A critique of Mil-Hdbk-217E reliability prediction methods

Some limitations on the use of Mil-Hdbk-217E models within the design process are discussed. Reliability was predicted for three printed wiring boards representative of those used for avionic applications in order to evaluate the inherent variability. Parts count and parts stress analyses were conducted in three environments using Mil-Hdbk-217E models. In addition, parts stress analyses were conducted at various temperatures, assuming that components were thermally isolated and the thermal interactions resulted from the characteristics of the cooling system. The results suggest that reliability can be predicted only when the layout of the components and exact thermal mapping are known. In practice this is not acceptable, since some measure of reliability prediction is necessary in determining electrical, thermal, and mechanical design tradeoffs early in the design process