GB2576895A

GB2576895A - Thick film resistor

Info

Publication number: GB2576895A
Application number: GB1814445.1A
Authority: GB
Inventors: Johannes Kloppers Gradus
Original assignee: FERRO TECH BV; Ferro Techniek BV
Current assignee: FERRO TECH BV; Ferro Techniek BV
Priority date: 2018-09-05
Filing date: 2018-09-05
Publication date: 2020-03-11
Anticipated expiration: 2038-09-05
Also published as: GB201814445D0; GB2576895B

Abstract

A thick film resistor, such as a thick film heater 1, comprises a metal substrate 6 having first and second opposite surfaces, a first dielectric layer 5 applied on or over the first surface of the substrate 6, a thick film heating track 2 applied on or above the first dielectric layer 5, and a second dielectric layer 7 applied on or over the second surface of the substrate 6. The first and second dielectric layers 5,7 are in balanced compression within the normal operating temperature range of the resistor, so as to avoid or reduce stress on the substrate. This allows a thin or flexible substrate to be used. The resistor 1 includes temperature sensitive dielectric material having a resistivity that changes rapidly above a predetermined temperature, so as to allow a temperature condition of the resistor to be sensed. The first and second dielectric layers 5,7 may be formed of ceramic or enamel.

Description

(54) Title of the Invention: Thick film resistor

Abstract Title: Thick film resistor having stress balancing layers (57) A thick film resistor, such as a thick film heater 1, comprises a metal substrate 6 having first and second opposite surfaces, a first dielectric layer 5 applied on or over the first surface of the substrate 6, a thick film heating track 2 applied on or above the first dielectric layer 5, and a second dielectric layer 7 applied on or over the second surface of the substrate 6. The first and second dielectric layers 5,7 are in balanced compression within the normal operating temperature range of the resistor, so as to avoid or reduce stress on the substrate. This allows a thin or flexible substrate to be used. The resistor 1 includes temperature sensitive dielectric material having a resistivity that changes rapidly above a predetermined temperature, so as to allow a temperature condition of the resistor to be sensed. The first and second dielectric layers 5,7 may be formed of ceramic or enamel.

1/2

Fig. 1

2/2

Fig. 3

Thick film resistor

Field of the Invention [0001] The present invention relates to a thick film resistor, such as a thick film heater, and its method of manufacture.

Background of the Invention [0002] One type of electric heaters or heating elements are commonly referred to as thick film heaters. These heating elements generally comprise one or more heating tracks that are printed as an ink or paste onto an insulating substrate and fired to form tracks of high electrical resistivity. Connecting tracks may be printed in a separate layer, and fired to form connecting tracks of low resistivity. The insulating substrate may be of an electrically insulating material, such as ceramic, or may be metallic with an insulating surface layer. One example of such a heating element is disclosed in EP-A-0286217.

[0003] Thick film heating elements are typically used in liquid heating vessels, flow-through heaters, electric irons and other domestic appliances. In many of these applications, the substrate is typically a substantially flat steel plate with the tracks deposited on one side, to form a heating element.

[0004] The insulating layer of a thick film heating element is typically a glass or glass ceramic type of material. The insulating layer can be applied by a screen printing technique or by electrostatic spraying or any other suitable process. The insulating layer is then dried and fired to produce a homogenous electrically insulating layer firmly adhered to the substrate. Resistor tracks, sensor tracks and possibly other insulating layers may be added on top of the insulating layer.

[0005] EP-B-0848894 discloses a heating element having a dished portion with a concave face on which an insulating layer and heating element are provided. This ensures that the insulating layer is under permanent compression.

