GB2324200A

GB2324200A - Strengthening and venting of multi-layer printed circuit boards

Info

Publication number: GB2324200A
Application number: GB9707153A
Authority: GB
Inventors: Kenneth Dobson
Original assignee: OPTION TECHNOLOGIES INT LIMITE
Current assignee: OPTION TECHNOLOGIES INT LIMITE
Priority date: 1997-04-08
Filing date: 1997-04-08
Publication date: 1998-10-14
Also published as: GB9707153D0

Abstract

A multi-layer printed circuit board comprises a plurality of internal core layers 1 which are separated from each other by interleaved dielectric layers 10. Each of the core layers 1 is provided around its periphery with a strengthening frame 2 in the form of a venting pattern. These venting patterns 2 on opposed surfaces of the cores 1 are arranged to form positive and negative interlocking versions 2A and 2B of the same pattern. Preferably, the positive version of the pattern comprises a plurality of upraised patches 9A and the negative version of the pattern defines a plurality of complementarily shaped hollows 9B which surround the patches 9A and into which they nestle. Advantageously, the positive and negative venting patterns 2A,2B interlock to leave a venting clearance of up to 5 thousandths of an inch (0.127 mm) therebetween, allowing air and excess resin to be expelled during the lamination process.

Description

IMPROVEMENTS TO MULTI-LAYER PRINTED CIRCUIT BOARDS The present invention relates to multi-layer printed circuit boards and in particular to the construction of the internal cores which make up the various layers of such a board.

A multi-layer printed circuit board comprises a plurality of internal cores each of which comprises an electrical circuit on at least one side thereof.

Typically, circuits are formed on both sides of each core and adjacent cores are separated from one another within the board by interposed layers of a dielectric material.

During the conventional manufacture of such cores, copper is coated onto both sides of a dielectric material which is typically a resin-impregnated glass fibre cloth.

The resulting "copper" panel is then coated with a photosensitive film which when, it has been photographically exposed, is resistant to specific chemicals which can be used for etching away the surface of the copper beneath the unexposed areas of the film. The circuit design which it is desired to form on the core is then photographically transferred onto the panel by exposing the photo-sensitive film to the pattern. The panel is then etched so that the only copper remaining on the panel is that in the form of the circuit pattern, thus producing a core.

To produce a conventional multi-layer circuit board, a plurality of cores with appropriate patterns are stacked appropriately and in register with one another. There may be as many as eight cores within the stack to produce sixteen circuit layers, although more commonly four or five cores are stacked, and registration between them is achieved by known tooling systems. Such tooling systems comprise, for example, a series of appropriately shaped and positioned registration holes and slots which are formed in each core during manufacture prior to its coating with the photo-sensitive "resist" layer. Between adjacent cores is located a dielectric layer comprising a resin-impregnated glass cloth. The stack of cores is then subjected to heat and pressure in a press as a result of which the resin which impregnates the cores and the dielectric layers melts, flows throughout the stack, and cures. After cooling, the result is a multi-layer board which can then be drilled and punched to form a conventional printed-circuit board.

One of the problems with the production of multilayer printed circuit boards as described above is that the cores tend to lack mechanical strength and it is difficult to achieve registration of the stack. This problem has been solved in the past by leaving a border of copper around the periphery of the core which does not form part of the circuit but comprises a strengthening frame for the core. However, the disadvantage with this arrangement is that during pressing, the frames tend to retain air and excess melted resin within the board rather than permitting these to escape from the stack. This can cause surface blisters and other problems with the finished board. Such problems can be overcome either by leaving a series of spaced slots in the frame through which air and resin can flow outwardly from the stack or by making the frame in the form of an openwork pattern through which the air and resin can flow. Such frames are usually termed "venting patterns" within the industry.

However, a further problem can arise during the pressing of the stack of cores to form the circuit board and this is that the various layers in the stack can slip sideways out of register. If the slippage is significant, then the resulting board is useless and must be discarded.

It is an object of the present invention to provide a means whereby the aforementioned slippage problem during production of a multi-layer printed circuit board can be either overcome or substantially mitigated.

