EP0410766A1

EP0410766A1 - Improvement in or relating to electrical devices incorporating electrical connectors

Info

Publication number: EP0410766A1
Application number: EP90308227A
Authority: EP
Inventors: Colin Ivan Terry
Original assignee: Thorn EMI PLC
Current assignee: Thorn EMI PLC
Priority date: 1989-07-26
Filing date: 1990-07-26
Publication date: 1991-01-30
Also published as: JPH03149792A; CA2021937A1; AU5983690A; GB8917077D0

Abstract

An electrical device comprises a substrate 2,4 supporting a film-type track 8, such as a heater track formed by depositing and curing a conductive polymer ink. A connector member 5 is provided which is bonded to the substrate 2,4, by means of a suitable adhesive 7, such as epoxy. The film-type track 8 is arranged to overlie at least an end portion 6 of the connector member 5 to provide a low electrical resistance contact therebetween. The arrangement permits the connector member to be mechanically bonded directly to the substrate 2,4, and not via the film-type track 8. The device may be used as a heater for mirrors, such as in car mirrors or mirrors for other applications, such as dentistry, where misting of the viewable mirror surface can be expected to occur.

Description

The present invention relates to electrical devices in the form of electrically resistive tracks made from conductive inks, especially inks comprising a polymeric binder and an electrically conductive constituent, and in particular to such a track for use as a heating track. The present invention also provides a novel method of making an electrical connection to such an electrically resistive track and is particularly applicable to the conductive polymer ink disclosed and claimed in our co-pending British Patent Application No. 8917078.1.
Conductive polymer inks are known in the art, eg as disclosed in Japanese Patent Application No. 60-218689 laid open under No. 62-74975 (Nihon Shastin Insatsu) which relates to electrically conductive inks for use in manufacturing printed circuit boards and hybrid boards for electronic parts. The document discloses that generally inks in which the binder is a thermoplastic resin show no problems with electrical characteristics or suitability for printing onto a substrate. However, when an attempt is made to solder an electrical connection to the printed electrical part, solder bubbles and blisters can be produced on the surface of the printed electrical part making it unsuitable for its intended use. The solution to this problem proposed in Japanese application 62-74975 is to use a thermoplastic resin chosen from a group termed 'heat resisting engineering plastics' which are heat-resistant and so to which soldered connections can be made. However, 'heat-resisting engineering plastics' are not produced in high volume and consequently are relatively expensive.
For electrical devices formed from conductive polymer inks, which have a relatively low melting point, the methods available for making an electrical connection to the device are further limited. In one example of a prior art connector system for an electrical device formed from such an ink printed onto a supporting substrate, an eyelet is fixed through the electrical part formed from the polymer ink and through the substrate on which the electrical part is printed. However, such a connector system can only be used with substrates through which suitable holes can be made and, furthermore, it can be difficult to ensure that a low resistance connection to the device has been achieved.
The requirements for a good connector system are:

1) A low resistance connection between the connector and electrical part;
2) A good bond strength between the connector and the substrate on which the electrical part has been printed.

The present invention seeks to provide an electrical device in the form of a film-type track in which both of the above requirements are achieved.
Accordingly, there is provided an electrical device comprising an electrically resistive film-type track supported on a substrate, and a connector member for providing an electrical connection to the film-type track, wherein the connector member is bonded to the substrate, thereby to provide a mechanical bonding connection between the connector member and the substrate, and the film-type track is arranged to overlie at least a portion of the connector member.
Preferably, the film-type track comprises a conductive polymer ink including a polymeric binder and an electrically conductive constituent.
Advantageously, the connector member comprises a sheet-like metallic element which, preferably, comprises nickel.
The film-type track may comprise a heating element and the connector element may be adhesively bonded to the substrate, such as by epoxy resin.
The substrate may comprise a mirror which, for example, may be an automotive vehicle mirror or a mirror for use in dentistry.
The present invention also provides a method of forming an electrical connection to a film-type track supported on a substrate, the method comprising bonding a connector member to the substrate and forming the film-type track on the substrate in a position such that the film-type track is arranged to overlie at least a portion of the connector element bonded to the substrate.
The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 shows a schematic cross-sectional representation of an electrical device in accordance with the present invention; and
Figure 2 shows a schematic cross-sectional representation of a heatable mirror incorporating an electrical device as shown in Figure 1.

