CN220755080U

CN220755080U - Thick film heater

Info

Publication number: CN220755080U
Application number: CN202321214614.0U
Authority: CN
Inventors: 罗布·马特坎普; 瑞纳杜斯·赫曼努斯·伯纳杜斯·迪嫩; 詹姆斯·罗; 安德鲁·亨特
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2022-05-17
Filing date: 2023-05-17
Publication date: 2024-04-09
Anticipated expiration: 2033-05-17
Also published as: GB2618803A8; GB2618803A; GB202207193D0

Abstract

A thick film heater comprises a substrate, one or more heating traces, and one or more other traces, wherein the heating traces and the other traces are separated from each other by an intermediate insulating layer and may at least partially overlap. The other trace may be a connection trace for connecting to a heating trace, a sensor trace, or other heating trace. There may be a second heating trace overlaying the first heating trace such that the heating trace sections of the second heating trace overlie the gaps between the heating trace sections of the first heating trace. These arrangements can provide more uniform heating of the substrate and effectively utilize space on the substrate.

Description

Thick film heater

Technical Field

The present application relates to thick film heaters and methods of manufacture.

Background

Thick film heating elements typically include one or more heating traces that are screen printed as an ink or paste onto an insulating substrate and fired to form high resistivity traces. The connection traces or pads may be printed with different types of inks or pastes and fired to form low resistivity connection traces and pads.

The insulating substrate may be an electrically insulating material (such as ceramic) or may be a metal with an insulating surface layer. Thick film heating elements having a metal substrate are typically manufactured by applying an electrically insulating layer over the metal substrate and then forming heating traces on the surface of the insulating layer. The insulating layer may be a glass or ceramic material applied using screen printing techniques or a more conventional enamel process. The metal substrate is typically stainless steel. The firing temperature and other characteristics of the insulating material, the heating traces and the pads must be compatible with those of the metal. Furthermore, a protective ceramic or glass cover layer may be added, for example by spraying or screen printing and subsequently firing.

Additional details of Thick film technology are described, for example, in White n. (2017) thin Films pages 707-709 and 712: kasap s., cap p. (eds) Springer Handbook of Electronic and Photonic Materials (handbook of electronic and photonic materials, sapringer). The thick film paste may include an active material, a frit, and an organic carrier. The frit remains after firing and forms part of the structure of the thick film resistor. Thus, "thick film" refers to a specific type of resistor having a unique structure and properties, and not just a comparative term or to a product manufactured by a specific process.

The heating traces and bond 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 in the form of a paste by screen printing, and then dried and fired as described above. The connection trace or pad overlaps the heating trace to provide a low resistance connection with the heating trace. The connection pads may provide electrical connection to an external power source.

The connection tracks may provide electrical connections between the heating tracks or between individual sections of the heating tracks, as disclosed for example in EP-a-1905271. Using connecting traces or chains in this manner allows the heating traces to be more densely packed on the heater surface without causing the traces to follow paths with small radii, which would result in current crowding or accumulation along the interior of the traces due to reduced resistance compared to the exterior; this may lead to internal overheating and failure of the cabling.

The resistance of thick film heating traces depends on the resistivity of the resistive material as well as the thickness, length and width of the traces, as shown in the equations below.

R=ρ x L/A R =resistance (Ohms)

ρ=resistivity (ohm.mm)

L=trace length (mm)

A = trace cross-sectional area (mm) ² )

R=ρ x L/(w x t) w=track width (mm)

t=thickness of trace (mm)

For a given trace material and thickness:

ρ/t= K K =constant (Ohms)

Then:

R＝K x L/w

the ratio L/w is dimensionless and can be considered as the number of blocks that make up the trace.

The number of blocks is used when designing the layout of thick film resistor traces. The designer must determine the optimal length of the sides of the square. The larger the square, the more resistive trace material used. The material is typically made of silver, platinum or palladium and is therefore relatively expensive. Small cubes are desirable to minimize cost. The result is a short, narrow trace. However, the service life of the heater is determined by the temperature of the heating trace during use. This is a function of the application of the heater and the area of the heating traces.

