GB2101411A - Flexi-rigid printed circuit boards - Google Patents

Abstract

An improved method of making a flexi-rigid printed circuit board including the steps of laminating a plurality of flexible sheets or layers of insulating and conducting materials (1, 2, 3) between sheets of rigid materials, interposing layers of bonding material (4) between the rigid and flexible materials in selected areas only, and interposing layers of non- adherent material (5) between the rigid and flexible materials in the remaining areas. The rigid sheets comprise a composite structure including a first thick rigid sheet portion (6a, 6b) which is apertured (8a) or sub-divided to define both the selected and remaining areas and a second thin continuous rigid sheet portion (10) which is coextensive with both the selected and remaining areas and is laminated with the thick portion, the thick portion being placed adjacent the flexible sheets or layers, bonding the laminated rigid and flexible sheets and layers together and subsequently parting the thin rigid portions along boundaries corresponding to those of the selected areas (6a) so that the remaining areas (6b) of the rigid sheets can be removed from the board to leave flexible board areas extending from rigid board areas. <IMAGE>

Description

SPECIFICATION Flexi-rigid printed circuit board This invention relates to improvements in the methods of making composite flexi/rigid printed circuit boards, commonly known as flexi-rigid boards.

Flexi-rigid boards are herein defined as composite wiring panels which have two or more individual insulating layers laminated and bonded together in the manner of a conventional multilayer board such that certain areas of the board contain both rigid and flexible layers bonded to produce substantially rigid areas, and other areas in which the rigid layers are omitted to leave only the flexible layers and their associated copper track patterns.Such flexible areas may form a hinge between two rigid areas which can be mounted into an electronic equipment such that one rigid area can move relative to the other, or alternatively the flexible only areas may form "tails" which project around the periphery of a rigid area to provide connections between the rigid area and components mounted non coplanar to the rigid area and either fixed in relation to the rigid area or free to move.

Most usually, but not essentially, the interconnections between the metal tracks on the flexible layers and further metal tracks on the rigid layers are made by plated through holes.

One of the known difficulties in the manufacture of such composite panels lies in the sequence of bonding all the rigid and flexible layers together and then in the subsequent processes of drilling, plating, etching, cleaning and soldering.

Known ways in which these processes are achieved are methods in which prior to bonding all the rigid material and the bonding agent therefor are removed from over all areas which are to be flexible only in the finished structure.

After bonding, the flexible areas and the edges of the rigid areas adjacent to these flexible areas are sealed over with some form of resist or skin which protects these areas from attack by the chemicals used in cleaning, plating, etching and soldering.

These known methods are difficult to achieve with any reasonable precision, and have the disadvantage that if a cover-coat or protective plastic film has been bonded over the tracking in the flexible areas prior to the final bonding of the whole flexi-rigid assembly, the bond between that covercoat or protective film and the flexible substrate may be weakened or damaged during the final bonding of the whole assembly.

The disadvantages inherent in these processes described above can be overcome by another known method illustrated in Fig. 1. The core of the flexi-rigid board is formed of a flexible insulating substrate film 1 clad, on one side at least, with a copper track or pattern 2. A cover coat or protective film 3 is applied over the track or pattern 2. A sheet 4 of bonding material is then placed in contact with the flexible structure over those areas which are to form the rigid part of the board. A sheet 5 of similar thickness to the sheet 4 but made of, e.g. PTFE is placed over the areas which are to remain flexible. Whatever material is used for the sheet 5 it must be such that neither the bonding material nor the flexible material nor any rigid material will adhere to it under the temperatures and pressures used in bonding.Next a sheet 6 of rigid material, e.g. a conventional rigid dielectric substrate, is laid on the composite bonding/non-adherent layer 4/5. The rigid sheet 6 is usually clad on one surface (the outer surface) with a copper skin 7 or conductor pattern. The inner side of the rigid sheet 6 has grooves 8 formed in it before it is laminated with the flexible structure, these grooves extending only partway through the thickness of the rigid sheet and defining the boundaries of the eventual flexible areas corresponding to the outlines of the nonadherent layer 5. The whole structure is then subjected to the required bonding procedures in a heated press. After bonding has been completed the rigid sheet is cut into from the outside along the boundaries defined by the grooves 8 and the portions of rigid material overlying the nonadherent material 5 can then be removed.This process requires that the rigid sheet 6 is preformed with the requisite grooves to ensure that the cutting step does not damage the flexible structure.

