In the course of a testing program to evaluate double-drift-region silicon IMPATT diodes more than 400X-band diodes from 3 different wafers have been processed and operated under accelerated conditions. A representative number of failures from this testing program have been analyzed and 3 chip failure mechanisms have been identified. These are: short-circuiting recrystallized channels in the 1) interior, or on the 2) surface of the mesa, and 3) metal bridges over the p-n junction. Examples are shown in a number of SEM micrographs. The different failure mechanisms can be correlated in a systematic way to mechanical, electrical, and thermal stressing of the diodes and to differences in wafer properties.