GB2157487A - Housing for an electrical component - Google Patents

Abstract

A housing for an electrical component uses an annular ceramic collar 7 as part of a sealed chamber within which the component 1 is mounted. A spring washer 8 urges the collar into contact with a disc electrode 3 which acts upon the upper surface of the component. Pressure seals at areas 11, 12 and 13 close the chamber containing the component 1. The chamber is purged and filled with dry nitrogen through tube 15. A plastics cover 16 provides protection for the upper face of the collar. The housing can be very compact and is economical in its use of ceramic material. <IMAGE>

Description

SPECIFICATION Housing for an electrical component This invention relates to a housing for a semiconductor component which is mounted within a sealed envelope in such a manner asto permit itto withstand the application of high voltages. In such a housing it is customary to mount the component between electrodes, at least one electrode of which is interior to the envelope which is primarily constituted by a large insulating wall memberformed of a ceramic such as alumina orthe like, typically in theform of an elongate hollowtube.

The present invention seeks to provide an improved housing which is inherently less bulky and is more economical in its use of ceramic insulating material.

According to a first aspect of this invention a housing for an electrical component includes an electrode held in pressure contact with the component by means of a ceramic body which forms part of a hermeticallysealedchamberwithinwhichthecompo- nent is located.

According to a second aspect of this invention a housing includes a planar semiconductor component held in pressure contact between two electrodes so as to have good electrical paths therewith, the pressure being exerted on one of the electrodes via a ceramic plate by a resilient member, both the resilient member and the ceramic plate defining part ofthe wall of a hermetically sealed envelope within which said component is mounted.

The invention is further described by way of example with reference to the accompanying drawing which shows a housing for a semiconductor component.

Referring to the drawing, the housingforthe semiconductor component is shown in an elevation section view which is largely circularly symmetrical about the axis XY. The semiconductor component 1, which typically is a high powerthyristor or the like, is held in close thermal and electrical contact with the upper surface at a relatively massive electrode 2 which also acts as a heat sink. During operation, the component can generate a great deal of heat as it may be a very high power device, and even if its operation is efficient it will inevitably dissipate heat, particularly during turn on and turn off periods. In orderto hold the componentata safe operating temperature it is importantto provide an efficient thermal path to a low temperature heat sink.So, that the electrode 2 is held atthe required temperature it is convenientlyformed with a series of heat shedding fins along its lower surface, although these are not illustrated. Whilstthe component is operative in an electrically non-conductive state it can be required to withstand an extremely high potential difference applied across its two major faces, one face of which is in contact with the electrode 2. In orderto permit this voltage to exist without causing electrical breakdown or tracking, it is necessarytomountthecomponentwithin aninertenvironment.

Electrical connection is made to the upperface of the component 1 by means of an electrode 3 which has a lower contact surface in the shape of a circular disc. It is provided with a central elongate extension 4 which extends to make contact to an elongate conductor bar 5. Athin disc 6 is placed between the lower surface of electrode 3 and component 1. This disc provides both a mating hard surface and a thermal match to the surface of the component.

The electrode 3 is annealed and can accommodate slight variations in surface condition so that an even pressure can be applied to the semiconductor component 1 itself, which may be of a relatively fragile and mechanically weak nature. Typically disc 6 is made of molybdenum sometimes suitably coated to permit friction-free movement during thermal cycling, yet generating a low resistance electrical path between component 1 and elecrode 3.

The electrode 3 is held in place by means of a ceramic collar7 which is provided with a central aperture through which the stem 4 ofthe electrode 3 passes. The collar 7 is urged into close contact with the base of the electrode 3 by means of a dished spring washer 8 which is retained by a clamping ring 9 which acts upon the outer circular periphery of the washer.

During assembly, downwards pressure is applied to the clamping ring 9 until the correct loading force is experienced bythe semiconductor 1. When this correct loading force has been reached, the perimeter ofthe clamp ring 9 is welded to the upper edge of a sleeve 10, the lower end of which is securely fixed, eg by brazing, to the electrode 2. The sleeve 10 is of course in this case formed of a metal material which can be readily brazed to form a hermetic seal.

Thus the washer8, the clamp ring 9, and the sleeve 10 are all at the same potential as the electrode 2, and are insulated from the electrode 3 by means of a ceramic collar7.