This known process of using grooved outer layers suffers from the disadvantages that the grooves must be very precisely controlled in depth, and must be cut individually in each outer layer. Even with tight control over cutting depth, the actual thickness of material left to be cut through as a final operation will vary with the tolerance on the thickness of the outer layer laminate as well as that on the depth of the grooves. The grooves must have a width such that under the temperature and pressure of bonding any flow of resin from the bonding sheets can not bridge the grooves to provide unwanted bonding in the flexible areas.The necessary width of the grooves allows a small portion of the covercoat or protective layer bonded over the flexible areas to be free of pressure during the final bonding operation, such that the bond between the covercoat or protective film and the flexible substrate may be weakened in these areas.

According to the present invention there is provided a method of manufacturing a flexi-rigid printed circuit board, as hereinbefore defined, including the steps of laminating a plurality of flexible sheets or layers of insulating and conducting materials between sheets or rigid materials, interposing layers of bonding material between the rigid and flexible materials in selected areas only, interposing layers of nonadherent material between the rigid and flexible materials in the remaining areas, characterized in that the rigid sheets comprise a composite structure including a first thick rigid portion which is apertured or sub-divided to define both the selected and remaining areas and a second thin continuous portion which is coextensive with both the selected and remaining areas and is laminated with the thick portion being placed adjacent the flexible sheets or layers such that the thin portions form the exterior of the laminated structure, bonding the laminated rigid and flexible sheets and layers together and subsequently parting the thin portions along boundaries corresponding to those of the selected areas so that the remaining areas of the rigid sheets can be removed from the board to leave flexible board areas extending from rigid board areas.This allows tight control to be maintained over the thickness of the rigid material and high reliability of bonding in the finished flexirigid board.

Embodiments of the invention will now be described with reference to Figs. 2 to 4 of the accompanying drawings, in which:~ Fig. 2 is an illustrative cross-section of a flexirigid board according to the invention, Figs 3a-3c illustrate a method of removing scrap rigid material from a board, and Figs. 4a 4b illustrate a method of making a flexi-rigid board having tails.

The flexi-rigid board illustrated in Fig. 2 comprises a flexible substrate film 1, copper track 2, cover coat 3, bonding sheet 4 and nonadherent PTFE sheet 5 as in the flexi-rigid board of Fig. 1. The rigid sheet comprises a composite structure made up of thick sheet portion or portions 6a, 6b which are bonded to a thin continuous sheet portion 10 by means of a bonding layer 1 The channels or grooves 8 of the rigid sheet which define the boundaries between the rigid and flexible areas extend through the entire thickness of the thick sheet 6a, 6b. This enables the remaining thickness which is to be cut through after bonding to be accurately controlled as it is variable only by the manufacturing tolerance on the thin sheet of material 10, instead of being variable by the tolerance on the thicker sheet 6 plus the tolerance on the depth of grooving.The thin outer copper cladding layer 7 is the same as before and is ungrooved. The bonding layer 11 between the two rigid sheets 6a(6b) and 10 may be continuous, or it may be terminated (not shown) at the boundary between the rigid and the flexible areas in exactly the same manner as the sheets 4 and 6 if the thick layer 6a (6b) is so thin that there is a risk of bonding resin flowing down the sides of the spaces 8a to reach the flexible layers. This improvement allows all the boundary to be cut right through the thick sheets of rigid laminate, so that several sheets can be stacked at one time in the profiling machine and be profiled simultaneously, thus substantially reducing the cutting costs. It should be noted that although not explicity illustrated in Fig. 2 the same method of fabrication of the rigid portions may be used on both sides of the flexirigid board.The same remark applied to all the other improvements to be described below.

A second embodiment of the invention is to fill the boundary gaps 8a with PTFE, e.g. a rectangular cross-section strip. This will apply pressure to the flexible layers at the bottom of the gap during bonding.

