuit wiring pattern on both surfaces of an insulating base material, wherein the flexible circuit board is repeatedly bent in one direction. The present invention relates to a flexible circuit board having a bent portion to be made. The flexible circuit board also includes a flexible circuit board in which an electromagnetic shield layer is provided on a surface protection layer on the outer side in the bending direction.
[0002]
[Prior art]
It has been recognized that the bending of the flexible circuit board causes a tensile strain caused by repeated bending, thereby causing a break in the circuit wiring pattern of the bent portion.
[0003]
In this regard, regarding a single-sided flexible circuit board, as described in Japanese Patent Application Laid-Open No. 7-283494, as a countermeasure against repeated use in bending in both front and back directions, a conductor circuit is provided with a flexible circuit board. There has been proposed one configured to be located at the center line of the thickness of the substrate.
[0004]
According to this configuration, since the tensile strain applied to the circuit wiring pattern can be minimized, the bending resistance of the flexible circuit board is improved.
[0005]
On the other hand, for a so-called double-sided flexible circuit board having circuit wiring patterns on both sides of an insulating base material, measures are taken as described in JP-A-11-121881.
[0006]
This is because, in a flexible circuit board that is repeatedly bent and used in one direction, the surface protection layer of the circuit wiring pattern positioned outside in the bending direction is more than the surface protection layer of the circuit wiring pattern positioned inside in the other bending direction. By making the circuit wiring pattern located outside the bending direction closer to the center line of the thickness of the flexible circuit board, the thickness is reduced to reduce the tensile strain applied to the circuit wiring pattern located outside the bending direction. This is to improve the bending resistance of the circuit board.
[0007]
However, in the case of a double-sided flexible circuit board, if the circuit wiring pattern located on the outside in the bending direction is to be brought closer to the center line of the thickness of the flexible circuit board, the circuit wiring pattern located on the inside in the bending direction becomes acceptable. The position must be largely shifted inward in the bending direction with respect to the center line of the flexible circuit board.
[0008]
Incidentally, according to the configuration shown in FIG. 1 of Japanese Patent Application Laid-Open No. H11-121881, the bending surface inside surface of the circuit wiring pattern located inside the bending direction with respect to the entire thickness of the flexible circuit board, The ratio of the amount of deviation from the center line in the thickness direction of the flexible circuit board is 30.7%. Further, according to the configuration shown in FIG. 2 of the publication, this ratio is 31.7%. The measure against disconnection of the circuit wiring pattern focused on such tensile strain has a certain effect, but the effect is not so high.
[0009]
Then, the inventor of the present invention analyzed the disconnection phenomenon to improve the flexibility of the circuit wiring pattern, and as a result, a large compression strain acted on the circuit wiring pattern largely shifted inward in the bending direction. I discovered the fact that the circuit wiring pattern was broken by being stretched later.
[0010]
In particular, a flexible circuit board with a shield sheet that has begun to be widely used in recent portable communication devices, that is, a shield circuit in which an electromagnetic shield sheet is disposed on a surface protection layer located outside the bending direction of the flexible circuit board. It has been found that in a flexible circuit board, a disconnection occurs due to a compressive strain in a circuit wiring pattern positioned inside in a bending direction.
[0011]
FIG. 3 shows a double-sided flexible circuit board having a shield layer using a conventional adhesive-free double-sided copper-clad laminate. This flexible circuit board is formed on a first surface of a flexible insulating base material 31 made of a flexible insulating film such as a polyimide having a thickness of 25 μm by an etching technique using a photofabrication of a copper foil having a thickness of 18 μm. And a second circuit wiring pattern 33 formed on the second surface by photofabrication etching on a copper foil having a thickness of 18 μm.
[0012]
Then, on the first circuit wiring pattern 32, a flexible resin film 34 of polyimide or the like having a thickness of 25 μm is adhered by an adhesive layer 35 having a thickness of 20 μm to form a first film for the first circuit wiring pattern. A surface protection layer 36 is formed, and a flexible resin film 37 of polyimide or the like having a thickness of 25 μm is adhered on the second circuit wiring pattern 33 with an adhesive layer 38 having a thickness of 20 μm to form a second circuit wiring pattern. A second surface protection layer 39 for a wiring pattern is formed.
