GB2078448A

GB2078448A - Electrical printed circuits

Info

Publication number: GB2078448A
Application number: GB8020073A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1980-06-19
Filing date: 1980-06-19
Publication date: 1982-01-06
Also published as: GB2078448B

Abstract

A method of making a multilayer printed circuit board comprises the steps of forming a first conductive pattern on a substrate, applying a curable insulating ink and then applying a curable conductive ink to form a second conductive layer. The resulting structure is then cured. The inks consist of I.R. or U.V. curable binder material and an insulating filler or particulate metallic filler. The insulating and conductive inks may be deposited by screen printing or from a pressurized nozzle.

Description

SPECIFICATION Electrical printed circuits This invention relates to electrical printed circuits and a method of making the same, and to electrical components embodying such circuits.

The production of circuit boards requiring more than one conductor layer has traditionally relied upon subtractive etching of single or double sided Cu clad circuit board for conductor definition plus an additive copper plating technique for through-board connections. An additioned lamination process may also be required for multilayer boards. Both methods are only economically viable for large production quantities, neither lending itself to small volume or custom design applications.

As an alternative to the basic method of making single sided printed circuit boards by the aforementioned printing and etching technique it has been proposed, in British patent application No. 25200/78 (T.M. Jack son-R .J. Hodges-G . L. Ashcroft-S. L. Am phlett-61-38-2-1), to screen print or otherwise deposit patterns of conductor material in conjunction with insulator material. The structure and formulation of suitable screen printable materials is the subject of British patent application No. 7903119 (G.L. Ashcroft-3). Printed circuit boards using the materials and techniques of the aforementioned patent applications can form the basis of components such as rotary switches. These techniques are particularly suited for small production runs or prototype fabrication.

According to the present invention there is provided a method of making electrical printed circuits comprising the steps of forming on a substrate successively a first pattern of an electrically conducting material, a second pattern of an electrically insulating ink material, as hereinafter defined, and a third pattern of electrically conducting ink material, as hereinafter defined, said second pattern having apertures therein whereby electrical contact may be established between the first and third patterns, and finally curing the structure so formed.

An electrically conducting ink material is defined as a thermal ultraviolet or infra red curable liquid binder material having a proportion of a particulate metallic filler material sufficient to impart adequate electrical conductivity to the cured material for use as in electrical circuits. Typically the binder material is a bisphenol dihydroxy diphenyl methane resin with an aromatic amine hardener and the particulate metallic filler is a silver flake material, though polyester, polyamide, phenol- resols or phenolics may be used to advantage.

An electrically insulating ink material is defined as a l.R. or ultra violet curable liquid binder material having a proportion of a particulate solid insulating filler material sufficient to impart adequate electrical insulation to the cured material for use in electrical circuits.

Typically the binder material is a bisphenol dihydroxy diphenyl methane resin with an aromatic amine hardener and the particulate insulating filler is a powdered silica material, or U.V. curable adducts.

In a preferred embodiment of the invention the substrate is a copper foil clad laminate having said first conductor pattern formed thereon by etching.

In yet a further embodiment of the invention the method includes the step of depositing in the apertures of the insulating pattern electrically conducting ink material to provide a substantially wholly planar surface of the deposition of the succeeding pattern of electrically conductive ink material.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:~ Figure 1 illustrates an etched copper foil laminate, Figure 2 illustrates a pattern of insulating ink material to be deposited on the laminate of Fig. 1, Figure 3 illustrates a pattern of conducting ink material to be deposited on the pattern of Fig. 2, Figure 4 illustrates a further pattern of conducting ink material to be deposited on the patterns of Figs. 2 and 3 together.

The drawings illustrate generally the steps required according to the invention to fabricate a coded rotary switch as a co-planar element incorporated into a P.C.B. support.

A single or double sided copper foil clad laminate is used. This will act as a support to the electrical circuit and provide a means by which component attachment by soldering can be achieved. The single sided board is lithographically etch resist printed and then etched according to traditional subtractive copper techniques to produce a base pattern, Fig. 1, of conductive tracks and pads as shown in Fig. 1. The copper remaining must be oxide free and chemically clean.

An insulating pattern, Fig. 2, is then printed, the pattern being defined as so-called 'via' holes in the cured insulation layer. This is accomplished by screen printing the insulating ink material onto the etched copper foil laminate. This insulation pattern may be termed the "initial" pattern. This pattern is then cured as required by the particular ink constituency utilised. The exposed copper in the via holes may then be de-oxised prior to the next step.

