WO1991009511A2

WO1991009511A2 - Electrical conductors of conductive resin

Info

Publication number: WO1991009511A2
Application number: PCT/GB1990/001973
Authority: WO
Inventors: John Michael Lowe
Original assignee: Technology Applications Company Limited
Priority date: 1989-12-19
Filing date: 1990-12-18
Publication date: 1991-06-27
Also published as: AU6966191A; WO1991009511A3; GB8928640D0

Abstract

Methods of applying an electrical conductor of conductive epoxy resin comprise applying the resin to a substrate, applying a resist over part of the conductors (14) leaving exposed contact pads (16), applying e.g. by printing polymer ink, temporary conductors (20) to the substrate (10) connected to the areas (16), immersing the substrate in an electroplating bath and passing an electric current through the conductors (2) to deposit a conductive layer of metal to which solder will adhere on the contact pads (16). The conductors (20) are then removed. In through hole plating to connect conductors on opposite sides of the substrate (10) a first resin layer (26) is applied over a hole (24), resin material (26a) blown into the hole (24) and a second conductive resin layer (28) applied on the opposite surface of the board and material (28a) from that second layer blown into the hole (24) into contact with the material (26a) providing an electrically conductive path through the hole (24).

Description

ELECTRICAL CONDUCTORS OF CONDUCTIVE RESIN

This invention is concerned with electrical conductors of conductive resin, in particular epoxy resin , and especially methods of providing such electrical conductors on a substrate for example a printed circuit board.

In the manufacture of printed circuit boards it is necessary to provide conductors on the surface of the board. One way in which it has been proposed to apply electrical conductors to the surface of the substrate, for example a printed circuit board, is by printing, for example screen printing, of suitable materials. Certain electrically conductive polymer inks, for example silver loaded, have proved to be satisfactory and can be applied to a surface in desired patterns by a screen printing process whilst in a fluid form, the ink subsequently being cured to a solid form. However, such conductive inks are unable to withstand elevated temperatures met during the manufacture of printed circuit boards, for example in soldering e.g. in wave- soldering apparatus or when a board is passed into reflow soldering apparatus. Such conductive inks are only able to withstand temperatures of up to about 180°C, above which temperature there is a tendency for the conductors formed of such ink to be destroyed. Electrical conductors can also be printed in desired patterns, e.g. by screen printing, from so-called "conductive resins" and especially conductive epoxy resins. Materials currently available known as conductive epoxy resins are in fact epoxy resin materials loaded with a sufficient quantity of finally divided conductive material, for example finely divided silver, to impart good conductivity to the cured epoxy resin material. Conductive epoxy resins cure from a viscous fluid state to a solid state when exposed to a temperature of about 200°C for about three seconds and will withstand temperatures as high as 300°C for a considerable period of time of up to about two hours and temperatures of about 250°C continuously; conductive epoxy resins cure slowly in any event to a solid condition, for example they will reach a solid condition in about a week at room temperature. The temperatures to which the cured epoxy resins are resistant are sufficient that they will not be damaged by the temperatures normally encountered in manufacture of printed circuits and in particular will not be damaged by contact with molten solder, for example in wave soldering apparatus. Conductive epoxy resins have considerable mechanical strength, in addition, and may be used on substrates for printed circuits which substrates themselves will not withstand soldering temperatures. Conductive epoxy resins may also be used in conjunction with polymer inks which, as mentioned above, likewise will not withstand soldering temperatures. However, solder commonly used in printed circuit manufacture will not adhere satisfactorily to cured conductive epoxy resin materials which can cause problems in the use of printed conductors of conductive epoxy resin so that their utility is much reduced compared with the potential usage.

In order to print conductors which may be exposed to solder, polymer inks may be used and protected against direct contact with solder e.g. in wave-soldering apparatus, by coating them with a suitable protective compound eg. a resist or, where an electrical connection is to be made to the polymer ink, by applying to the connection region a conductive coating for example including carbon black, to protect the polymer ink. The carbon-including coating is used where an electrical pressure type contact is required. Such protective coatings will only give short-term protection against the effects of excessive heat and the polymer ink conductors axe still destroyed by exposure to elevated temperatures over a period of time. Such coatings will also protect against silver migration. However, if it is essential to make solder connections to silver-loaded polymers, it is possible to plate the contact with a suitable metal using an electroless plating process which is extremely difficult to control reliably

It is commonly necessary to make electrical connections between opposite surfaces of a circuit board, for example where circuits are printed on both sides of the board. For this purpose, holes have been drilled or otherwise formed through the board and connecting wires passed through and attached at either side or polymer inks (which are of low viscosity) have been caused to flow through the holes to make the necessary electrical connections. However, connecting wires are bulky and inconvenient to use and, as mentioned above, such polymer inks are adversely effected by elevated temperatures and thus neither connecting system is very satisfactory. Conductive epoxy resin materials, whilst being fluid are nevertheless of high viscosity and, so far as the applicants are aware, no method of making such through board connections utilising conductive epoxy resins have been proposed.

