GB2051474A - Mounting arrangements for electrical components - Google Patents

Abstract

Thermal stresses which could lead to fracture of a plate 6 of boron nitride forming part of a thermally conductive electrically insulating mounting for an electrical component 1 such as a high power thyristor, are minimised by providing thin sheets 7, 8 of a material of relatively low coefficient of expansion (eg Mo or W) between plate 6 and adjacent massive bodies 4, 5 which typically are of Cu. A casing 9, 10, 11 may surround the boron nitride plate to contain an electrically insulating oil in which it is immersed. <IMAGE>

Description

SPECIFICATION Mounting arrangements for electrical components This invention relates to mounting arrangements intended for use in an assembly by means of which an electrical component can be mounted on a heat sink. It is common to mount high power electrical components on heat sinks so as to prevent the components reaching excessively high temperatures when operating. Certain kinds of electrical components can be seriously damaged or their performance greatly impaired if they are allowed to become too hot. The maximum operating temperature is particularly critical for semiconductor and other electronic components, and great difficulties can be encountered in mounting high power semiconductor components in a manner which enables them to be operated at very high power levels without overheating.

For this reason it is common to mount a component on a heat sink in such a way that a very good thermal path exists from the component to the heat sink, and this can be a relatively straightforward matter to arrange in those cases where the heat sink need not be electrically insulated from that portion of the electrical component which is adjacent to it.

However it is extremely difficult to provide a good thermally conductive path which at the same time is an electrically insulating path, since the use of conventional electrical insulating materials seriously degrades the thermal properties. The use of boron nitride has been proposed, since this material is an excellent electrical insulator, and is an acceptably good thermal conductor if its thickness is sufficiently small. Practical difficulties have prevented the adoption of this material on a wide scale since it has proved unexpectedly liable to mechanical fracture and consequent electrical breakdown, and the present invention seeks to provide an improved mounting arrangement for electrical components.

According to this invention a mounting arrangement for an electrical component includes a plate of boron nitride positioned between two relatively massive bodies composed of a material which is electrically and thermally conductive, and sheets of material positioned on both sides of the plates so as to be interposed between the plate and the respective bodies, the material having a coefficient of thermal expansion the value of which is between the corresponding values for boron nitride and the material of which said bodies are composed.

Although one of said bodies may be a heat sink or part thereof, this is not preferred; instead preferably both of said bodies are formed of copper. In this case one of said bodies is, in use, mounted in contact with said heat sink, and an electrical component is mounted in thermal contact with said other body. An electrode may be positioned between the electrical component and said other body.

Preferably the sheets of material are thinner than the thickness of said plate. This minimises thermal impedance of the thermal path from the component to the heat sink. The material is chosen with its strength in mind so as to enable it to withstand thermal stress.

Preferably the material is molybdenum or tungsten.

Preferably the lateral dimensions of boron nitride are greater than that of either body.

This provides long electrical creep paths and minimises the risk of electrical tracking or breakdown The presence of the additional layers reduces thermally induced mechanical stress in boron nitride when the mounting arrangement becomes hot. However, to prevent an electrical short circuit occurring even if the boron nitride plate cracks or otherwise becomes damaged by thermally induced mechanical stresses preferably the boron nitride plate is mounted in an electrically insulating liquid. If a crack should appear the liquid will immediately fill it, thereby maintaining the electrical insulation. Preferably the liquid is a silicone oil.

Preferably each layer is located in shallow recesses in respective ones of said bodies.

This prevents undesirable movement of the layers relative to the bodies and facilitates the assembly of the mounting arrangement.

The invention is further described by way of example with reference to the accompanying drawing which illustrates a mounting arrangement in accordance with the present invention.

Referring to the drawing, a high power semiconductor device, in this case a thyristor 1, is mounted so as to be in good thermal communication with a finned heat sink 2. The thyristor 1 is positioned between a top copper electrode 3 in the form of a bus bar and a lower electrode 12 which is also in the form of a bus bar. In operation, a very large potential difference can exist between the electrodes 3 and 12 and when the tyristor is rendered conductive by means of a control signal applied to a control electrode (not shown), the thyristor can conduct very large currents. In so doing it can become extremely hot and in order to prevent the thyristor overheating the heat must be conducted rapidly to the large finned heat sink 2.

Many kinds of electrical components can be seriously damaged if they are allowed to over heat and this applies particularly to semiconductor devices. The performance can be seriusly impaired or degraded if its operating temperature exceeds a certain value and furthermore it can be permanently damaged or destroyed if it is allowed to overheat. For this reason the thermal environment of the thyristor is of great importance. The heat flows from the thyristor 1 to the heat sink 2 via two relatively massive copper bodies 4 and 5, the latter being in close contact with the heat sink.

