Environmental Stress Screening (ESS)
(a) ESS is the tailored application of electrical and environmental stresses to electronic equipment to identify & eliminate defective, abnormal parts and manufacturing defects. It is a production screen conducted on 100% of the manufactured items to minimize infant mortality. ESS is intended to simulate infant mortality failures via accelerated aging without causing damage/inducing wear out to the product under manufacture. ESS weeds out latent defects arising out of weak parts, workmanship and other non-conformances, which would otherwise cause failure during higher-level testing or early field service.
(b) ESS is a product-specific programme and therefore, specific screen strength needs to be defined for type of product. Stress screening is a part of manufacturing process in which the simulated environmental stresses are used to screen out those failures that would otherwise occur in the field. The stress should be closely tailored to the equipment’s design capability to provide an effective screen without damaging good components. It is necessary to conduct ESS at the earliest possible stages where it is possible to reveal latent defects and initiate necessary corrective actions.
Screens for Conduct of ESS.
A tailored screen requires identifying specific parameters of equipment being screened, so that the screening levels should not exceed design limits, but they must be of sufficient strength to precipitate failures due to weak parts and manufacturing defects at the earliest time such that corrections are most cost-effective. The following two types of screens are usually used for ESS:-
(a) Thermal Cycling Stress Screening(TCSS).
TCSS is the process of cycling through two temperature extremes, typically at relatively high rates of change. It is used to expose latent defects by inducing failures through thermal fatigue. The number of thermally induced stress reversals, the temperature extremes, and the thermal rate of change of the hardware are the principal parameters associated to expose thermally sensitive manufacturing defects.
(b) Random Vibration Stress Screening(RVSS).
In RVSS, the equipment under test is randomly vibrated from 20 Hz to 2000 Hz. It exposes mechanical fatigue or failure, loose solder joints, improper bonding, or chip level mechanical flaws.
ESS Process Sequence
(a) The electronic hardware is recommended to be screened as per the following sequence:-
Stage-1: Perform thermal cycling screening at PCB level
Stage-2: Perform random vibration at PCB level or higher indenture
Stage-3: Repeat thermal cycling screening at an indenture higher than PCB level
(b) The conduct of thermal cycling prior to random vibration pre-stresses potential defects which can then be surfaced more effectively by random vibration. Random vibration also conditions some defects to the point of failure which are detected by subsequent thermal cycling with performance monitoring. All ESS requirements must be accomplished at the lowest feasible level of assembly.
(c) The ESS is to be conducted on 100% electronics as per latest guidelines promulgated.