Abstract:

In the use of Mil-Hdbk-217E models, some limitations on their applicability within the design process have surfaced. Reliability was predicted for three printed wiring boards representative of those used for avionic applications in order to evaluate the inherent variability. A parts count and parts stress analysis were conducted at three environments using Mil-Hdbk-217E models.  In addition, parts stress analyses were conducted at various temperatures, assuming that components were thermally isolated and also assuming that thermal interactions resulted from the characteristics of the cooling system.

The result of this investigation suggests that reliability 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. In particular, reliability prediction gives a parameter against which the thermal engineer can select the appropriate cooling mechanism, the degree of redundancy, and the extent of acceptable stresses.
 

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