GB2209292A - Application of conductive metallic film to a glass ceramic support surface - Google Patents

Description

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1 APPLICATION OF CONDUCTIVE METALLIC FILM TO A GLASS CERAMIC SUPPORT SURFACE 09MA-17029 This Invention relates generally to the application of metallic patterns on glass ceramic substrates and particularly to the application of conductive metallic strips to the underside of glass ceramic support surfaces for use In glass ceramic cooktop appliances.

Use of a glass ceramic material for the support surface In electric cooktops of both conventional conduction type and the radiant or infrared type Is well known. When using such material as a cooktop support surface, provision must be made to avoid overheating the support surface. One such temperature limiting arrangement Is disclosed in our prior application No. 8728736. In that Serial No. 000,684 filed January 5, 1987, by Payne et al. In this arrangement the temperature of the glass ceramic cooktop is measured using a sensor comprising a pair of parallel conductive strips formed on the underside of the cooktop. The strips are arranged to extend over a portion of the glass ceramic platb which overlies a heating unit. The strips comprise a noble metal paste bonded to the underside of the glass ceramic support surface.

Such conductive strips are normally applied to the glass ceramic plate prior to its assembly into the cooktop. The plate may go through a series of additional transportation and handling steps after appplication of the conductive strips but prior to assembly Into a cooktop appliance. Consequently the strips must be resistant to abrasive forces to which they may he exposed during such steps.

In order to achieve good adherence, suppliers of noble metal pastes used in such applications conventionally add a binder or flux material to the paste. The paste is screen printed on the glass ceramic and fired. When exposed to firing temperatures on the order of 700-870 OC, the flux in the paste rl-acts with the glass ceramic 209292 A material to bond the metal film to the glass ceramic surface. Very good adherence and abrasion resistance can be achieved in this fashion. Rowever, with some commercially available pastes this reaction reduces the impact resistance of the glass ceramic material in the area of the strips. In addition to'the Initial weakening effect, the reaction may continue when exposed to the normal cooktop operating temperatures. This continuing reaction would gradually reduce the Impact resistance of the glass material In the area of the conductive strips yet further.

Thus, there Is a need for an improved method of applying conductive strips to the glass ceramic material which provides the desired abrasion resistance for the conductive strips without adversely affecting the impact resistance of the glass ceramic material or the performance of the conductive strips when provided as sensors.

A method of applying a metal film on a glass ceramic plate in accordance with the present invention comprises the following steps. A thin layer of a metal paste comprising an organo-metallic compound 1 -Y 1 1 preferably substantially devoid of flux material is applied to the Class ceramic plate surface in the desired pattern. The plate Is then slowly heated to a temperature sufficient to burn off the organic components of the paste, typically a temperature on the order of 600-700' F. After cooling, the resultant strips of metallic film are covered with a thin protective layer of an organic resin material Preferably characterized by a vaporization temperature In the normal operating temperature range to which the glass is exposed when In use. On the first operational use of the glass the organic resin cleanly vaporizer. away. Since the bonding reaction of the flux laaterial with the glass ceramic Is not relied upon for abrasion resistance, the glass ceramic material need only be heated sufficiently to burn off the organic components of the paste. Thus, high temperature firing Is not necessary, reducing the cost and complexity of the fabrication steps.

The thin protective layer of organic resin material compensates for the reduced bonding with pastes having little or no flux by protecting the metal film from abrasion prior to and during assembly of the cooktop appliance. Once assembled, the Inner or lower cooktop surface Is no longer subject to abrasion. Use of a protective material which vaporizes away when exposed to typical cooktop operating temperatures insures that the protective coating does not interfere with proper sensor operation.

In a preferred form of the Invention the paste is a noble metal paste containing a substantial percentage of gold and the organic resin material Is a clear acrylic resin.

Though described in the context of a ceramic glass cooktop appliance, the method of the invention has utility in any application Involving applying a metallic film pattern to a glass ceramic -4substrate,.which pattern Is thereafter subject to abrasion prior to and during assembly of a finished product Incorporating the substrate but which Is not subject to abrasion once the finished product Is fully assembled, where normal operation of the end product exposes the substrate to a high temperature environment.

