GB2116370A

GB2116370A - Flexible circuit board

Info

Publication number: GB2116370A
Application number: GB08302378A
Authority: GB
Inventors: Katsuyuki Yatabe; Seiichi Tobe
Original assignee: Nippon Mektron KK
Current assignee: Nippon Mektron KK
Priority date: 1982-01-30
Filing date: 1983-01-28
Publication date: 1983-09-21
Also published as: GB2116370B; FR2520965B1; DE3302993A1; GB8302378D0; JPS58116272U; DE3302993C2; FR2520965A1

Abstract

A flexible circuit board comprises a base film with a circuit pattern 2 thereon, a cover film 3 over part of the circuit pattern which leaves an exposed connection portion 2A of the circuit pattern, and stress relief apertures 5, 6 in the cover film and/or the base film adjacent the edge 4 of the cover film to prevent stress concentrations which cause breaking of the circuit pattern. <IMAGE>

Description

SPECIFICATION Flexible circuit board This invention relates to the field of flexible circuit boards. More particularly, this invention relates to printed circuit flexible circuit boards provided with a partial cover film which leaves exposed end portions of the circuit pattern for connection to other circuits or components.

Flexible circuit boards are known in the prior art which have the basic structure of a plastic base film, a circuit pattern on the base film, and a plastic cover film which covers most, but not all of the circuit pattern, the cover film leaving exposed some of the circuit pattern so that the circuit pattern may be col ,nected to another electrical circuit. Prior art circuit boards of this type have a stress concentration problem in that there are different bending characteristics or different flexure strengths in the circuit board between the portion which has the cover film and the portion which does not have the cover film. The practical results of this difference in flexure or bending strengths is that a stress concentration line exists across the circuit pattern at the location of the boundary line of the cover film.This stress concentration feature is most undesirable because it may result in breaking of the circuit pattern which will, of course, destroy the effectiveness and usefulness of the circuit board.

The above-discussed and other problems of the prior art are overcome or alleviated by the flexible circuit board of the present invention. In accordance with the present invention, stress relief means are provided in the cover sheet and/or in the base sheet to dissipate or mitigate to a large extent the concentration of stresses due to bending moments or otherwise which would exist in the prior art. The stress relief means are in the form of stress relief apertures in the cover sheet and/or in the base sheet near and in alignment with the edge line defined by the junction of the cover sheet with the base sheet. These stress relief apertures may be in rows of uniform size, or they may be of varying size.Since the cover sheet and base sheet are typically a flexible plastic material, it is particularly convenient and easy to form the cover sheet and the base sheet with the desired patterns of operations during preliminary manufacturing steps.

The above-discussed and other advantages of the present invention will be apparent to and understood by those skilled in the art from the following detailed description and drawings.

Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES: FIGURE 1 is a top plan view of a flexible printed circuit board in accordance with the prior art.

FIGURE 2 is a top plan view of a flexible printed circuit board in accordance with the present invention.

FIGURE 3 is a side elevation view of the flexible printed.circuit board of FIGURE 2.

Referring first to FIGURE 1, a flexible printed circuit board in accordance with the prior art is shown. A base sheet 1, typically of thin flexible plastic, has a series of conductors 2 thereon forming a circuit pattern. The circuit pattern may be formed by any conventional printed circuit or similar technique. A cover film 3, also typically of thin flexible plastic, covers and is bonded to the base sheet or film 1 up to à boundary line 4. End portions 2A of the conductors are left exposed to provided for interconnection portions to connect the conductors to another circuit or to circuit components.This structure shown in FIGURE 1 is conventional and well known in the art, with the cover film 3 serving to protect against unintentional and unwanted circuit connections to the conductors or to protect against short circuiting by accidential interconnection of the conductors, and also to protect against environmental hazards.

In a flexible circuit board such as shown in FIGURE 1, the portion covered by cover film 3 and the portion having the exposed conductors 2A naturally have different flexure or bending strengths or characteristics. It is also known that structures such as shown in FIGURE 1, bending stresses are concentrated along the edge or boundary line 4. Hence, edge or boundary line 4 defines an area of stress concentration where breaking of the conductors 2 tends to occur.

Naturally, breaking of the conductors is a very serious matter, and especially so if it should occur in a device or instrument subsequent to installation of the circuit board, because operation of the device or instrument of which the circuit board forms a part may be totally interrupted or seriously impaired. All of this may lead to serious problems, especially in the field of flexible circuit boards where it is very important to have highly reliable flexible circuit board products which are free from problems of this type and which are required to undergo large numbers of flexing cycles in normal use.

