HEATING UNIT

Abstract: An electrical connector (2, 4) for making a connection to a thick film track comprises a plurality of conductive fibres (3) braided together, each of the fibres having a diameter so as to provide sufficient stiffness to the connector and to permit adhesion of the connector to a thick film track.

Description: This invention relates to a heating unit comprising electrical connectors making connections to thick film conductor tracks applied to an insulative substrate. In particular, though not exclusively, this invention relates to such heating units, where the electrical connectors must have a high current carrying capacity to be able to carry the high power required of the heating element. It is known, for example from 'Hybrid Circuits', Number 15, January 1988, to make connections to thick film conductor tracks using gold or aluminium wire having a diameter of the order of tens of micrometers. The wire is welded to the track by ultrasonic welding. The use of copper wire, also having a diameter of the order of tens of micrometers, has also been proposed. However, such connectors are expensive and do not have sufficiently high current carrying capability and temperature stability. It is an object of the present invention to provide a heating unit in which at least some of the aforementioned problems are alleviated. According to the present invention there is provided a heating unit comprising a thick film track applied to an electrically insulative substrate, and an electrical connector connected to the thick film track by an adhesive, the connector comprising a plurality of conductive fibres braided together, the diameter of each fibre being in the range of from 10µm to 300µm. The inventor has found that connectors provided in accordance with the invention can be bonded to a thick film heater track using a suitable adhesive and producing a strong bond. This high bond strength is thought to be due to the high surface area of the connector, provided by the plurality of fibres, resulting in excellent wetting by the adhesive. Furthermore the plurality of fibres provides a structure with a sufficiently large cross-sectional area for a required current-carrying capability but with a degree of flexibility to be effective as a connector. The cross-sectional area of the connector is preferably greater than the cross-sectional area of the track. The electrical and mechanical properties of the connector are determined by the number of fibres, which could be of the order of thousands, the diameter of the fibres and the material of which they are made. Various metals may be used to make the connector, the most suitable metal for a particular application depending in part on the material of the thick film track to which the connector is to be adhered. Suitable metals include stainless steel, nickel and copper. An advantage of stainless steel is that it is not easily oxidised, an important consideration in view of the fineness of the individual fibres. The temperature stability of the connector is determined by the resistance to oxidation of the connector material - a stainless steel (430 grade) braided connector is stable to at least 750°C. The current-carrying capability of an electrically conductive structure is determined at least in part by its cross-sectional area. Hence, in a heating unit provided in accordance with the present invention, the current-carrying capability of the connector is less likely to be a limiting factor. Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings of which: Figure 1 shows one end of a connector provided in the heating unit of the present invention; and Figure 2 shows part of a heating unit incorporating such a connector. In one embodiment, the connector is stainless steel and formed of 1000 individual fibres, each fibre being 12 µm in diameter. Such a connector has a very low resistance and is capable of carrying a current of tens of Amperes at 240V.A.C. Figure 1 shows one end of a connector 2 and illustrates how the fibres 3 are braided together. A glass/metal adhesive can be used to adhere the connector to the thick film track. Advantageously, the adhesive may be the same conductive ink as used to form the thick film track. This permits the same firing process to be used to set the adhesive and form the track. An example of a suitable adhesive for a nickel thick film track comprises nickel and glass in the proportion of 80% to 20% by weight. A suitable, but non-limiting, method of forming a bond between a connector and a thick film heater track will now be described hereinafter. A conductive ink is formed comprising nickel and glass in the proportion of 80% to 20% by weight together with a suitable screen printing medium such as ESL 400. This ink is screen printed in the desired pattern onto a substrate. A braided connector formed of a plurality of stainless steel fibres, each fibre being of diameter 12 µm, has one end flattened. This end is dipped into the aforementioned conductive ink to be wetted by the ink and is then held in position on the track pattern using a suitable positioning means such as a jig. The whole is then dried at 150°C and fired at a temperature in the range of from 750°C to 1100°C, producing a connector bonded to a thick film track. The firing procedure is preferably carried out in a nitrogen atmosphere to prevent oxidation of the metal. The jig may hold a number of connectors in position on the track pattern during the manufacturing process. Further electrical connections can be made to the connector by crimping, soldering or welding another piece of wire onto the other end. The connector may be insulated using a glass or ceramic sleeve.

Claims: A heating unit comprising a thick film track (8) applied to an electrically insulative substrate (10), and an electrical connector (2;4) connected to the thick film track by an adhesive (6), the connector comprising a plurality of conductive fibres (3) braided together, the diameter of each fibre being in the range of from 10µm to 300µm. A heating unit according to Claim 1, characterised in that the cross-sectional area of the connector is greater than the cross-sectional area of the track. A heating unit according to Claim 1 or 2, characterised in that the diameter of each fibre is 12µm. A heating unit according to any one of the preceding claims, characterised in that the connector is formed of stainless steel.

Inventors: HONOUR, ANDREW MICHAEL
Applicants: THORN EMI PLC
Application year: 1989
Main classification: H01R4
IPC codes: H01R4, H01B5, H05K3