This chapter determines the thermal stress limits on electronic packages for safe operation which ensures a desired useful mission life. The mechanism of wire fatigue involves the flexure of the wire about the reduced wire cross-section at the heel during temperature cycling. The mechanism has a dominant dependence on the magnitude of the temperature cycle and is independent of steady-state temperature. The critical intermetallic layer thickness is calculated based on the intermetallic compounds that form the fastest for the bimetallic combination. A horizontal line on the graph at a thickness equal to the wire thickness at the bond pad gives the time-temperature product that will result in failure after time equal to the abscise. Reversion is a steady-state temperature-dependent phenomenon that actuates the reversion temperature for the encapsulant or molding compound.