GB2147460A

GB2147460A - Printed circuit board manufacture

Info

Publication number: GB2147460A
Application number: GB08424755A
Authority: GB
Inventors: Roderick Wilson Froud; Howard Terence Mills
Original assignee: International Research and Development Co Ltd
Current assignee: International Research and Development Co Ltd
Priority date: 1983-09-29
Filing date: 1984-10-01
Publication date: 1985-05-09
Also published as: GB2147460B; GB8424755D0; GB8326169D0

Abstract

A method of forming a wiring pattern in a conductive layer 15 involves charging a photo- conductive surface of a drum 2 at a charging station 4, and turning the drum to an imaging station 6, where an image representing the wiring pattern is projected onto the surface to discharge illuminated portions, to a development station 8, where charged masking material deposits on the remaining charged areas of the surface, and to a transfer station 10 where a bias voltage is applied to transfer the material to the conductive layer 15. The masking material is fixed to the layer by heater 10 and the layer is etched. <IMAGE>

Description

SPECIFICATION Printed circuit board manufacture This invention relates to the formation of a conductive wiring pattern of a printed circuit board.

According to the present invention there is provided a method of forming a wiring pattern in a layer of conductive material, comprising: forming a pattern on a surface in charged particulate material, the particulate material depositing on the surface under the influence of an electric field; Bring the surface into engagement with the layer of conductive material; applying an electric field between the surface and the layer of conductive material to transfer the particulate material to the layer; fixing the material to the layer of conductive material; and subjecting the layer to an etching step, the pattern of the fixed material defining the areas to be etched.

In one embodiment the surface is a photoconductive surface and the step of forming the pattern on it involves electrically charging it, projecting onto it an image representing the wiring pattern to discharge illuminated portions, and applying the charged particulate material to it. The image may be so projected using a lamp, a cathode ray tube with a fibre optic face plate, a scanning laser or a light gate array. Thus the required circuit pattern may be generated electronically from a computer or computer assisted design system.

In an alternative embodiment the steps of charging a photoconductive surface and then projecting onto it an image representing the wiring pattern to discharge illuminated portions may be replaced by a single step whereby an uncharged, dielectric surface is charged directly in a representation of the pattern using, for example, a cathode ray tube with an electrode faceplate, an electrode array, or an ion deposition array. The required pattern or charge may be generated using data fromn a computer or a computer assisted design system.

In another alternative embodiment the surface is a photoconductive surface carrying a representation of the wiring pattern, and the step of forming the pattern on the surface in transferable particulate material involves electrically charging the surface, illuminating it to discharge exposed portions of the surface and applying charged particulate material to the surface, to deposit on the permanent representation of the wiring pattern.

It is preferred, in this further alternative embodiment, that the permanent representation of the wiring pattern is formed on the photoconductive surface as a result of the deposition of charged particulate material thereon under electrical influence and the permanent fixing of that material on the surface.

It is desirable, in each of the alternative embodiments of the invention, though not essential, that the particulate material, which forms a pattern on the surface representing the wiring pattern, serves, after the fixing step, as masking material during the etching step. An alternative would be to lay down a second material after the fixing step, and then to remove the originally transferred material The second material would then serve as the masking material during the subsequent etching step.

Preferably the surface on which the wiring pattern is represented as a charge pattern is on an endless belt or a drum, the surface moving from apparatus at which the pattern is formed to apparatus at which the surface is brought into engagement with the layer of conductive material.

The invention will now be further described, by way of example, with reference to the accompanying drawing.

The apparatus shown comprises a revolvable drum 2 which has a resilient surface coated with a layer of photoconductive material such as zinc oxide, with a conductive backing. Processing stations are disposed around the drum, adjacent its surface. These are, in the order in which they operate in the printing cycle: a charging station 4; an imaging station 6; a mask developing station 8; a transfer station 10; and a cleaning station 12.

A master copy 14 of a wiring pattern to be formed is placed on a plate 16 which forms part of the imaging station. The printing cycle is initiated and the drum turns. As it does so the charging station 4, a corotron, applies a uniform surface charge to the photoconductive drum surface as it passes beneath it. The drum continues to turn and at the imaging station 6 an image is projected onto the charged section of the drum surface, from the master copy 14. The illuminated areas of the surface are thereby discharged, and the remaining charge on the surface is in a pattern corresponding to the wiring pattern. The drum continues to turn and charged surface moves to the developing station, where the charged pattern is developed by a charged, high resistivity powder (which is to serve as masking material), being resistant to the solution which is to be used as the etching solution.

The masking material may, for example, be a one or two component magnetic powder, a one component non-magnetic powder or a polymer dispersed in a liquid.