[0013]
A 9 μm-thick polyimide or polyethylene is formed on the surface protection layer positioned outside in the bending direction, that is, on the first surface protection layer 36 formed on the first circuit wiring pattern 32 above the drawing. On one surface of a flexible insulating film 40 such as terephthalate, a shield sheet 42 having a thin shield layer 41 formed of a conductive metal such as aluminum, copper, or silver by a sputtering method or a vapor deposition method has a thickness of 23 μm. Are bonded by an adhesive 43.
[0014]
The following is a description of the compression strain of the circuit wiring pattern in the flexible circuit board configured as described above, based on the thickness of each layer. The total thickness of the flexible circuit board is 183 μm, and the amount of deviation between the inner surface in the bending direction of the second circuit wiring pattern 33 located inside the bending direction and the center line of the thickness of the flexible circuit board Is 46.5 μm, and its ratio to the total thickness is 25.4%. This is a remarkable decrease as compared with 30.7% to 31.7% in the configuration disclosed in Japanese Patent Application Laid-Open No. H11-121881.
[0015]
However, in the flexible circuit board having this configuration, many of the flexible circuit boards in which the circuit wiring pattern located inside the bending direction first breaks occur, and the breakage due to the compressive distortion of the inner pattern causes the flexible circuit board to break. As a result, the bending resistance of the substrate is impaired. Therefore, further measures are needed.
[0016]
The present invention has been made in view of the above points, and has a double-sided flexibility capable of preventing disconnection of a circuit wiring pattern caused by compressive strain in a flexible circuit board repeatedly used in one direction. It is an object to provide a circuit board.
[0017]
[Means for Solving the Problems]
To achieve the above object, the present invention provides:
A flexible insulating base material has a first circuit wiring pattern on a first surface, a second circuit wiring pattern on a second surface, and a flexible circuit on both the first and second circuit wiring patterns. A flexible circuit board provided with a surface protection layer composed of a flexible resin film and an adhesive layer, wherein the first circuit wiring pattern is bent outward in a bending direction with respect to a center line in a thickness direction of the flexible circuit board. And the second circuit wiring pattern is disposed so as to be located inside in the bending direction, and the position of the inner side surface in the bending direction of the second circuit wiring pattern is set to the entirety of the flexible circuit board. A flexible circuit board, wherein the circuit wiring pattern and the surface protective layer are configured to be close to a thickness center position;
Is provided.
[0018]
Embodiment
FIG. 1 is a perspective view showing an external appearance shape of a flexible circuit board with a shield to which the present invention is applied when mounted. In the illustrated case, the flexible circuit board 1 is formed by bending so that the side surface forms a U-shape, and the shield layer 3 is provided on the upper surface of the surface protection layer 2 located outside the bending direction. Terminals 4 for connection to an external substrate are formed at both ends. Although not shown, the degree of bending of the flexible circuit board 1 may be further increased than that of the U-shape, and the flexible circuit board 1 may be formed into an α-shape by bending.
[0019]
When the electronic device is mounted on the electronic device, the inner portion of the U-shaped (or α-shaped) is compressed and elongated each time the bent portion of the electronic device is bent, and the U-shaped (or α-shaped) shape is formed. A compressive strain is generated inside the part.
[0020]
FIG. 2 is a diagram showing a laminated cross section of the flexible circuit board shown in FIG. In FIG. 2, the upper side in the drawing shows the outside in the bending direction, and the lower side shows the inside in the bending direction.
[0021]
In the embodiment shown in FIG. 2, a circuit wiring pattern is formed on each of the first and second surfaces of a flexible insulating base material 11 made of a flexible insulating film such as polyimide having a thickness of 25 μm. That is, the first surface is provided with a first circuit wiring pattern 12 formed by an etching method by photofabrication on a copper foil having a thickness of 18 μm, and the second surface is similarly provided with a copper foil having a thickness of 18 μm. A second circuit wiring pattern 13 formed by an etching method based on photofabrication is provided.
