Quantification of screen duration permits controlled purification of systems by the elimination of early failures to a predetermined degree of purity, consistent with reliability requirements at time of delivery for field use, or consistent with budgetary cost constraints, or both.

In an earlier paper the authors developed a method for quantifying screen duration of Environmental Stress Screening (ESS) of homogeneous parts or component populations.1 This is briefly summarized under "Case of a Parts Population" and extended under "Mixed Parts Systems," to systems consisting of a mix of many different parts with different failure rates.

In the case of parts, each failure constitutes the end of life of that part, while in the case of systems, a failure usually results in repair or "renewal" and the system continues operation (except for one-time systems). For parts, time-to-failure is observed, while for systems, times-between-failures is observed.

The section titled "A Numerical Example" contains an example of calculating screen duration (Ts) for different predetermined degrees of purity, characterized by the screen residues that constitute the complements of purity. The Appendix describes the computer program used for the numerical solutions.

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