Thick film heating elements

Abstract

A multi-layer thick film heating element comprises a substrate 1, 2, one or more heating tracks 3, 4, and one or more further tracks 6, 7, wherein the heating track(s) and further track(s) are mutuall

Description

Field of the Invention

[0001] The present invention relates to thick film heating elements and methods of manufacture. Background of the Invention

[0002] Thick film heating elements generally comprise one or more heating tracks that are screen printed as an ink or paste onto an insulating substrate and fired to form tracks of high electrical resistivity. Connecting tracks or pads may be printed, with a different type of ink or paste, and fired to form connecting tracks and pads of low resistivity.

[0003] The insulating substrate may be of an electrically insulating material, such as ceramic, or may be metallic with an insulating surface layer. Thick film heating elements with metal substrates are typically manufactured by applying an electrical insulating layer onto a metal substrate and subsequently forming heating tracks onto the surface of the insulating layer. The insulating layer can be a glass or ceramic material applied using a screen printing technique or a more conventional vitreous enamelling process. The metal substrate is most commonly stainless steel. The firing temperature and other characteristics of the insulating materials, heating tracks and pads have to be compatible with the characteristics of the metal. In addition, a protective ceramic or glass covering layer may added, for example by spraying or by screen printing, and subsequently firing.

[0004] Further details of thick film technology are described for example in White N. (2017) Thick Films pages 707-709 &712 in: Kasap S., Capper P. (eds) Springer Handbook of Electronic and Photonic Materials. The thick film paste may comprise an active material, a glass frit and an organic vehicle or carrier. The glass frit remains after firing and forms part of the structure of the thick film resistor. Hence, 'thick film' refers to a specific type of resistor with a characteristic structure and properties and is not merely a comparative term or a reference to a product when manufactured by a particular process.

[0005] The heating tracks and the connection pads comprise metal particles, typically silver, platinum or palladium or a mixture of two or more of these, and glass. They are applied to the insulating layer by screen printing in the form of a paste which is then dried and fired as described above. The connection tracks or pads overlap the heating tracks so as to provide a low resistance connection to the heating tracks. Connection pads may provide an electrical connection to an external power source.

[0006] Connection tracks may provide electrical connections between heating tracks or between separate sections of the heating track, as disclosed for example in EP-A-1905271. Using connecting tracks or links in this way allows heating tracks to be more densely packed on the heater surface without causing the tracks to follow paths with small radii, which would lead to current crowding along the inside of the track due to reduced electrical resistance compared with the outside; this can lead to overheating and failure at the inside of the track.

[0007] The resistance of a thick film heating track depends on the resistivity of the resistor material and the thickness, length and width of the track, as shown by the equations below. R = pxL / A R = resistance (Ohms) p = resistivity (Ohm.mm) L = track length (mm) A = track cross section area (mm2) R = pxL / (wxt) w = track width (mm) t = track thickness (mm) For a given track material and thickness: p / t = K K = constant (Ohms) It follows that: R=KxL / w The ratio L / w is dimensionless and can be thought of as the number of squares which constitute the track.

[0008] When designing the layout of thick film resistor tracks, the number of squares is used. The designer has to determine of the optimum length of sides of the squares. The larger the squares, the more resistor track material is used. The material is typically made of silver, platinum or palladium and therefore is relatively expensive. To minimise the cost a small square is desired. The result is a short, narrow track. However, the service life of the heater is determined by the temperature of the heating tracks during use. This is a function of the application of the heater and the area of the heating track.

[0009] The ratio of power to track area, referred to as power density expressed in W / cm2, is an important design parameter. Limiting the power density can result in a long, wide track. The power density can be reduced by reducing the gaps between the tracks. The screen printing process and the electrical insulation required between adjacent sections of track limit the minimum gap width. It can therefore be difficult to design a heating track which provides a low power density and an acceptable gap between adjacent sections of heating track.

