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Reliability and its requirement Environmental Stress Screening

4. Accelerated Tests

4.1 Accelerated Life Testing
4.2 Reliability Enhancement Testing
4.3 Environmental Stress Screening

4.0 Accelerated Tests:

There are three basic types of accelerated tests:

1. Accelerated life tests
2. Reliability enhancement tests, and
3. Environmental stress screening

As shown in the figure 1, Environmental Stress Screening (ESS) is aimed at exposing infant mortality failures which would otherwise occur early in the life of the product. Reliability Enhancement Testing (RET) is conducted to find early failures related to the product design, but is also used to determine the robustness of the product with respect to the random failures of the useful life period.

The purpose of accelerated life testing is to find out how, when, and why wear out failures occur in the product.

Fig-2  A take from paper on "Acceleratedtesting for product reliability assurance" by Lioyd  W.coundra

Figure 2 shows how accelerated life testing, reliability enhancement testing, and environmental stress screening fit into the product design, development, and manufacturing cycle. The on-line processes are shown as rectangles. On-line processes are those which are part of the product design and production cycle. They are conducted with samples of the actual product. Off-line processes are those which are not part of the design and production cycle. They are usually not conducted on actual product samples, but on generic samples representing the materials, components, or processes used to manufacture the product. Two out of the three types of accelerated tests ( reliability enhancement testing and ESS) shown above are on-line processes and the third (accelerated life testing) is an off-line process.

4.1 Accelerated Life Tests:

Accelerated life tests are conducted on components, materials, and manufacturing processes (e.g. Formation of a plated through hole) to determine their useful life in the required product application. Their purpose is not to expose defects, but to identify and quantify the failures and failure mechanisms which cause products to wear out at the end of their useful life. Because of this, accelerated life tests must last long enough to cause the samples under test to fail by wear out. The test time may typically vary from a few weeks to a few months.

In addition, separate accelerated life tests are conducted for each potential wear out mechanism, since the stresses which produce failures are different for each mechanism. Traditional accelerated life test methods have involved the application of single stresses( for example only sine vibration or only temperature cycle). However, it is increasingly felt many potential failure mechanisms result from, or are accelerated by, combinations of environmental conditions (e.g. random vibration + high temperature).

Accelerated life tests are commonly called qualification tests, because they are used to qualify a component, materials, or processes for all applications. A more realistic approach is to collect accelerated life test data for a given set of test conditions, and then to extrapolate these results for each application.

Accelerated life tests usually take too long to be conducted on-line, as part of any product development cycle. Therefore, they must be conducted off-line, well before the components, materials, or processes are needed for a given application. For these reasons, ALT are usually conducted generically, using generic samples which represent the materials, components, and processes used for a variety of products.

The benefits of ALT are:

1. The ability to estimate useful life of the product.

2. It also allows designer/manufacturer of the products to identify and improve and control the critical components, materials, and processes so that the final product is robust and mature.

Potential failure mechanisms must be known; and the stress environment of the product must be understood. Specific acceleration models must be available for each failure mechanism; and the results must be interpreted properly.

4.2 Reliability Enhancement Testing:

The purpose of RET is to determine the types and levels of environmental stresses which cause failures in a product, given that there are no defects in the materials and components used in manufacturing it. In this sense, RET is a type of inspection test for the product design, processes. Because RET is not directed toward finding infant defects, the sample size can be very small. The ideal time to conduct RET is at the end of the design cycle, when the expected design, materials, components, and manufacturing processes are available, production has not yet begun. RET is not a qualification test, since its purpose is to find weak spots and correct them before production begins.

RET is usually conducted by applying the expected environmental and operating stresses (either singly, sequentially, or simultaneously) initially at low levels, and then increasing them in steps until one of the three things occur:

1. all samples fail
2. stress levels are reached which are well above those expected in service, or
3. irrelevant failures occur

An important benefit of RET is to survey and determine the product upper and lower distruct limits. This is useful in determining the robustness of the product design. The ESS levels should not be changed to reflect theses limits as it could overkill the product. Test, Analyze and Fix (TAAF) is crucial to a well developed RET.

4.3 Environmental Stress Screening:

Environmental Stress Screening (ESS) is useful in minimizing the early failures of manufactured product by screening latent defects. ESS is one of the most used reliability tests. Its purpose is to precipitate latent defects, which are detectable only with the application of stress. The defects are ideally are those introduced into the product during manufacturing, since design-related defects should have been detected and eliminated by reliability enhancement testing during the design phase. ESS is effective only for a product with an infant mortality region, which is indicated by degreasing initial failure rate in fig.1. ESS should be based on an understanding of the potential types of latent defects in the product, the failure mechanisms, and the stresses that cause them. ESS conditions are set up to precipitate those defects and the data are used to determine their causes and taking preventive actions.

Reliability and its requirement Environmental Stress Screening
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