A further embodiment is to remove completely the areas 6b of the thick portion and to fill the entire void comprised of the gaps 8a and the area 6b with a solid block of PTFE. The blocks of PTFE must be cut to sizes which are smaller than those of the voids to be filled only by the cutting tolerances on the rigid layers. The thickness of the solid blocks of PTFE must be selected to match the bonded thickness of the rigid layers 6a. The use of solid blocks reduces the costs of assembly because they are very much easier to insert than the thin pieces of PTFE 5 (Fig. 1) and also the solid blocks ensure that during the bonding process pressure is applied substantially equally over the whole of the flexible area. Furthermore the closely fitting solid blocks act to prevent resin flowing out from the bonding layers over the flexible areas.

Customarily, the completed flexi-rigid boards are cut to profile and the portions of the outer rigid layers which lie over the flexible areas are cut away either by knife cuts or by controlled depth routing as part of the board manufacturing process. It may be desirable to leave these outer rigid areas in place, either to protect the flexible area during mass soldering operations orto avoid any possible stressing of the flexible areas through over-flexing prior to final installation. The use of the composite outer rigid layers with the thin ungrooved outermost layer described in the first embodiment facilitates cutting, e.g. with a knife, after the assembled and soldered flexi-rigid board has been installed in position.

Yet another embodiment facilitates removal of the outer rigid layers after final assembly.

Normally, after bonding, drilling and through hole plating, the outer copper cladding is etched away to leave a pad or pad-and-track pattern over the rigid areas only. In the embodiment shown in Fig.

3a the outer copper layer 7 is etched to a pattern such that, in addition to the pad only or pad-andtrack pattern normally formed on the outer surfaces, bars 12 of copper are left along the edges of the rigid areas, separated by narrow grooves 13 from copper bars or sheet 14 on what will become the scrap rigid material over the flexible areas. The copper bars 12 and 14 can be plated with additional copper 1 2a and 1 4a to provide extra strength. The grooves 13 are kept as narrow as the printing, etching and plating processes will allow, in practice a width of approximately 0.1 mm is desirable. The grooves must be positioned so that under worst possible manufacturing misalignment not more than a quarter of the width of a groove can be over the bonded rigid area. With customary manufacturing tolerances this means the grooves 13 must be of such a width that more than substantially 80% of the width of the bar 12 lies over the bonded rigid area under worst possible misalignment tolerances. When the flexi-rigid board has been cut to profile all the inner rigid material, e.g. PTFE blocks 6b is pushed out, Fig. 3b, leaving the rigid areas of the board joined only by the flexible area and two outer thin layers. If now one of the rigid areas is raised and lowered relative to the other rigid area the effect of the stiffening copper bars 12 and sheet 14, which make those areas of the thin outer layers thicker than the original thin layers in the grooves 13, will be to cause cracking and breaking of the thin layers in the bottom of the grooves 13, Fig. 3c.This method of breaking out the outermost thin layers does not leave a very clean edge as would a cutting knife, but it eliminates the risks of cutting into the flexible layers and allows the final removal of the outer layers to be performed by semi-skilled workers, It should be noted that whilst the above embodiments have been described applied to a hinged type of flexi-rigid board in which a flexible area interconnects two rigid areas, similar techniques can be applied to a flying tail type of board in which flexible portions without rigidising ends project from the periphery of a rigid area as shown in Figs. 4a and 4b. When such boards are made, it may be necessary to cut the flexible tails 1 5 to shape prior to bonding.This is particularly the case when tails of different shapes on different flexible layers leave the rigid area, one over the other, to make final profiling of the tails by routing impossible. When such partial profiling of the tails prior to final bonding is done, the transition 16 between unbondable filler sheets 5 and the bonding sheets 4 in the scrap border of the blank must be kept a centimetre or more beyond the profiled edge of the tails 17.

An improvement to known conventional manufacture of such flex-rigid boards can be made if the pre-profiling of each of the tails, which may contain holes which have been plated through the flexible layers prior to bonding, is performed such that the cut 17 round the periphery of each tail is left complete, leaving a length 1 8 to be cut through on final profiling of the bonded assembly. This uncut portion 18 forms a small tab which holds the flexible tail firmly in position in its correct place in the blank during lay-up prior to bonding. On tails of complex shapes (e.g. an elongated L shape) it may be desirable to leave a multiplicity of such small uncut tabs to ensure that no portion of a tail can become folded over during lay-up.The final routing operation may be designed to cut these tabs off to final profile, or it may leave a portion of the tab projecting from the flexible tail to be trimmed off by knife or shears after routing and removal of the scrap material surrounding the board. In cases where a small flexible tail at one level in the flexi-rigid board projects from an edge of the rigid area over or under a larger tail at a different level such that the profile of the smaller tail lies totally within the profile of the larger tail, then the tabs left holding the smaller tail to the outside scrap border of its flexible layer will have to be left long enough to project beyond the profile of the larger flexible tail, and they will be cut through by routing only at the profile of the larger tail, leaving the surplus length of tab to be trimmed off by knife or shears.Note that as shown the pre-profiled edges have been shown with wide slots 17 as would be left by a router, but these edges may be cut by knife, laser, steel rule die or presstool or etched to leave only a single line cut.

