WO1993011652A1

WO1993011652A1 - Printed circuit combination and process

Info

Publication number: WO1993011652A1
Application number: PCT/GB1992/002171
Authority: WO
Inventors: Robert Banfield
Original assignee: Northumbria Circuits Limited
Priority date: 1991-11-27
Filing date: 1992-11-25
Publication date: 1993-06-10
Also published as: GB9125173D0

Abstract

A flexible-rigid printed circuit combination comprises a rigid sheet (1) of electrically insulating pre-impregnated bonding material divided with an intervening flexible polyimide or polyester sheet (3) joined by a narrow region of edge overlap to each part. Sheets of copper foil are adhered to each face of the three sheets and subjected to photoresist patterning and etching processes to produce conductor tracks (5a, 5b). Overcoats (6) each with an adhesive layer (7) are applied over the flexible sheet to cover the tracks thereon. The ends are built up by sheets (8) of said bonding material and laminate sheets (11) with metal conductors (12) thereon. Thus, plated through-holes or pillars can be provided through the rigid ends, without touching the flexible material, to connect the metal tracks.

Description

PRINTED CIRCUIT COMBINATION AND PROCESS This invention relates to printed circuits and mor particularly to flexible-rigid printed circui combinations. Such combinations are well known an comprise rigid circuit portions interconnected by one o more flexible circuit portions. They are particularl advantageous in that they can be manufactured in on plane, including the mounting of electronic components o the rigid portions. After manufacture, they can be folde about the one or more flexible portions to provide a ver compact assembly, with the rigid portions in stacke relationship or located in different planes at a desire angle to one another.

The flexible portion generally comprises a differen electrically insulating material from the rigid portions. For example, the flexible portion may comprise polyimide material and the rigid portions may comprise a glass reinforced epoxy material. This has create problems in the manufacture of flexible-rigid circui board combinations of the prior art. In the method of the prior art, the flexible material with electricall conductive tracks, usually of copper, extending along its length has end regions formed of a shape and size matching those of the required rigid circuit portions. Rigid circuit board material provided with conductor tracks, also usually of copper, is then applied in contact with one or both sides of the flexible material at the end regions thereof such that the flexible material is secured to or sandwiched between rigid material. The rigid and flexible materials are laminated together, involving heat and high pressure and an appropriate adhesive bonding means. It is then necessary to drill holes thorugh the resulting rigid portions at desired locations where the conductive tracks are provided and form a plated metal layer on the walls of the through-holes to interconnect the conductive tracks on the flexible and rigid materials. Leads of electronic components may be required to be soldered into some of these plated holes. The flexible material has certain different physical and mechanical properties from the rigid material. For example, the materials possess different coefficients of thermal expansion and different moisture absorption properties.

As a result, during the lamination process precise registration between the flexible material and the rigid material layers is very difficult to effect and when the through-holes are subsequently drilled at conductor locations on the rigid material, it cannot be guaranteed that they will pass through the conductor track material on the flexible material with the necessary accuracy and in the worst case may miss one or more of the conductor tracks altogether, resulting in expensive products having to be rejected.

It is an object of the present invention to overcome this problem by providing a flexible-rigid printed circuit combination manufactured in such a way that it does not involve the flexible material extending into the rigid portion where the through-hole connections are required while arranging for the conductor tracks to extend integrally from the flexible material into the rigid portion.

The present invention provides a flexible-rigid printed circuit combination comprising a flexible portion extending from and secured to at least one rigid portion, said combination being characterised in that a narrow region of edge overlap is provided between the flexible portion and the rigid portion and at least one integrally formed electrical conductor track is provided adhered to the flexible portion and directly to the rigid portion and extending therebetween.

The said at least one rigid portion has suitably been formed by bonding an appropriately rigid layer of electrically insulating material to an electrically insulating sheet, said sheet being arranged in edge to edge relationship with the flexible portion, the layer of insulating meterial being arranged to extend beyond the insulating sheet in such a way as to provide the narrow region of edge overlap with the flexible portion, the or each said conductor track having been provided adhered to the flexible portion and the insulating sheet and traversing the junction therebetween, prior to effecting the bonding thereto of the rigid layer of insulating material. If desired, the said at least one rigid portion may include a said appropriately rigid layer of the electrically insulating material bonded to opposite faces of the said electrically insulating sheet and the said conductor tracks may, optionally, have been provided adhered to both opposite faces of the flexible portion and the insulating sheet.

Electrical conductors may be suitably provided on a surface of the or each rigid layer.

The or each said conductor track suitably comprises a metal foil, preferably copper. Suitably a sheet of the metal foil has been bonded, e.g. by a lamination process, to at least one face of the flexible portion and the insulating sheet, followed by etching to remove selected regions of the foil to provide the conductor track or tracks.

