Environmental Stress Screening Revision A

Environmental Stress Screening is now being employed throughout the defense industry as a means of precipitating latent part and workmanship defects prior to fielding, with the expectation that improved field reliability and reduced support costs will result. The objective of this study was to develop a quantitative technique for planning, monitoring and controlling the cost effectiveness of stress screening programs for electronic equipment and to prepare a draft military standard based on the developed technique. Latent defects are introduced in equipment through defective parts and deficiencies in the manufacturing assembly process. Design related defects, though not insignificant, are not considered in the techniques developed. Stress screens act on part and workmanship defects with resulting fallout, but since screens are less than 100% effective, some defects escape in fielded equipment. A method for estimating the number of defects initially present was developed and is described later. The effectiveness of stress screens is measured by test strength determined from screening strength equations developed in a previous study and updated in this study based on acutal screening results and test detection efficiency. The question of what is a reasonable number of defects to escape is addressed based on equipment complexity and a tradeoff of stress screening cost and resulting field reliability.