[0022]
At the same time, a flexible resin film 14 of polyimide or the like having a thickness of 12.5 μm is adhered on the first circuit wiring pattern 12 with an adhesive layer 15 having a thickness of 20 μm to form the first circuit wiring pattern. The first surface protective layer 16 is formed, and a flexible resin film 17 such as polyimide having a thickness of 25 μm is adhered on the second circuit wiring pattern 13 with an adhesive layer 18 having a thickness of 20 μm. A second surface protection layer 19 for a second circuit wiring pattern is formed.
[0023]
Then, polyimide, polyethylene terephthalate, or the like is provided on the surface protection layer located outside in the bending direction, that is, on the first surface protection layer 16 formed on the first circuit wiring pattern 12 above the drawing. On one surface of a flexible insulating film 20 (9 μm thick) having a thin-film shield layer 21 formed by sputtering or vapor deposition of a conductive metal such as aluminum, copper, or silver, and having an electromagnetic shielding function. The held shield sheet 22 is adhered with an adhesive layer 23 (23 μm thick).
[0024]
The flexible circuit board configured as described above has an overall thickness of 170.5 μm, and has a surface inside the bending direction of the second circuit wiring pattern 13 located inside the bending direction and the flexible circuit board. The deviation from the center line of the thickness is 40.25 μm, and the ratio (= 40.25 / 170.5) to the total thickness is 23.6%.
[0025]
In the above embodiment, the thickness of the flexible resin film 14 such as polyimide constituting the first surface protection layer 16 located on the outer side in the bending direction is 12.5 μm, and the second surface located on the inner side in the bending direction. The thickness of the flexible resin film 17 made of polyimide or the like constituting the protective layer 19 is made thinner than 25 μm. Thereby, the amount of deviation between the inner surface in the bending direction of the second circuit wiring pattern 13 located on the inner side in the bending direction and the center line of the thickness of the flexible circuit board is relative to the entire thickness of the flexible circuit board. The ratio was set to 24% or less.
[0026]
Examination of the tendency of disconnection occurring in the flexible circuit board configured in this way revealed that the flexible circuit board in which the circuit wiring pattern located inside the bending direction reaches the disconnection and the circuit located outside the bending direction. The number of flexible circuit boards in which the wiring patterns are disconnected is almost the same, and only the inner circuit wiring patterns are no longer a factor in determining the bending resistance.
[0027]
In the above configuration, the ratio can be further reduced by reducing the thickness of the first circuit wiring pattern. When the thickness of the copper foil for the first circuit wiring pattern is 12 μm, The total thickness is 164.5 μm, the deviation amount of the second circuit wiring pattern is 37.25 μm, and the above ratio can be 22.64%.
[0028]
Also, by reducing the thickness of the second circuit wiring pattern, the ratio shown in FIG. 1 can be reduced. If the copper foil for the second circuit wiring pattern is 12 μm, The total thickness is 164.5 μm, the deviation amount of the second circuit wiring pattern is 37.25 μm, and the above ratio (= 37.25 / 164.5) can be 22.64%.
[0029]
Further, even if the thickness of the adhesive layer 15 for bonding the first surface protection layer 16 is smaller than the thickness of the adhesive layer 18 for bonding the second surface protection layer 19, the above ratio is reduced. be able to.
[0030]
In the above embodiment, the first and second circuit wiring patterns are formed using a non-adhesive double-sided copper-clad laminate in order to approach the center line of the entire thickness of the flexible circuit board. However, furthermore, an adhesive-free double-sided copper-clad laminate in which the thickness of a flexible insulating base material is reduced to, for example, 12 μm, and an adhesive-free adhesive in which the copper foil for a circuit wiring pattern is 12 μm on both sides The use of a mold double-sided copper-clad laminate can also contribute to lowering the above ratio, and the bending resistance can be further improved.
