12.1 INTRODUCTION

There have been two basic approaches to the application of environmental stress screening. In one approach, the government explicitly specifies the screens and screening parameters to be used at various assembly levels [1]. Another approach is to have the contractor propose a screening program which is tailored to the specific product and is subject to the approval of the procuring activity. Although both approaches suggest that the screen parameters and levels of each screening program should be tailorable to the equipment to be screened to suit existing requirements, design and factory capabilities, there are some unknowns in the implementation of stress screening. Some of these are:

1. How effective is the stress screen?

2. Has the reliability goal been achieved after a stress screen?

3. What is considered acceptable or unacceptable fallout from a screen?

4. Is an ESS inadequate or too excessive?

To answer these questions quantitatively, some key factors should be known prior to the planning of an ESS program. These key factors are:

1. Part initial latent defect level.

2. Defect level introduced during production.

3. Effectiveness of the screen.

4. Acceptable level of the number of latent defects which escape the screen.

The ESS programs developed in [2] and [3] provide quantitative approaches for the following:

1. 1. Assessing the number of part defects; i.e., incoming defect density.

2. Evaluating the effectiveness of screening for various stress screening parameters; e.g., random vibration, temperature cycling, etc.

3. Designing an ESS procedure to remove and thereby reduce the defects to an acceptable level...