Figure 1 shows a first substrate 2 and a second substrate 4 formed of insulating material. The second substrate 4 is required if the first substrate 2 is electrically conductive. A connector member 5 is provided which, in the embodiment shown is a short length of sheet metal, eg of nickel, cut to shape and cleaned, one end 6 of which is fixed flat to the substrate 4 by a high strength adhesive layer 7, eg epoxy.
An electrical part, such as an electrically resistive film-type heater track 8 is supported on the second substrate 4 and arranged so as to overlap the end 6 of the connector member 5. As the adhesion of the connector member 5 to the substrate 4 is achieved solely by the adhesive 7, the strength of the mechanical bond achieved between the connector member 5 and the heater track is not important. It is only important to ensure that good low resistance electrical communication is achieved between the track 8 and the connector member 5. The heater track 8 can be deposited by a variety of methods, such as screen-printing.
A low contact resistance between the heater track 8 and the connector member 5 is achieved by ensuring that a sufficient area of the connector member 5 is overlapped by the heater track 8. Thus the resistance between the connector member 5 and the track 8 is considerably less than the resistance of the heater track 8, alleviating the risk of 'burn out' due to local overheating at the point of connection. The track material so deposited is then dried or set. Finally, the distal end 9 of the bonded connector member 5 is shaped to produce a tag contact to which electrical supply wires can be fixed, such as by soldering or crimping.
In contrast to known connector systems, the present invention appreciates that a good mechanical bond strength between a connector and electrical part, such as the heater elements, can be achieved by adhering the connector directly to the substrate instead of via the heater track or other electrical part. The invention is particularly applicable to systems in which the electrical part to which connection is made is formed from a conductive polymer ink as it obviates the need for soldered connections to the electrical part and can be used on substrates, flexible or rigid, in which it is not possible to use eyelet connectors.
The present invention makes possible the manufacture of units in which a heating track formed from a conductive polymer ink is applied to a rigid substrate such as a mirror 10, as shown in figure 2.
Known units for heating mirrors are relatively costly and complex to manufacture. In one method, widely used in the car industry, a laminated assembly is applied to the rear of the mirror substrate. This laminated assembly consists of a sandwich of polymer sheets between which are a multitude of resistors formed from bands of high resistance pastes. A number of silver interdigitative fingers connect the resistors in parallel giving a net resistance of the required value. In another method, conductors and resistors are formed from an etched foil material on the rear surface of the mirror.
As can be seen from Figure 2, the mirror 10 comprises a glass layer 12 coated with an aluminium reflective layer 14. The aluminium layer 14 is covered with a film 16 of thickness 0.08mm. The film 16 is made of a material such as a pressure sensitive polyester film which will withstand temperatures of up to around 120^oC. The film 16 serves two purposes: a) to provide an insulating layer between the aluminium layer and a heater track; b) (in the case of car mirrors) to provide a layer which prevents the glass layer 12 from fragmenting if it should be broken.
Connectors 17, only one of which is shown in Figure 2, are adhered to the film 16 after which a conductive polymer ink is deposited onto the film 16 by a printing process, such as screen printing. The resistance of the track provided and hence its heating effect can be varied considerably by choice of track material and design. The ink is then dried or set to form an electrically conductive track 18 having the required resistance. An insulating top layer 20 may be applied if necessary which can serve to protect the conductive track lB and also to improve the heating effect of the track 18 on the glass layer 12, i.e. the visible surface of the mirror.
A suitable polymer ink for screen printing to provide a heating element is disclosed in our co-pending British Patent Application No. 8917078.1. Preferably, this ink contains equal volumes of nickel powder and carbon black in a binder comprising a solution of poly (vinyl butyral) in N-methyl-2-pyrrolidone. The screen printing medium used is Blythe Medium type 63/182 combined with unaccelerated unsaturated polyester in styrene.
By adopting the principles of the present invention a heatable mirror may be provided in which the heating element and the mirror are in much closer contact than in known arrangements. Thus the heating effect of the heating element on the mirror is improved. In addition, the mass of material constituting the heating element and, therefore, the mass of material to be heated (other than the mirror) is reduced, hence providing a faster response. In comparison with prior art heatable mirrors, the heatable mirrors of the present invention are able to provide the same heating performance, in terms of ultimate thermal gain, in one third of the time using one half of the power.
Although the present invention has been described with respect to a particular embodiment it should be realised that modifications may be effected whilst remaining within the scope of the invention. For example, other forms of connector member may equally be adopted, as can other forms of conductive ink.

Claims

1. An electrical device comprising an electrically resistive film-type track supported on a substrate, and a connector member for providing an electrical connection to the film-type track, wherein the connector member is bonded to the substrate, thereby to provide a mechanical bonding connection between the connector member and the substrate, and the film-type track is arranged to overlie at least a portion of the connector member.

2. An electrical device according to claim 1 wherein the film-type track comprises a conductive polymer ink including a polymeric binder and an electrically conductive constituent.

3. An electrical device according to claim 1 or claim 2 wherein the connector member comprises a sheet-like metallic element.

4. An electrical device according to any one of claims 1 to 3 wherein the connector member comprises nickel.

5. An electrical device according to any one of the preceding claims wherein the film-type track comprises a heating element.

6. An electrical device according to any one of the preceding claims wherein the connector element is adhesively bonded to the substrate.

7. An electrical device according to claim 6 wherein the connector element is adhesively bonded to the substrate by epoxy adhesive.

8. A method of forming an electrical connection to a film-type track supported on a substrate, the method comprising bonding a connector member to the substrate and forming the film-type track on the substrate in a position such that the film-type track is arranged to overlie at least a portion of the connector element bonded to the substrate.

9. A method according to claim 8 wherein the connector member is bonded to the substrate using an epoxy adhesive.

10. A mirror assembly comprising an electrical device according to any one of claims 1 to 7 or fabricated by a method according to claim 8 or claim 9.