Following the deposition and curing of the base pattern an intermediate pattern of electrically conducting ink material is deposited, Fig.

2, to fill the via holes and make a substantially planar surface for the next step. This intermediate layer is then cured.

After the deposition of the intermediate pattern as electrically conducting pattern of tracks and pads, Fig. 4, is screen printed. This pattern is then also cured.

Finally the remaining exposed copper areas are again de-oxised prior to component attach- ment by conventional soft soldering techniques e.g. wave or hand soldering.

It may be that further insulating and conducting layers are required to make up a multilayered printed circuit board. The same screen printing technique can be used, the number of satisfactory layers achievable being limited probably only by the thermo mechanical properties of the various layers, and any thermally induced mismatch between board inks.

For screen printing of the types of ink outlined in patent application No. 7903119 above a polyester screen will provide good prints over rough or undulating surfaces, such as are left by the etching of copper foil clad laminate or previous screen printing operations. If definition is not a problem a nylon screen may be used where deeper via holes have to be filled, e.g. 100 lim deep by 1 mm square holes.

This method can also be used to fabricate the equivalent of double sided multilayer boards, using single sided foil clad laminate as the raw stock. It would thus be possible to print keyboards with printed insulator and conductors forming crossovers on the same side of the board.

Whilst screen printing techniques have so far been referred to it is also possible to use other deposition techniques for the inks referred to. Thus it is possible to use a 'writing' technique in which the ink is dispensed from nozzles (preferably phenumatically fed) which traverse the surface of the substrate. Suitable nozzle dimensions would depend on the circuit track widths and spacings, but generally the nozzle will be approximately 10% smaller in diameter than the required track. For insulating layers a wider, specially shaped nozzle orifice may be more suitable. Minimum track dimensions are dictated only by the diameter ratio between nozzle and conductor filler used in the ink, typically a 2:1 ratio allows for a free flowing dispensing system, with minimum line widths of the order of 0.1 mm being possible.Nozzle dispensation of circuit elements has the additional advantage of not being susceptible to conductor breaks at and over irregularities such as the edges of via holes in an insulating layer.

The curing steps referred to above depend on the nature of the materials used. For a bisphenol A + amine hardener 30 minutes at 1 505C in a ventilated oven may be required.

On the other hand ultra violet curing materials will require only several seconds under an ultra violet lamp source, such as a 2 KW high pressure mercury lamp. In order that pinhole and other insulator defects in an insulating layer may be minimised it may be advantageous to deposit two insulating layers successively, either printed in different directions or, in the case of screen printing, using a second, different screen rotated through 90 When using a nozzle writing technique the large areas to be covered by the insulating layer can be covered by causing the nozzle to travel in a closely spaced meander pattern so that the ink tracks on successive traverses abut one another, or even overlap slightly.

Alternatively banks of individually controlled closely spaced nozzles can traverse the surface in parallel.

Claims

1. A method of making electrical printed circuits comprising the steps of forming on a substrate successively a first pattern of an electrically conducting material, a second pattern of an electrically insulating ink material, as hereinbefore defined, and a third pattern of electrically conducting ink material, as hereinbefore defined, said second pattern having apertures therein whereby electrical contact may be established between the first and third patterns, and finally curing the structure so formed.

2. A method according to claim 1 wherein the substrate is a copper clad laminate having said first conductive pattern formed thereon by etching.

3. A method according to claim 1 or 2 wherein electrically conducting ink material is deposited in the apertures in the insulating pattern to form a co-planar surface therewith prior to the deposition of the succeeding pattern of electrically conductive material.

4. A method according to any preceding claim including further alternating patterns of insulating and conducting ink material respectively.

5 A method according to any preceding claim wherein the ink materials are deposited by a screen printing technique.

6. A method according to any one of claims 1 to 4 wherein the ink materials are deposited by writing from a pressurized nozzle which traverses the surface on which the ink is to be deposited.

7. A method according to claim 6 wherein an insulating pattern is deposited by a closely spaced meander traverse of the nozzle.

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

9. An electrical printed circuit structure when made by the method of any preceding claim.

1 0. A coded rotary electrical switch including a printed circuit made by the method of any one of claims 1 to 8.

11. A keyboard printed circuit structure including printed insulators and conductors made by the method of any one of claims 1 to 8 which form crossovers on the same side of the board.