The component 1 itself is located in an inert gas, usually dry nitrogen, inside an hermetically sealed chamber. In orderto achieve sealing of the chamber, which is of an annular shape and completely surrounds the peripheries of the component 1, the base ofthe electrode 3 and the ceramic collar 4, three separate compression joints are reformed. The lowest joint, or seal 11, is formed between the flat lower surface ofthe ceramic collar 7 and three concentric ridges formed in the upper surface of the electrode 3.

This upper surface is tin-plated so that a sufficiently good hermetic seal is formed when the ceramic collar 7 is forced into intimate contact with the upper edges of these three rings as the tin deforms slightly to comply with the surface of the ceramic. Similarly the second seal 12 is formed between the inner circular edge ofthe dished washer8 and the uppersurface of the ceramic collar7. The washer itself is formed of a spring steel so as to provide the necessary resilience and loading force, but its surface is tin-plated so as to provide a conformal surface which makes the hermetic seal to the ceramic material.Thethird seal 13 is formed by the tin-plating on the outer perimeter of the washer 8 being in contact with the bearing surface of the clamp ring 9.

The pressure required to form these seals is ofthe order of 60 Newtons per square millimetre buta higher mounting pressure may be used if desired, dependent of course on the maximum loading force which the component 1 can safelywithstand.The electrical insulation is largely determined bythe surface properties ofthe ceramic collar 7 and it will be seen thatthere are two electrical leakage paths between the seals 11 and 12, one being around the outer surface ofthe collar 7, and the other along the central aperture through which the stem 4 protrudes.

Thus the position oftheseal 12 should not be too close to the inner surface ofthe stem 4.

The electrode 3 is centrally located with respect to the sleeve 10 by means of a silicone rubber iocator 14 which fits firmly inside the sleeve 10. When the three compression seals have been made the interior of the annularchambersoformed is purged andfilled with dry nitrogen via an out-gassing tube 15, the end of which is subsequently sealed. A plastic cover 16 is then inserted over the stem 4so that it provides some measure of protection against dust and dirtforthe upper surface ofthe components, particularlythe otherwise exposed surface ofthe ceramic collar 7.

Finally a cap 16, connected to the conductor bar 5, is inserted overthetop end of the stem 4 and crimped in place so as to retain the plastic cover 16.

Although in the drawing the plastic cover 16 is shown as having a substantial height and size, it can in practice be made very much smaller since it has no structural, electrical or hermetic sealing function to perform. The entire electrical insulation function is provided bythe ceramic collar7 which is relatively small and of a simple shape, therefore being relatively inexpensive to produce.

Claims (11)

1. Ahousingforan electrical component including an electrode held in pressure contact with the component by means of a ceramic body which forms part of a hermetically sealed chamberwithin which the component is located.

2. A housing including a planarsemiconductor component held in pressure contact between two electrodes so as to have good electrical paths therewith, the pressure being exerted on one of the electrodes via a ceramic plate by a resilient member, both the resilient member and the ceramic plate defining partofthewall of a hermetically sealed envelope within which said component is mounted.

3. A housing as claimed in claim 2 and wherein separatehermeticsealsareformed between the ceramic plate and the resilient member, and between the ceramic plate and said electrode.

4. A housing as claimed in claim 2 or 3 and wherein the hermetic seals are formed by pressure contact of the resilient member and the electrode respectively with the ceramic plate.

5. A housing as claimed in claim 4, and wherein the resilient member and the electrode each carry a coating of a malleable metal which deforms to constitute the hermetic seals.

6. A housing as claimed in claim 4 or and wherein the electrodesurface adjacentto the ceramic plate is provided with at least one raised ridge which defines the position ofthe hermetic seal.

7. A housing as claimed in any of claims 3 to 4, and wherein the electrode is provided with a central conductive stem around which the ceramic plate in the form of a collar is positioned.

8. A housing as claimed in claim 7 and wherein the electrode, and the ceramic plate are circular, and the surface of the electrode adjacenttothe ceramic plate carries a plurality of raised circular ridges which are concentric with said stem and which contact the ceramic plate to form a hermeticsealtherewith.

9. A housing as claimed in any of claims 3 to 8, and wherein the resilient member is fixed to a tubular memberthe base of which is secured to theotherof said electrodes in a hermetic manner.

10. A housing as claimed in claim 9 and wherein the resilient member is fixed to such a position of the tubular member asto determine the magnitude of the pressure applied to the component.

11. A housing for an electrical componentsub stantiaily as illustrated in and described with refer encetotheaccompanying drawing.