by Lloyd W. Condra, Hanse Environmental, Inc.
Environmental stress screening (ESS) is one of the most widely used of all accelerated reliability tests. It precipitates latent defects, which are detectable only with the application of stress. Latent defects are introduced into the product during manufacturing, since design-related defects should have been detected and eliminated by reliability-enhancement testing during the design phase.
Figure 1 illustrates the ESS concept. ESS is effective only for a product with an infant-mortality region, which is indicated by a decreasing initial failure rate in Figure 1. The optimum ESS time is t0, since at that point, all the infant-mortality defects have been screened out.
If ESS ends before t0, the product still contains infant-mortality defects which will be found by the user of the product. If ESS ends after t0, useful life is consumed without improving the failure rate.
The failure rate may not be zero even after t0. The failures occurring after t0 are not infant-mortality failures though, and they must be dealt with in ways other than ESS.
Many attempts have been made to prescribe standard ESS processes, but since ESS processes are product-specific, the most effective ones are based on a knowledge of the product, its potential defects and the stresses that cause them.1,2,3,4 An effective ESS process generates valuable data which can be used to improve the product as well as to screen out defects. Unfortunately, when ESS is viewed only as a requirement imposed by the customer or the market, its full benefits are not realized.
The compliance-based approach treats ESS like a cookbook process, in which the product is exposed to a standard set of stresses, at standard levels, for standard lengths of time. Little attention is given to the failure mechanisms, to how they are distributed in time, or to how the failure data can be used to improve the product. Compliance-based ESS provides few benefits other than satisfying a customer-imposed requirement.
Compliance-based ESS users can incur unnecessary expense. Table 1, 2 shows a typical ESS program implemented by a manufacturer of aerospace electronics equipment.
From a physics-of-failure point of view, these conditions are practically identical and, with minor modification, they could all be conducted in a single environmental test chamber. Since the compliance-based approach does not bring this level of understanding to the process, each condition was implemented as stated, and a separate test chamber was required for each one.
The physics-of-failure approach to ESS is based on an understanding of the potential types of latent defects in the product, the failure mechanisms and the stresses that cause them.5,6,7 The ESS conditions are set up to precipitate those defects, and the data is used to determine their causes and distributions.
Failure data is communicated to the appropriate design and manufacturing personnel and used to make changes to improve the product. If it is properly set up and operated, a physics-of-failure ESS process can be extremely cost-effective.
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