The one or more conductor tracks have preferably been adhered to the insulating sheet by means of inherent adhesive properties of the sheet. The sheet conveniently comprises a known pre-impregnated bonding material, e.g. a glass cloth material impregnated with an uncured or partly cured adhesive organic resin. Such materials are known in the art as 'pre-pregs'. One or more similar adhesive sheets may also be used to effect the bondings of the one or more said rigid layers of material.

The said flexible portion suitably comprises a polyimide or polyester material and has also been provided with a surface coating of an adhesive to enable the conductor track or tracks to be adhered thereto. Such an adhesive may, for example, comprise an acrylic composition. A cover coat, suitably of the same material as the flexible portion, may be provided over the conductor track or tracks on the flexible portion.

The said appropriately rigid layers of the electrically insulating material suitably comprise a well-known filled or reinforced plastics material, e.g. an epoxy, phenolic or polyimide material reinforced or filled with glass or paper or reinforced with aramid fibre material.

One or more plated through-holes or pillars may be provided in the rigid portion or portions to provide electrical connection from conductors on the rigid portion or portions to the said conductor track or tracks, and optionally to enable electronic components to be soldered thereto. The said appropriately rigid layers may be provided with an appropriate pattern of conductors thereon, interconnection between the conductors on the various layers being conveninetly provided by means of plated through-holes or pillars.

In a particular embodiment, at least two said rigid portions are provided, interconnected by a said flexible portion. Such an embodiment is conveniently formed by starting with a said insulating sheet provided with one or more openings or windows therein into each of which a said flexible portion is located, the conductor track or tracks being appropriately formed by etching a sheet of metal foil laminated onto the surface or surfaces of the sheet and the flexible portion or portions, rigid portion material with one or more corresponding openings or windows being suitably applied and the resulting product being cut or perforated to enable a required flexible-rigid combination or combinations to be isolated and which can be subsequently folded, or hinged, as required about the flexible portion or portions.

The present invention also provides a process for the production of the flexible-rigid printed circuit combination.

In the present invention, absence of material of the flexible portion extending through the rigid portion or portions obviates the registration problem of the prior art, which resulted from mismatch of the sandwiched flexible and rigid materials, when forming plated through-holes in the rigid portion or portions to interconnect with the said conductor track or tracks. In the invention, the conductor tracks, e.g. of copper, are formed as integral entities on, and extending across the interface between, the flexible and rigid portions. There is consequently none of the flexible material between the conductor track or tracks and the rigid material, in the rigid portion or portions apart from at the said narrow region of overlap.

The invention is now described, by way of example only, with reference to the accompanying drawings in which:

Figures 1, 2 and 3 illustrate process sequence in the production of a flexible-rigid printed circui combination according to the invention, and Figure represents a cross-sectional view of a complete flexible rigid printed circuit combination according to th invention.

Referring to Figure 1A, an electricall insulating sheet 1 comprising a well known form of pre impregnated bonding material, such as Fortin epoxid resin glass fabric pre-preg bonding material, has a opening 2 provided therein into which is located a portion of flexible polyimide material 3 having an adhesive layer 4 on opposite surfaces thereof. An example of such a polyimide material is Du Pont Pyralux (Registered Trade Mark) Bond Ply, provided with an acrylic adhesive. Sheets 5 of copper foil are laminated onto opposite surfaces of the assembly 1, 3 as illustrated in Figure IB, using well known heat and pressure laminating equipment. The sheets 5 of copper foil are then subjected to well known photoresist patterning and etching processes to produce copper conductor tracks 5a, 5b of integral form adhered to the flexible portion 3 and the sheet 1 and extending therebetween.

Referring now to Figure 3, covercoats 6, comprising films of flexible polyimide material coated on one face with an adhesive layer 7, are located over opposite faces of the flexible portion 3 and extending slightly onto the sheet 1. A suitable covercoat material is Du Pont Pyralux (Registered Trade Mark) Coverlay. Further sheets 8 of preimpregnated bonding material, suitably of the same type as used for the sheet 1, are located and arranged such that openings 9 are provided to leave the flexible portion and its covercoats 6 exposed, but such that there is a narrow region of overlap 10 between each sheet 8 and the flexible portion 3. Sheets 11 of rigid laminate material having copper conductors 12 thereon and of the same dimentions as the sheets of pre- impregnated bonding material 8, are located in contact with the sheets 8. The sheets 11 suitably comprise 5 copper-clad epoxy-glass laminates and are well known in the art. The entire assembly of Figure 3 is then subjected to heat and pressure in a known laminating press.

Referring now to Figure 4, this illustrates the

10 resulting finished flexible-rigid printed circuit combination, plated through-holes 13 having been formed through the rigid portions to interconnect the conductor tracks 5a, 5b traversing the flexible portion 3 with the conductors 12 on or in the rigid portions. It is an

15 important feature of the invention that the plated through-holes are provided entirely through the rigid material, i.e. there is no material of the flexible portion intervening where the through-holes are provided. There are therefore no problems with materials mismatch,

20 as occurred in the prior art and precise registration of the plated through-holes with the various conductor layers is obtained. The flexible portion 3 serves as a hinge, as well as an electrical interconnection, to allow the resulting combination to be folded into a compact

25 arrangement.