[0010] It may also be desirable to produce a thick film heater with uniform power distribution across the surface. Typically stainless steel is used as the substrate for thick film heating elements, but stainless steel is a relatively poor thermal conductor so that lateral conduction of heat from the heating tracks to the portion of the heater between the tracks is low, resulting in uneven heating. One solution to this problem is to attach a diffuser plate to the thick film heater as disclosed in EP-A-1177708, in which an aluminium diffuser plate is brazed to the stainless steel substrate. However the brazing of the two dissimilar metals is difficult because of the formation of inter-metallic compounds which can produce brittle joints. The resulting aluminium heating surface is not acceptable for some applications; for example is not desirable to have aluminium in contact with food. In other applications a hard wearing surface may be required.

[0011] In some applications is desirable to divide the heated area into a plurality of zones which can be controlled independently of each other. Separate connections to each zone are needed; it may be possible to have one common connection between the zones, but at least one separate connection is required for each zone. In conventional heaters these connections take up valuable space on the heater and produce unheated areas on the heater which may be detrimental to the operation of a device incorporating the heater.

[0012] It may be desirable to include a sensor on the surface of the thick film heater, such as a temperature sensor which changes in resistance with temperature, with either a positive or negative temperature coefficient. The sensor may be a discrete component which is attached to the heater or printed onto an insulating layer on the heater, in the same way as the heating tracks. An electrical connection to the sensor may be produced by printing and firing. The sensor and in particular the connection to the sensor takes up space on the heater. Summary

[0013] Aspects of the invention are defined by the accompanying claims.

[0014] At least some embodiments of the invention, one or more connections to or between one or more heating tracks are printed on different layers from the heating track(s), separated by one or more layers of insulating material. The separating insulating material may be similar to the insulating material applied to the metal substrate. This allows the heating track(s) and the connection(s) to overlap when viewed orthogonally to the heater surface.

[0015] The electrical connection(s) may overlap at least part of the heating track(s) when viewed orthogonally to the heater surface and may connect to the heating tracks(s) through one or more apertures in the separating insulating layer to allow the electrical connection between the tracks. The aperture(s) may be produced during the process of printing the insulating layer.

[0016] In embodiments including at least one sensor, such as a positive or negative coefficient temperature sensor, one or more connections to the sensor(s) may be printed on a different layer from the heating track(s). If the sensor is a discrete component, one or more apertures may be provided in the insulating layer, through which the connection(s) may be made. If the sensor is printed, this can either be on the same layer as the connection(s) or on the same layer as the heating track(s). If the sensor is on the same layer as the heating track(s), an aperture in the intermediate insulating layer is provided for the connection between the sensor and a connection track for connection to the sensor.

[0017] In some embodiments, a thick film heater includes a plurality of heating tracks printed as different layers separated by one or more intermediate electrically insulating layer and connected to each other or to an external power source either through one or more apertures in the intermediate insulating layer(s) or around an edge of the intermediate insulating layer(s), for example by extending the tracks beyond the edge(s) of the intermediate insulating layer(s). One or more connecting tracks or pads of low resistance material may be used to make the connections; these may reduce the voltage difference between adjacent heating tracks to reduce the possibility of failure of the insulating layer between the adjacent heating tracks. Brief Description of the Drawings

[0018] Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an exploded view of a conventional two zone thick heater. Figure 2 is a plan view of the thick film heater of Figure 1. Figure 3 is an exploded view of a two zone thick film heater in a first embodiment of the invention. Figure 4 is a plan view of the thick film heater of Figure 3. Figures 5a to 5e show the arrangement of heating tracks in a thick film heater in a second embodiment, in which: Figures 5a and 5b are plan views of first and second heating tracks respectively; Figures 5c-5e are respectively plan, exploded and cross-section views of the first and second heating tracks overlaid on each other. Detailed Description of Embodiments

[0019] Figures 1 and 2 show a conventional thick film heater or heating element comprising a substrate 1 with an electrically insulating layer 2 of enamel and first and second heating tracks 3, 4 formed on the substrate insulating layer 2. An electrical connection to one pole of the first heating track 3 may be made by a connection pad 5, and an electrical connection to one pole of the second heating track 4 may be made by a first busbar 6. Second poles of the first and second heating tracks 3, 4 are connected together by a second busbar 7. In this way, electrical power may be supplied independently to the first and second heating tracks 3, 4.

