Environmental Stress Screening: Basic Steps in Choosing an ESS Profile

Environmental Stress Screening (ESS) is the process of exposing a newly manufactured product to environmental stresses in order to identify and eliminate latent defects introduced during the manufacturing process. It is part of the manufacturing process and is therefore performed on 100% of the items manufactured. After ESS, the remaining product population will have a higher reliability than a similar unscreened population due to the resultant elimination of infant mortality defects. The technique was first developed by the military to reduce the number of defects introduced during the manufacturing process of complex electronic assemblies. The screening process can range from providing a basic check of an electronic system prior to shipment (commonly called burn-in) to the application of much more severe environmental profiles, often involving many temperature and vibration cycles at extreme limits. The benefits of screening depend on the maturity of the manufacturing process, with more mature processes expected to introduce fewer defects. One estimate of the potential benefit is provided by the Telcordia reliability prediction standard (SR-332, Issue 1), which estimates that the failure rate of a system population is four times higher at initial deployment than it will eventually be after one year, as shown in Figure 1, the bathtub curve. This means that if no ESS is performed, it will take up to one year for the population to naturally reach the expected steady state failure rate observed during the useful life period.