Although hard defects may be detectable in factory tests, weak products may exhibit failures or degrade only under certain stress conditions. Without stress testing, these weak products may be shipped to customers and cause early failures in the field. These failures usually come from product weaknesses resulting in the presence of weak populations and the lower end of a broad main population for the product strength distribution. The requirements of the product strength distribution are determined by the reliability criteria and by the distribution of the lifetime maximum stress of the product. Environmental stress testing (EST), which includes corrective actions, aims at eliminating product weaknesses to achieve robustness. Enviromental stres screening (ESS) may augment EST, but is applicable only under certain conditions. Its applicablity also depends on whether the dominating failure may be stimulated by threshold stress, cumulative stresses, or their combination.

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