The temperature dependencies of the failure mechanisms existing at various package elements have been investigated in terms of steady state temperature, temperature cycle, temperature gradient, and time dependent temperature change. The use of a simple Arrhenius expression to model microelectronic device reliability at all steady state temperatures is not correct because the temperature dependencies of the device are different at different steady state temperatures. Failure mechanisms such as reversion or depolymerization, contact spiking, and metallization migration occur at high temperatures encountered during fabrication or assembly are well normal operating temperatures. A physics of failure based device derating criteria has been proposed to aid the designer to enhance device life by means other than lowering steady state. The derating approach allows the user to examine the critical stresses in multichip module packages and reduce their value to obtain a desired mission life.