CN115515260A - Thick film heating element and method of manufacture - Google Patents

Abstract

The thick film heating element comprises a metal substrate (2), an insulating layer (3) and conductive tracks (5) and contact pads (4) formed on the insulating layer using a thick film manufacturing process. The substrate (2) may comprise a plurality of layers (2 a, 2b, 2 c) of different metallic materials, for example a copper layer (2 b) between two layers (2 a, 2 c) made of steel. This may provide a substrate with good thermal conductivity and a thermal expansion coefficient compatible with the material of the insulating layer (3). The layers are bonded together using a rolling process such as cold rolling bonding.

Description

Thick film heating element and method of manufacture

technical field

The present invention relates to a thick film heating element and a method of manufacturing the same.

Background technique

Thick film heating elements typically include one or more heating tracks that are screen printed as ink or paste onto an insulating substrate and fired to form High resistivity traces. The connection traces or pads can be printed in separate layers using 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 with metal substrates are typically fabricated by applying an electrically insulating layer to the metal substrate and subsequently forming heater traces onto the surface of the insulating layer. The insulating layer can be a glass or ceramic material applied using screen printing techniques or the more traditional glass enamel process. The metal base is most commonly stainless steel. The firing temperature and other properties of the insulating material, heater traces and pads must be compatible with those of the metal.

More details of thick film technology are described eg in Kasap S., Capper P. (eds) Springer Handbook of Electronics and Photonic Materials, White N. (2017) Thick Films pp. 707-709 and 712. A thick film paste may include an active material, a glass frit, and an organic carrier or vehicle. The glass frit remains after firing and forms part of the structure of the thick film resistor. Accordingly, "thick film" refers to a specific type of resistor having characteristic structures and properties, and is not merely a comparative term or designation to a product when manufactured through a specific process.

Thick film heating elements have many applications including kettles and cooking appliances where it is desirable to increase the watt density of the heating element to reduce the size and potentially cost of the heating element, or to reduce the noise, or provide very even heat. In some applications, such as cooking machines, it is desirable that the temperature of the heating surface be uniform. Hot spots can cause overheating of the material being heated. One of the features that limits the ability to achieve these goals is the material of the substrate on which many thick film heating elements are fabricated. Stainless steel has a relatively low thermal conductivity; for 440 steel, the thermal conductivity is 24.2 W/mK. A substrate with low thermal conductivity will not conduct heat laterally as much, resulting in high temperatures at the trace locations and low temperatures between the traces. A solution to this problem is to provide a diffuser plate attached to the steel substrate by brazing, as described in GB-A-2547148. However, brazing two dissimilar metals is not easy, and the brazing temperature must be compatible with the firing temperature of the insulation and traces.

Stainless steel is used because it provides a durable, corrosion-resistant surface that can be polished or textured depending on the application. Stainless steel is excellent for heating elements used in kettles, water heaters, cookers and irons.

The coefficient of thermal expansion of the substrate material is also important. Ideally, the substrate has a coefficient of expansion slightly greater than that of the insulation and heater trace materials so that when the heater cools after firing, the insulation and traces are subjected to as much tension as the insulation and traces can withstand. Different tensile stresses than compressive stresses. The temperature at which the material will be subjected to tensile stress due to thermal expansion is above the normal operating temperature of the heating element.

Stainless steel and especially ferritic or martensitic steels have an expansion coefficient of approximately 10×10 ⁻⁶ /K. The expansion coefficient of glass is about 8.5×10 ^-6 /K. Copper has a coefficient of 17 x 10 ^-6 /K, which makes copper incompatible with heating element materials.

Contents of the invention

According to one aspect of the invention there is provided a composite or laminated metal substrate for a thick film heating element comprising two or more layers bonded or secured together by a rolling process.

One or more of the layers may provide the desired properties of the outer surface, and one or more may provide increased heat transfer. This aspect of the invention can provide a substrate with good thermal conductivity and a coefficient of thermal expansion compatible with insulating materials.

Differences in the coefficients of thermal expansion between the various layers making up the heating element (especially the insulating layer and the laminate substrate) can lead to distortion or bowing of the substrate when the element cools after the firing process. The insulating layer will be on the raised side of the element. This effect can be reduced or eliminated by having the two outer layers of the substrate have unequal thicknesses and/or by making the outer layers consist of different materials. The thickness and/or material may be chosen such that the deformation or bending is reversed so that the insulating layer is on the concave side of the heating element.

The need to provide layers with good thermal conductivity increases when the substrate is thinner. For example, it may be desirable to manufacture flexible thick film heating elements. One such method of making a thin flexible heating element is described in GB-A-2576895, in which a dielectric layer is formed on opposite surfaces of the substrate to balance the compressive forces on the surface of the substrate. This method can also be applied to embodiments of the present invention, especially those having thin substrates, for example below 0.5 mm in thickness.

