GB2087654A

GB2087654A - Printed Circuit Boards

Info

Publication number: GB2087654A
Application number: GB8036530A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-11-13
Filing date: 1980-11-13
Publication date: 1982-05-26
Also published as: GB2087654B

Abstract

A method for making printed circuit boards. An insulating substrate is coated with an even layer of fine copper powder. Local heating, either through a mask or by direct scanning of a laser beam, is applied to the copper particles to cause the copper particles to bond to the substrate surface in selected areas to form a conductive pattern. The unbonded copper particles are removed and the remaining conductive pattern is electroplated.

Description

SPECIFICATION Printed circuit boards This invention relates to a method for making printed circuit boards, and to boards made by the method.

The most widely used method for making printed circuit boards is the so-called "subtractive" method. A copper clad insulating laminate is coated with a photoresist material which is then exposed through a mask. After exposure the resist is removed in the areas where no copper conductor is required and the unwanted copper is removed by etching. Finally the remaining etch resistant resist is cleaned off the final copper pattern.

Alternative "additive" methods are known in which the copper, or other conductive material, is deposited on selected areas of an insulating substrate. One such method involved screen printing of a conductive ink, which, after suitable treatment, is electroplated to provide the final pattern.

The present invention provides an "additive" method of fabricating printed circuits.

According to the present invention there is provided a method for making printed circuits comprising the steps of coating the surface of an insulating substrate with a layer of particulate conductive material so as to render the whole surface electrically conducting, locally heating the coated surface to cause the particulate material to bond to the substrate surface in selected areas to form a conductive pattern and subsequently removing the unbonded particulate material from the substrate surface.

In a preferred embodiment of the invention the conductive pattern is subsequently plated with metal.

The above and other features of the invention will become more readily apparent from the following description of embodiments of the invention.

The invention provides not only conductive tracks on a substrate but also other conductive areas such as pads, lands and even vias (holes interconnecting different conductor layers in a double-sided or multilayer structure).

Typically a plastics material substrate is coated on one face with an even thickness layer of finely divided copper powder, so as to render the whole surface electrically conductive. This is similar to the initial coating applied in the practice of electroforming. A mask is then placed over the coated substrate to define the desired conductive pattern and the uncovered areas (the pattern) are exposed to continuous or pulsed radiant heat which has been preset to a power level sufficient to cause bonding between the copper particles and the substrate surface, e.g. by heating the plastics material and softening or melting the surface to a state where the copper particles are bonded once the substrate surface cools down.

The mask is then removed and the unbonded copper particles are cleaned off the substrate and recovered for further use. The substrate surface now has an accurately defined electrically conductive pattern on it and this pattern can be built-up if desired by conventional electroless and/or electroplating processes.

We have found that, at least with copper, the particle size and type are important. Particular attention must be paid to the shape of the particles. Flake shaped particles have been found to be unsuitable, although when loosely heaped they have a good electrical conductivity the resultant tracks formed by the above process have extremely high if not infinite impedance. On the other hand dendritic copper powder forms an extremely conductive track. The copper powder may be applied dry but is preferably applied in the form of a slurry, for example in a binder consisting of a sodium alginate solution or a methoxy cellulose solution. The binder should be one which is burnt off during the heating step, or at least leaves a carbon residue. A slurry can be applied in many obvious ways, e.g. by painting by brush or roller, or by dipping.Dry powder can be applied, for example, by first depositing discrete piles of powder on a level surface and then subjecting the surface to a controlled vibration.

The radiant heat source is conveniently a laser.

The operating wavelength should be chosen so that a satisfactory heat absorption occurs. For copper the preferred operating wavelength would be in the region of 1.06 microns, and a suitable laser would be a neodymium doped yttriumaluminium-garnet (Yag) laser.

However, it may be that the wavelength is chosen so that primary heating occurs in the binder material and this is sufficient to cause bonding to take place. Once the binder material has been burnt off the radiant energy is reflected from the metal particles, thus providing an automatic control of-the heating process, if the wavelength used is reflected. For copper, a CO2 laser, 10.6 micrometres wavelength, would be usable.

As an alternative to making the powder coated surface the laser beam can be focused to the required resolution and simply scanned over the surface, or the surface can be carried on an x-y table and moved beneath the laser beam.

Reference has been made to the use of copper powders. The invention is not limited to making electrically conductive patterns in copper, it can be used with other powdered materials, e.g. Nickel, Titanium, Molybdenum. However, it may well be that for some metals the heating operation is carried out in an inert atmosphere to prevent undesirable oxidation of the heated powder.

Claims

1. A method for making printed circuits comprising the steps of coating the surface of an insulating substrate with a layer of particulate conductive material so as to render the whole surface electrically conducting, locally heating the coated surface to cause the particulate material to bond to the substrate surface in selected areas to form a conductive pattern and subsequently removing the unbonded particulate material from the substrate surface.

2. A method according to claim 1 wherein the particulate conductive material is deposited on the substrate in the form of a slurry.

3. A method according to claim 1 or 2 wherein the particulate conductive material is copper powder.

4. A method according to any preceding claim wherein the local heating is effected by a source of radiant heat.

5. A method according to claim 4 wherein the radiant heat source is a laser.

6. A method according to any preceding claim including the step of electroplating the conductive pattern.

7. A method for making printed circuit boards substantially as described.

8. A printed circuit board made by the method claimed in any preceding claim.

New claims or amendments to claims filed on 1 9 May 1 981.

Superseded claim 1.

New or amended claim

1. A method for making printed circuits comprising the steps of coating the surface of an insulating substrate with a layer of particulate dendritic conductive material locally heating the coated surface to cause softening or melting of the substrate surface to a state where the particulate material will bond to the surface in selected areas when the surface cools to form a conductive pattern and subsequently removing the unbonded particulate material from the substrate surface.