Combined Acceleration Factor with Distributional Field Stress Levels and Multiple Test Stress Levels

Accelerated life test significantly reduces test time, given that the underlying failure mechanisms do not change in the test from the field. The reliability demonstrated in the test can be converted to the reliability under field stress condition through an acceleration factor. This acceleration factor is defined as the lifetime ratio between lower (or field) stress level and higher (or test) stress level. Many mathematical models have been developed over the years, most of which are physics based or a combination of empirical model fitting with a sufficient amount of previous experience in testing similar units [1]–[3], especially for the constant stress level and step-stress level in the test. This paper assumes such acceleration equation or factor is known. However, the distributional field stress level, not single stress level, is considered. In other words, the objective here is to evaluate field reliability with distributional field stress level. It is achieved through an combined acceleration factor derived in this paper, considering distributional stress level in the field and single or multiple stress level in the test. The combined acceleration factor can be used to convert the distributional field stress level and the multiple test stress levels to their equivalent constant stress levels, respectively, which simplifies the test design and the data analysis.