Thermophysics of Reliability Screening

The thermal behavior of a transistor is very important to its reliability. However, this behavior is complicated by the nonuniformity of the operating temperature and its dependence on operating conditions. An understanding of this behavior is necessary to accurately interpret reliability data and to establish more effective reliability screening procedures. The measurement of thermal impedance or thermal operating stress of a transistor is also complicated by the junction temperature nonuniformity and dependence upon operating conditions. Great care must be taken in using electrical methods to measure thermal impedance since it is not easy to determine what the indicated value represents. Because of the interdependence of most parameters used for reliability measurements and testing, it appears necessary to use a multi-parameter analysis on reliability data to obtain effective screening. This analysis should determine not only maximum value limites of each parameer but also limits relative to the values of other parameters. Reliable operation would correspond to regions in multiparameter space which are not necessarily bounded by a specific value of each parameter.