GB2202093A - Making printed circuit boards - Google Patents

Abstract

An electrical circuit is formed on an insulating substrate (1) by selectively etching a thin copper foil (2,3). The foil is masked by means of an etch resist layer (4,5) which is sensitive to ultra violet light. The layer of resist (4,5) is initially continuous and is covered by a coating (6,7) of a photo-sensitive emulsion on which a circuit pattern is written by a visible or infra-red beam of light to selectively render regions of the coating opaque or light transmissive. The underlying photo-resist layer is then exposed via the light transmissive regions using a high energy ultra violet light source (8) to form the desired circuit pattern in the resist. The unhardened regions of the resist, together with the overlying coating (6,7), are chemically removed and the exposed copper is etched away to form the required circuit. <IMAGE>

Description

ELECTRICAL CIRCUITS This invention relates to electrical wiring and/or circuits of the kind which comprise a pattern of electrically conductive material carried by an insulating substrate. One common form of electrical wiring is a so-called printed circuit board or printed wiring board in which a thin foil of metal, usually copper, which is supported by an electrically insulating substrate, (usually termed a board) is selectively removed to leave metal tracks in the required pattern. In some printed circuit manufacturing systems the metal is selectively removed by means of an etchant, most commonly a wet chemical etchant, and those areas of the metal which are to remain are protected by an in-contact layer of etch resistant material.The etch resistant material - the resist - is formed into the required pattern by exposing it to an illuminated image of the pattern, the resist being such that illuminated portions respond differently to unilluminated portions so that the illuminated portions become soluble or insoluble allowing the soluble portions to be chemically removed.

In other printed circuit manufacturing systems, the photo sensitive material is exposed to an iluminated image of the pattern so that exposed portions become soluble or insoluble allowing the soluble portions to be chemically removed. The areas left exposed are plated with metal, usually tin-lead, which will act as a resist. The photo sensitive material is removed and the areas which have not been covered by the plating are then removed by etching.

In order for such a process to be performed it is currently necessary to first make an optical mask of the pattern from which the illuminated image can be projected onto the photo sensitive surface. It has been proposed to write the pattern directly onto the photo sensitive layer by steering a light beam such as a laser. The optical energy needed to change the resist from a soluble to insoluble state or vice versa is high, and accordingly the power of the laser would have to be high. Using currently available materials an ultra violet laser is needed, but such lasers are expensive, short lived and potentially hazardous.

The present invention seeks to provide an improved process for manufacturing electrical wiring and circuits.

According to this invention a method of preparation for making an electrical circuit in the form of a metallic circuit pattern on an insulating substrate from an initially continuous metallic foil includes the steps of coating the metallic foil with an ultra violet sensitive resist material; itself covered by a photo-sensitive material which is sensitive to visible or infra-red light; writing a circuit pattern with said light on the surface of the photo-sensitive material and selectively rendering the photo-sensitive material opaque and light transmissive to partially mask the underlying resist material; illuminating the unmasked resist material with ultra violet light to locally make it soluble or insoluble, and removing the photo-sensitive material and soluble resist material; and utilising the remaining resist material to selectively remove the metallic foil to leave a required circuit pattern.

The exposed foil may be etched by suitable chemicals or protected from etchant by suitable plating Thus the invention provides additional steps which avoid the need to trace out the circuit pattern with a high energy localised beam of ultra violet light.

Instead a relatively inexpensive and safe infra-red or visible laser beam is used, and despite the additional steps the method is overall beneficial since it permits complex circuit patterns to be directly written without the need to produce a conventional photographic mask.

Preferably the metallic foil is copper. Although not limited to a rigid substrate, the invention is particularly suited for use with a rigid fibre glass substrat-e. Such a substrate can readily carry a c# ciii t Pi0t tern on both its surfaces, and both surfaces can be processed simultaneously although the initial t l a 1 steps of writing the circuit patterns may be performed sequentially so as to permit the use of just a single visible or infra-red laser.

Such a visible or infra-red laser can be readily steered by an optical deflector, or the laser source can be moved relative to the substrate by means of a mechanically mounted carriage or combination of the two. The beam of laser light can be made sufficiently narrow as to permit the writing of circuit patterns having a very high resolution.

The invention is further described by way of example with reference to the accompanying drawings in which Figures 1 to 4 show different stages in the manufacture of an electrical circuit.

