| Basics of environmental stress screening |
Basics of environmental stress screening
- for electronics test and production environments
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 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.
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