The ratio of power to wire area (referred to as power density, in W/cm ² Representation) is an important design parameter. Limiting the power density results in long, wide traces. The power density can be reduced by reducing the gap between the traces. The minimum gap width is limited by the screen printing process and the electrical insulation required between adjacent sections of the tracks. It is therefore difficult to design a heating trace that provides a low power density and acceptable clearance between adjacent sections of the heating trace.

It may also be desirable to produce thick film heaters with uniform power distribution across the surface. Typically stainless steel is used as the substrate for the thick film heating element, but stainless steel is a relatively poor thermal conductor, so that the lateral conduction of heat from the heating traces to the portion of the heater between the traces is low, resulting in uneven heating. One solution to this problem is to attach a diffuser plate to a thick film heater as disclosed in EP-a-1177708, wherein an aluminum diffuser plate is brazed to a stainless steel substrate. However, brazing of these two dissimilar metals is difficult because the formation of intermetallic compounds can create brittle joints. The resulting aluminum heated surface is unacceptable for certain applications; for example, applications where aluminum is in contact with food are undesirable. In other applications, a wear resistant surface may be desired.

In some applications, it is desirable to divide the heating zone into multiple zones, which can be controlled independently of each other. Each area requires a separate connection; there may be one common connection between the regions, but each region requires at least one separate connection. In conventional heaters, these connections take up valuable space on the heater and create unheated areas on the heater that may be detrimental to the operation of the device containing the heater.

It may be desirable to include a sensor on the surface of the thick film heater, such as a temperature sensor where the resistance varies with temperature with either a positive temperature coefficient or a negative temperature coefficient. The sensor may be a separate component attached to the heater or printed onto the insulating layer on the heater in the same manner as the heating traces. The 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.

Disclosure of Invention

Aspects of the present application are defined by the appended claims.

In at least some embodiments of the present application, one or more heating traces or one or more connections between one or more heating traces are printed on different layers than the heating trace(s), the different layers being separated by one or more layers of insulating material. The isolation insulating material may be similar to the insulating material applied to the metal substrate. This allows the heating trace(s) to overlap with the connection(s) when viewed perpendicular to the heater surface.

The electrical connection(s) may overlap at least a portion of the heating trace(s) when viewed perpendicular to the heater surface, and may be connected to the heating trace(s) through one or more holes in the isolation insulating layer to allow electrical connection between the traces. The hole(s) may be created during the printing of the insulating layer.

In embodiments that include at least one sensor (such as a positive or negative temperature sensor), one or more connections of the sensor(s) may be printed on a different layer than the heating trace(s). If the sensor is a discrete component, one or more holes may be provided in the insulating layer through which the connection(s) may be implemented. If a sensor is printed, the sensor may be on the same layer as the connection(s), or may be on the same layer as the heating trace(s). If the sensor is on the same layer as the heating trace(s), the holes in the intermediate insulating layer are provided for connection between the sensor and the connection trace to connect the connection trace to the sensor.

In some embodiments, the thick film heater comprises a plurality of heating traces printed as distinct layers separated by one or more intermediate electrically insulating layers and interconnected or connected to an external power source through one or more holes in or around the edge(s) of the intermediate insulating layer(s), for example by extending the traces beyond the edge(s) of the intermediate insulating layer(s). One or more connection traces or pads of low resistance material may be used to make the connection; these may reduce the voltage difference between adjacent heating traces to reduce the likelihood of insulation failure between adjacent heating traces.

Drawings

Specific embodiments of the present application will now be described with reference to the accompanying drawings, in which:

fig. 1 is an exploded view of a conventional dual zone thick heater.

Fig. 2 is a plan view of the thick film heater of fig. 1.

Fig. 3 is an exploded view of a dual zone thick film heater in a first embodiment of the present application.

Fig. 4 is a plan view of the thick film heater of fig. 3.

Fig. 5 to 9 show an arrangement of heating traces in a thick film heater in a second embodiment, in which:

fig. 5 and 6 are plan views of a first heating trace and a second heating trace, respectively;

fig. 7 to 9 are a plan view, an exploded view and a sectional view of the first heating trace and the second heating trace, respectively, which are overlapped with each other.