Referring to Figure 1 there is shown therein an insulating substrate 1 composed of an insulating fibreglass rigid board, both surfaces of which carry a thin copper foil 2 and 3, the foil being firmly bonded to the surface of the substrate 1. Although the foil is initially continuous, it is subsequently to be selectively removed so as to leave conductive tracks in position on the two surfaces of the substrate 1 and it is these tracks whch define the circuit pattern onto which electrical components are typically added. In order to permit the selective removal of the copper foils 2 and 3, a thin coating of photo-resist material 4 and 5 is overlaid onto each copper foil so as to completely cover it. This resist material is now very well known in the technology and it is resistant to the action of etchants and the like which are capable of dissolving and removing copper.

It is increasingly desirable to write the circuit patterns directly onto the substrate using a steered laser beam of light rather than to first produce a photographic image in the shape of the circuit pattern.

However, the resist material 4 and 5 is sensitive primarily to ultra violet light, whereas it is not very sensitive to visible light. According, therefore, to the present invention, the resist material 4 and 5 is coated with a photo-sensitive emulsion layer 6 and 7, this emulsion being conventional photo-sensitive material which is responsive to visible and/or infrared light.

In Figure 1, a laser 10 is shown mounted for transverse movement in two dimensions in a plane which is parallel to that of the substrate 1. It can be moved rapidly by means of a mechanical carriage so as to trace out a required circuit pattern. The laser is energised as and when necessary so that whilst the carriage is correctly positioned over a region of the emulsions 6 and 7, the laser can be activated so as to only leave unexposed areas which will become tracks or pads or other required features. The laser 10 is usch as to generate a fairly high energy narrow beam of laser light.

In practice, a single laser 10 might be used with the two emulsion layers 6 and 7 being exposed sequentially, the substrate 1 being inverted after the first emulsion has been exposed so as to permit exposure of the second emulsion.

After the circuit pattern has been traced out using the laser 10, the exposed areas of the emulsion 6 and 7 are treated with a suitable developer to make opaque those portions of the emulsion which have been exposed to the laser. This stage is illustrated in Figure 2 in which those regions of the emulsion which do not correspond to required copper tracks and so on have been made opaque. Subsequently, the whole of the structure is exposed under a strong collimated ultra violet source 8 of light to harden the exposed portions of the resist layers 4 and 5 beneath 11 and 12 of the emulsion 6 and 7. The unhardened regions of the resist layer, together with the overlying portions of the emulsion 6 and 7 are then subsequently removed in a suitable chemical bath as of the kind commonly used in printed wiring board processing. This step is illustrated in Figure 3 and it will be seen that the copper foil 2 and 3 is now exposed except for very localised regions covered by resist material 13 and 14, which remain. This resist material is resistant to the etchants which are used to etch and remove copper and the exposed copper is now removed in the conventional manner to leave a required circuit pattern of the kind shown in Figure 4.

Since, apart from the step involving the laser 10, and the ultra violet light 8 the processing of the materials can be performed in a series of chemical baths or the like, processing of both sides of a double-sided printed circuit board can proceed simultaneously rendering the process relatively simple and economical. Furthermore, since the resist material is exposed under a collimated source of ultra violet light, which does not necessarily originate from a laser, a very strong and intense source can be readily provided in a simple and economical manner. Despite this the precision and ease with which a circuit pattern can be written with the use of a fine beam laser 10 is retained.

Claims (7)

1. A method of making an electrical circuit in the form of a metallic circuit pattern on an insulating substrate from an initially continuous metallic foil including the steps of coating the metallic foil with an ultra violet sensitive resist material; itself covered by a photo-sensitive material which is sensitive to visible or infra-red light; writing a circuit pattern with said light on the surface of the photo-sensitive material and selectively rendering the photo-sensitive material opaque and light transmissive to partially mask the underlying resist material; illuminating the unmasked resist material with ultra violet light to locally make it soluble or insoluble, and removing the photo-sensitive material and soluble resist material; and utilising the remaining resist material to selectively remove the metallic foil to leave a required circuit pattern.

2. A method as claimed in claim 1 and wherein the circuit is written with visible or infra-red light in the form of one or more narrow beams of light.

3. A method as claimed in claim 2 and wherein the narrow beam of light is laser light.

4. A method as claimed in claim 2 and wherein the laser light is generated by a laser which is movable in plane parallel to that of the substrate, and is arranged to trace out a required circuit pattern as it moves.

5. A method of making an electrical circuit substantially as illustrated in and described with reference to the accompanying drawings.

6. An electrical circuit arrangement when made in accordance with any of the preceding claims.

7. A method as claimed in claim 1 and wherein the circuit is written with visible or infra-red light in the form of narrow beams of light.