Flat body, particularly for use as a heat sink for electronic po

Details Flat body, particularly for use as a heat sink for electronic po Flat body, particularly for use as a heat sink for electronic po

1989-02-24

1991-04-30

Hille, Rolf

357 80, H01L 2336

Patent

active

050123246

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a flat body having the features recited in the prior art part of claim 1. Such flat bodies may consist, e.g., of a thin alumina slab, which is covered on both sides by respective metal plates, and are known as heat sinks for electronic power components, e.g. in igniting devices for Otto engines in automobiles. At such locations said bodies must have a high resistance to thermal shock. The resistance of the previously known bodies to thermal shock is inadequate because the coefficients of thermal expansion of the ceramic slab, on the one hand, and of the metal plates, on the other hand, differ greatly so that temperature changes in the body will give rise to correspondingly strongly changing tensile and compressive stresses which are parallel to the surface and may result in the ceramic slab in a fracture which is parallel to the surface (conchoid fracture). The resistance to thermal shock will be inadequate particularly if the metal plates are joined to the ceramic slab without an interlayer (direct bonding process). The thermal stresses which occur may be compensated in part by an interlayer of a solder alloy which has a high ductility.
It is an object of the invention to provide flat bodies which are of the kind described first hereinbefore and have an improved resistance to thermal shock.
That object is accomplished by flat bodies having the features recited in claim 1. Advantageous further developments of the invention are a subject matter of the dependent claims.
It has been found that a substrate which is covered by metal plates on both sides will have a much higher resistance to temperature shock if the metal plates have marginal strips reduced in thickness. To provide such marginal strips, the metal plates may be beveled and a bevel at an angle of about 20.degree. to the surface of the ceramic slab is particularly suitable. But it is preferred to use metal plates which are reduced in thickness at the edge by being stepped. Marginal strips defined by a single step will be sufficient to effect a substantial increase of the resistance to temperature shock. But the resistance to temperature shock can be further improved if the thickness of the metal plates is reduced toward the edge in a plurality of steps. Where only a single step is provided, it will be recommendable to provide a marginal strip in a thickness of 40 to 50%, most preferably a thickness of about 45%, of the thickness of the metal plates.
The width of the marginal strips which are reduced in thickness is suitably 1 to 3 times, most preferably 1.5 to 2 times, the thickness of the metal plates.
The greatest improvement of the resistance to thermal shock will be achieved if the metal plates are provided with marginal strips which are reduced in thickness at all edges, i.e., all around their periphery. If this is not desired, however, for the sake of a less expensive manufacture, it will be recommendable to provide the metal plates at mutually opposite edges with marginal strips reduced in thickness. From the aspect of manufacturing technology it will be particularly suitable to use rectangular metal plates which are stepped at two parallel edges; such metal plates may continually be made from a metal strip which is milled at its two longitudinal edges to provide the thinner marginal strips and which is subsequently divided into individual metal plates.
In order to achieve a high resistance to thermal shock it is also recommended to provide identical and coextensive metal plates on both sides of the ceramic slab so that the ceramic slab will not be subjected to also to flexural stresses in case of a temperature change.
In accordance with the invention it is particularly possible to make bodies which have a high resistance to thermal shock and consist of a thin ceramic slab which is directly bonded to metal plates made of a material which has a particularly high thermal conductivity, particularly copper. The disadvantage which is involved in known bodies laminated by direct bonding relative to soldered bodies as rega

REFERENCES:
patent: 3717798 (1973-02-01), Kahn
patent: 3829598 (1974-08-01), Darnell
patent: 4556899 (1985-12-01), Kurihara et al.

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