A clear understanding of environmental stress screening requires a good definition as a baseline. The following definition addresses the key aspects of ESS:
Environmental stress screening of a product is a process which involves the application of one or more specific types of environmental stresses for the purpose of precipitating to hard failure, 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 capabilities.
ESS isolates manufacturing problems caused by poor workmanship, faulty and/or marginal parts. It also identifies design problems if the design is inherently fragile and if qualification and reliability growth tests are too benign. The most common stimuli used in ESS are temperature cycling and random vibration.
ESS is a process rather than a test in the normal accept/reject sense. Those participating in the effort, including the contractor, should never be led to believe that a "failure" is bad and would be held against them. ESS is intended to stimulate defects, not to simulate the operating environment, and therefore, factory "failures" are encouraged. The root causes of ESS failures need to be found and corrected before there is a complete process.
Initially, ESS must be applied to 100% of the units manufactured, including repaired units. By using a closed loop feedback system, one will be able to eventually determine if the screening program should be modified.
A viable ESS program must be dynamic - the screening program must be actively managed, and tailored to the particular characteristics of the equipment being screened. This includes conducting a survey to determine the mechanical and thermal characteristics of the equipment and refining the screening profiles as more information becomes available and/or designs, processes, and circumstances evolve.