Alloys with very low coefficients of expansion can be used in the construction of metal substrates. These materials include iron/nickel alloys and iron/nickel/cobalt alloys. It should be noted that these alloys can be used as the substrate itself, rather than as part of a composite, and these alloys can be used to achieve finished heating elements with little or no deformation, especially when the substrate is very thin, e.g. 0.5mm the following.

The use of the above alloys in metal substrates for thick film heating elements is considered to be independently inventive. Thus, according to another aspect of the present invention there is provided a metal substrate for a thick film heating element, the metal substrate comprising an iron/nickel alloy or an iron/nickel/cobalt alloy.

Description of drawings

In the following only by way of example, specific embodiments of the present invention are described in conjunction with the accompanying drawings, wherein:

Figure 1 is a perspective view of a thick film heating element in a first embodiment of the invention;

Figure 2 is an exploded perspective view of the thick film heating element in the first embodiment;

Figure 3 is a cross-section of the thick film heating element in the first embodiment;

Figure 4 shows the bonding process for the thick film heating element in the first embodiment;

Figure 5 is a flow chart of the bonding process; and

Figure 6 is a cross section of a thick film heating element in a second embodiment.

detailed description

Figures 1-3 show a thick-film heating element 1 comprising a substrate 2 consisting of three layers 2a, 2b, 2c having an insulating layer 3 with enamel. The outer layers 2a, 2c may be made of steel such as ferritic stainless steel. The intermediate layer 2b can be made of copper.

The outer layers 2a, 2c may have a thickness in the range between about half the thickness of the middle layer 2b and equal to the thickness of the middle layer 2b. For some applications, the thickness of the middle layer 2b can be between 1mm and 2mm, but in the case where the thick film heating element 1 is required to be flexible, the total thickness of the layers 2a, 2b, 2c can be in the range of 0.1mm-0.2mm range, such as 0.15mm, and the thickness of each layer is equal, such as 0.05mm.

In the method of manufacture of the substrate 2, the layers 2a, 2b, 2c may be joined together by any of a number of methods such as welding, riveting or brazing. Methods such as hot or cold rolling for the manufacture of isothermal bimetallics are well suited for this application and can be used instead.

The method of bonding the layers 2a, 2b, 2c together by rolling (eg cold roll bonding) is schematically illustrated in Figures 4 and 5 . Cold roll bonding is a solid phase cold welding process in which the layers 2a, 2b, 2c are forced together under sufficient pressure to reduce the overall thickness of said layers 2a, 2b, 2c. This severe plastic deformation creates a metallurgical bond as the atomic lattices of the different metals fuse into a common structure. Bond strength between different metals can be increased by heating the material to induce diffusion at the interface. The rolling process may include the following steps.

Cleaning (step S1 ) - the surfaces of the layers 2a, 2b, 2c are cleaned before bonding is produced. Preferably, substantially all traces of contaminants, in particular oil or grease, are removed. Suitable cleaning processes include solvent cleaning, detergent, alkaline cleaning; including electrocleaning or anodic cleaning.

Mechanical abrasion (step S2) - In addition to one or more cleaning processes, mechanical abrasion of the surface (usually by scratch brushing) is used to remove metal oxides.

Heating (step S3) - Although the process is described as cold rolling bonding, sometimes one or more of the materials to be bonded is heated above ambient temperature to increase the ductility of the materials. The temperature tends to be lower than that which would cause material diffusion at the interface, and therefore the process is still classified as a cold rolling process.

Rolling (step S4) - the layers 2a, 2b, 2c are then put together and run through a rolling mill. The rollers 10 in the rolling mill are arranged such that the gap distance between the rollers is smaller than the combined thickness of the layers 2a, 2b, 2c so that the layers 2a, 2b, 2c are plastically deformed. The friction at the surface interface between the layers 2a, 2b, 2c is sufficient to destroy any remaining oxide layer, and the forcing of the surfaces together will be sufficient force to weld the layers 2a, 2b, 2c together.

Heat treatment (step S5) - after rolling, the material properties of the substrate 2 can be enhanced by heat treatment, such as an annealing process, to relieve the stresses induced during the rolling process, and also by promoting The diffusion of the respective materials at the interface between them increases the weld strength.

Further processing (step S6) - if desired, the substrate 2 can then be treated by conventional metalworking procedures. For example, the substrate 2 may be cut to a desired shape, straightened through a tension leveling process, and/or pressed or blanked/blanked in a pressing tool.

Other methods of forming the substrate include electrochemical or electroless plating, sputtering or thin film techniques, flame spraying or screen printing.