The bonding of flexi-rigid boards made incorporating any of the improvements described herein will be better than that on boards made without such improvements, but the quality of the bonding can be further enhanced by the known technique of placing the whole assembly of laid up boards and tooling in vacuum bags prior to placing in the press, such that all bonding is done under vacuum. Because of the stability problems on thin flexible laminates during processing, it is customary to leave a fairly wide unetched copper border all around the required board pattern.

When bonding is to be done under vacuum, which ensures the exclusion of air without the use of excessive compliant sheeting and whilst using low flow adhesives, it is common practice to etch gas vent slots through the wide border.

Whilst these gas vent slots do allow any entrapped air to escape from the patterned area of the flexible laminate, they also decrease the effectiveness of the broader copper border in maintaining stability of the laminate. An improvement according to this invention is to etch the gas vent slots to include bends, preferably but not essentially four sharp corners such that the slot has a profile which introduces a shearing element into any changes in stress patterns within the laminate during processing, and reduce the distortions associated with the purely tensile or compressive stress changes inherent with the use of straight vent slots.

It is to be understood that whilst the thick portions 6a of the rigid sheet have been described simply as a rigid dielectric sheet since they may equally well comprise rigid printed circuits, either single or multi-layer with plated through connections to the flexible circuits which extend into the rigid areas.

Claims (1)

1. Claims

   1. A method of manufacturing a flexi-rigid printed circuit board, as hereinbefore defined, including the steps of laminating a plurality of flexible sheets or layers of insulating and conducting materials between sheets of rigid materials, interposing layers of bonding material between the rigid and flexible materials in selected areas only, interposing layers of nonadherent matedal between the rigid and flexible materials in the remaining areas, characterised in that the. rigid sheets comprise a composite structure including a first thick rigid portion which is apertured or sub-divided to define both the selected and remaining areas and a second thin continuous portion which is coextensive with both the selected and remaining areas and is laminated with the thick portion being placed adjacent the flexible sheets or layers such that the thin portions form the exterior of the laminated structure, bonding the laminated rigid and flexible sheets and layers together and subsequently parting the thin portions along boundaries corresponding to those of the selected areas to that the remaining areas of the rigid sheets can be removed from the board to leave flexible board areas extending from rigid board areas.

   2. A method according to claim 1, wherein the selected areas and the remaining areas in the thick portion of the composite rigid sheet are separated by a gap or gaps, the method including the steps of filling said gap(s) with non-adherent material prior to laminating the composite rigid sheets with the flexible sheets or layers.

   3. A method according to claim 1 , wherein the remaining areas of the thick portion of the rigid sheet are made of non-adherent material, each thin portion of a composite rigid sheet is provided with an outer metal cladding, the method including the step of removing the outer metal cladding along boundaries corresponding to the boundaries between the selected and the remaining areas prior to laminating the rigid and flexible sheets and subsequent to the bonding step removing first the non-adherent parts of the thick portion and then parting the thin portions by misaligning the bonded selected areas to break the thin portions along the lines of the boundaries.

   5. A method according to any preceding claim, including the step of partial pre-profiling of the flexible.materials to define areas destined to become tails by cutting through the flexible materials for part only of the tail profile periphery prior to laminating and bonding of the flexible and rigid materials.

   6. A method according to any preceding claim including the step of providing in at least one layer of the flexible materials gas vent slots which are shaped to include bends or corners such that the slot profile introduces a shearing element into any changes in stress patterns within the laminate during processing.

   7. A method of manufacturing a flexi-rigid printed circuit board substantially as hereinbefore described with reference to the accompanying drawings.