Insulated metal substrates (IMS) constructed with three layers a metal base plate, a thin insulation layer and a copper conduction foil have been applied to power circuits of 100-400 V. In such IMS, the insulation layer is stressed under a high strength electric field of 1-3 kV/mm. This paper describes the results of enhanced thermal humidity bias tests (THB) (85/spl deg/C,70%RH,DC800V,1250 V) for the insulation layer. It was clarified that copper ion migration occurred in the insulation layer and that its occurrence was related to the amount of ionic impurity in insulation material. The new IMS which the authors have developed has increased resistance to such copper ion migration.
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