After fabrication, the substrate can be used as a component in the fabrication of thick film heating elements. In the manufacture of a thick film heating element, an insulating layer 3 is formed on a substrate and a contact pad 4 and one or more heating traces 5 are formed on the insulating layer 3 using a thick film printing and firing process as described above. . One or more overglaze layers (not shown) may be formed over the heating traces 5 such that the contact pads 4 are exposed.

In embodiments where the flat substrate 2 is made using two similar layers of steel sandwiching different metals, such as copper, the resulting substrate can remain flat when the temperature is changed, such as during a thick film firing process. The coefficient of lateral expansion of the substrate will depend on the coefficients of expansion of the two materials (steel and dissimilar metal), the Young's modulus of the materials and the relative thickness of the materials. In the example of stainless steel and copper, the Young's modulus of the material is:

Copper 121GPa

Steel 444 220Gpa

The total thermal expansion may be proportional to the product of the coefficient of expansion, thickness and Young's modulus of the individual layers. For example, in a substrate where the copper layer has the same thickness as each of the steel layers, the coefficient of expansion will be:

Ce＝(2×220×10 ^-6 )+(121×17×10 ^-6 )/(2×220+121)

Ce＝11.51×10 ^-6 /K

If the stainless steel is half the thickness of the copper, the coefficients will be:

Ce＝12.50×10 ^-6 /K

A factor of 12.50×10 ^-6 /K is compatible with insulation and trace materials typically used for steel substrates.

FIG. 6 shows a second embodiment, which differs from the first embodiment in that the substrate 2 comprises a single layer metal with a very low coefficient of thermal expansion, such as an iron/nickel alloy or an iron/nickel/ cobalt alloy. For example, alloy 1.3981, which is an iron/nickel/cobalt alloy sold under the trademark Dilver, has an average coefficient of expansion between 30°C and 600°C of 7.9×10 ⁻⁶ . The coefficient for a nickel/iron alloy with 36% nickel is 1-3 x 10 ^-6 , although the coefficient increases above about 200°C due to phase transformation.

The thick film heating element 1 may be fitted in a domestic appliance, such as a milk frother, kettle, cooking machine or iron. A power source is connected to contact pad 4 and is controlled to control the heat provided by the thick film heating element. Depending on the type and size of the appliance, the diameter of the base 2 may be between 70mm and 130mm and may eg be square, rectangular or circular in plan. For the flexible element 1 the dimensions may be eg 5 mm x 250 mm.

In some appliances, the surface of the substrate 2 opposite the heating trace 5 provides the heating surface for contacting the liquid or other material to be heated; this may be referred to as the wetted side of the thick film heating element. The surface of the substrate 2 closest to the heating trace 5 is referred to as the dry side.

alternative embodiment

In an alternative embodiment, the substrate 2 may comprise only two layers 2a, 2b, eg provided by the copper layer 2b in case a heated copper surface is required. The insulating layer 3 will then be formed on the single steel layer 2a.

Alternatively or additionally, the substrate 2 may have an insulating or dielectric layer formed on the side opposite to the insulating layer 3 (for example, on the outer surface of the outer layer 2c (or 2b in the case of only two layers) ) to balance the compression applied by the insulating layer 3 to the surface of the substrate 2. This may allow the substrate 2 to be thin (eg <0.5mm thickness) and/or flexible.

Alternative embodiments that may be apparent to those of skill in the art after reading the above disclosure may still fall within the scope of the appended claims.

Claims (14)

1. A method of manufacturing a thick film heating element having a metal substrate comprising two or more layers of metal or metal alloy, each of which is different, the method comprising:

bonding the layers together to form the metal substrate by using a rolling process;

forming a dielectric layer or an insulating layer on at least one surface of the substrate; and

one or more thick film heating traces are formed on the dielectric or insulating layer.

2. The method of claim 1, wherein the rolling process comprises cold rolling bonding.

3. The method of claim 1, wherein at least one of the layers has a different thermal conductivity coefficient than another of the layers.

4. The method of claim 1, wherein the substrate comprises at least three of the layers.

5. The method of claim 4, wherein an intermediate one of the layers has a higher thermal conductivity coefficient than an outer one of the layers.

6. The method of claim 4, wherein outer ones of the layers have equal thicknesses.

7. The method of claim 4, wherein outer ones of the layers have unequal thicknesses.

8. The method of claim 1, wherein the substrate remains substantially flat after the thick film heating trace is formed.

9. The method of claim 4, wherein the outer layers are composed of mutually different materials having different coefficients of thermal expansion.

10. The method of claim 4, wherein at least one of the layers comprises copper.

11. The method of claim 10, wherein an intermediate one of the layers comprises copper.

12. The method of claim 4, wherein at least one of the layers comprises steel.

13. The method of claim 12, wherein an outer one of the layers comprises steel.

14. The method of claim 1, wherein the substrate has a thickness of less than 0.5 mm.

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