A formulation of environmental stress testing and screening

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 often be shipped to customers causing early failures in the field. Early 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 on 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, aim at eliminating product weaknesses to achieve robustness. Environmental stress screening may augment EST, but is applicable only under certain conditions. Its applicability also depends on whether the dominating failure may be stimulated by threshold stress, cumulative stresses or their combination