GB2130019A

GB2130019A - Flexible circuit board

Info

Publication number: GB2130019A
Application number: GB08302377A
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: 1984-05-23
Also published as: FR2520964B1; FR2520964A1; GB8302377D0; JPS58116271U; GB2130019B; DE3302994A1; DE3302994C2

Abstract

A flexible circuit board comprises a base film 1 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 reinforcing elements 5 attached to or forming a part of the base film in alignment with the edge 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 connected to another electrical circuit. Prior art circuit boards of this type have a stress concentration problem in that there are different bending charcteristics 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, reinforcing structure is attached to 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 reinforcing structure is in the form of protruding parts from the base sheet at the ends of the edge line defined by the junction of the cover sheet with the base sheet. These reinforcing protrusions may consist of elements integrally formed in the base sheet, or they may be elements separately formed and suitably attached to the base sheet.Since the base sheet is typically a flexible plastic material, it is particularly convenient and easy to form the base sheet with integral protrusions; or the protrusions may be added to the base sheet in a separate manufacturing step.

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 a boundary line 4. End portions 2A of the conductors are left exposed to provide for interconnection portions to connect the condutors 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 numbersof flexing cycles in normal use.

Reffering now to a joint consideration of FIGURES 2 and 3, the structure of the present invention is shown wherein reinforcing structure is incorporated into the printed circuit board to largely dissipate or mitigate the stress concentrations 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, reinforcing elements 5 form a part of lower circuit film 1 and protrude from the lower circuit film 1 at either one or both ends of boundary line 4. FIGURE 2 shows the reinforcing elements 5 protruding from base sheet 1 at both ends of line 4, and that is a preferred configuration; but the invention also contemplates structure having only one of the reinforcing elements 5.The protruding reinforcing elements 5 may be in the form of semi-circular elements as shown in FIGURE 2, or they may be in other forms or shapes such as rectangles or in other oval or arcuate shapes. The particular shape is not important, as long as the reinforcing elements 5 form part of base sheet 4 and protrude from base sheet 4 at one or both ends of boundary line 4 and are positioned so that they overlap or extend on both sides of boundary line 4. Reinforcing elements 5 may be formed by any convenient shaping process, such as punching, and they may be formed as an integral part of base sheet or film 1 in the same punching or forming process in which the base sheet is formed.Alternately, the reinforcing elements 5 may be formed as separate members and then bonded, such as by thermosetting or pressure sensitive resin material, to the appropriate positions at the edge of base film 1. In this latter case, it is possible, and perhaps particularly convenient in some manufacturing process, to form the entire flexible circuit structure other than the reinforcing elements 5, and then at a suitable time during the manufacturing process position the reinforcing elements 5 at the appropriate locations and bond them to the base sheet 1.

Because of the presence of the reinforcing elements as an integral part of base sheet 1 (either by being formed as a part of the base sheet or being integrally bonded thereto), and because of the location of the reinforcing elements 5 whereby they straddle the ends of boundary 4, 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 a highly reliable and improved product wherein the risk of conductor line breaking 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 reinforcing means on said base sheet at at least one end of said boundary.

2. A flexible circuit board as in claim 1 wherein: said reinforcing means straddles the end of said boundary.

3. A flexible circuit board as in claim 2 wherein: said reinforcing means is an integrally formed part of said base sheet.

4. A flexible circuit board as in claim 2 wherein: said reinforcing means is bonded to said base sheet.

5. 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 reinforcing means on said base sheet at opposite ends of said boundary.

6. A flexible circuit board as in claim 5 wherein: said reinforcing means straddle the ends of said boundary.

7. A flexible circuit board as in claim 6 wherein: said reinforcing means are integrally formed parts of said base sheet.

8. A flexible circuit board as in claim 6 wherein: said reinforcing means are bonded to said base sheet.