Basics of environmental stress screening

- for electronics test and production environments

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Environmental stress screening (ESS) is an important element of many electronics production and test programmes. With the growing importance of the reliability of electronic equipment, environmental stress screening or ESS has become established in many production lines. The idea is that by stressing the equipment during the early stages of its like, any problems will be highlighted and resolved. As such environmental stress screening (ESS) is an established part of many production processes.

Equipment reliability

It has long been known that all equipment obeys what is termed the product reliability. This has the familiar bath tub curve that indicates that failures are likely to occur during the initial operation of a product. This phase, known as the infant mortality phase has a high failure rate as defects caused by the manufacturing process occur. Once these failures have occurred the product failure rate drops during the operational life of the product. After a period of time, typically many years, the failure rate starts to rise as the product ends its useful life and enters the wear out phase.

 

Product reliability curve

Product reliablity curve

Each phase of the curve arises from different causes. In reverse the wear out phase results from the general design, and robustness of the design. The use of components that will not wear or fail after long periods of use is key. The level of failures during the useful life results again from the design, and the use of components of sufficient quality.

The infant mortality period results mainly from the production process. One of the chief causes of failure in any production process arises from the solder joints. With surface mount technology (SMT) used in most products these days, and the complexity of most assemblies there can be many thousands of solder joints, each of which is a possible cause of failure. In addition to this there are other possible reasons for failure. Although the standard of components these days is very high, there is still a risk that they may fail after use. Additionally a failure in the production process may result in undue stress being placed on a component which later fails. Static damage is another cause of latent defects. These and many more reasons can lead to the level of infant mortality.

Stress screening

While the production process should aim to provide as high a quality product as possible, many production organisations see the benefit in trying to speed up the occurrence of any defects. In this way they can be corrected easily while still within the bounds of the production environment, and costly warranty call out charges and repairs can be avoided.

The use of environmental stress screening can be used to hasten the occurrence of the infant mortality failures. A number of techniques are used to ensure that the majority of failures occur within the time span of the production and test process, and that the equipment has reached the operational phase of its life when it enters service.

The techniques employed in environmental stress screening include running the equipment, temperature cycling, and also vibration. Under some circumstances other techniques including humidity may be used.

Profiling

When running an ESS programme it is necessary to ensure that the optimum conditions are used. This is a careful balance between ensuring that the equipment defects are brought out, while not causing any damage to the equipment. Time factors are also important because stress screening programmes may take many hours, and when large volumes of equipment are being produced this can necessitate large numbers of equipments passing through the ESS programme. Typically these programmes are run 24 hours a day and are automated along with having many equipments running at the same time.

It is also necessary to collect data to ensure that the optimum programme has been adopted. If the programme is too long then it uses valuable production resource and keeps up too many units within the stress screening programme. If it is too short then failures will be seen in the field. Thus data collection is essential to check that the programme is optimum.

Summary

Once running correctly an ESS programme can pay dividends, although by the fact that it takes time to run, it is normally only applied to smaller quantity high value items.

By Ian Poole

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