GB2148608A - Forming conductive regions in polymeric materials - Google Patents

Abstract

Conductive regions in a plastics body are formed by selectively implanting the body with positive ions, e.g. N<+>, to a concentration of 3 x 10<15> to 3 x 10<16> per cm<3>. These conductive regions may be plated with copper to form a printed circuit board. The body may alternatively be implanted with negative ions.

Description

SPECIFICATION Forming conductive regions in polymeric materials This invention relates to processes for forming conductive regions in insulating plastics materials, e.g. in the manufacture of printed circuit boards.

The formation of conductive regions on plastics material has in the past involved either the selective printing of conductive inks or electroless plating followed by selective etching. However these processes are time consuming and, particularly in the latter case, somewhat expensive in the use of materials.

Also with the present trend towards miniaturisation, the manufacture of circuit boards by these techniques is becoming increasingly more difficult as the conductor track spacing is reduced.

The object of the present invention is to minimise or to overcome these disadvantages.

According to one aspect of the invention there is provided a process for forming electrically conductive regions in a bulk plastics material, the process including selectively implanting positively charged ions in those regions of the plastics that are to be rendered conductive, wherein the ions are implanted in said regions to a concentration of 3 x 1015 to 3X 1016.

According to a further aspect of the invention there is provided a process for fabricating a printed circuit board, the process including selectively implanting a laminar plastics body with positively charged ions to a concentration of 3 X 1015 to 3 X 1016 per cm3 thereby rendering the plastics conductive in the implanted regions, and plating a conductive metal on the implanted regions.

An embodiment of the invention will now be described with reference to the accompanying drawing in which: Figure 1 is a schematic view of an ion implantation apparatus; and Figure 2 illustrates the relationship between applied voltage and current in an ion implanted polymer.

Referring to Fig. 1, a plastics body 11 to be selectively implanted is placed in a vacuum chamber 12 coupled via an accelerator tube 13 to an ion source 14. The body 11 is bombarded with positive ions from the source 14 to provide a doping concentration of 3 x 10'5 to 3 X 1016 in those regions that are to be rendered conductive. The exposure to the ions may be effected via a mask, e.g. of alumina, by directional control of a focussed beam to 'write' the desired pattern on the body 11. Typically we employ an accelerating voltage of 80 to 100 KV and a beam current of 2 to 4 microamp/cm2, these conditions being chosen to reduce heating effects.Advantageously the implanted ion is singly charged nitrogen, but other positive ions such as C+ and OC can of course be employed. it will also be appreciated that, if a suitable accelerator is available, the plastics material may be implanted with negative ions.

We have found that polyetherimide plastics treated by this process show a conductivity in the implanted regions six orders of magnitude above that of the untreated polyer. Similar results have been observed with glass reinforced epoxy resin boards of the type employed in the manufacture of printed circuits.

In particular we have treated polyphenylene sulphide polymers by this technique. Such materials comprise a chain of benzene rings linked in the para position by sulphur bridges and have the structure:

Conductive tracks 25mm long by 2mm wide were implanted in this material to a doping level of 3 X 1016 N+ions/cm3. The tracks were found to have a mean resistance of 10##. From a calculated implantation depth of 0.1 Smicrons this corresponds to a sensitivity of the order of 1 ;2cm.

The relationship between current and applied voltage for nitrogen implanted polyphenylene sulphide is shown in Fig. 2. The upper line corresponds to a doping level of 1 x 1016 ions/cm3 whilst the lower line corresponds to a level of 3 X 1 1015 ions/cm3. As can be seen from Fig. 2, the relationship between current and voltage is linear, indicating that the conductivity is substantially ohmic.

The technique can be extended to the manufacture of printed circuit boards. The board substrate material is implanted in those regions where conductor trials are to run to render those regions conductive. A conductive metal, e.g. copper, is then electroplated, or electroless plated, on to the conductive regions to form the devised conductor pattern.

The board is then finished by drilling the necessary through holes. Since the metal is not deposited over the whole of the board surface it will be appreciated that a considerable saving in material is effected. Also, as the ion implantation technique provides a very high definition, relative complex and high density conductor patterns can be formed without difficulty.

Claims (9)

1. A process for forming electrically conductive regions in a bulk plastics material, the process including selectively implanting positively charged ions in those regions of the plastics that are to be rendered conductive, wherein the ions are implanted in said regions to a concentration of 3 X 1015 to 3 X 1016.

2. A process as claimed in claim 1, wherein the ions are accelerated to a voltage of 80 to 100 kilovolts.

3. A process as claimed in claim 1 or 2 wherein the plastics material is exposed to an ion beam current of 2 to 4 microamps per cm2.

4. A process as claimed in claim 1, 2 or 3, wherein the ions are positively charged nitrogen ions.

5. A process as claimed in claim 1, 2, 3 or 4, wherein the plastics material is a polyetherimide.

6. A process as claimed in claim 1, 2, 3 or 4, wherein the plastics material is a polyphenylene sulphide.

7. A process for fabricating a printed circuit board, the process including selectively implanting a laminar plastics body with positively charged ions to a concentration of 3 X 1015 to 3 X 1016 per cm3 thereby rendering the plastics conductive in the implanted regions, and plating a conductive metal on the implanted regions.

8. A process for forming electrically conductive regions in a bulk plastics material, which process is substantially as described herein with reference to the accompanying drawings.

9. A printed circuit board or substrate made by a process as claimed in any one of the preceding claims.