[0031]
Further, while a flexible insulating resin film was conventionally bonded to form the surface protective layer, a surface protective layer for the first circuit wiring pattern is formed by electrodeposition of an electrodepositable insulating resin. The first surface protective layer can be made thinner, and the above ratio can be further reduced, which is effective in improving the bending resistance.
[0032]
【The invention's effect】
As described in detail above, according to the present invention, in a double-sided flexible circuit wiring board that is repeatedly bent in one direction and used, the pattern located on the inner side in the bending direction can reduce the compression distortion due to bending. It is possible to improve the bending resistance of the flexible circuit board by improving the bending resistance of the pattern.
[0033]
In addition, since it is not particularly necessary to reinforce with a new member, the bending resistance can be improved without impairing the flexibility required for the flexible circuit board.
[Brief description of the drawings]
FIG. 1 is a diagram showing one embodiment of the present invention.
FIG. 2 is a sectional view showing an embodiment of the present invention.
FIG. 3 is a cross-sectional configuration diagram of a conventional shielded circuit board.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Flexible circuit board 2 Surface protection layer 3 Shield layer 4 Terminal 11 Flexible insulating base material 12 First circuit wiring pattern 13 Second circuit wiring pattern 14 Flexible resin film 15 Adhesive layer 16 First Surface protective layer 17 Flexible resin film 18 Adhesive layer 19 Second surface protective layer 20 Flexible insulating film 21 Shield layer 22 Shield sheet 23 Adhesive layer 31 Flexible insulating base material 32 First circuit wiring pattern 33 second circuit wiring pattern 34 flexible resin film 35 adhesive layer 36 first surface protection layer 37 flexible resin film 38 adhesive layer 39 second surface protection layer 40 flexible insulating film 41 shield layer 42 Shield sheet 43 Adhesive layer

Claims (7)

The flexible insulating base material has a first circuit wiring pattern on a first surface, a second circuit wiring pattern on a second surface, and a flexible circuit board on both the first and second circuit wiring patterns. In a flexible circuit board provided with a surface protection layer consisting of a flexible resin film and an adhesive layer,
When the flexible circuit board is bent along the center line in the thickness direction, the first circuit wiring pattern is disposed outside in the bending direction and the second circuit wiring pattern is disposed inside in the bending direction. At the same time, the thickness of each element in the circuit wiring pattern and the surface protection layer is set such that the position of the inner surface of the second circuit wiring pattern in the bending direction is closer to the center of the entire thickness of the flexible circuit board. A flexible circuit board, comprising: The flexible circuit board according to claim 1,
When the total thickness is A and the distance between the inner surface in the bending direction of the second circuit wiring pattern, which is inside in the bending direction at the time of bending, and the thickness center position of the flexible circuit board is B,
B / A × 100 ≦ 24 (%)
A flexible circuit board that satisfies the ratio indicated by. The flexible circuit board according to claim 1,
A surface protection layer comprising a flexible resin film and an adhesive layer is provided on both the first and second circuit wiring patterns, and further on the surface protection layer located on the outer side in the bending direction among the surface protection layers. , A flexible circuit board provided with an electromagnetic shield layer. The flexible circuit board according to claim 3,
In the surface protective layer, at least one of a flexible resin film, an adhesive layer, and a circuit wiring pattern constituting the surface protective layer located on the outer side in the bending direction has a flexibility located on the inner side in the bending direction. A flexible circuit board characterized by being thinner than a resin film, an adhesive layer, and a circuit wiring pattern. The flexible circuit board according to claim 3,
A flexible circuit board, wherein the electromagnetic shield layer is a shield sheet including a flexible insulating film, a thin film shield layer, and an adhesive layer. The flexible circuit board according to claim 1,
The surface protection layer of the first circuit wiring pattern is formed by electrodeposition of an electrodeposition insulating resin. The flexible circuit board according to claim 1,
A flexible circuit board, wherein a thickness of a circuit wiring pattern located inside the bending direction in the circuit wiring pattern is smaller than a thickness of a circuit wiring pattern located outside the bending direction.

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