The failure rate designation (M, P, R, S, etc.) identifies the failure rate level for which a component is qualified. The designation and quality factor varies by component type. For example, for a CDR capacitor, the failure rate levels and PI Q factors are as follows:
Failure-rate level Failure rate PI Q Factor
designation percent/1,000 hrs
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M……….. 1.0 PI Q = 1.0
P……….. 0.1 PI Q = 0.3
R……….. 0.01 PI Q = 0.1
S……….. 0.001 PI Q = 0.03
The appropriate failure rate designation for mil-spec qualified parts is often included in the mil-spec part number. The pertinent mil specification document would provide details for making this determination.
Parts that are not procured to military specifications, including automotive parts, are more difficult to assess. A “by the book” prediction analysis would treat these parts as having a commercial or unknown screening level with the requisite PI Q factor. However, it may possible to justify “adjustments” to this factor if you have experience data on specific components or can determine how a commercial part is manufactured and screened as compared to the military specification. Depending upon your requirements, adjustments may have to be reviewed and approved by your customer.
Additionally, 217Plus may be considered as an alternative approach to reliability predictions. Based upon more recent field data obtained on a variety of components, including those of commercial grade, it eliminates the component quality factors in favor of process grading and a reliance on actual operating and non-operating stresses.
