Hilaire Ananda Perera P Eng

Adaptive Environmental Stress Screening Handbook Hilaire Ananda Perera P Eng

Environmental Stress Screening (ESS) is a process which involves the application of one or more specific types of environmental stresses for the purpose of precipitating to failure, any latent, intermittent, or incipient defects or flaws which would cause product failure in the use environment. The stress may be applied in combination or in sequence on an accelerated basis but within product design limits. ESS detects manufacturing problems caused by poor workmanship or by faulty and/or marginal parts. It also identifies design problems if the design is inherently marginal and if qualification and engineering tests were too benign. ESS is based on the adjustment of stress screens in response to previously observed screening results to minimize Outgoing Defects. Stress screening is a closed-loop process and relies upon information from monitoring to improve processes and screens; that is, it is an iterative process. Only through this can Adaptive ESS be effective in terms of latent defect removal, and hence be cost effective. Stress screening should be monitored to a depth which ensures that all failure modes cause an 'item failure' indication. In this connection continuous monitoring of items during the screen should be undertaken where possible, since certain failures may only appear under stress and not at ambient conditions With no firm failure mechanism/mode information, Random Vibration followed by Thermal Cycling with few Power On/Off cycles is a good default condition. Screening should not stress the equipment such that fatigue failures are precipitated.