For nonrepairable devices, it is possible to evaluate and optimize Environmental Stress Screening (ESS) effectiveness using the well-known Weibull analysis techniques. For repairable modules or systems, these techniques cannot be applied directly; the equipment may involve more than one failure, and after each repair, during the early life period, its behavior changes (improves). From a statistical point of view, it is a mistake to consider all the failures in the aggregate. We should study separately the distribution laws of the first failure, the second failure, the third failure, etc. A comparison of these laws gives the average number of early failures per module or system. The knowledge of the distributions parameters yields the optimal screening duration. From this information we can easily define the optimal ESS process. This article reports how this method was actually used on a real case. Moreover, it demonstrates ESS effectiveness at the final assembly level, although the boards, equipped with high-reliability components, were already screened.

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