Environmental Stress Screening and Strength of Screen Evaluation for Dormant-Storage Components

We present an approach to the development and evaluation of environmental stress screening (ESS) for a dormant-storage, multi-shot component. The ESS is developed to precipitate and detect latent manufacturing defects without significantly degrading the component's probability of successful function under normal operating environments. The evaluation of the ESS is achieved by using an additional strength of screen (SOS) operation to test for escapes from the screen. The resulting data are pass/fail data only, because the characteristics of this type of component do not allow a standard “time to failure” analysis. The calculated SOS efficiency f is then used to estimate initial field “reliability.” We illustrate the use of the methodology with a case study involving an electrical component manufactured within the Nuclear Security Enterprise (NSE). In development and qualification, twelve failures were detected by the ESS, and the SOS operations detected one escape. The resulting analysis showed the SOS efficiency to be approximately 92%, adequate for the component reliability goal. The resulting initial field reliability was estimated to be 99.3%, acceptable for this electrical component. Failure investigations were conducted to determine the root cause of each of these failures. Information from these investigations resulted in changes to the manufacturing process to eliminate or minimize the reoccurrence of these failures. The number of ESS failures have been reduced, and no additional failures have been observed at the SOS operation.