According to a first aspect of the present invention there is provided a multi-layer printed circuit board comprising a plurality of internal core layers which are separated from each other by interleaved dielectric layers and which are each provided with a strengthening frame in the form of a venting pattern, and characterised in that the venting patterns on apposed surfaces of the cores within the board form positive and negative interlocking versions of the same pattern.

It has been found that if adjacent core layers within the stack making up to board are provided with venting patterns which are capable of interlocking, that despite the fact that a dielectric layer is interposed between them, the patterns still tend to interlock to some extent during the pressing process. This interlocking tends to prevent relative slippage between the core layers and thus retains them in registration with one another.

Preferably, the positive version of the pattern comprises a plurality of upraised patches and the negative version of the pattern defines a plurality of complementarily shaped hollows which surround the patches and into which the upraised patches nestle.

Positive and negative patterns wherein upraised patches nestle within complementarily shaped hollows are advantageous as they tend to prevent slippage of the core layers over one another in all directions. Patterns which consist simply of interlocking parallel bars and channels, whilst within the scope of the present invention, do not exhibit this advantage as they may permit slippage along the longitudinal axes of the channels.

Preferably also, each core layer comprises a venting pattern on each side thereof, which patterns comprise positive and negative versions of the same pattern.

Preferably also, the positive and negative venting patterns on each side of the core layer are in register with one another relative to a registration system provided for the layer.

Preferably also, the positive and negative venting patterns interlock to leave a venting clearance of up to 5 thousandths of an inch (0.127 mm) therebetween.

Advantageously, the positive and negative venting patterns interlock to leave a venting clearance of substantially up to 3 thousandths of an inch (0.0762 mm) therebetween.

Preferably also, the venting pattern comprises a regular crisscross pattern of crossing bands defining interspersed rhomboid shapes.

According to a second aspect of the present invention there is provided a core layer for use in the production of a multi-layer printed circuit board comprising on each side thereof a strengthening frame in the form of a venting pattern, and characterised in that the venting patterns on each side thereof comprise positive and negative versions respectively of the same pattern.

The present invention will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a plan view of part of the borders of one side of each of two core layers, which sides are intended to be apposed to one another in a completed printed circuit board; and Fig. 2 is an enlarged cross-section along the line X-X of Fig. 1 showing the apposed surfaces of the two core layers when stacked together with an interleaved dielectric layer.

In the present invention, each core layer 1 of a multi-layer printed circuit board comprises a core of substantially conventional construction as described above but which is also provided with strengthening frames 2 in a novel and inventive form. One example of a pattern of venting frame 2 is described below but it should be appreciated that this pattern is not the only one which could be used in the invention.

As shown in Fig. 1, each core layer 1 comprises a sheet of resin-impregnated glass fibre cloth which has been coated with copper that has been etched away on each side 3 thereof to leave a circuit pattern (not shown) substantially centrally of the sheet and the strengthening frame 2. A further strengthening frame of conventional pattern may also be provided as an inner frame between the circuit pattern and the outer frame 2. Such a frame may comprise a venting pattern comprising a regular series of upraised circular copper patches 4.

Conventional tooling system holes 5 for rivets or pins and the like may also be provided along with markers and targets such as those labelled with numerals 6 and 7 whereby the various layers 1 may be distinguished, for example for alignment purposes, both during and after production.

However, the frames 2 on each side 3 of the core layers 1 which are intended to be apposed to one another in a completed printed circuit board comprise positive and negative interlocking versions 2A, 2B of the same pattern.

It can be seen in Fig, 1 that in this example the venting pattern 2 comprises a regular crisscross pattern of crossing bands 8 defining interspersed rhomboid shapes 9. In the positive pattern 2A of the pair, the crossing bands comprise channels 8A and the rhomboid shapes comprise upraised patches 9A whereas in the negative pattern 2B the crossing lines comprise upraised bands 8B defining hollows 9B of rhomboid shape interspersed therebetween. Preferably, the depth of the venting pattern 2 is such that it accommodates at least one whole rhomboid shape 9 and advantageously along at least two opposite edges of the layer 1 accommodates at least three whole rhomboid shapes 9.

In will be appreciated that in order to interlock as shown in Fig. 2, the positive and negative patterns 2A and 2B must be dimensioned so that the crisscross channels 8A are slightly wider than the upraised crossing bands 8B which fit therein and that the dimensions of the rhomboid patches 9A must therefore be similarly of slightly smaller dimensions that those of the rhomboid hollows 9B in which they nestle.