[0020] In this example, each of the first and second heating tracks 3, 4 comprises a plurality of heating track sections or portions separated by gaps and connected in series by connecting portions or sections of low electrical resistance, so as to avoid current crowding. The heating track sections or portions may be parallel and linear as shown, although other alternative configurations may be used depending on the geometry of the substrate 1 and the required heating area.

[0021] In a method of manufacture of the thick film heating element, the substrate insulating layer 2 is formed on the substrate 1 and the heating tracks 3, 4, connection pad 5 and busbars 6 and 7 are formed on the substrate insulating layer 2 using a thick film printing and firing process, for example as described above. An overglaze layer or layers (not shown) may be formed over the heating tracks 3a, 3b, leaving the contact pad 5 and ends of the busbars 6, 7 exposed.

[0022] The heating tracks 3, 4 have a high electrical resistance, for resistive heating, while the connection pad 5 and busbars 6, 7 typically have low electrical resistance.

[0023] The substrate 1 may be made of steel, such as ferritic stainless steel. Alternatively, the substrate 1 may be made of an insulating material such as ceramic, in which case the substrate insulating layer 2 may not be required and the thick film tracks or other parts may be printed directly onto the substrate 1.

[0024] As shown particularly in Figure 2, the busbars 4, 5 occupy part of the width of the substrate 1, which reduces the space available for the heating tracks 3, 4. Hence the heating density over the substrate 1 is limited, and the heating across the width of the substrate 1 may be uneven.

[0025] A first embodiment as shown in Figures 3 and 4 differs from the example of Figures 1 and 2 in that an intermediate insulating layer 10 is formed over the first and second heating tracks 3, 4, in a pattern that includes apertures lla-lld. The busbars 6, 7 are formed on the intermediate insulating layer 10 and connect to the first and second heating tracks 3, 4 through the apertures lla-lld. Where the thick film material of the busbars 6, 7 is printed over one or more of said apertures lla-lld, some of the thick film material passes through the respective aperture lla-d and makes an electrical connection to the portion of the first or second heating track 3, 4 beneath the aperture lla-d, at least after firing.

[0026] In this embodiment, no separate connection pad is necessary, but in other embodiments one or more connection pads may be printed over respective aperture(s) and thereby make a connection through the aperture(s) to the track(s) beneath. The first and second heating tracks 3, 4 may be connected together in series or in parallel, and may be independently or collectively switchable or connectable.

[0027] As shown in Figure 4, at least some portions of the busbars 6, 7 overlie or overlap parts of the heating tracks 3, 4, when viewed perpendicular to the substrate 1, rather than being arranged alongside the heating tracks 3, 4; this allows a higher proportion of the surface area of the substrate 1 to be used for heating, thus giving a higher and / or more even heating density across the surface area of the substrate 1.

[0028] The arrangement of the first embodiment may be reversed such that the busbars 6, 7 are formed on the substrate insulating layer 2 and the first and second heating tracks 3, 4 are formed on the intermediate insulating layer 10 so as to overlie at least part of the busbars 6, 7.

[0029] In a second embodiment of the invention, the first heating track 3 is formed on the substrate insulating layer 2 and the intermediate insulating layer 10 is formed over the first heating tracks. The second heating track 4 is then formed on the intermediate insulating layer 10. This arrangement allows the second heating track 4 to overlie at least part of the first heating track 3, thus increasing the heating density of the thick film heater.

[0030] In one particularly advantageous embodiment illustrated in Figures 5c-5e, heating track sections of the second heating track 4 overlie gaps between heating track sections of the first heating track 3. Preferably, the widths of the gaps between heating track sections of the first heating track are substantially equal to the width of the respectively overlying heating track sections of the second heating track 4. Hence, as shown in Figure 5c, a heating area of the substrate 1 is substantially evenly covered with the heating track sections of the first and second heating tracks 3, 4 so that the substrate 1 is evenly heated by the heating tracks 3, 4 without the need for lateral thermal conduction in the substrate 1, which may be limited in material such as stainless steel.

