GB2217919A

GB2217919A - Pulse absorbent resistor

Info

Publication number: GB2217919A
Application number: GB8809502A
Authority: GB
Inventors: Duncan George Brodie
Original assignee: Crystalate Electronics Ltd
Current assignee: Crystalate Electronics Ltd
Priority date: 1988-04-22
Filing date: 1988-04-22
Publication date: 1989-11-01
Also published as: GB8809502D0

Abstract

A pulse-absorbing planar thick film electrical resistor comprises a ceramic substrate (4), a screen-printed and fired thick film electrical resistance element (1) and thick film electrically conducting terminations (2, 3). The resistance element (1) is covered with a layer (5) of a fired-on dielectric material which is vitreous material incorporating a particulate ceramic filler (eg alumina) or which is a glass-ceramic material. The dielectric material has a coefficient of thermal expansion appropriately matched to that of the substrate (4) and has been fired on at a temperature substantially the same as that used for firing the thick film resistance element (1). <IMAGE>

Description

ELECTRICAL RESISTOR This invention relates to planar film electrical resistors comprising a thick film resistance element on an electrically insulating substrate. More particularly it relates to resistors of this form intended for operation under conditions where a voltage pulse of short duration (typically less than 0.5 second duration) may be applied and where, during the period of application of the pulse, a high level of power dissipation occurs in the thick film resistance element. Such resistors are known as pulse absorbing resistor An applied pulse, which may occur when the resistors are used in telecommunications equipment and there is a power cross fault or lightning strikes a cable associated therewith, may result in the surface temperature of the resistors rising by several hundred degrees during the period of application of the pulse.

It is essential that pulse absorbing resistors do not suffer catastrophic failure when subjected to such pulse conditions and it is also generally specified that they must be able to endure a predetermined number of pulses in test conditions without the resistance value, measured between terminals of the resistor, changing very significantly. It is usual practice to provide a protective electrically insulating coating over the film resistance element and this coating must be able to withstand the high temperature which occurs during application of an electrical pulse to the resistor. The thick film resistance element is usually provided by screen-printing and firing a suitable paste onto a ceramic substrate and applying electrically conductive terminations in similar manner.

Screen-printable glaze compositions are available for screen printing as cover coats or over-glazes over such resistance elements and these are normally fired at a temperature (eg about 500 C) which is lower than that used to fire the electrical resistance composition onto the substrate 0 (eg 850 C). When such typical cover coat or over-glaze compositions are used on pulse absorbing resistors it is found, however, that during test under pulse conditions significant and unpredictable permanent changes in electrical resistance value occur and catastrophic failures sometimes occur in a test batch.

We have now found that superior performance of pulse absorbing electrical resistors of the type described results if, instead of applying the intended cover coat or over-glaze compositions, the resistors are covered with a layer of dielectric material of the type which is known to be used specifically as a multilayering dielectric in thick film hybrid circuit manufacture. Such a dielectric is suitably formulated to be fired at a temperature substantially the same as that used for thick film resistance and conductor elements (eg 8500C) and has a coefficient of thermal expansion which is appropriately matched to that of the substrate (eg alumina ceramic) normally used with such elements.

Examples of such dielectric materials, which are available for application as screen-printable compositions, are vitreous materials incorporating one or more particulate fillers of ceramic materials such as alumina.

Glass-ceramic materials may also be applied as the dielectric material.

The following example illustrates the invention, reference being made to the accompanying drawing which shows a cross sectional view of a pulse-absorbing electrical resistor according to the invention.

A pulse-absorbing planar thick film electrical resistor is constructed as follows. A thick film electrical resistance element 1 and thick film electrically conducting terminations 2, 3 for it are provided in known form on an alumina ceramic substrate 4 using well-known screen-printing and firing techniques An example of a suitable composition for providing the resistance element 1 is Du Pont type 4133, and the conducting terminations may suitably be formed from Du Pont type 6120. Both of these compositions are fired onto the substrate at 850 C.

A dielectric coating 5 is applied to cover the resistance element and suitably comprises a ceramic-filled glass composition. Such a composition is commercially available in a form for application by screen-printing and then firing. One such composition is Du Pont type 5704, which is 0 screen-printed and fired onto the resistor at 850 C and provides an alumina-filled glass coating.

The resistance element and terminations may be fired separately from, or co-fired with, the dielectric coating.

The resulting resistor is found to exhibit smaller changes in electrical resistance value and reduced incidence of catastrophic failure compared with resistors of the prior art employing lower-temperature firing glaze cover coat materials

Claims

CLAIMS 1 A pulse-absorbing planar thick film electrical resistor comprising an electrically insulating ceramic substrate having thereon a screen printed and fired thick film electrical resistance element and electrically conducting terminations therefor, said resistance element being provided with a covering of a layer of a fired-on dielectric material which has a coefficient of thermal expansion appropriately matched to that of the substrate and which has been fired on at a temperature substantially the same as that used for firing the thick film resistance element.
2 An electrical resistor according to Claim 1 in which the said dielectric material is of a type used as a multilayering dielectric in thick film hybrid circuit manufacture.
3 An electrical resistor according to Claim 1 or 2 in which the dielectric material comprises a vitreous material incorporating a particulate filler of a ceramic material.
4 An electrical resistor according to Claim 3 in which the said particulate filler comprises alumina.
5 An electrical resistor according to Claim 1 or 2 in which the dielectric material comprises a glass-ceramic.
6 An electrical resistor according to any preceding Claim, in which the said insulating ceramic substrate comprises an alumina ceramic material.
7 An electrical resistor according to any preceding Claim in which the said layer of dielectric material has been applied by screen printing.
8 A pulse absorbing planar thick film electrical resistor constructed and arranged substantially as hereinbefore described with reference to the accompanying drawing.