Substrates for supporting electrical tracks and/or components

A substrate for supporting electrical components, such as thick film resistive heating elements, comprises a plate member, such as a metallic plate member, coated on one or both of its flat surfaces with a glass ceramic material. It has been found that the problems of (a) electrical breakdown between the metallic plate member and the thick film resistive heating element and (b) lack of adhesion between the thick film and the glass ceramic material can be substantially reduced or eliminated by reducing the porosity of the glass ceramic material. Methods of producing a glass ceramic layer having a low porosity, involving a two-stage heating process, are described.

This invention relates to substrates intended to support electrical 
components, for example thick film resistive heating elements, and it 
relates especially, though not exclusively, to such substrates which 
comprise a metallic plate member coated on one or both of its flat 
surfaces with a glass ceramic material. The invention also provides a 
method of manufacturing such substrates. 
Such substrates are known, one being available under the trade name 
KERALLOY from Wade Potteries plc, and have been proposed for use in 
supporting resistive heating elements applied, for example, as thick films 
by screen printing, and intended for domestic usage, for example as hob 
heating elements. 
GB No. 990023 (Associated Electrical Industries Limited), for example, 
discloses a printed electrical heater assembly comprising a metal backing 
member, a heat resistant electrically insulating coating formed of e.g. a 
ceramic on at least one surface of said metal and a conductive coating 
formed on said insulating layer or layers of a material having a suitable 
conductivity and pattern to form an electrical heater circuit or circuits. 
The metal backing member having a heat resistant electrically insulating 
coating on at least one surface provides the substrate for the conductive 
coating. 
Difficulties arise in practice, however, with the use of such substrates 
under the exacting operational conditions associated with hob units. In 
particular, it has been found that electrical breakdown can occur between 
the thick film resistive heater and the metallic plate member included in 
the substrate, which is generally held at earth potential, when mains 
voltage is applied to the track. Furthermore, the thick film resistive 
heater track can exhibit lack of adhesion to the glass ceramic material. 
It has been determined by the inventor that both the above-identified 
difficulties can be substantially reduced or eliminated by ensuring that 
the percentage porosity of the glass ceramic coating material, as defined 
hereinafter, is rendered less than or equal to 2.5 and the invention 
provides a substrate having a glass ceramic coating of such low porosity 
and a method of producing such a substrate. 
According to the present invention, there is provided a substrate for 
supporting electrical components, said substrate comprising a plate member 
having on at least one surface a layer of a glass ceramic material wherein 
the percentage porosity of the glass ceramic layer, as defined 
hereinafter, is equal to or less than 2.5. 
By percentage porosity is meant the porosity at a random cross-sectional 
plane through the substrate perpendicular to the plate member expressed as 
the percentage ratio of the cross-sectional area of pores on the plane to 
the cross-sectional area of the remainder of the glass ceramic layer on 
that plane.