One of the various objects of the present invention is to provide improved methods of using conductive resin conductors.

In one aspect the invention may be considered to provide a method of making an electrical conductor of conductive resin to which solder connections can be made comprising applying conductive resin to which solder will not normally adhere to a substrate in a desired pattern, applying a layer of electrically insulating material over part of the pattern leaving exposed those areas of the pattern which are to be solderable, applying temporary conductors to the substrate connected with each of said areas, immersing the substrate in an electroplating bath, and passing an electric current through said temporary conductors whereby to cause a conductive layer to which solder will adhere to be deposited on said areas of the pattern.

Preferably in a method in accordance with the invention the resin is an epoxy resin and the conductive layer is a suitable metal, for example nickel. Conveniently the temporary conductors all lead to a connection portion which is also temporary, and to which an electrical supply may be connected to provide one electrode of the electroplating bath. Suitably the temporary conductors and connecting portion are formed by printing an electrically conductive ink in a suitable configuration.

After the said areas have been plated in carrying out a method in accordance with the invention, the temporary conductors are removed from the substrate; conveniently, where the temporary conductors are formed of electrically conductive ink, this is only partially cured before plating and in that event the temporary conductors may readily be wiped from the substrate. Alternatively, the temporary conductors may be removed by exposure to elevated temperatures to destroy them, e.g. when the substrate is passed through a wave-soldering process. Suitably the substrate is a printed circuit board.

In another aspect the invention may be considered to provide a method of making an electrical connection between opposite surfaces of a printed circuit board comprising forming a hole through the board, applying a layer of conductive resin to one surface of the board over said hole and, while the conductive resin is in a fluid condition, causing the resin to pass through the hole to provide an electrically conductive path to the opposite face of the board, the resin thereafter curing to a solid condition.

Preferably in carrying out a method as set out in the last preceding paragraph a first layer of conductive resin is applied to a first surface of the board and a portion thereof caused to pass into the hole, a second layer of conductive resin is applied to the second, opposite, surface of the board and a portion thereof caused to pass into the hole into contact with said portion from the first layer. Suitably the resin is caused to pass into the hole either by blowing or sucking it into the hole.

In another aspect the invention may be considered to provide a printed circuit comprising electrical conductors of conductive resin in a desired pattern on at least one surface of a substrate, selected areas of said conductors carrying a conductive metal layer to which solder is adherent. In yet another aspect the invention may be considered to provide a printed circuit board comprising electrical conductors of conductive resin on opposite surfaces of the board and at least one hole passing through the board through which conductive resin extends connecting conductors on the opposite surfaces of the board.

There now follows a detailed description to be read with reference to the accompanying drawings of a method of making a printed circuit board utilising electrically conductive epoxy resin. It will be realised that this method and the boards made thereby has been selected for description to illustrate the invention by way of example.

In the accompanying drawings:-

Figure 1 is a plan view showing part of a printed circuit board; and

Figure 2 is a view in section with part broken away on the line II-II of Figure 1.

In Figure 1 is shown a substrate 10 which suitably comprises a rigid printed circuit board 12 for example of polymerised epoxy materials containing glass fibres, or paper fibres mixed with synthetic resins, for example phenolic materials to produce a paper bound by impregnated synthetic resin, commonly known as synthetic-resin-bonded paper. The boards may if desired have an initial layer provided by a conventionally deposited copper sheet etched to provide copper conductors and subsequently having further conductive layers applied by printing conductive epoxy resin, or conductive epoxy resin conductors may be printed as a first layer, in a desired pattern by screen or other suitable printing techniques.

In Figure 1 a number of discrete conductors 14 are shown, each produced by screen printing using a conductive epoxy resin. The resin is cured after having been applied to the surface of the substrate 10 and thereafter a layer of electrically insulating material (commonly known as "resist" - suitable materials include cross-linking polymers sold under the Trade Mark "Photocoat" and various types are flexible, and curable by heat or ultraviolet radiation) is applied over parts of the conductor pattern leaving exposed areas of the pattern, namely contact pads 16. After the resist 18 (indicated in the drawing by stippling) has cured, a plurality of temporary conductors 20 (indicated in the drawings by dashed lines) are applied to the substrate, mainly over the resist layer 18 but each terminating at one of the contact pads 16. The temporary conductors are interconnected and ultimately lead to a connecting portion, namely a plating bar 22. The temporary conductors 20 and plating bar 22 are provided by a silver-loaded polymer ink (of composition known to those skilled in the art - a polymer thick film ink containing silver flakes) applied in a desired pattern by screen printing, so that electrical connections are made to each of the discrete conductors 14 by way of one of the contact pads 16. Where one of the discrete conductors 14 is long, more than one temporary conductor may be connected to that printed conductor 14.

