GB2207006A

GB2207006A - Laser trimmed resistor

Info

Publication number: GB2207006A
Application number: GB08716404A
Authority: GB
Inventors: Duncan George Brodie
Original assignee: Crystalate Electronics Ltd
Current assignee: Crystalate Electronics Ltd
Priority date: 1987-07-11
Filing date: 1987-07-11
Publication date: 1989-01-18
Anticipated expiration: 2007-07-11
Also published as: GB2207006B; GB8716404D0

Abstract

A planar electrical resistor comprises a flat substrate (7) of electrically insulating material, an elongate film resistance element (8) on its surtace and electrically conductive terminals (9) in contact with opposite ends of the element (8). A plurality of laser-formed cuts are provided through the element (8) to form a plurality of elongate closed loops (10A, 10B, 10C) spaced across the element in side-by-side relationship, whereby a desired resistance value between the terminals (9) is obtained. When a high energy electrical pulse of short duration is applied to the terminals (9) uniform thermal energy distribution is obtained over the surface of the substrate (7) and risk of fracture thereof is minimised. <IMAGE>

Description

ELECTRICAL RESISTOR This invention relates to planar film electrical resistors comprising a 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) is applied and where during the period of application of the pulse, a high level of power dissipation occurs in the film resistance element. Such a pulse may result in the surface temperature of the resistance element rising by between 1000C and 5000C during the period of application of the pulse.

It is widely known to provide planar film resistors for this purpose in which the film resistance element and electrically conductive terminals for it comprise screen-printed and fired thick film materials. it is also well-known that after providing the resistance element (which may often occupy almost the entire area of a major surface of the substrate) and its terminals, it is necessary to effect a trimming operation on the element to achieve a precisely required resistance value measured between the terminals.An approach which has hitherto been adopted for this purpose is shown in Figure 1 of the accompanying drawings and comprises forming, with a laser, a U-shaped cut 1 (sometimes also referred to as a box cut) through resistance element 2 provided on a substrate 3, the cut 1 extending from an edge of the element intermediate a pair of terminals 4 such that a portion 5 of the element 2 bounded by the cut is separated from the remainder of the element. By this means the resistance path between the terminals 4 is effectively narrowed along 50% or more of its length.Where the resistance element 2 occupies a large area of a major surface of the substrate 3 there is a risk that when a high-energy electrical pulse is applied to the terminals 4, unless the resulting thermal energy is distributed uniformly over the entire area of the substrate 3 fracture of the substrate 3 will occur as a result of localised overheating. When the resistor is trimmed using a U-shaped cut 1 as shown in Figure 1, the portion 5 of the resistance element bounded by the cut 1 carries no current and is therefore not directly electrically heated. The thermal energy resulting from the applied high-energy electrical pulse will therefore not be distributed uniformly over the surface of the substrate 3 and there is a serious risk of fracture of the substrate 3 occurring eg along the dotted line 6.

It is an object of the present invention to overcome or minimise this problem.

The present invention provides a planar electrical resistor comprising: a substantially flat substrate of electrically insulating material: an elongate film electrical resistance element on a major surface of said substrate; a pair of electrically conductive terminals disposed on said substrate in electrical contact with opposite ends of said element; said resistor being characterised by a plurality of laser-formed cuts provided through said element to form a plurality of elongate closed loops spaced across said element in side-by-side relationship, whereby a desired resistance value between said terminals is obtained.

The plurality of elongate closed loops are preferably uniformly spaced across the element.

By means of the invention, when a high-energy electrical pulse of short duration is applied to the terminals of the resistor, more uniform thermal energy distribution over the surface of the substrate is obtained, compared with the arrangement of the prior art.

The electrical resistance element and the conductive terminals suitably comprise screen-printed and fired thick film materials, such materials being well-known in the art.

The substrate suitably comprises a ceramic material, eg an alumina ceramic material.

One or more further cuts may be laser-formed transversely across said loops through portions of the resistance element which are bounded by the loops and isolated thereby from the remainder of the resistance element. Such further cuts serve to increase the voltage-withstanding properties of the resistor.

The invention is now described by way of example with reference to Figures 2 and 3 of the accompanying drawings which represent plan views of alternative embodiments of electrical resistor constructed according to the invention.

Referring to Figure 2, an embodiment of planar electrical resistor according to the invention comprises a substantially flat rectangular substrate 7 of an electrically insulating material, such as alumina ceramic material. An elongate thick film electrical resistance element 8 is provided by well-known screen-printing and firing techniques and covering a large proportion of the area of a major surface of the substrate 7. A pair of electrically conductive terminals 9 of thick film foam are likewise provided on the substrate 7 and in electrical contact with opposite ends of the resistance element 8.In order to trim the resistance element to provide a precisely required resistance value measured between the terminals 9, a plurality of cuts lOA, 10B, lOC are provided through the element 8 by means of a programmed laser, to form a plurality of elongate closed loops, uniformly spaced across the element 8 in side-by-side relationship.Although it would be preferred for each of the loops formed by the cuts lOA, 10B and lOC to be of substantially the same length, in practice this will be unlikely to be achievable and whereas two of the three loops (lOA, 10B) shown in Figure 2 may be formed of substantially the same length, the third (ie final) loop (lox) will usually be of different length, ie shorter or longer than the others, since it has to be adjusted so that it is completed when precisely the required resistance value is achieved, measured between the terminals 9.

When a high energy electrical pulse of short duration is applied to the terminals 9 of the resulting resistor, the resulting thermal energy is distributed across the surface of the substrate much more uniformly than in the prior art arrangement shown in Figure 1 and localised overheating and risk of fracture of the substrate 7 is minimised.

In practice it may be difficult to programme a laser to produce perfect closed loops of the form shown in Figure 2. A more practical arrangement is shown in Figure 3 which resembles that of Figure 2 in so far as resistance element 8 and terminals 9 are provided on substrate 7 and a plurality of cuts lOA, 10B, lOC, are provided through the element 8 by a laser to form a plurality of elongate closed loops. In this case however, it is found to be more practically convenient to provide a lead-in cut llA, lIB, llC with the laser for each loop, the laser being traversed in the direction of the arrows.

Generally the loops will be closed short of the starting point of the laser cut.

If required, a series of transverse cuts 12 may be provided with a laser across the loops lOA, 10B, lOC through those portions 13A, 13B, 13C of the resistance element which are bounded by the loops and are isolated thereby from the remainder of the resistance element. The use of such cuts is described in Applicants' co-pending Patent Application No 8712628 and they serve to increase the voltage-withstanding capability of the resistor by reducing voltage stress across the cuts forming the loops lOA, 10B, lOC.

Claims

1. A planar electrical resistor comprising: a substantially flat substrate of electrically insulating material: an elongate film electrical resistance element on a major surface of said substrate; a pair of electrically conductive terminals disposed on said substrate in electrical contact with opposite ends of said element; said resistor being characterised by a plurality of laser-formed cuts provided through said element to form a plurality of elongate closed loops spaced across said element in side-by-side relationship, whereby a desired resistance value between said terminals is obtained.

2. A resistor according to Claim 1 in which the plurality of elongate closed loops are uniformly spaced across the element.

3. A resistor according to Claim 1 or 2 in which the electrical resistance element and the conductive terminals comprise screen-printed and fired thick film materials.

4. A resistor according to Claim 1, 2, or 3 in which the substrate comprises a ceramic material.

5. A resistor according to any preceding Claim in which one or more further cuts are laser-formed transversely across said loops through portions of the resistance element which are bounded by the loops and isolated thereby from the remainder of the resistance element.

6. A planar electrical resistor constructed and arranged substantially as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.