Temperature cycling environmental stress testing (EST) • Humidity and pollutants (corrosive gases) • Mechanical, e.g., vibration (sinusoidal or random), shock, or impact • Operational, e.g., variations in rated voltage, current or power Stimuli may be applied individually, consecutively or concurrently. For PSEST, temperature cycling is most commonly employed.EST procedures are time-consuming and energyintensive, however, and there is a clear imperative for test optimization in order to reduce costs. In general, the application of comprehensive and lengthy PSEST is advisable for new products or manufacturing procedures in order to detect and correct design or process flaws. As the maturity of the product or process IntroductionAccelerated environmental stress tests (EST) are commonly applied during the design and manufacture of telecommunications equipment in order to ensure reliability [1,6]. Lucent Technologies conducts design EST (DEST) and production sampling EST (PSEST) procedures for network products. DEST is usually applied during the product development process, with the objective of correcting design flaws, and thereby "ruggedizing" the hardware [7]. PSEST is typically used as an end-of-line manufacturing screen to precipitate and detect latent defects in order to obviate so-called infant mortality failures. A range of stimuli can be applied during EST [2,7,12] improves, the duration of PSEST may be decreased, eventually yielding to a short test performed on a sampling basis. Conversely, it may be necessary to increase test duration if design or process modifications are implemented. In the case of modifications, the equipment manufacturer could be at risk of incurring the increased warranty costs associated with the release of marginal products to customers. Optimization of PSEST for contemporary telecommunications equipment is challenging, primarily because it is common for many mechanisms to cause failure during the tests. A single, dominant mechanism-or, at most, a small number of mechanisms-would facilitate optimization based on the analyses of root causes. For complex systems with multiple failure mechanisms, optimization is usually achieved by the analysis of time-to-failure data. The relationship between time-to-failure and the applied stress is the basis for the optimization of the profile and duration of the stress-for temperature cycling, the profile is described in terms of hot and cold soak temperatures, ramp rates, soak durations, and number of cycles. Furthermore, time-to-failure data from PSEST can be extrapolated in order to estimate the number of escapes-the marginal components not precipitated to failure and detected during PSEST-in order to evaluate the probability of occurrence of early-life failures. Finally, the criterion for the optimization of PSEST is a balance of costs [4,5,23]: on the one hand, the cost of screening, which include test resources, scheduling, and time; and on the other, the costs associated with field repair, warranties and damage to customer satisfaction. Th...