A very unusual transistor failure,caused by a solenoid

By a customer providing industry electronics, we were faced to repeated failures of a small power transistor. However, nearly all samples were burnt and no conclusion could be drawn from device analysis. We took the next step forward towards system analysis. The electronic board with the transistor was mounted at the end of a big solenoid. The metal baseplate of the board holder also served as a heatsink for the power transistor. The board was mounted into a plastic housing, which has been damaged in many cases. It turned out that, when the big solenoid's core was running against its mechanical end stop, the mechanical impulse knocked the electronic board to the end of the plastic housing, thus deforming the heatsink plate in a manner that it lost the thermal contact to the transistor after some time of use. In result, the transistor overheated and failed. This case study is a nice example showing the need of a system-related failure analysis for root-cause finding.