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Since ESS does not add value to the product, it should be eliminated or reduced as quickly as possible. This can not be done without proper justification, which requires relevant data. Therefore, ESS process must be set up so that it provides data which can be used to eliminate defect causes and thereby reduce or eliminate the ESS step. Following is a step-wise process which can be used for this purpose:
1. Collect failure rate data during the ESS process: Failure data must be collected as and when the failure occurs. The time of occurrence of failure must also be noted. Data from all the items under test must be collected and recorded whether or not there was a failure.
2. Prepare a plot of failure rate vs. Time: If, 1. The failure rate decreases with time, there is an opportunity to reduce it further by taking corrective actions on the product failures.
3. If the failure rate is constant then it will mean i)no more improvement can be made because the product has already entered the flat region(only random failures occur) of the bath-tub curve. or ii) The applied stress level has not reached the flaw location, and therefore, applied stress levels are to be modified (intensified) to bringout latent defects.
4. If the failure rate increases with time, this may be due to one of the following reasons:
I) That the stress levels are high and inducing unnatural defects into the product. Applied stress levels are to be modified to bring out only latent defects.
ii) That the product has already reached wear out phase. Some other means of improving the product reliability are to be followed as the ESS will not be effective.
5. Analyze failures and separate them according to failure mechanism: All failures must be analyzed in order to take corrective action. Many ESS operations do not include any structured method to analyze the failures. Most frequently used one is simple Pareto Analysis for classification of defects.
6. Improve the Product: Improve the product, including design, components, materials, and processes by using the ESS data. If those responsible for the ESS process are not those responsible for designing and manufacturing the product, it is important that good communication take place between them.
7. Collect and analyze ESS data for the improved product: If proper steps have been taken to improve the product, then the area under the infant mortality region of the failure rate vs. time curve should be smaller. This may result from either a reduced slope of the curve, or from a shorter time in which it reaches a constant failure rate.
8. Modify ESS conditions to reflect the new failure rates: As failure mechanisms are eliminated, then the stresses that precipitate them may be eliminated. In some cases, additional stresses or increased levels may have to be introduced to detect failure mechanisms which were not exposed. If this is the case, care must be taken to avoid introducing irrelevant failures.
9. Reduce or eliminate ESS as warranted: If the ESS process has been set up properly, and if the proper data are collected and used effectively, it will result in a continuously improving product. Eventually, a point will be reached where the ESS process may be reduced significantly or eliminated entirely. It may also be possible to reduce the frequency of ESS by going from a 100% screen to a sample screen.
ESS costs include the cost of capital equipment, the recurring cost of conducting the process, the cost of analyzing and repairing failures, and the risk of actually introducing new failures into the product.
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