Appendix D
PART QUALIFICATION APPROACHES: GENERAL GUIDELINES
Part qualification includes verification that a part meets all requirements for a particular intended applica- tion. This verification requires detailed knowledge of the application, such as the life-cycle environmental and operating profiles, and the failure and degradation mechanisms of the part under these conditions, in order to define the necessary data to verify qualification. The qualification data may already exist from previous tests or use in another application, or new data may be required. In either case, the acceleration factors of the failure and degradation mechanisms of test or use conditions relative to the target applica- tion use conditions must be characterized. The part qualification rationale for a particular application typ- ically requires analysis and/or test results from multiple sources, such as part manufacturer testing, equipment manufacturer testing, and in-service use of another application. Developing the strategy for deriving this qualification rationale requires tradeoffs of various goals, restrictions, and cost.
The following list highlights some of the areas that must be considered when developing a part qualifi- cation approach:
• The data sample sizes must reflect the required reliability and confidence level for the target applica- tion.
• The part- or assembly-level data assessment must consider the environmental and operational stress conditions of the data for comparison to the target application conditions, taking into account all part failure and degradation mechanisms and their stress condition acceleration factors.
• Compared with part-level testing, higher assembly-level test data provide greater application-specific functional performance insight and verification of design compatibility with other parts. However, the test asset cost can be very high, which can limit sample sizes to the point of losing sufficient confidence in the reliability characterization.
• Using in-service data requires full characterization of the in-service environmental and operational stress conditions for comparison to the target application conditions. It also requires confirmation that in-service failures have been analyzed to their root cause at the part level to ensure that the data reflect the true performance of the part.
• In some cases, higher assembly-level design may not allow sufficient insight into a particular part's performance, so part testing should supplement the higher level testing.
• Developing test software for complex parts (microprocessors, digital signal processors, etc.) may rep- resent the major cost in part testing.Therefore, leveraging the ability to test the parts at higher assembly levels would avoid the difficult and costly task of developing part-level test capability if the part manufacturer will not support such an effort.These parts also tend to have low frequency of use (e.g., one per circuit card assembly), which generally suggests that part-level testing would provide better characterization.These competing constraints require detailed assessment to determine the optimum approach.
• Testing at higher assembly levels requires consideration of the larger number of failure and degrada- tion mechanisms with different acceleration factors compared to the part level, which can often require limiting test stress levels, and thus requiring very long test times, to not cause failure of
assembly before assessment of lower acceleration factor failure mechanisms.
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