Beautiful Failure

These fascinating blue-white tendrils of electricity, forming a ring of tiny lightning bolts all along the edge of a ceramic substrate, while interesting to look at, represent failure. Semiconductor power modules must pass a high potential (or hipot) test, in which high voltage (much higher than the module’s operating rating--in this case the hipot test is conducted at about 10,000 volts) is applied to the current carrying components. If a module is built correctly, this high voltage will not find a conductive path through the insulation layer to the metal base plate beneath—no bright arc of current will zap across a gap. A passing hipot test is boring to watch. Sometimes, the exact nature and location of an unwanted conductive path, leading to isolation (or hipot) failure, is hard to define. Photos such as this can sometimes help provide the necessary clues to solve the mystery.

(This image was taken through a half-inch thick Lexan safety housing, which is interlocked so no human can come into contact with dangerous current. The arcing is taking place within an inch-thick layer of silicone dielectric potting gel, which is used to fill most power modules.)

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