[0006] EP-A-1828068 discloses a temperature sensor, which may comprise the insulating layer described above, a conductive layer applied to the insulating layer and a second insulating layer applied to the conductive layer. The resistive heating tracks are then applied on top of the second insulating layer. The composition of the second insulating layer is selected so that at a predetermined temperature the resistance of the layer falls substantially. This change in resistance can be detected using a suitable electronic circuit connected between the resistive tracks and the conductive layer and the power to the resistor tracks may be disconnected in response to the detection, to prevent excessive temperatures being reached. This type of sensor is referred to by the proprietor of EP-A1828068 as Electronic Full Area Sensing Technology or E-FAST (TM).

[0007] EP-A-2163130 discloses an alternative temperature sensor arrangement, in which a sensor track is applied running substantially parallel to the resistive tracks. The sensor track and resistive tracks are covered with an insulating layer selected so that the resistance falls substantially at a pre-determined temperature.

Statements of the Invention [0008] According to one aspect of the present invention, there is provided a thick film resistor according to claim 1. According to another aspect of the present invention, there is provided a method of manufacture according to claim 18.

[0009] Embodiments of the present invention relate to a thick film heating element or thick film resistor comprising a metal substrate. The substrate may be steel, preferably ferritic stainless steel, but any other suitable metal may be used.

[0010] An insulating layer applied to the substrate may be strong when the applied stress is compressive but quite weak when subjected to tensile stress. It is preferable that the insulating layer of the finished heater is under compressive stress both at room temperature and at the resistor's operating temperature. This may be achieved by selecting the substrate material and the insulating material so that the coefficient of thermal expansion of the substrate is higher than that of the insulating layer. The result is that as the heating element cools after the insulating layer is fired the substrate contracts more that the insulating layer, hence applying a compressive stress to the insulating layer. The maximum operating temperature of the heating element is below the firing temperature used during the manufacturing process, so even at the maximum operating temperature the resulting stress is compressive.

[0011] In certain applications a thin and/or flexible substrate may be required, for example where there is a requirement for a heater or resistor of extremely low thermal mass or a heater or resistor with a degree of flexibility. If the cooling of the heater during the manufacturing process produces a compressive stress in the insulating layer there may be a tendency for the substrate to distort on cooling. The deformation may render the heater unable to be further processed by printing, drying and firing the resistive tracks, or result in a heater which is not planar.

[0012] In at least some embodiments, this issue is solved by applying a correcting layer or layers of insulating material or other material to the reverse side of the substrate to apply similar stresses to the substrate as the insulating layer, including any further layers on or above the insulating layer. This may be achieved by selection of the material (such as the coefficient of thermal expansion of the material), thickness and/or number of the correcting layer(s), which may differ from those of the insulating layer. The resultant heating element may be flat enough to be further processed and to be used in the intended fashion.

[0013] In the prior art, the thickness of a metal substrate used in thick film heaters is 1mm to 4mm. By adopting the above technique of applying a correcting layer or layers to the reverse side of the substrate, the thickness of the metal substrate can be reduced to 0.5 mm or less.

[0014] The heater or resistor includes temperature sensitive dielectric having a resistivity that changes rapidly above a predetermined temperature. This change in resistivity may be detected by detecting a leakage current through the temperature sensitive dielectric. The temperature-sensitive dielectric may be provided in a dielectric layer between a sensor layer and the heater track. Alternatively, the insulating layer between the heater track and the substrate may comprise material having a resistivity that changes rapidly above a predetermined temperature. In another alternative, a sensor track may be provided on running substantially parallel to at least part of the heater track, and an insulating layer may be provided over the sensor track and the heater track, the insulating layer being of temperature sensitive dielectric material.

Brief Description of the Drawings [0015] Specific embodiments of the present invention are described below by way of example only, with reference to the drawings, in which:

Figure 1 shows a thick film heater according to an embodiment;

Figure 2 shows an exploded view of the same heater; and

Figure 3 is a flowchart of a method of manufacture of the heater.

Description of Embodiments [0016] In the following description, the terms glass, ceramic and enamel may be interchangeable. The term insulating layer may be understood to comprise more than one layer, not all of which are insulating per se. The terms heater and resistor may be interchangeable, although certain embodiments may be designed specifically for heater or resistor applications.