Further, in accordance with the present Invention a heating apparatus such as, for example. a household cooking appliance of the type having a glass ceramic support surface and temperature sensing =cans comprising a pair of generally parallel conductive strips for each heating unit formed on the underside of the glass ceramic support surface is provided with an Improvement wherein the conductive strips are formed of a noble metal paste containing no significant amounts of flux material, and a thin protective layer of organic resin material such as a clear acrylic material covers the conductive strips prior to first use of the beating unit. The material comprising the protective layer cleanly vaporizes away when the heatink unit Is heated to a temperature in its normal operating range.

The. invention may be better understood and appreciated from the following detailed description taken in conjunction with the drawings, in which

FIG. 1 is a front perspective view of a portion of a cooktop appliance Illustratively embodying the present invention; FIG. 2 is a sectional side view of a portion of the cooktop of Fig. 1 taken along lines 2-2 of Fig. 3 showing details of one of the heating =its and the temperature sensor; 7 i 1 FIG. 3 is a top view of a portion of the cooktop of Fig. 1 showing details of one of the heating units and Its associated temperature sensor; and FIG. 4 is an enlarged view of a portion of Fig. 2 with portions exaggerated to Illustrate details thereof.

Fig. 1 Illustrates a glass ceramic cooktop appliance designated 10. Cooktop appliance 10 has a generally planar glass ceramic cooking surface 12. A metallic appearance trim member 11 borders cooktop surface 12. Circular patterns 13(a)-13(d) on surface 12 delineate the relative lateral positions of each of the four heating units (not shown) located directly underneath surface 12. A control and display panel generally designated 15 Includes a complete set of touch control keys 17 and a seven-segment digital LED display element 19 for each beating unit.

The term "glass ceramic" as used heicin refers to a re-crystallized glass material characterized by virtually zero thermal expansion such as the Ceran family of materials manufactured by Schott Claswerke and similar materials manufactured by Nippon Electric Glass Co. In particular, in the illustrative embodiment the glass ceramic material Is an infrared transmissive glass ceramIc material designated Ceran-85 manufactured by Schott Glaswerke.

A heating unit Is disposed beneath each of the circular patterns 13(a)13(d). In the discussion to follow the designator 14(a) shall be understood to refer to the heating unit disposed under pattern 13(a). Eeating unit 14(a) is shown in greater detail in Figs. 2 and 3. For purposes of Illustration. only one of the heating units Is shown In such detail. It will he understood that heating units similar in structure to that shown in Figs. and 3 are disposed beneath patterns 13(b)-13(d) as well.

1 Referring again to rigs. 2 and 3, beating unit 14(a) comprises an open coil electrical resistance element 16 of spiral configuration which Is designed when fully energized to radiate primarily In the - - Infrared (1-3 micron) region of the electromagnetic energy spectrum'.

Element 16 Is arranged In a concentric coil pattern and staked or otherwise secured to a support disk 18 formed of Micropore material which Is available from Ceramaspeed under the iname Microtherm. Disk 18 Is supported In a sheet metal support pan 20 by an Insulating liner 22 formed of an aluminum oxide, silicon oxide composition. This Insulating liner Includes an annular upwardly extending portion 22A which serves as an Insulating &pacer between base 18 and the ceramic glass cooktop 12. When fully assembled, pan 20 is spring loaded upwardly forcing the annular portion 22A of Insulating liner 22 into abutting engagement with the underside of cooktop 12 by support means (not shown). A gap 22B Is provided in the upper surface of annular portion 22A to avoid abrasion contact between portion 22A and conduction strips 30 where the strips extend between cooktop 12 and portion 22A. Heating units of this type are manufactured and sold commercially by Ceramaspeed under the part name Fast Start Radiant Heater with Concentric Coil Pattern.

AS noted above, it is important to limit the operating temperature of the glass ceramic plate to avoid damage to the plate due to overheating. A temperature sensor for this purpose Is provided in the Illustrative embodiment in the form of four pairs of conductive strips 30 formed on the underside of glass ceramic plate 12. One pair is associated with each heating unit. Strips 30 serve as electrical conductors and the glass ceramic material In the gap 32 between the strips is a resistance, the value of which varies as a function of the temperature of theglass. The sensor portion 30(a) 1 A; 1 4 k of strips 30.extends over Its associated heating unit. The tail portion 30(b) extends from the periphery of the heating unit area to near the edge of the cocktop, terminating at terminal pads 30(c). Terminal pads 30(c) facilitate connection of the sensors for external circuitry. A thin protective layer of clear acrylic resin 34 covers strips 30.

