GB2160712A

GB2160712A - Forming conductive tracks on circuit boards

Info

Publication number: GB2160712A
Application number: GB08415865A
Authority: GB
Inventors: Wilbert Ridd George; Kenneth George Snowden
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1984-06-21
Filing date: 1984-06-21
Publication date: 1985-12-24
Also published as: GB8415865D0; GB2160712B

Abstract

Conductive tracks (22) on an insulating substrate (11), e.g. in the manufacture of a printed circuit board, are formed by coating the substrate with a conductive film, applying a powdered solder to the film, and directing laser light on to the solder to fuse a track pattern on to the film. The fused solder is then used as an etch mask for the removal of the uncoated portions of the film. <IMAGE>

Description

SPECIFICATION Forming conductive tracks on circuit boards This invention relates to processes for forming conductive tracks on insulating substrates, e.g. in the manufacture of circuit boards.

Conductive tracks on circuit boards are conventionally formed by coating the entire board surface with a conductive film, e.g. of copper, and selectively etching away the unwanted regions of the film to leave a conductor pattern. Typically the pattern is defined by a photocircuit material.

The pattern once formed may then be tinned with a solder composition to ensure a reliable electrical contact between the pattern and the components that are subsequently mounted on the board. The conductor density on the board, i.e. the minimum allowable spacing between adjustment conductors is limited by this tinning operation. If the spacing is too small then solder bridging of adjacent tracks can occur. This constraint limits the degree of complexity of the circuit mounted on the board.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a method of forming a conductive pattern on an insulating substrate, the method including coating the substrate with a conductive film, applying a powdered solder to the film, selectively exposing the powder to laser radiation whereby the exposed regions of the powder are fused to form an adherent coating on the conductive film, and removing the uncoated regions of the film so as to define the conductor pattern.

Typically the conductive film comprises copper and the solder is a tin/lead alloy.

An embodiment of the invention will now be described with reference to the accompanying drawings in which Figs. 1 to 3 show successive stages in the manufacture of a circuit board.

In a preferred process a plastics substrate board 11 (Fig. 1) is coated with a copper film 12, e.g. by electroless coating followed by electrolytic coating. The board is cleaned, if necessary, and is then coated with a fluxcoated tin/lead solder powder. This powder may be prepared from a solder alloy powder 13, preferably smaller than 200 mesh, which is treated with a solution of the flux, dried, and then passed through a screen, typically smaller than 170 mesh.

The powder coated structure is exposed to high intensity radiation from a laser whereby the powder is fused to 'wet' the copper and form an adherent coating thereon. The laser beam is traversed across the structure so as to 'write' the devised conductor pattern in the form of fused solder tracks 22 (Fig. 2). For this purpose a variety of lasers can be used, but we prefer to employ a 1.06 micron YAG laser as this wavelength is strongly reflected by the copper substrate.

When the devised pattern has been formed the laser is switched off and the unfused powder is removed from the board which is then exposed to an etchant, e.g. hydrochloric acid. The solder pattern provides a mask allowing the uncoated copper to be removed to define the conductor tracks on the finished board (Fig. 3).

As the process is dry the surplus powder is easily removed and there is no waste of solder. Also the flux is retained on the solder particles and there are thus no flux residues requiring removal.

We have successfully employed solder alloys in the range 60 Sn/Pb to 63 Sn/Pb having a particle size less than 80 microns.

These powders have been coated with a flux comprising adiphic acid, succinic acid or mixtures thereof at 1.5 to 2.5%, are preferably 2% by weightof solder. The fluxes were applied from solution in isopriopyl alcohol, but other alcohols can of course be used for this purpose. Also other organic fluxes can be employed. The coated powders were dried and screened through a 120 mesh sieve to eliminate powder agglometers. The powders were then dusted on to copper coated substrates through a 60 mesh sieve to ensure even coating.

Conductor tracks were 'written' in the coating by the application of radiation from a pulsed Nd/YAG laser having a wavelength of 1.06 microns and a pulse time of 10 ms. The samples were traversed relative to the laser beam at a rate of 0.1 mm/sec.

After formation of the pattern the remaining powder was removed. Solder prills attached to the edges of the fused pattern were removed by light brushing with a soft wire brush. The coated structures were then etched in dilute hydrochloric acid or acid ferric chloride solution to remove the untreated copper surface and leave the conductor pattern.

In some applications the solder coated board may be immersed in a hot vapour to improve bonding of the solder to the copper.

For example the board may be immersed in Fluoroinert FC 70 vapour at 21 5 C or FC 43 vapour at 1 74 C.

Claims

1. A method of forming a conductive pattern on an insulating substrate, the method including coating the substrate with a conductive film, applying a powdered solder to the film, selectively exposing the powder to laser radiation whereby the exposed regions of the powder are fused to form an adherent coating on the conductive film, and removing the uncoated regions of the film so as to define the conductor pattern.

2. A method as claimed in claim 1, wherein the conductive film is copper and the solder is a tin/lead solder.

3. A method as claimed in claim 2 wherein the solder composition is in the range 60 Sn/Pb to 63 Sn/Pb.

4. A method as claimed in claim 1, 2 or 3 wherein said solder powder is coated with a flux.

5. A method as claimed in claim 4, wherein said flux comprises adipic acid, succinic or mixtures thereof.

6. A method as claimed in claim 5, wherein the flux comprises 1.5 to 2.5% by weight of the solder.

7. A method as claimed in claim 5 or 6, wherein said flux is applied to the powder as a solution in an alcohol.

8. A method as claimed in any one of claims 1 to 7, wherein said solder powder is smaller than 200 mesh.

9. A method as claimed in any one of claims 1 to 8, wherein said laser is a Nd/YAG laser.

10. A method as claimed in claim 9, wherein said laser is a pulsed laser.

11. A method as claimed in any one of claims 1 to 8, wherein the solder coated board is immersed in an inert vapour at a temperature of 174 to 215"C.

12. A method of forming a conductive pattern on an insulating substrate, which method is substantially as described herein with reference to the accompanying drawings.

13. A printed circuit board made by a method as claimed in any one of the preceding claims.