[0017] Figures 1 and 2 show a heater 1 in an embodiment of the invention, made up of layers. The different layers will be described in the order in which they processed during the manufacture of the heater 1, with reference to the steps (SI to S8) shown in Figure 3. A metal substrate 6 is provided (SI), preferably of steel, such as ferritic steel. The substrate 6 is thin, preferably below 0.5 mm thick. The substrate 6 is preferably planar, with substantially planar and parallel opposite faces. The substrate 6 may be flexible, for example so that the substrate 6 may be flexed by hand and/or by assembly tools.

[0018] The substrate 6 is cleaned (S2) and first and second layers 5 and 7 are applied (S3) to respective opposite faces of the substrate 6. The first and second layers 5 and 7 are of an insulating material, such as enamel. The application can be by electrostatic spraying or screen printing or any other suitable process. The first and second layers 5, 7 are then fired (S4) at a high temperature.

[0019] The first and second layers 5, 7 are arranged so as both to be in compression after firing, as they cool to the normal operating temperature range of the heater 1. This may be achieved by selecting the material of both the layers 5, 7 to have a lower coefficient of thermal expansion than that of the substrate 6. The compression of the first and second enamel layers 5, 7 is balanced so as to reduce or eliminate stress applied to the substrate, for example by selection of the material, thickness and/or number of the second layer(s) 7, which may differ from those of the first layer 5.

[0020] In this embodiment, the heater 1 includes a temperature sensor, for example of the E-FAST type as described above, constructed as follows. A conductive layer 4 is applied (S5) to the first layer 5 by screen printing or other suitable process. The conductive layer 4 is shown as a solid layer but could be in the form of a mesh or grid. A temperature-sensitive dielectric layer 3 is applied (S6), comprising a dielectric having a resistivity which drops significantly above a predetermined temperature, such that a leakage current may flow therethrough. For example, the specific electrical resistance of the dielectric may be high at room temperature, for example 1.5 x 10¹¹ Ω-cm or higher, but may fall drastically as the temperature increases, for instance to a typical value of 1.5 x 10⁷ Ω-cm at 180-400° C. The specific electrical resistance of the dielectric and its change with temperature may be chosen according to the desired application. The specific electrical resistance of the temperature sensitive dielectric layer 3 is preferably substantially lower than that of the first layer 5, at least above the predetermined temperature, so that the leakage current flows only through the temperature-sensitive dielectric layer 3.

[0021] The temperature-sensitive dielectric layer 3 includes an aperture in which a contact pad 8 is later added to the conductive layer 4.

[0022] A heater track 2 is then applied (S7). This can be a serpentine shape as shown or any other suitable configuration. In an alternative arrangement, instead of having a continuous heater track 2 with bends of tight radius, discrete segments of heater track 2 may be joined by bridges comprising a material of low resistivity, as disclosed for example in EP-B-1905271.

[0023] Contact pads 9, 10 of a material with low resistivity are then added at each end of the heater track 2 and a contact pad 8 on the conductive layer 4 (S8). Optionally a further insulating (e.g. enamel) layer (not shown) may be applied to cover the heater 1 with the exception of the contact pads 8, 9, 10. This further layer protects the heater tracks 2 from abrasion and provides electrical insulation, should this be required.

[0024] The layers 2, 3, 4 and contact pads 8, 9, 10 may be applied by screen printing or other suitable process. The layers 2, 3, 4 and contact pads 8, 9, 10 may be dried and fired, either individually or together. It may be necessary to add a further layer or layers to the second layer 7, on the reverse side of the heater 1, to ensure that the heater 1 remains flat during firing of the layers 2, 3, 4 and contact pads 8, 9,10.