The temperature limiting control described and claimed in the aforementioned application No. 8728736, the disclosure in which is incorporated herein by reference, uses this type of temperature sensor arrangement.

In the illustrative embodiment the metallic strips 30 are formed by screen printing an organotallic paste commonly referred to as a resinate, preferably a noble metal resinate, on the glass ceramic surface In the desired pattern or configuration.

AS is also mentioned above, it is well-known to screen print organometallic pastes on glass ceramic surfaces and to fire the ceramic material at temperatures on the order of 700-870'C to bond the metallic film to the glass ceramic surface. The organo-metallic pastes or resinates commercially available and commonly used In such applications are typically proprietary formulations, typically having a significant amount of a flux material. As used herein the term flux or flux material refers to any one of many commonly used low melting point oxide binders which react with the glass ceramic substrate to form an adhesive band between the metal film and the glass ceramic when exposed to the high firing temperatures. Compounds of bismuth are frequently used as flux materials.

For applications of metallic patterns on ceramic glass In which the film portion of the glass will not normally be exposed to high temperatures on the order of 590 0 C' or higher, or In those 1 Z.

cases In wtLch the impact resistance of the glass surface is of little concern, the conventional approach utilizing a paste containing substantial amounts of flux works satisfactorily. However, In the Illustrative embodiment sensor portion 30(a) of the film which extends over the beating unit Is exposed to operating temperatures which may frequently exceed 590 0 C. In such an environment a flux constituent in the paste way continue to react with the glass ceramic material. This continuing reaction way reduce the Impact resistance of the glass ceramic substrate proximate the film. In the Illustrative embodiment the glass ceramic substrate Is the cooktop support surface. Impact resistance is of concern since the film pattern Is located precisely in the area where utensils are commonly supported and eh Is vulnerable to having utensils or other objects dropped onto the surface. Thus, use of pastes containing substantial amounts of flux for bonding Is undesirable at least for that portion of the conductive pattern which extends over the heating units.

However, without flux material to bond the film to the glass ceramic substrate, the metal film can be easily rubbed off. Typically, the film is applied to the glass ceramic cooktop support surface relatively early in the overall cooktop assembly process. Thus, the cooktop surface with the film applied may experience several handling and transportation steps prior to and during assembly of the cooktop appliance providing frequent exposure of the metal film to abrasive forces. Since removal of any portion of the film could significantly adversely affect the proper operation of the conductive strips as temperature sensors, abrasion of the film Is unacceptable.

The present Invention makes advantageous use of the fact that once the cooktop appliance is fully assembled the conductive film pattern on the lower inner face ofthe glass ceramic material Is no 6 longer exposed to abrasion. A high degree of adherence of the film to the glass ceramic, which is necessary to resist abasion prior to and during assembly, Is not needed after assembly.

Accordingly, In the method of the present Invention In its preferred form substantially f luxless organo-metallic paste is used, eliminating the potential adverse effect of the flux/ceramic glass reaction on Impact resistance. After application of the paste to the glass ceramic plate In the desired pattern, the plate is slowly heated to a temperature In the 315 0 C to 370 0 C range to decompose the organic components of the paste. This step leaves a bright metallic film on the glass surface in the desired pattern. Then after uffic-ient cooling, a thin protective layer of an organic resin, re-ferably a clear acrylic resin, is applied to the plate surface covering the metallic film and areas of the plate adjacent the film.

This thin protective coating when dry provides abrasion resistance for the metallic film prior to and during assembly of the cooktop appliance.

In order to prevent the protective coating from Interfering with proper sensor operation, a coating material is used which is characterized by a vaporization temperature low enough that during the first operation of the cooktop heating unit at its normal operating temperature, the resin layer cleanly vaporizes away, leaving only the metallic film on the surface of the glass ceramic plate.

In addition to providing the desired adhesion without compromising the structural integrity of the glass ceramic substrate, the conventional firing step can be omitted from the film deposition process. The heating step can be performed with a relatively low temperature heat source rather than a high temperature firing furnace enabling the entire process to be ftrried out with less costly equipment and fever processing steps.

a Essentially fluxless organo-metallic pastes or resinates suitable for use In the method of present Invention are readily commercially available. Satisfactory results have been achieved using pastes available from Engelhard Industries designated #7005 and #8041.

Normally these formulations contain flux but can he obtained from the manufacturer without flux by so specifying.