The substrate 10 on which the temporary conductors 14 have been printed but only partially cured is then immersed in an electroplating bath, an electrical supply being connected to the plating bar 22 so that the temporary conductors 20 and the contact pads 16 of the pattern provide one electrode in the electroplating process.

The electroplating bath comprises a solution (the composition of which is well known to those skilled in the art and which is selected according to the metal to be deposited) which, when an appropriate current is passed, causes a conductive layer of a suitable metal, for example nickel to be deposited on the contact pads 16. The conductive layer deposited is chosen to be such that solder will adhere to the deposited layer when conventional soldering techniques are used and the deposited layer will not be damaged by exposure to the temperatures and conditions encountered in the manufacture of the printed circuits.

SUBS^TITUTESHEET NOT FURNISHED UPON FILING

is likewise caused to pass into the hole, for example by blowing until it contacts the material 26a blown into the hole from the first layer, thus creating an electrically conductive path through the hole 24 to the opposite face of the board. The surface of the conductive epoxy resin in the hole will also be plated during the electroplating process to provide a through hole plated connection from a printed circuit on one surface to a further printed circuit on the opposite surface of the board.

The method described by way of example provides a simple and efficient way of printing patterns of electrically conductive resin on substrates which are resistant to elevated temperatures, sufficient to permit normal soldering techniques to be used, the conductors so applied having contact pads to which solder will adhere in normal soldering processes and through-hole connection is readily accomplished.

Claims

1. A method of making an electrical conductor of conductive resin to which solder connections can be made comprising applying conductive resin to which solder will not normally adhere to a substrate in a desired pattern, applying a layer of electrically insulating material over part of the pattern leaving exposed those areas of the pattern which are to be solderable, applying temporary conductors to the substrate connected with each of said areas, immersing the substrate in an electroplating bath, and passing an electric current through said temporary conductors whereby to cause a conductive layer to which solder will adhere to be deposited on said areas of the pattern.

2. A method according to Claim 1 wherein the resin is a conductive epoxy resin.

3. A method according to either one of Claims 1 and 2 wherein the conductive layer is metal.

4. A method according to any one of the preceding claims wherein the temporary conductors all lead to a connection portion to which an electrical supply may be connected to provide one electrode of the electroplating bath.

5. A method according to any one of the preceding claims wherein the temporary conductors are formed by printing an electrically conductive ink in a suitable configuration.

6. A method according to any one of the preceding claims wherein the temporary conductors are wiped from the substrate after electroplating.

7. A method according to any one of Claims 1 to 3 wherein the temporary conductors are removed by exposure to elevated temperatures to destroy them.

8. A method according to any one of the preceding claims wherein the substrate is passed through a wave soldering process in which the solder is at an elevated temperature and the temporary conductors are removed thereby.

9. A method according to any one of the preceding claims wherein the substrate is a circuit board through which a hole is formed, a layer of conductive resin is applied to one surface of the board over said hole and while the conductive resin is in a fluid condition caused to pass through the hole to provide an electrically conductive path through the hole to the opposite surface of the board, the resin thereafter curing to a solid condition.

10= A method of making an electrical connection between opposite surfaces of a printed circuit board comprising forming a hole through the board, applying a layer of conductive resin to one surface of the board over said hole and, while the conductive resin is in-a fluid condition, causing the resin to pass through the hole to provide an electrically conductive path to the opposite face of the board, the resin thereafter curing to a solid condition.

11. A method according to either one of Claims 8 and 9 wherein a first layer of conductive resin is applied to a first surface of the board and a portion thereof caused to pass into the hole, and a second layer of conductive resin is applied to the opposite, second, surface of the board and a portion thereof caused to pass into the hole into contact with said portion from the first layer.

12. A method according to any one of Claims 8 to 10 wherein the resin is caused to pass into the hole by blowing it into the hole.

13. A method according to any one of Claims 8 to 10 wherein the resin is caused to pass into the hole by sucking it into the hole.

14. A printed circuit made by a method according to any one of the preceding claims.

15. A printed circuit comprising electrical conductors of conductive resin in a desired pattern on at least one surface of a substrate, selected areas of said conductors carrying a conductive metal layer to which solder is adherent.

16. A printed circuit board comprising electrical conductors of conductive resin on opposite surfaces of the board and at least one hole passing through the board through which conductive resin extends connecting conductors on the opposite surfaces of the board.