[0025] The above embodiment illustrates one possible arrangement. A person skilled in the art will appreciate that other similar arrangements are possible. For example, if a temperature sensor is not required, the layers 3 and 4 and steps S5 and S6 may be omitted, as well as the contact pad 8. Alternatively, the first layer 5 may comprise material having a resistivity that changes rapidly above a predetermined temperature. The leakage current flowing through the first layer 5, between the heater track 2 and the substrate 6, may be detected. This arrangement can be used when the electrical supply to the heater track is low voltage or the substrate is suitably insulated to prevent the risk of electric shock.

[0026] In another alternative, a sensor track may be provided on the first layer 5, running substantially parallel to at least part of the heater track 2, and an insulating layer may be provided over the sensor track and the heater track 2, the insulating layer being of temperature sensitive dielectric material. A leakage current between the sensor track and the heater track may be detected.

[0027] In each of the above temperature sensing arrangements, a rapid increase in leakage current above the predetermined temperature may be detected, and used to turn off or reduce the power to the heater track 2, thus limiting the operating temperature of the heater. Hence, it is not essential that the precise temperature of the heater 1 be monitored, only that a temperature condition above a predetermined temperature be detected.

[0028] Additional layers may be provided between the layers described above. Hence, it may not be essential that each layer as described is applied directly onto the previous layer, but may be provided over the previous described layer.

[0029] The heater 1 may be incorporated in a domestic appliance, such as a kettle, flow through heater, coffee maker, food processor, or milk frother, or other types of appliance. The heater 1 may be used in medical equipment such as a cauterizing scalpel, as a heater in a printer, laminating machine, or as a heater for controlling the temperature of an injection moulding machine or mould tool.

[0030] When arranged as a resistor, the heater 1 may be used as a temperature sensor, strain gauge, angular transducer or other type of positional sensor.

[0031] When incorporated into an appliance, the heater or resistor 1 may be flexed so as to comply with an object to be heated and/or sensed, or to fit within a space. The heater or resistor 1 may be held permanently in a flexed position within the appliance, or may be flexed during assembly. The heater or resistor 1 may be flexed so that the side carrying the heater track 2 is convex, preferably such that the first layer 5 is still in compression.

Claims

1. A thick film resistor, comprising:

a. a metal substrate having first and second opposite surfaces;

b. a first dielectric layer applied on or over the first surface of the substrate;

c. a thick film heating track applied on or over the first dielectric layer; and

d. a second dielectric layer applied on or over the second surface of the substrate;

wherein the first and second dielectric layers are in compression, and stress on the substrate due at least to the compression of the first dielectric layer is reduced or balanced by stress on the substrate due to the compression of the second dielectric layer; and wherein the resistor includes temperature sensitive dielectric material having a resistivity that changes rapidly above a predetermined temperature, so as to allow a temperature condition of the resistor to be sensed.

2. The resistor of claim 1, wherein the metal substrate is flexible.

3. The resistor of claim 2, wherein the metal substrate is flexed so as to be non-planar.

4. The resistor of claim 3, wherein the first surface is convex.

5. The resistor of any preceding claim, wherein the metal comprises steel.

6. The resistor of any preceding claim, wherein the metal substrate is 0.5 mm thick or less.

7. The resistor of any preceding claim, wherein the material of the second dielectric layer differs from the material of the first dielectric layer.

8. The resistor of any preceding claim, wherein the thickness of the second dielectric layer differs from the thickness of the first dielectric layer.

9. The resistor of any preceding claim, wherein the second dielectric layer comprises a plurality of discrete layers.

10. The resistor of any preceding claim, wherein the temperature sensitive dielectric comprises a third dielectric layer arranged between the first dielectric layer and the heating track, the resistor including a sensor layer arranged between the first and third dielectric layers to allow the change in resistivity of the third dielectric layer to be detected.

11. The resistor of any one of claims 1 to 9, wherein the temperature sensitive dielectric comprises the first dielectric layer, such the rapid change in resistivity of the temperature sensitive dielectric may be detected by detecting a leakage current between the heating track and the substrate.