Referring now to a joint consideration of FIGURES 2 and 3, the structure of the present invention is shown wherein stress relief structure is incorporated into the printed circuit board to largely dissipate or mitigate the stress concentration due to bending moments or bending stresses. The flexible circuit board shown in FIGURES 2 and 3 has the same general components 1, 2, 2A, 3 and 4 as in the prior art structure of FIGURE 1. However, in accordance with the present invention, stress relief through hole apertures 5 are present in at least cover film 3. The apertures 5 are located in cover film 3 as close to the boundary edge 4 as is consistent with maintaining the integrity of the edge portion 4.

Preferably, apertures 5 are in a straight line, are all of the same diameter, and are positioned to be staggered relative to the conductors 2 (to prevent exposure of and possible contamination of conductors 2). Alternatively, the apertures 5 may be of different diameters, or may be in multiple rows. As long as the apertures 5 are positioned as in FIGURE 2 (i.e., staggered relative to conductors 2), the apertures 5 may also be extended through base sheet 1 to provide stress relief in both base sheet 1 and cover sheet 3.

FIGURE 3 shows another embodiment of the present invention wherein separate rows of through hole apertures are present in each of the films or sheets; apertures 5 being in cover sheet 3 and apertures 6 being in base sheet 1 and displaced from apertures 5 relative to the boundary edge 4.

Although it is preferable to provide the through hole apertures 5 at positions staggered relative to conductors 2, that may not be possible where the density of conductors 2 is high and there are only small spaces between the conductor lines. In that event, through hole apertures 5 in cover sheet 3 may overlap conductors 2; and if desired, the hole 5 may be filled with a flexible resin or other material to prevent exposure of the conductors 2.

Apertures 5 and/or 6 may conveniently and easily be formed by punching in the cover and base sheets at any suitable time in the manufacturing process.

Because of the presence of the stress relieving apertures 5 and/or 6 as an integral part of cover sheet 3 and/or base sheet 1, the bending stresses which would in the prior art tend to be concentrated along boundary line or edge 4 of the cover film are dissipated or mitigated, thus eliminating or significantly reducing the problem of conductor breakage because of bending stress concentrations in base film 1 along edge 4. The resulting circuit board of the present invention has the significant advantage over the prior art in that it provides at very low cost at a highly reliable and improved product wherein the risk of conductor line braking due to stress concentrations along boundary line 4 is virtually eliminated, while at the same time the basic flexible properties of the circuit board are maintained unimpaired. The invention also has the advantage of being an easily and economically implemented solution to a potentially serious problem.

While preferred embodiments have been shown and described various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.

Claims

1. A flexible circuit board having: a base sheet with electrically conductive circuit means thereon; a cover sheet on said base sheet covering less than all of said base sheet and leaving part of said circuit means exposed, said cover sheet having an edge portion defining a boundary beyond which the base sheet and circuit means extend: and stress relief means in at least said cover sheet adjacent said boundary.

2. A flexible circuit board as in claim 1 wherein: said stress relief means is a plurality of apertures in said cover sheet.

3. A flexible circuit board as in claim 1 wherein: said stress relief means is a plurality of apertures in said cover sheet aligned relative to said boundary.

4. A flexible circuit board as in claim 3 wherein: said conductive circuit means is a plurality of spaced conductors, and said apertures are staggered relative to said conductors to be opposed to the spaces between conductors.

5. A flexible circuit board as in claim 1 wherein: said stress relief means is a plurality of apertures in said cover sheet; and filler material in said apertures.

6. A flexible circuit board having: a base sheet with electrically conductive circuit means thereon; a cover sheet on said base sheet covering less than all of said base sheet and leaving part of said circuit means exposed, said cover sheet having an edge portion defining a boundary beyond which the base and circuit means extend; and stress relief means in said cover sheet and in said base sheet.

7. A flexible circuit board as in claim 6 wherein: said stress relief means is a plurality of apertures in said cover sheet and a plurality of apertures in said base sheet.

8. A flexible circuit board as in claim 7 wherein: said conductive circuit means is a plurality of spaced conductors, and said apertures in said cover sheet and said base sheet are in alignment and are staggered relative to said conductors to be opposed to the spaces between conductors.

9. A flexible circuit board as in claim 6 wherein: said stress relief means is a plurality of apertures in said cover sheet aligned relative to said boundary and a plurality of apertures in said base sheet aligned relative to said boundary and displaced relative to said apertures in said cover sheet.