However, as the patterns 2A and 2B are also required to be venting patterns through which air and resin can flow outwardly of the core during the pressing of a circuit board in its manufacturing process, it is also necessary for there to be sufficient clearance between the interlocking patterns for this to occur. In addition, in the finished product between the interlocking patterns 2A, 2B is located a dielectric layer 10 which typically comprises a resin-impregnated glass cloth and which must be accommodated by the clearance. It will be appreciated however that the clearance must not be so great that significant slippage can occur between adjacent core layers 1 sufficient to put them out of register.

Preferably, therefore, the difference in the dimensions between the positive and negative patterns 2A and 2B is such as to leave a venting clearance of up to 5 thousandths of an inch (0.127 mm) therebetween. However, advantageously the venting clearance is no more than 3 thousandths of an inch (0.0762 mm).

As most core layers 1 are provided with circuit patterns on both of their sides 3, preferably the venting patterns 2 on the opposite sides 3 of each of the layers 1 are made positive and negative versions respectively of the same pattern. Also, the positive and negative patterns 2A and 2B on each side 3 of each core layer 1 formed in register with one another relative to the registration system provided for the layers 1. Hence, when a series of such layers 1 are stacked during production of a circuit board, positive and negative patterns 2A, 2B can be layered apposed to each other.

As previously mentioned, despite the fact that dielectric layers 10 are interposed between the core layers 1, the patterns 2A and 2B of apposing layers 1 still tend to interlock during the pressing process. This interlocking tends to prevent significant relative slippage between the core layers 1 and thus retains them in registration with one another.

Whilst in this example the venting pattern 2 comprises a regular crisscross pattern of crossing bands 8 defining interspersed rhomboid shapes 9, it will be appreciated that other interlocking patterns can be used.

For example, the crossing bands could define circular shapes rather than diamond or rhomboid shapes.

Preferably, however, as previously stated the positive version of the pattern comprises a plurality of upraised patches and the negative version of the pattern defines a plurality of complementarily shaped hollows which surround the upraised patches and into which they nestle.

Claims

1. A multi-layer printed circuit board comprising a plurality of internal core layers which are separated from each other by interleaved dielectric layers and which are each provided with a strengthening frame in the form of a venting pattern, and characterised in that the venting patterns on apposed surfaces of the cores within the board form positive and negative interlocking versions of the same pattern.

2. A circuit board as claimed in Claim 1, wherein the positive version of the pattern comprises a plurality of upraised patches and the negative version of the pattern defines a plurality of complementarily shaped hollows which surround the patches and into which the upraised patches nestle.

3. A circuit board as claimed in Claim 2, wherein the depth of the strengthening frame is such that it accommodates at least one whole upraised patch or complementarily shaped hollow.

4. A circuit board as claimed in any one of Claims 1 to 3, wherein each core layer comprises a venting pattern on each side thereof, which patterns comprise positive and negative versions of the same pattern.

5. A circuit board as claimed in Claim 4, wherein the positive and negative venting patterns on each side of each core layer are in register with one another relative to a registration system provided for the layer.

6. A circuit board as claimed in any one of Claims 1 to 5, wherein the positive and negative venting patterns interlock to leave a venting clearance of up to 5 thousandths of an inch (0.127 mm) therebetween.

7. A circuit board as claimed in any one of Claims 1 to 6, wherein the positive and negative venting patterns interlock to leave a venting clearance of substantially up to 3 thousandths of an inch (0.0762 mm) therebetween.

8. A circuit board as claimed in any one of Claims 1 to 7, wherein the venting pattern comprises a regular crisscross pattern of crossing bands defining interspersed rhomboid shapes.

9. A core layer for use in the production of a multilayer printed circuit board comprising on each side thereof a strengthening frame in the form of a venting pattern, and characterised in that the venting patterns on each side comprise positive and negative versions of the same pattern.

10. A multi-layer printed circuit board substantially as described herein with reference to Fig.1 or Fig. 2 of the accompanying drawings.

11. A core layer for use in the production of a multilayer printed circuit board substantially as described herein with reference to and as shown in Fig.1 or Fig. 2 of the accompanying drawings.