For many applications it is important to electrically insulate the thyristor 1 from the heat sink 2 and for this purpose a thin plate of boron nitride is positioned between the two massive copper bodies 4 and 5. Boron nitride is a good electrical insulator, but possesses a thermal conduction property which is very much better than most electrical insulators, although, of course, it is not such a good thermal conductor as for example, copper. In order to minimise the thermal impedances the plate 6 is made as thin as possible and typically it is about 2 millimetres thick. Boron nitride is a relatively fragile material having a low coefficient of thermal expansion and it has been found that when the mounting assembly becomes hot the boron nitride plate can fracture.To reduce the likelihood of thermally induced mechanical stresses arising, additional sheets 7 and 8 are positioned on either side of the plate 6. These sheets are thin molybdenum plates having, for example, a thickness of between zmillimetre and 1 millimetre. Instead of molybdenum, thin sheets of tungsten could be used instead. The sheets 7 and 8 are in the form of flat discs which are located in respective circular recesses formed in the surfaces of the bodies 4 and 5.

The coefficient of thermal expansion for the various materials are approximately as follows Boron nitride 1 X 10-6 per'C Molybdenum S X 10-6 per'C Tungsten 4.5 X 10-6 per C Copper 16 x 10-6 per C It will thus be seen that the coefficient of expansion of molybdenum is very much less than that of copper and this significantly reduces the mechanically induced stresses which are applied to the boron nitride when the copper bodies 4 and 5 expand and contract.

Because relatively large potentials can exist between the two bodies 4 and 5, the diameter of the boron nitride plate 6, which is also in the form of a disc, is made significantly greater than the diameter of the molybdenum discs so as to provide a relatively long electrical tracking path. However, if the molybdenum plate should fracture electrical discharge may occur through the crack so formed, and to reduce the likelihood of this happening the boron nitride plate 6 is immersed in an electrically insulating oil such as silicone oil. The oil is held in an enclosure defined by a pair of annular discs 9 and 10 which respectively surround a body 4 or 5 and these two discs are joined together by an annular ring 11 formed of an electrical insulating ceramic.The outer surface of the ring 11 is provided with deep flutes so as to provide a relatively long electrical trackig path to reduce the likelihood of electrical break down. The enclosure so formed is sealed and contains the silicone oil In practice, a large clamping force is applied in the direction of the arrow A so as to firmly press the mounting assembly into close contact with the heat sink 2 and typically tie bolts would be used, although these are not shown.

Although the electrical component (the thyristor 1) is shown sandwiched between the upper electrode 3 and the lower electrode 12 this need not necessarily be the only practicable arrangement and a configuration similar to that shown in our U.K. patent specification 1,501,388 may be used. In this arrangement the body 4 would have a square section instead of a circular section and the electrical component would be sandwiched between one of the four edge surfaces of the body 4 and a further relatively massive body (not shown) .

Although the thickness of the molybdenum is less than that of the boron nitride, it is a relatively strong material and is more capable of withstanding the thermal stresses involved.

Claims (12)

1. A mounting arrangement for an electronic component including a plate of boron nitride positioned between two relatively massive bodies composed of a material which is electrically and thermally conductive, and sheets of material positioned on both sides of the plate so as to be interposed between the plate and the respective bodies, the material having a coefficient of thermal expansion the value of which is between the corresponding values for boron nitride and the material of which said bodies are composed.

2. An arrangement as claimed in claim 1 and wherein both of said bodies are formed of copper.

3. An arrangement as claimed in claim 2 and wherein one of said bodies is mounted in contact with a heat sink.

4. An arrangement as claimed in claim 2 and wherein the electrical component is mounted adjacent to and in thermal contact with the other of said bodies.

5. An arrangement as claimed in claim 4 and wherein an electrode is positioned between the electrical component and the other of said bodies.

6. An arrangement as claimed in any of the preceding claims and wherein the sheets of material are thinner than the thickness of said plate.

7. An arrangement as claimed in claim 6 and wherein said sheets of material are cmposed of molybdenum or tungsten.

8. An arrangement as claimed in any of the preceding claims and wherein the lateral dimensions of boron nitride are greater than that of either body.

9. An arrangement as claimed in any of the preceding claims and wherein each layer is located in a shallow recess in respective ones of said bodies.

10. An arrangement as claimed in any of the preceding claims and wherein the boron nitride plate is mounted in an electrically insulating liquid.

11. An arrangement as claimed in claim 10 and wherein the liquid is a silicone oil

12. A mounting arrangement for an electronic component, substantially as illustrated in and described with reference to the accom- panying drawing.