Referring now to FIG. 1, there is shown a substrate including a support 
plate 1, made of e.g. metal or a glass ceramic material of suitable 
thickness to provide rigidity, coated on either side with a glass ceramic 
material 2,3, such as a calcium magnesium alumina silicate. The glass 
ceramic coatings 2,3 are applied by screen printing powdered glass ceramic 
material on to the support plate, or by electrophoresis. It is a 
characteristic of glass-ceramic materials that they can be caused to 
crystallise by the application of heat, and it is usual in this field for 
the powdered coatings of amorphous glass to be caused to crystallise, thus 
converting them into continuous glass ceramic layers, by heating the 
entire substrate, in a single-stage process, up to a temperature in excess 
of 1000.degree. C., above the material's softening point, at which it 
crystallises rapidly. The material is then allowed to cool. 
Substrates prepared in this way, however, tend to exhibit an undesirably 
high degree of porosity, the percentage porosity value being determined 
e.g. as shown in FIG. 2 by making a random cross-sectional cut through the 
substrate perpendicular to the plane of the support plate. The ratio of 
the area of all pores such as 4 sliced through by the cut to that of the 
remainder of the glass ceramic layer in the plane of the cut is called the 
porosity ratio and is conveniently expressed as a percentage (P). It is a 
characteristic of this invention that the value of P is equal to or less 
than 2.5. This compares with values of P of 4.0 or more achievable by more 
conventional processing. 
The desirably low values of P required by the invention are achievable, the 
inventor has determined, by observing that the powdered glass ceramic 
coating can be converted into a continuous layer by means of a two-stage 
heating process, in the first stage of which the substrate is heated, not 
to the aforementioned temperature in excess of 1000.degree. C., at which 
crystallisation occurs rapidly, but rather to a temperature above the 
softening temperature of the glass ceramic material, but below the 
temperature at which rapid crystallisation occurs, e.g. in the range of 
from 800.degree. C. to 890.degree. C., preferably in the range of from 
800.degree. C. to 875.degree. C. for the aforementioned calcium magnesium 
alumina silicate, at which the material has softened appreciably but 
crystallises only slowly, for a time dependent upon the temperature 
concerned, but typically of the order of five to thirty minutes. This time 
is dependent upon the rate of crystallisation and the viscosity of the 
material in its softened state. At the lower end of this range, the 
viscosity of the coating material is high, but crystallisation is slow and 
an extended time may be allowed for pores to close. At the upper end of 
the range, the viscosity of the coating is markedly reduced, and, 
although, crystallisation is relatively rapid, the majority of pores are 
found to close before an appreciably crystalline layer is formed. For the 
aforementioned calcium magnesium alumina silicate, in the first stage of 
the process the material is preferably heated at 875.degree. C. for 7 
minutes. The mechanism of pore closure is believed to be primarily that of 
surface tension. 
The second stage of the process, which involves the rendering permanent of 
the glass ceramic state by heat treatment, similar to that conventionally 
used, and as mentioned above, is to raise the coating temperature to a 
value (e.g. in excess of 1000.degree. C. for the aforementioned calcium 
magnesium alumina silicate) at which rapid crystallisation occurs, but 
below that at which the crystals redissolve, the rapid crystallisation 
producing a glass ceramic layer. The end result is the production of a 
substrate in which the glass ceramic layers exhibit percentage porosities 
of 2.5 or less. This is found to reduce considerably the incidence of 
failure of heater units by electrical breakdown and also improves adhesion 
of the thick film resistive heater track to the glass ceramic material. 
In another method the substrate is produced by the application of a 
plurality of glass ceramic layers to the support plate, each individual 
layer being produced by the two-stage heating process. The inventor has 
found that the electrical breakdown characteristics of the substrate 
depend markedly on and are improved by the number of glass ceramic layers 
used, even if the overall thickness of the composite is the same. The 
reason for this appears to be that pinholes may be produced during the 
formation of a layer which are too large to be completely closed during 
the first stage of the two stage heating process, but that there is a very 
small chance that pinholes in successive layers will coincide to provide a 
complete path from the electrical component to the metallic support plate. 
It is also possible to produce the substrate by applying a plurality of 
glass ceramic layers, each individual layer being treated using the first 
stage of the heating process before the next layer is applied. The 
composite layer may then be rendered permanent using the second stage of 
the two-stage heating process. Substrates produced using this method do 
exhibit some improvement in their electrical characteristics. 
The use of screen printing to apply glass ceramic coatings to produce the 
substrate is particularly applicable to the methods as described in 
accordance with the present invention. To provide a glass ceramic layer of 
suitable thickness, e.g. 100 .mu.m, four coatings of glass ceramic 
material are printed onto the support plate, the whole then being fired 
using the two-stage heating process. Alternatively, the two-stage heating 
firing is used to produce a first glass ceramic layer after two coatings 
have been printed, following which a subsequent two coatings are printed 
and fired by the two-stage heating process. The resulting glass ceramic 
layer produced in this method is of the same thickness as that produced by 
the aforementioned method but has significantly improved electrical 
breakdown characteristics. 
In another method using screen printing, two coatings are printed and then 
fired using the two-stage heating process. This is repeated a further two 
times to produce a glass ceramic layer of greater thickness e.g. 150 
.mu.m. The further significant improvement in electrical breakdown 
characteristics for the glass ceramic layer produced by this method is 
believed to be caused by the combination of multiple firings and the 
greater thickness of the glass ceramic layer. 
In producing substrates using screen printing, it has been found that, 
provided that the composite glass ceramic layer on the substrate is of 
suitable thickness, two is the optimum number of coatings to be printed 
and then fired at the same time using the two-stage heating process. The 
advantage of this may be in the production of a glass ceramic layer of 
sufficient thickness whose state, including the position of any pinholes, 
has been rendered permanent, before the next layer is applied. It is 
possible that, if an individual glass ceramic layer, applied and fired 
using the two-stage heating process, is not of sufficient thickness, the 
benefit of using multiple firings is lessened. 
FIGS. 3a and 3b show typical thick film resistive heating tracks 10 and 20 
printed in known manner on to the coated surface 2 of a substrate of the 
kind shown in FIG. 1. The track can be of precious metal or any other 
suitable material known to those in the art and the entire unit as shown 
in FIG. 3a or 3b is preferably overglazed with glass ceramic material. 
In use, a unit such as that shown in FIG. 3a or 3b, or a larger substrate 
containing, say, four individually energisable heating tracks may be 
deployed either beneath a conventional glass ceramic hob top to provide 
the heater units of a domestic hob or cooker, or as a hob unit itself. 
Heater units so provided have low thermal mass, and correspondingly a 
thermal response which is considerably faster than that of conventional 
cooker elements and can approach that of the recently developed technology 
which utilises halogenated tungsten filament lamps as heat sources. 
Clearly, the invention's use is not restricted to hobs and cookers. There 
are many domestic and industrial heating applications for which the 
invention would be suitable. Some non-limitative examples are kettle jugs, 
electric irons, space heaters, tumble dryers, and ovens. 
It will be appreciated that the heater units need not be formed as, or 
retained in the form of, a flat plate and other substrate configurations, 
such as cylinders and cones, can be used for certain applications if 
desired. Air can be forced over and/or through a suitably shaped heater 
unit, if desired, to distribute heated air to locations other than the 
immediate vicinity of the heater unit itself. 
The invention can also be used in low-power applications, where for 
example, resistive components desposited on a substrate need to be laser 
trimmed to a predetermined value of resistance. The low porosity exhibited 
by the glass ceramic on a substrate in accordance with the invention is 
beneficial because it reduces the incidence of uncontrolled rupture of a 
component being trimmed by a laser beam which can occur if the beam 
punctures a pore in the vicinity of the component. Such rupture usually 
causes the resistance value of the component to depart from tolerance and 
thus necessitates the scrapping, or at least reprocessing, of the unit.