IMPROVEMENTS TO THICK FILM ELEMENTS

Abstract: A method of manufacturing a thick film resistive heating element (1) comprising the application of one or more thick film resistive tracks (3, 3A, 3B) to the surface of an electrically insulative substrate (2) includes the application of at least one compensating thick film resistive track (6) to the surface of the substrate (2) which is electrically disconnected from said one or more thick film resistive tracks (3, 3A, 3B) at the time of application. By this means, when the resistance of said one or more resistive tracks (3, 3A, 3B), excluding the one or more compensating tracks (6), are measured should this measured resistance vary from a predetermined required value by more than a permitted tolerance then at least one of the compensating resistive tracks (6) can be electrically connected to said one or more resistive tracks (3, 3A, 3B) to bring the total resistance of the tracks (3, 3A, 3B, 6) within the permitted tolerance range. Preferably, each compensating thick film track (6) is printed in parallel with said one or more of the compensating tracks (3, 3A, 3B) whereby electrical connection of one or more of the compensating tracks (6) to said one or more resistive tracks (3, 3A, 3B) decreases the resistance of the electrically connected resistive tracks (3, 3A, 3B, 6) as a whole.

Description: IMPROVEMENTS TO THICK FILM ELEMENTS The present invention relates to thick film resistive heating elements such as can be used particularly but not exclusively in liquid heating appliances such as water boilers, kettles and the like. Conventionally, thick film resistive heating elements are manufactured by printing a thick film resistive track on to the surface of an electrically insulative substrate. The length, width and thickness of the printed track and the resistivity of the track material determines its resistance. However, whilst the length and width of the track are relatively simple to control, the thickness is more difficult to control, for example because of irregularities in the surface of the underlying substrate. As a result, variations in the printing and resistivity of such thick film resistive tracks can cause the measured resistance of any given track layout to vary by up to ± 15% from a desired level. British Standards and other similar standards permit resistive tolerances of -5% and +10% for elements when fitted to domestic appliances such as water boilers and kettles. Equally, other standards are set for elements in other industrial or domestic apparatus. However, it will be appreciated that given the variance of the resistance of thick film resistive elements from desired levels, a high wastage of manufactured elements which do not meet the required standards can result. The object of the present invention is provide a thick film resistive heating element and a method of producing same which overcomes or substantially mitigates the aforementioned problem. According to a first aspect of the present invention there is provided a method of manufacturing a thick film resistive heating element comprising the steps of applying one or more thick film resistive tracks to the surface of an electrically insulative substrate; applying at least one compensating thick film resistive track to the surface of the substrate but electrically disconnected from said one or more thick film resistive tracks at the time of application; measuring the resistance of said one or more resistive tracks excluding the one or more compensating tracks; determining whether the measured resistance varies from a predetermined required value by more than a permitted tolerance; and electrically connecting at least one of the compensating resistive tracks to said one or more resistive tracks to vary the resistance of the tracks in the event of the measured resistance falling outside said permitted tolerance. Preferably, each compensating thick film resistive track is printed in parallel with said one or more resistive tracks whereby electrical connection of one or more of the compensating tracks to said one or more resistive tracks decreases the resistance of the electrically connected resistive tracks as a whole. Preferably also, the width, length and thickness of said one or more thick film resistive tracks are predetermined so that the measured resistance will on average be higher than the predetermined required value, and the width, length and thickness of each compensating track is predetermined so that its resistance is on average such that when electrically connected in parallel to said one or more thick film resistive tracks the measured resistance of the electrically connected resistive tracks as a whole will only vary from the predetermined required value by an amount no more than the permitted tolerance. Preferably also, the method comprises the additional step of applying an encapsulating insulating layer over all the resistive tracks and the substrate to protect and insulate same. According to a second aspect of the present invention there is provided a thick film resistive heating element comprising at one or more thick film resistive tracks applied to the surface of an electrically insulative substrate and characterised in that at least one compensating thick film resistive track is applied to the surface of the substrate in parallel with said one or more resistive tracks but electrically disconnected therefrom at the time of application. An example of the present invention will now be described with reference to the accompanying drawing which shows a thick film resistive heating element according to the invention in combination with a temperature sensitive control device. A thick film resistive heating element 1 is formed by applying a first dielectric adhesion layer to a rigid substrate 2, such as a stainless steel plate, the adhesion layer being selected to have a coefficient of thermal expansion approximately equal to that of the steel. One or more further separate coatings are then separately applied such that the final coating has a coefficient of thermal expansion approximately equal to a thick film ink. A thick film circuit layout is then applied by silk- screen printing in which a conductive track 3 constituting the heating element is printed. The track is preferably formed of palladium silver but may alternatively be made of other conducting materials such as nickel, platinum, silver, or carbon, for example. Preferably, the track 3 follows a tortuous path of predetermined length over the majority of the area of the substrate 2 to maximise the heated area of the element 1. At its ends, the track 3 terminates in respective contact portions 4 and 5 which are adapted to make electrical connection with an electrical control device for the element 1. Thick film resistive tracks such as the track 3 are usually deposited on the insulated substrate 1 at a constant thickness of between approximately 10 to 14 microns. However, the width of the track may be varied to vary its resistance. Its resistance is reduced by increasing the width of the track and correspondingly increased by reducing the width of the track. In this particular example, and as shown in the drawings, the track 3 is formed by a pair of parallel tracks 3A and 3B. It will be appreciated that for any given circuit layout, a track 3 can be printed with a predetermined length and width and with a predetermined rate of application of the resistive ink from which it is formed. However, inaccuracies in printing and irregularities in the surface of the dielectrically coated substrate can locally affect the width and thickness of the track 3. Overall, therefore, the resistance of the track 3 will tend to vary from a predetermined value calculated from an ideal length, width and thickness of track. Some methods of applying the dielectric layers to the substrate 2 are better than other in producing a smooth surface on which the track 3 is then printed. However, even with the best methods of production, it is difficult to produce consistently dielectrically coated substrates that can be accurately printed with resistive tracks, which have measured resistances which are always within permitted tolerances. To this end, a thick film resistive heating element 1 according to the present invention is additionally printed with at least one compensating thick film resistive track 6. Each track 6 is applied to the surface of the substrate in parallel with the tracks 3A and 3B but electrically disconnected therefrom at the time of application by means of a gap 7. It will be appreciated that the track 6 and the tracks 3A and 3B are all printed at the same time but, as will be described, the gap 7 is thereafter either closed or left open. After printing of the tracks 3 and 6, the electrical resistance of the track 3 comprising the tracks 3A,3B is measured. It can then be determined for each element 1 whether the measured resistance varies from a predetermined ideal value based on the length, width and thickness of the tracks 3 presumed to have been printed by more than a permitted tolerance. As previously mentioned, typically the resistance of such thick film resistive tracks can vary by up to + 15% from a desired level but safety and other standards can require resistive tolerances no more than - 5% and + 10% from a fixed value. If it is found that the measured resistance of the track 3 is within the permitted tolerances, then the gap 7 can be left open. However, if the measured resistance is found to be higher than the permitted tolerances, the gap 7 can be closed to decrease the overall resistance of the tracks 3. The gap 7 is closed by screen printing a short section of the track 6 with the same thick film ink as used previously for the tracks 3 and 6. If more than one compensating track 6 has been printed initially, then as many of these as required can be completed to decrease the overall resistance of the track 3 to within the tolerance required. It will be appreciated that although one or more compensating tracks 6 could be printed to be connected in series with the tracks 3A,3B to increase the overall resistance of the track 3 when electrically connected thereto, this is unlikely to be practical in most circumstances because the contact portions 4 and 5 require special treatment during production of the element and must usually always be placed in a fixed position for connection to the control device. Hence for this reason, the track or tracks 6 are normally printed in parallel with the tracks 3A, 3B to reduce their resistance. To this end, the expected average resistance of the track 3 based on its ideal thickness, width and length can be arranged to be slightly higher than the required value. Likewise, the width, length and thickness of each compensating track 6 can be predetermined so that its resistance is on average such that when electrically connected in parallel with the tracks 3A, 3B the measured resistance of the resistive track 3 as a whole will only vary from the required value by an amount no more than the permitted tolerance. As shown in the drawing but not of particular relevance to the present invention, at least part of the tracks 3A,3B can be split into a plurality of thinner parallel tracks 8. This spreads the temperature of the tracks 3A and 3B locally and is useful where the element 1 is to be used in conjunction with an electromechanical control device 9. The effect of such track splitting can, however, be taken into account when determining the required resistance of such an element. After the gap or gaps 7 have been either closed or left open, an encapsulating insulating layer can then applied over the completed circuit and the substrate to protect the circuit. However, this coating is interrupted in the regions of the contract portions 4 and 5 so that electrical connection can be made thereto. Hence the invention provides a simple means of improving the viability of thick film resistive heating elements by enabling them to meet required standards.

