3. Analyze failures and separate them according to failure mechanism.
All failures must be analyzed in order to take corrective action. It is truly amazing that many ESS operations do not include any structured method to analyze the failures and to provide the results to those who can take the proper corrective action.
4. Prepare plots of failure rate vs time for each failure mechanism.
After this is done, the criteria of Step 2 must be applied to each failure mechanism. Again, only failure mechanisms with decreasing failure rates can be attacked with ESS.
5. Improve the product.
Without using the data generated by ESS to improve the product, including design, components, materials and processes, there is no hope of reducing or eliminating the ESS process. If the staff responsible for the ESS process is not the staff responsible for designing and manufacturing the product, it is important that good communication take place between the two groups.
6. Collect and analyze ESS data for the improved product.
If the 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.
7. Modify ESS conditions to reflect the new failure rates.
As failure mechanisms are eliminated, the stresses that precipitate them may be eliminated. If they occur in shorter times, then the duration of the ESS process may be shortened.
In some cases, additional stresses or increased levels may have to be introduced to detect failure mechanisms which were not expected. If this is the case, care must be taken to avoid introducing irrelevant failures.
8. Reduce or eliminate ESS as warranted.
If the ESS process has been set up properly, and if the proper data is 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.
The effectiveness of ESS ultimately must be evaluated economically. This analysis is based on the cost to conduct ESS, the cost of field failures, and the frequency of occurrence of field failures.7,9,10,11,12,13,14 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. The benefit is in the reduced costs of field failures.