We have studied the behavior of ionic impurities in the insulation layer of metal-base PWBs and carried out nondestructive detection of copper ionic migration using the pulsed electroacoustic method for measuring the space charge. Space charge polarization was observed initially and after pre-absorption under high-temperature, high-humidity conditions. It appears to be attributable to the ions present initially in the insulation layer. The space charge measurement detected a conductive region formed in the insulation layer near the anode after bias testing under hightemperature, high-humidity conditions (THB test). Element distribution analysis verified, it as a region of copper ionic migration. Growth of copper ionic migration in the insulation layer was detected nondestructively.

Since a metal-base printed wiring board (PWB) with superior thermal conductivity has been widely used in power electronics products, high reliability is required at high electric field. Many reports have already covered surface' copper ionic migrations on PWBs which occur short-circuits on the board. The migration growing into the depth direction of a insulation layer has been clearly observed recently, and it is revealed that the growth depends on the ionic impurities of the resin.We measured space charge behaviour in the insulation layer to know the relationship between the internal space charge and the ionic migration. The results suggest that internal negative charges are generated at high temperature with high humidity, and that a conductive region is formed near a copper electrode under dc electric field.

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