The surface then passes on, to the transfer station 10. At the transfer station the image of the wiring pattern, defined in masking material, is transferred to a copper surface on an insulating board 15. The board 15 is mounted on a conveyor belt 16, or, in an alternative embodiment, on a belt between reels, which runs in synchrony with the drum 2, such that at the transfer station 10 the copper surface engages the image in masking material on the surface of the drum 2. A transfer roller 18 is provided on the underside of the conveyor belt 16 at the transfer station 10 to ensure intimate contact between the board and the drum surface. A bias voltage is applied between the conductive backing of the photoconductor surface on the drum and the copper layer on the board to bias the charged masking material to transfer to the copper surface.

The board, with its copper surface now carrying the wiring pattern in masking material, is conveyed through a radiant heater 20, to pre-heat the board, and then through a hot roller unit 22, to fuse the masking material. The board then passes automatically to a copper etching machine. The masking material is not dissolved by the etchant but is removed thereafter, leaving the wiring pattern in the remaining copper, on the insulating substrate. Whilst the board 35 passes to the heating apparatus 20 and 22 the drum continues to turn and the portion of the surface on which the image in masking material was defined passes through the cleaning station 12, where it is electrically discharged and cleaned, and thence back to the charging station 4, for the start of the next cycle.

An alternative embodiment uses a different technique to form the pattern in masking material on the transfer surface. A 'master sheet' is formed, comprising a photoconductive layer with a conductive backing and, on the photoconductive layer, a permanent representation corresponding to the wiring pattern.

The permanent representation is in fixed material, formed by charging the photoconductive layer, illuminating the layer with a negative image of the wiring pattern, to leave a charge pattern which is a positive representation of the wiring pattern, applying charged particulate material to develop the remaining charged areas, and fixing the material, using heat.

The master sheet is used as follows: the photoconductive surface is charged and the sheet is then subjected to flood illumination.

The charge on exposed portions of the master sheet is dissipated leaving only those areas carrying fixed material charged. Charged particulate material (which is to serve as masking material during etching) is then applied and this material is transferred to the conductive surface in which the wiring pattern is to be formed. The master sheet carrying the permanent, fixed material passes on for the next cycle.

Thus, in this embodiment the cycle involves charging, flood illumination, development and transfer without the need for exposure to the light image of the pattern at the imaging station 6, and the cycle time can be reduced.

When it is required to change the pattern the master sheet is discarded and replaced with an 'unused' master sheet, comprising the layer of conductive material with the conductive backing. Again the first cycle is used to form the permanent represention of the wiring pattern on the photoconductive surface, without transfer, and subsequent cycles involve the transfer of further particulate material which is to serve as masking material.

The embodiments described above involve the formation of a positive representation of the pattern, in particulate material which is to serve as masking material, on the transfer surface. It will be appreciated that a negative representation could be formed in a charged material, and transferred to the conductive board, and a subsequent negative-to-positive processing stage employed before etching.

Claims

CLAIMS:

1. A method of forming a wiring pattern in a layer of conductive material, comprising: forming a pattern on a surface in charged particulate material, the particulate material depositing on the surface under the influence of an electric field; bringing the surface into engagement with the layer of conductive material: applying an electric field between the surface and the layer of conductive material to transfer the particulate material to the layer; fixing the material to the layer of conductive material; and subjecting the layer to an etching step, the pattern of the fixed material defining the areas to be etched.

2. A method according to claim 1 wherein the particulate material serves, after the fixing step, as masking material during the etching step.

3. A method according to claim 1 or 2 wherein the surface is a photo-conductive surface and the step of forming the pattern on it involves electrically charging it, projecting onto it an image representing the wiring pattern to discharge illuminated portions, and applying the charged particulate material to it.

4. A method according to claim 1 or 2, wherein the surface is a photo-conductive surface carrying a permanent representation of the wiring pattern, and the step of forming the pattern on the surface in transferable particulate material involves electrically charging the surface, illuminating it to discharge exposed portions of the surface and applying charged particulate material to the surface, to deposit on the permanent representation of the wiring pattern.

5. A method according to claim 3 wherein the permanent representation of the wiring pattern is formed on the photoconductive surface as a result of the deposition of charged particulate material thereon under electrical influence and the permanent fixing of that material on the surface.

6. A method according to claim 1 or 2 wherein the step of forming a pattern on the surface in particulate material involves charging a dielectric surface in a charge pattern representing the wiring pattern and applying the charged particulate material to the dielectric surface.

7. A method according to any of claims 1 to 6 wherein the surface on which the wiring pattern is represented in particulate material is on an endless belt or a drum, the surface moving during an operating cycle from apparatus at which the pattern is formed in transferable particulate material to apparatus at which the surface is brought into engagement with the layer of conductive material.

8. A method substantially as hereinbefore described with reference to the accompanying drawing.