WO1996007301A1

WO1996007301A1 - Long flex circuits

Info

Publication number: WO1996007301A1
Application number: PCT/US1995/010842
Authority: WO
Inventors: Bharat B. Pant; Richard K. Spielberger
Original assignee: Honeywell Inc.
Priority date: 1994-08-29
Filing date: 1995-08-25
Publication date: 1996-03-07

Abstract

A flexible circuit (10) has laterally spaced apart conductor zone panels (12, 14) each having a set of parallel conductors (16, 18) which are connected at end regions (20, 24) of the zone panels (12, 14). A slit (48) separates the zone panels (12, 14) except at the end regions (20, 24) a first fold line (50) continues from the end of the slot (48) between the panel zones (12, 14) and separates the panel zones (12, 14). A second fold line (60) is at the end zone of one panel (12). Folding a zone panel (12) along the fold lines (50, 60) extends the length of the circuit (10).

Description

LONG FLEX CIRCUITS

BACKGROUND The present invention relates generally to flexible circuits, and more particularly to an improved flexible circuit assembly that allows the length ofthe flexible circuit that can be made using a given photolithography system to be extended.

Manufacturers of flexible circuits use photolithography and etching processes to build flexible circuits using, for example, a polyimide dielectric substrate with copper conductors. Typically manufacturers of flexible circuits buy their substrate material in panels of a length that is related to the capability of their photo exposure system. Light exposure equipment, sources of light, and lenses for transmitting light all are readily available only for a limited physical length of exposure. A common maximum length of such exposure systems is 30 inches. This means that the longest continuous flexible circuit that can be made is approximately 30 inches. While this length is adequate for most flexible circuit applications, there are many other applications where longer flexible circuits are needed. In addition to the need for longer length flexible circuits, there is a need to be able to make these same flexible circuits with very fine conductor lines, for example .002 inches in width or less. The resolution ofthe exposure system and cleanliness ofthe environment are critical since a spot of dust can create an open or a short. The exposure system resolution or precision is more easily obtainable with exposure systems having shorter physical lengths of exposure.

Thus a need exists for long length flexible circuits having fine lines that can be fabricated using existing photo exposure equipment having a limited physical exposure length.

SUMMARY The present invention solves these and other needs by providing a flexible circuit that can be fabricated in lengths that are greater than the length ofthe photo lithography exposure length. An integral flexible substrate has laterally spaced apart panel zones with each panel zone having a set of parallel conductors. Conductors in adjoining end regions of each panel zone have conductors which connect the two sets of parallel conductors. A slit extends between the panel zones except at the end regions. One fold line is formed as an extension ofthe slit between the panel zones and extends through the end region between the zones. A second fold line is at the end region of one panel zone and perpendicular to the conductors within the panel zone. A panel zone is folded along both fold lines in either order so that the folded panel zone extends in a direction opposite the direction in which the other panel zone extends and the length of the flexible circuit is approximately twice the length of a panel zone. By continuing with a third panel zone having a third set of conductors and an end section at the opposite side ofthe substrate from the first two panel zones, the length ofthe flexible circuit is further extended.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of an integral substrate and conductors. FIG. 2 is a plan view ofthe integral substrate of Fig. 1 folded according to the principles ofthe present invention.

FIG. 2a is an enlarged cross-sectional drawing of a portion of FIG. 2 showing a first folding arrangement.

FIG. 2b is an enlarged cross-sectional drawing of a portion of FIG. 2 showing an alternative folding arrangement.

FIG. 2c is a cross-sectional drawing through a portion of a panel zone.

DETAILED DESCRIPTION A flexible circuit is shown in the drawings and generally designated 10. Circuit 10 includes conductor bearing panel zones 12 and 14 which are formed from an integral flexible substrate 15 and are laterally spaced apart. Panel zone 12 has elongated traces such as conductors 16 and panel zone 14 has elongated traces such as conductors 18. Panel zone 12 has an end region 20 having front surface 32, back surface 34, and remaining region 22 having front surface 36 and back surface 38. Panel zone 14 has an end region 24 having front surface 40, back surface 42, and remaining region 26 having front surface 44 and back surface 46. and a remaining region 26. End regions 20 and 24 are located adjacent one another and conductors 28 connect conductors 16 of panel zone 12 with conductors 18 of panel zone 14. A slit 48 in substrate 15 extends between panel zones 12 and 14 except at end regions 20 and 24. Panel zones 12 and 14 have a length, 1, related to the particular exposure system being used. In accordance with the principles ofthe present invention, the length of flexible circuit 10 can be multiples of the length 1. This is accomplished through the use of fold line 50 and fold line 60 and can be explained as follows. Zone 12 is folded along fold line 50 with front surfaces 32 and 36 facing front surfaces 40 and 44 respectively and with adhesive 52 between surfaces 32 and 40. Zone 12 is then folded along fold line 60 so that a part of surface 36 is facing back surface 34 with adhesive 52 between the part of surface 38 that is facing surface 34. Folding as just described results in flexible circuit 10 as shown in FIG.2 and 2a which has a length of 2 1 minus the length of an end section. For example, assume that flexible circuit 10 of FIG. 1 had a length 1 of 30 inches and slit 48 were stopped 0.030 from the left end of FIG. 1. The total length of flexible circuit 10 of FIG. 2 would then be 59.97 inches.