Detailed Description

Fig. 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 a first heating track 3 and a second heating track 4 formed on the substrate insulating layer 2. The electrical connection to one pole of the first heating trace 3 may be made through the connection pad 5 and the electrical connection to one pole of the second heating trace 4 may be made through the first bus bar 6. The second poles of the first heating trace 3 and the second heating trace 4 are connected together by a second bus bar 7. In this way, power may be supplied to the first heating trace 3 and the second heating trace 4 independently.

In this example, each of the first and second heating traces 3 and 4 includes a plurality of heating trace sections or portions separated by gaps, and are connected in series by a low-resistance connection portion or section to avoid current crowding. The heating trace sections or portions may be parallel and linear as shown, but other alternative configurations may be used depending on the geometry of the substrate 1 and the heating area required.

In the method of manufacturing a thick film heating element, for example as described above, a substrate insulating layer 2 is formed on a substrate 1, and heating traces 3, 4, connection pads 5 and bus bars 6 and 7 are formed on the substrate insulating layer 2 using thick film printing and firing processes. One or more overglaze layers (not shown) may be formed over the heating traces 3a, 3b, exposing the ends of the contact pads 5 and the bus bars 6, 7.

The heating tracks 3, 4 have a high resistance for resistive heating, while the connection pads 5 and the bus bars 6, 7 typically have a low resistance.

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 thick film traces or other portions may be printed directly onto the substrate 1.

As shown in particular in fig. 2, the bus bars 4, 5 occupy a portion of the width of the substrate 1, which reduces the space available for heating the tracks 3, 4. Therefore, the heating density on the substrate 1 is limited, and the heating may not be uniform over the width of the substrate 1.

The first embodiment as shown in fig. 3 and 4 is different from the embodiment of fig. 1 and 2 in that an intermediate insulating layer 10 is formed in a pattern including holes 11a to 11d over the first and second heating traces 3 and 4. The bus bars 6, 7 are formed on the intermediate insulating layer 10 and connected to the first and second heating traces 3, 4 through the holes 11a to 11 d. In the case of thick film material of the bus bars 6, 7 printed on one or more of said holes 11a to 11d, at least after firing, some of the thick film material passes through the corresponding holes 11a to 11d and is electrically connected with the portion of the first heating trace 3 or the second heating trace 4 below the holes 11a to 11 d.

In this embodiment, no separate connection pads are required, but in other embodiments one or more connection pads may be printed on the corresponding hole(s) to connect to the underlying trace(s) through the hole(s). The first heating trace 3 and the second heating trace 4 may be connected together in series or in parallel, and may be switched or connected independently or jointly.

As shown in fig. 4, at least some portions of the bus bars 6, 7 overlie portions of the heating traces 3, 4 or overlap portions of the heating traces 3, 4 when viewed perpendicular to the substrate 1, rather than being disposed alongside the heating traces 3, 4; this allows a higher proportion of the surface area of the substrate 1 to be used for heating, giving a higher and/or more uniform heating density over the surface area of the substrate 1.

The arrangement of the first embodiment may be reversed such that the bus bars 6, 7 are formed on the substrate insulation layer 2 and the first and second heating traces 3, 4 are formed on the intermediate insulation layer 10 so as to overlie at least part of the bus bars 6, 7.

In the second embodiment of the present application, the first heating trace 3 is formed on the insulating substrate layer 2, and the intermediate insulating layer 10 is formed on the first heating trace. A second heating track 4 is then formed on the intermediate insulating layer 10. This arrangement allows the second heating trace 4 to overlie at least part of the first heating trace 3, thereby increasing the heating density of the thick film heater.

In a particularly advantageous embodiment illustrated in fig. 7 to 9, the heating trace sections of the second heating trace 4 overlie the gaps between the heating trace sections of the first heating trace 3. Preferably, the width of the gap between the heating trace sections of the first heating trace is substantially equal to the width of the respectively covered heating trace sections of the second heating trace 4. Thus, as shown in fig. 7, the heating area of the substrate 1 is substantially uniformly covered by the heating trace sections of the first heating trace 3 and the second heating trace 4, such that the substrate 1 is uniformly heated by the heating traces 3, 4 without requiring lateral heat conduction in the substrate 1, which may be limited by materials such as stainless steel.

