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Scanning electron microscopy was employed to characterize the structure of the soft-soldered joints in Si power transistors (type 2N3055), which failed electrically during thermal cycling. Cracks were observed in the soldered joint of many failed devices. Crack propagation seems to be accelerated by the presence of voids in the joint region. The importance of the reactions between the Ni layers and Sn of the .solder also affects the long-term reliability of the devices. Fractographs obtained from the failed devices reveal ductile fracture of the soft solder as well as creep or fatigue failure of the solder at the elevated temperature. Based on the structural studies of both failed and uncycled devices, some possibilities for improving device performance are discussed.