Alternatives to ESS
ESS is effective only when the product has an infant-mortality region. If this is not the case, other methods must be used. Some other methods which also involve the application of stresses are ongoing reliability testing (ORT), ongoing accelerated life testing and periodic requalification.
ORT exposes a small sample, for example, less than 1% of production on a regular basis, to stresses at or slightly above the operating range for periods ranging from a few days to a few weeks. All failures are analyzed, and the data is used to improve the product. At the conclusion of the test, the surviving samples are shipped as regular product.
Ongoing accelerated life testing is similar to ORT, except that the stresses are somewhat higher, and the test is continued until the samples fail. Since this is a destructive test, the sample sizes may be somewhat smaller than those of ORT, especially if the product is an expensive one.
Periodic requalification involves the repetition of the qualification procedure, or an abbreviated version of it, on a periodic basis (usually once or twice per year). This type of test had its beginning in some of the U.S. military standards. Since periodic requalification does not involve a wide range of sample lots and since it is expensive, it is losing popularity.
The overall purpose of ESS is to assure that, once a product is qualified, there will be no uncontrolled variations in the individual items during the production phase. The application of stresses is necessary to detect some defects which cannot be observed by functional or visual observation.
The only realistic way to develop and operate an effective ESS process is to use the physics-of-failure approach. This requires an understanding of the product, and knowledge of the types of defects and the types of stresses which precipitate them.
Almost by definition, a significant amount of trial and error is associated with developing efficient ESS processes; but once the basic knowledge is gained, it can be applied to a wide range of products. In most cases where ESS has been implemented, it has proven to be quite effective in reducing overall product costs.