Claims: CLAIMS 1. A method of manufacturing a thick film resistive heating element (1) comprising the steps of applying one or more thick film resistive tracks (3,3A,3B) to the surface of an electrically insulative substrate (2) ; applying at least one compensating thick film resistive track (6) to the surface of the substrate (2) but electrically disconnected from said one or more thick film resistive tracks (3,3A,3B) at the time of application; measuring the resistance of said one or more resistive tracks (3,3A,3B) excluding the one or more compensating tracks (6) ; determining whether the measured resistance varies from a predetermined required value by more than a permitted tolerance; and electrically connecting at least one of the compensating resistive tracks (6) to said one or more resistive tracks (3,3A,3B) to vary the resistance of the tracks (3,3A,3B) in the event of the measured resistance falling outside said permitted tolerance. 2. A method as claimed in Claim 1, wherein each compensating thick film resistive track (6) is printed in parallel with said one or more resistive tracks (3,3A,3B) whereby electrical connection of one or more of the compensating tracks (6) to said one or more resistive tracks (3,3A,3B) decreases the resistance of the electrically connected resistive tracks (3,3A,3B) as a whole. 3. A method as claimed in Claim 1 or Claim 2, wherein the width, length and thickness of said one or more thick film resistive tracks (3,3A,3B) are predetermined so that the measured resistance will on average be higher than the predetermined required value, and the width, length and thickness of each compensating track (6) is predetermined so that its resistance is on average such that when electrically connected in parallel to said one or more thick film resistive tracks (3,3A,3B) the measured resistance of the electrically connected resistive tracks as a whole will only vary from the predetermined required value by an amount no more than the permitted tolerance. 4. A method as claimed in any one of Claims 1 to 3 , which comprises the additional step of applying an encapsulating insulating layer over all the resistive tracks (3,3A,3B,6) and the substrate (2) to protect and insulate same. 5. A thick film resistive heating element comprising at one or more thick film resistive tracks (3,3A,3B) applied to the surface of an electrically insulative substrate (2) and characterised in that at least one compensating thick film resistive track (6) is applied to the surface of the substrate (2) in parallel with said one or more resistive tracks (3,3A,3B) but electrically disconnected therefrom at the time of application.

Inventors: LITTLE PETER JAMES, LITTLE, PETER, JAMES
Applicants: LITTLE PETER JAMES, PIFCO LTD, LITTLE, PETER, JAMES, PIFCO LIMITED
Application year: 1995
Main classification: H05B3
IPC codes: H05B3