For manufacturing convenience, the sheets 1 and 8 of pre-impregnated bonding material and the sheets 11 of rigid laminate may be applied in the form of sheets with rectangular windows therein corresponding to the

30 openings 2 and 9 as appropriate. At the completion of processing, redundant surrounding material is removed to isolate the required flexible-rigid combination.

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Claims

CLAIMS:

1. A flexible-rigid printed circuit combinatio comprising a flexible portion joined to a comparativel rigid portion with at least one electrical conducto track extending over surfaces of both portions, th flexible portion being arranged so that the combinatio can be bent, characterised in that a narrow region o edge overlap is provided between said flexible portio (3) and said rigid portion (1) and at least on integrally formed electrical conductor track (5a or 5b) is adhered to surfaces of said flexible portion and said rigid portion so as to extend therebetween from said edge overlap in opposite directions.

2. A flexible-rigid printed circuit combination according to Claim 1, characterised in that a second integrally formed electrical conductor track (5a or 5b) is adhered to faces of said flexible portion and said rigid portion that face oppositely to those faces to which said at least one electrical conductor track adheres, said second electrical conductor track also extending from said overlap in opposite directions.

3. A flexible-rigid printed circuit combination according to Claim 1 or Claim 2, characterised in that said rigid portion is divided to provide a space between two parts thereof in which said flexible portion is mounted with a narrow region of edge overlap at each junction between said flexible portion and a part of said rigid portion, said electrical conductor track or each such track being mounted to extend over said flexible portion and each said part of said rigid portion.

4. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that a flexible overcoat (6) with an adhesive layer (7) thereon is applied over the face or each face of said flexible portion to cover said one conductor track or each said conductor track.

5. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that said flexible portion consists of a polyimide or polyester material coated with an adhesive to retain said conductor track or tracks thereon.

6. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that said rigid portion is composed of a pre-impregnated bonding material having inherent adhesive properties serving to hold said flexible portion and said conductor track or tracks in position.

7. A flexible-rigid printed circuit combination according to Claim 6, characterised in that said pre- impregnated bonding material is a glass cloth material impregnated with an uncured or partly cured adhesive organic resin.

8. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that a further layer of rigid insulating material (8) is applied to one or each of said surfaces of said rigid portion in a position to leave the location of said flexible portion exposed.

9. A flexible-rigid printed circuit combination according to Claim 8, characterised in that a sheet (11) of rigid laminate material having at least one electrical conductor (12) having at least one electrical conductor (12) thereon and of substantially the same area as said further layer of insulating material is mounted on said further layer or each such further layer.

10. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that one or more plated through-holes or pillars are provided through the rigid material, free from the flexible material, to provide for electrical conduction between opposite faces of the combination.

11. A flexible-rigid printed circuit combination according to any one of the preceding claims, characterised in that said electrical conductor track or each such track is formed to shape from metal foil when bonded to said flexible portion and said rigid portion.

12. A flexible-rigid circuit combination substantially as hereinbefore described with reference to the accompanying drawings.

13. A process for manufacturing a flexible-rigid printed circuit combination according to Claim 1, characterised in that a rigid sheet of electrically insulating pre-impregnated bonding material (1) is located with a narrow region of edge overlap in relation to a flexible sheet of polyimide or polyester material (3) having an adhesive layer (4) on at least one surface thereof, laminating a sheet of metal foil on adjacent faces of said flexible sheet and said rigid sheet, subjecting said metal foil to photo-resist patterning and etching processes to produce a metal track (5a or 5b) adhered to said flexible sheet and said rigid sheet and extending therebetween, bonding a flexible overcoat (6) of polyimide or polyester material coated on one face with an adhesive layer (7) to said flexible sheet thereby covering said metal track, and submitting the assembled combination to heat and pressure to effect the required adhesion between the elements of the combination.

14. A process according to Claim 13, characterised in that said rigid sheet of electrically insulating pre- impregnated bonding material is formed from a glass cloth material impregnated with an uncured or partly cured adhesive organic resin.

15. A process according to Claim 13 or Claim 14, charcterised in that a rigid laminate sheet (11) having at least one metal conductor thereon is fixed, with or without an intervening further rigid sheet of electrically insulating pre-impregnated binding material (8) in edge to edge relation with said overcoat (6) , over said rigid sheet (1) in a position such as to leave the location of said flexible sheet exposed, and providing one or more plated through-holes or pillars through said laminate sheet and said further rigid sheet, if provided, 5 to enable an electrical connection to be made with said metal track free from said flexible sheet.

16. A process for manufacturing a flexible-rigid printed circuit combination formed according to Claim 1, substantially as hereinbefore described with reference to 10 the accompanying drawings.

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