Alternatively fold lines 50 and 60 may be used as follows. Panel zone 12 is first folded along fold line 60 so that a part of front surface 36 of remaining portion 22 is against front surface 32 of end 20. Next panel zone 12 is folded along fold line 50 so that a part of back surface 38 of panel zone 12 is against front surface 40 of end portion 14. The folding arrangement is illustrated in FIG. 2b with adhesive 52 between surface 40 and part of surface 38, and between surface 32 and part of surface 36. Copper conductors 16 and 18 are deposited and patterned using well known techniques. Insulating layers are provided to allow conductors to overlay the other conductors when folded.

In defining fine lines of copper, e.g., .002 inches (2 mils) wide, it is desirable to maintain a high ratio of line width to line thickness. For a line width of approximately 2 mils it is believed that a thickness of .5 oz. copper (.7 mil) or less is satisfactory, although the invention is not at all limited to this range. A good representative working range for thickness in these types of circuits is believed to be in the range of 6 to 18 microns.

While the invention has been described with reference to copper conductor and polyimide dielectric, other materials may be used. For example, aluminum conductor may be used and materials such as mylar and polyester may be used as dielectric.

It is to be noted that the flexible circuit 10 may have separate circuits patterned on both sides of substrate 15. That is, panel zone 12 could have a set of conductors on back surface 38, and panel zone 14 could have a set of conductors on back surface 46, joined by conductors on back surfaces 34 and 42 of end regions 20 and 24 respectively. An additional advantage ofthe present invention is that it allows the use of older existing photolithography systems to fabricate longer flexible circuits than in the past.

Claims

CLAIMS We claim:

1. An elongated flexible circuit comprising: an integral flexible substrate comprising at least first and second laterally spaced apart conductor bearing panel zones, with each said panel zone having an end region and a remaining region; a first plurality of parallel conductors in said first panel zone remaining region; a second plurality of parallel conductors in said second panel zone remaining region; a third plurality of conductors in said end regions connecting said first plurality of parallel conductors to said second plurality of parallel conductors; said substrate having a slit extending between said first panel zone and said second panel zone except at said end regions, said slit parallel to said first and second plurality of conductors; a first-fold line extending through said end regions and parallel to said first and said second pluralities of conductors; a second fold line at said end region of said first panel zone and perpendicular to said first plurality of conductors; said first panel zone folded along said first fold line and said second fold line with said first panel zone remaining region extending in a direction opposite a direction of said second panel zone remaining zone region; and means for securing said first panel zone to said second panel zone at said end regions.

2. Flexible circuit of claim 1 wherein said first panel zone end region abuts said second panel zone end region and a portion of said first panel zone remaining region overlays said first panel zone end region.

3. Flexible circuit of claim 1 wherein a portion of said first panel zone remaining region is sandwiched between said first panel zone end region and said second panel zone end region.

4. Flexible circuit of claim 1 wherein said flexible substrate is selected from the group consisting of polyimide, mylar and polyester, and said conductors are selected from the group consisting of copper and aluminum.

5. Flexible circuit of claim 4 wherein said means for securing is adhesive means.

6. Flexible circuit of claim 1 wherein said conductor bearing panel zones have a first side and a second side with said first, said second and said third pluralities of conductors located on said first side and said flexible circuit further comprises: a fourth plurality of conductors on said second side of said first panel zone remaining region; a fifth plurality of conductors on said second side of said second panel zone remaining region; a sixth plurality of conductors on said second side of said end regions connecting said fourth plurality of conductors to said fifth plurality of conductors.

7. A method of forming a flexible circuit comprising the steps of: providing an integral flexible substrate having at least first and second laterally spaced apart conductor bearing panel zones, with each said panel zone having an end region and a remaining region with a first plurality of parallel conductors in said first panel zone remaining region, a second plurality of parallel conductors in said second panel zone remaining region, and a third plurality of conductor, and in said end regions connecting said first plurality of parallel conductors to said second plurality of parallel conductors; forming a slit extending between said first panel zone and said second panel zone except at said end regions, said slit parallel to said first and second plurality of conductors; folding said first panel zone along a first fold line extending through said end regions and parallel to said first and said second pluralities of conductors; folding said first panel zone along a second fold line at said end region of said first panel zone and peφendicular to said first plurality of conductors; securing said first and second panel zones at said end regions.

8. Method of claim 1 wherein said step of securing said first and second panel zones comprises applying adhesive at said end sections.