12. The resistor of any one of claims 1 to 9, including a sensor track running substantially parallel to at least part of the heating track, wherein the temperature sensitive dielectric material is provided as an insulating layer covering the sensor track and the heating track, such that the sensor track is arranged to sense the change in resistivity of the temperature sensitive dielectric material.

13. The resistor of any preceding claim, comprising a thick film heater.

14. An electrical appliance including the thick film heater of claim 13.

15. The electrical appliance of claim 14 comprising a kettle, flow through heater, coffee maker, food processor, or milk frother.

16. Electrical apparatus comprising a printer, laminator, cauterizing scalpel, injection moulding tool or machine, including the resistor of any one of claims 1 to 13.

17. A method of manufacturing the thick film resistor of any one of claims 1 to 1212, comprising:

a. applying the first dielectric layer on or over the first surface of the substrate;

b. applying the second dielectric layer on or over the second surface of the substrate; and

c. applying the thick film heating track on or above the first dielectric layer; wherein the first and second dielectric layers are fired at a high temperature, and the substrate contracts more than the first and second dielectric layers on cooling, such that the first and second dielectric layers are in compression and stress on the substrate due to the compression of the first dielectric layer is reduced or balanced by stress on the substrate due to the compression of the second dielectric layer.

Intellectual

Property

Office

Application No: GB1814445.1

Examiner: Thomas Britland

17. A sensor including the resistor of any one of claims 1 to 12.

18. A method of manufacturing the thick film resistor of any one of claims 1 to 13, comprising:

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS;

Claims

1. A thick film resistor, comprising:

a. a metal substrate having first and second opposite surfaces;

d. a second dielectric layer applied on or over the second surface of the substrate; wherein the first and second dielectric layers are in compression, and stress on the substrate due at least to the compression of the first dielectric layer is balanced by stress on the substrate due to the compression of the second dielectric layer;

wherein the resistor includes temperature sensitive dielectric material having a . resistivity that changes rapidly above a predetermined temperature, so as to allow a temperature condition of the resistor to be sensed;

and wherein the metal substrate is flexible.

2. The resistor of claim 1, wherein the metal substrate is flexed so as to be non-planar.

3. The resistor of claim 2, wherein the first surface is convex.

4. The resistor of any preceding claim, wherein the metal comprises steel.

5. The resistor of any preceding claim, wherein the metal substrate is 0.5 mm thick or less.

6. The resistor of any preceding claim, wherein the material of the second dielectric layer differs from the material of the first dielectric layer.

7. The resistor of any preceding claim, wherein the thickness of the second dielectric layer differs from the thickness of the first dielectric layer.

8. The resistor of any preceding claim, wherein the second dielectric layer comprises a plurality of discrete layers.

9. The resistor of any preceding claim, wherein the temperature sensitive dielectric comprises a third dielectric layer arranged between the first dielectric layer and the heating track, the resistor including a sensor layer arranged between the first and third dielectric layers to allow the change in resistivity of the third dielectric layer to be detected.

10. The resistor of any one of claims 1 to 8, wherein the temperature sensitive dielectric comprises the first dielectric layer, such the rapid change in resistivity of the temperature sensitive dielectric may be detected by detecting a leakage current between the heating track and the substrate.

11. The resistor of any one of claims 1 to 8, including a sensor track running substantially parallel to at least part of the heating track, wherein the temperature sensitive dielectric material is provided as an insulating layer covering the sensor track and the heating track, such that the sensor track is arranged to sense the change in resistivity of the temperature sensitive dielectric material.

12. The resistor of any preceding claim, comprising a thick film heater.

13. An electrical appliance including the thick film heater of claim 12.

14. The electrical appliance of claim 13 comprising a kettle, flow through heater, coffee maker, food processor, or milk frother.

15. Electrical apparatus comprising a printer, laminator, cauterizing scalpel, injection moulding tool or machine, including the resistor of any one of claims 1 to 12.

16. A sensor including the resistor of any one of claims 1 to 11.