[0031] Connections to the heating tracks 3, 4 may be made by one or more busbars 6, 7 and / or connection pads 5, for example through apertures 11 in the intermediate insulating layer 10 as in the first embodiment, or in a further insulating layer.

[0032] The heating tracks 3, 4 may be arranged so that live connections are at the top and neutral connections at the bottom, with reference to Figures 5a and 5b; if the thick film heater is supplied by a DC voltage the connections would be positive and negative. The result is that the voltage between adjacent sections of the tracks respectively in the first and second heating tracks 3, 4 are a small fraction (in this specific embodiment, one seventh) of the total voltage across the heating tracks 3, 4. This reduces the risk of failure of the intermediate insulating layer 10.

[0033] In a third embodiment, not shown in the drawings, there is provided at least one heating track 3, 4 and at least one sensor, such as a positive or negative coefficient temperature sensor, with one or more electrical connections to the sensor(s) printed on a different layer from the heating track(s) 3, 4, the layers being separated by an intermediate 5 insulating layer 10. Where the sensor is a discrete component, one or more apertures 11 may be provided in the intermediate insulating layer 10, through which the electrical connection(s) may be made. Where the sensor is printed, this can either be on the same layer as the connection(s) or on the same layer as the heating track(s). Where the sensor is on the same layer as the heating track(s), an aperture in the intermediate insulating layer 10 is provided for 10 the connection between the sensor and a connection track for connection to the sensor.

[0034] In variants of the above embodiments, there may be more than two layers of thick film components, each separated by corresponding intermediate insulating layers.

[0035] Alternative embodiments which may occur to the skilled person on reading the above description may nevertheless fall within the scope of the following claims. 17 06 25

Claims

1. A thick film heater comprising a substrate, one or more thick film heating tracks, and one or more further tracks, wherein one or more portions of the thick film heating tracks are separated from one or more portions of the further tracks by an5 intermediate insulating layer, wherein said one or more heating tracks comprise afirst thick film heating track and the one or more further tracks comprise a second thick film heating track, and wherein each of the first and second thick film heating tracks comprises a plurality of heating track sections mutually separated by gaps, and at least some of the heating track sections of the first thick film heating track overlie10 respective gaps between track sections of the second thick film heating track,wherein the one or more further tracks comprises at least one sensor track.

2. Thick film heater of claim 1, wherein the one or more portions of the thick film heating tracks and the one or more portions of the further tracks mutually overlap.

3. Thick film heater of claim 1 or claim 2, wherein the one or more further tracks are15 electrically connected to the one or more thick film heating tracks.

4. Thick film heater of claim 3, wherein at least one said further track is electrically connected to one or more said thick film heating track through one or more apertures in the intermediate insulating layer.

5. Thick film heater of claim 3 or claim 4, wherein at least one said further track is20 electrically connected to one or more said thick film heating track around an edge ofthe intermediate insulating layer.

6. Thick film heater of any preceding claim, including a plurality of said thick film heating tracks interconnected by said one or more further tracks.

7. Thick film heater of claim 6, wherein the thick film heating tracks are connected in25 series.

8. Thick film heater of claim 6, wherein the thick film heating tracks are connected in parallel.17 06 259. Thick film heater of any preceding claim, further including at least one discrete sensor, where the further tracks comprise one of more connecting tracks for electrical connection to the at least one discrete sensor.

10. Thick film heater of any preceding claim, wherein the one or more thick film heating 5 tracks are formed on the substrate and the one or more further tracks are formed onthe intermediate insulating layer.

11. Thick film heater of any preceding claim, wherein the substrate comprises electrically conductive material having a substrate insulating layer formed thereon.

12. A method of manufacturing the thick film heater of any preceding claim, comprising10 forming the one or more thick film heating tracks on the substrate, forming theintermediate insulating layer over the one or more thick film heating tracks, and forming the one or more further tracks on the intermediate insulating layer.

13. A method of manufacturing the thick film heater of any one of claims 1 to 11, comprising forming the one or more further tracks on the substrate, forming the 15 intermediate insulating layer over the one or more further tracks, and forming theone or more thick film heating tracks on the intermediate insulating layer.

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