The connection to the heating tracks 3, 4 may be achieved by one or more bus bars 6, 7 and/or connection pads 5, for example through holes 11 in the intermediate insulating layer 10 or in other insulating layers as in the first embodiment.

Referring to fig. 5 and 6, the heating traces 3, 4 may be arranged with live connections at the top and neutral connections at the bottom; if the thick film heater is powered by a DC voltage, the connection will be positive and negative. As a result, the voltage between adjacent sections of the tracks in the first heating track 3 and the second heating track 4, respectively, is a fraction (in this particular embodiment, a seventh) of the total voltage across the heating tracks 3, 4. This reduces the risk of failure of the intermediate insulating layer 10.

In a third embodiment, not shown in the drawings, at least one heating track 3, 4 and at least one sensor (such as a positive or negative temperature sensor) are provided, wherein one or more electrical connections of the sensor(s) are printed on a different layer than the heating track(s) 3, 4, which layers are separated by an intermediate insulating layer 10. In case the sensor is a separate component, one or more holes 11 may be provided in the intermediate insulating layer 10, through which hole(s) electrical connection(s) may be made. The printed sensor may be located on the same layer as the connector(s), or may be on the same layer as the heating trace(s). In case the sensor is on the same layer as the heating trace(s), the holes in the intermediate insulating layer 10 are provided for connection between the sensor and the connection trace to connect the connection trace to the sensor.

In variations of the above embodiments, there may be more than two layers of thick film components, each layer being separated by a respective intermediate insulating layer.

However, alternative embodiments, which may occur to those skilled in the art upon reading the foregoing description, may fall within the scope of the appended claims.

Claims

1. A thick film heater comprising a substrate, one or more heating traces, and one or more other traces electrically connected to the one or more heating traces, wherein one or more portions of the heating traces are separated from one or more portions of the other traces by an intermediate insulating layer, and at least one of the other traces is electrically connected to one or more of the heating traces around an edge of the intermediate insulating layer.

2. The thick film heater of claim 1, wherein one or more portions of the heating trace overlap one or more portions of the other trace.

3. A thick film heater comprising a substrate, a plurality of heating traces, and one or more other traces electrically connected to the plurality of heating traces, wherein one or more portions of the heating traces are separated from one or more portions of the other traces by an intermediate insulating layer.

4. A thick film heater according to claim 3, wherein a plurality of said heating traces are interconnected by said one or more other traces.

5. The thick film heater of claim 4, wherein the one or more other traces are electrically connected to the heating trace through one or more holes in the intermediate insulating layer.

6. The thick film heater of claim 4, wherein the heating traces are connected in series.

7. The thick film heater of claim 4, wherein the heating traces are connected in parallel.

8. A thick film heater according to claim 3, wherein the one or more other traces comprise at least one sensor trace.

9. A thick film heater according to claim 3, further comprising at least one discrete sensor, wherein the other traces comprise one or more connection traces for electrically connecting to the at least one discrete sensor.

10. A thick film heater according to claim 3, wherein the one or more heating traces are formed on the substrate and the one or more other traces are formed on the intermediate insulating layer.

11. A thick film heater according to claim 3, wherein the substrate comprises a metallic material having a substrate insulating layer formed thereon.

12. A thick film heater comprising first and second heating traces separated from each other by an insulating layer, characterized in that each of the first and second heating traces comprises a plurality of heating trace sections separated from each other by gaps, characterized in that at least some of the heating trace sections of the first heating trace overlie corresponding gaps between the trace sections of the second heating trace.

13. The thick film heater of claim 12, wherein the width of the heating trace section is equal to the width of the gap covered by the heating trace section.

14. The thick film heater of claim 12, wherein the first and second heating traces are interconnected to one another by one or more holes in an intermediate insulating layer.