While the above-mentioned pastes have been found to provide satisfactory results in the Illustrative embodiment, It will be appreciated that the Invention herein Is not limited to use of any particular paste. There are many commercially available noble metal resinates which could be used in accordance with the present Invention provided the paste formulation contains no significant amount of flux material.

Similarly, the manner of applying the paste to the glass ceramic surface is not critical. In the illustrative embodiment the conductive strips were applied by screqn-prlnting the desired pattern through a 160 mesh nylon screen. It will be appreciated that other application processes could be similarly employed to apply the paste to the glass In the desired pattern, such as pad printing using a silicone rubber pad as a stamp, stenciling, or decal transfer.

Satisfactory protection against abrasion has been achieved using a sprayable acrylic resin commercially available from Borden, Inc. under the designation Krylon Crystal Clear Acrylic Spray Coating for layer 34. This coating decomposes cleanly with little or no visible smoke when exposed to cocktop operating temperatures which typically are in the range of 315 0 C to 540 0 C. A coating less than one mil thick has been found to provide sufficient abrasion resistance in the illustrative embodiment. As shown din Fig. 4, in which the relative thickness of the -1 protective layer of acrylic resin 1 1 1 -11is exaggerated for purposes of Illustration, the layer need not extend over the entire lower surface of the cooktop but merely needs to completely cover the conductive strips which are to be protected. While satisfactory results are achieved with the Krylon spray, the Invention Is not limited to this particular coating material.

Due to thinness and poor adherence, the conductive strips applied as hereinbefore described are not normally connected directly to external wiring. In the Illustrative embodiment terminal pads 30(c) which can be easily soldered are provided to facilitate connection of the strips to the external sensor circuitry. Terminal pads 30(c) are formed of a noble metal paste of the particulate type which is thicker than the aforementioned organo-metallic pastes. Such pastes preferably comprise gold particles In a glass binder. For the terminal pads, good adherence takes precedence over Impact resistance since the integrity of the electrical connection is critical. Rowever, since the pads are small, nominally one quarter inch square., theeffect on Impact resistance is small. This effect can be further reduced by strategically locating the pads in areas of the plate where Impact resistance is less critical and where the temperature is less extreme.

In the Illustrative embodiment terminal pads 30(c) are located at the edge of the cooktop. By locating the pads near the edge, the impact resistance over the central area of the support surface Is not compromised. Furthermore, the edge of the cooktop is supported by the countertop so that Impact resistance of the glass ceramic alone at this location is less critical; and the edge of the cooktop is sufficiently remote from the heating units that the pads are not exposed to high temperatures which might cause the reaction between binder and glass to continue after the firing process, as could occur If located In closer proximity to the heating unit.

The application of terminal pads to the glass ceramic plate requires some additional fabrication steps. The basic thin conductive strips are applied to the glass ceramic surface In the desired pattern as hereinbefore described. After drying the strips at approximately 120 0C, terminal pads are overprinted at the desired locations by silk screen printing or pad printing or other suitable technique, using a particulate paste. Next the pattern Is slowly heated to approximately 700- 870 0 C and held at that temperature for approximately 10 minutes before cooling. Finally, after cooling, the protective coating of clear acrylic resin Is sprayed over the pattern area to complete the process. In the illustrative embodiment the terminal pads are formed using the particulate paste available commercially from Englehart Industries under the designator A-2290.

Since the terminal pads must be fired to provide the desired 1 _5 adhesive bonding, the advantage of eliminating the firing step is lost. Alternatively, a paste requiring a lower firing temperature in combination with a joining means other than soldering could be used.

For example, silver paste E222B from Engelhard was fired satisfactorily at about 315 0 C.

Though most advantageously used with pastes which contain no flux as hereinbefore described, the temporary protective coating to resist abrasion can also be beneficially used with pastes having some flux In applications where even with the flux present the resultant abrasion resistance of the film is inadequate.

For example, a commercially available paste having otherwise desirable characteris tics may have sufficiently low flux content that the effect on Impact resistance of the glass ceramic is acceptable but the adhesive bond Is Inadequate. A clear organic resin which vaporizes cleanly away at operating temperatures could be applied over the film -1 4 1.

1 -13just as with no flux pastes, to provide the needed abrasion resistance prior to and during assembly of the end product.

