Reliability and thermal impedance studies in soft soldered power transistors

This paper discusses the reliability of soft soldered silicon power transistors under various thermal cycling conditions. A thermal cycling system is described which provides electrical testing and thermal impedance measurements of the devices until they fail. The thermal impedance measurement technique and the cycling parameters of the devices tested are discussed. Differences between computed and measured steady-state thermal resistance indicate interface problems associated with the fabrication of soft soldered devices. The study is focused on 2N3055 power transistors from a variety of manufacturers. It is shown that the thermal impedance is sensitive to changes in the Si-header joint and will increase upon cycling due to the thermal fatigue process. Information on mechanical failure of joints is obtained from scanning electron microscopy studies on the failed devices. The important parameters that affect the lifetime of the devices are indicated, and their effects are shown quantitatively. It is concluded that thermal impedance measurements can provide a reliable probing technique for imperfections in the soft soldered contacts found in many power devices.