The existing Environmental-Stress-Screen(ESS)plans precipitate defects by stressing all products for specified durations.For high reliability and long life products

the plans usually require long screen durations to allow nearly all defective items to fall

and thus generate excessive aging effects of good items.For some products

failures are defined in terms of performance characteristics exceeding their critical values.This paper describes the principles of using degradation measurements of performance characteristics to screen the products.In an ESS

the performance characteristics of defective items degrade considerably faster than those of good ones

yielding a bimodal distribution of the characteristics.As screen duration increases

the 2 modes of the distribution shift apart.It is possible to find a tightened critical value to weed out the defective items before they fail.This paper

based on these principles

by choosing optimal screen durations and tightened critical values

designs the optimal screen plans which meet the screening efficiency and which simultaneously meet the reliability requirement.Because the ESS regime of this paper allows defective parts to be screened out before they fail

the developed ESS plans can shorten screening duration and alleviate aging effects of good products.A numerical example is given to illustrate the effectiveness of the method.