Though the method of the invention as described herein is applied to a glass ceramic cooktop appliance, the utility and applicability of the method Is not so limited. The method could readily be employed for other products in which a thin metallic film pattern is to be applied to a glass ceramic surface using a no flux or low flux organo-metallic paste, when abrasion resistance is needed prior to and during assembly of the end product but is of no concern when the product Is fully assembled.

It is realized that numerous modifications and changes will occur to those skilled in the art. For example, the ceramic support surface could be incorporated in a gas cocktop in which the heating units comprise gas burners rather than electric elements. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true scope of the invention.

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Claims (10)

CLAIMS:

1. A method of applying a thin metal film to a glass ceramic plate comprising the steps of: depositing a thin layer of a metal paste onto the glass ceramic plate surface in a predetermined pattern; slowly heating the plate to a temperature sufficient to burn off the organic components of the paste; and covering the resultant pattern of conductive metal film with a thin protective layer comprising an organic resin material.

2. The method of Claim 1 wherein the metal paste comprises an organometallic compound substantially devoid of flux material.

3. The method of Claim 2 wherein the organic resin material is characterized by a vaporization temperature less than the temperature to which the glass ceramic plate is exposed in normal use -whereby the resin material cleanly vaporizes away when the glass plate is put in use.

4. The method of Claim 3 wherein the organic resin material comprises a clear acrylic resin.

5. A method of applying a thin metal film to a glass ceramic plate comprising the steps of: depositing a thin layer of a noble metal paste onto the glass ceramic plate surface in a predetermined pattern, said metal paste comprising an organo-metallic compound substantially devoid of flux material; I -is- slowly beating the plate to a temperature sufficient to burn off the organic components of the paste; and covering the resultant conductive metal film with a thin protective layer of a clear organic resin material to protect the metal film against abrasion said clear organic resin material characterized by a vaporization temperature less than the temperature to which the Class ceramic plate Is exposed In normal use whereby the resin material cleanly vaporizes away when the glass plate is put in use.

6. The method of Claim -5 wherein the clear organic resin material comprises an acrylic resin.

7. A glass ceramic support surface for use In a heating apparatus, said support surface comprising:

is a glass ceramic plate member for supporting objects to be heated, said plate having one or more heating areas which overlie corresponding heating units when fully assembled in a heating apparatus; a pair of parallel conductive strips formed on the lower surface of said plate member beneath each beating area, said strips extending from the periphery of the beating area Inwardly toward the center thereof and adapted for connection to external circuitry for measuring the resistance of the glass between said strips as a measure of the temperature of the glass in the beating area, said strips being formed of a noble metal paste, said paste being substantially devoid of any flux material; and a thin protective layer of an organic resin material adhering to the lower surface of said ceramic plate member covering raid conductive strips, said layer being removable by vaporization when heated to the normal operating temperature of the support surface when fu2-ly assembled and operating in a heating apparatus, whereby raid conductive ttrips are protected from abrasion during handling of the glass ceramic plate prior to and during assembly of the heating apparatus.

8. A household cooking appliance of the type having a glass ceramic plate for supporting loads placed thereon to be heated, at least one heating unit disposed beneath the glass ceramic support surface to heat loads supported thereon, and temperature sensing means comprising a pair of generally parallel. conductive strips for each beating unit formed on the underside of the glass ceramic plate and extending across at least a portion of the support surface overlying the heating unit, wherein: ' the conductive strips are formed of a noble metal paste applied to the glass ceramic surface, said paste being devoid of any flux material; and a thin protective layer of organic resin material is applied to the underside of the cooking surface covering the conductive strips, said resin material vaporizing away when the heating unit is heated to its normal operating temperature range; whereby said conductive strips are protected from abrasion during handling of the glass ceramic plate prior to and during assembly of the cooking appliance.

9. An appliance as in Claim 8 wherein said organic resin material comprises a clear acrylic resin.

10. A glass ceramic plate, or apparatus incorporating such plate, provided with an applied conductive metallic film substantially as hereinbefore described with reference to the accompanying drawings.

Published 1988 C1 The Patent 051oe- St-- House. 66 71 Hign Holborn. London WC1R 4TP- Further copies may be obta=ed frOM The Patent Office. Sales Brancl, St Ma:7 Cray. Orpington, Kent BRE 3RD. PnMed by Multiplex tecll=cries ltd. St Mai7 Cray. Kent. Con 167 k