Accelerated Lifetest Results on Submarine-cable Transistors

A computerized analysis is outlined; it uses a lognormal distribution that is fitted to the observed data by the maximum likelihood method. A typical computer output illustrates the failure levels at operational temperatures. The statistical curve fitting technique aids the analysis of accelerated lifetest data. Failure estimates at junction temperatures, much lower than those under accelerated lifetest, are obtained by extrapolation. However, it is up to user whether such an extrapolation is justified. Due to the quantized nature of the data, an optimum exists for the time of device inspection, whereby statistical information is obtained at minimum testing expense. For example, inspecting the 250°C sample at 80 and 160 hours would be a waste of time since no failures should occur.