Environmental stress screening (ESS) has become one of the primary approaches in the modern electronic industry to precipitate and eliminate latent or hidden defects in electronic products which are introduced mainly during the manufacturing, assembling and packaging processes. Temperature cycling, plus random vibration (shaking and baking) are the primary processes of ESS. This text presents coverage of the subject, from basic concepts and the historical evolution of ESS, to the statistical and physical quantification of ESS.
Environmental Stress Screening is now being employed throughout the defense industry as a means of precipitating latent part and workmanship defects prior to fielding, with the expectation that improved field reliability and reduced support costs will result. The objective of this study was to develop a quantitative technique for planning, monitoring and controlling the cost effectiveness of stress screening programs for electronic equipment and to prepare a draft military standard based on the developed technique. Latent defects are introduced in equipment through defective parts and deficiencies in the manufacturing assembly process. Design related defects, though not insignificant, are not considered in the techniques developed. Stress screens act on part and workmanship defects with resulting fallout, but since screens are less than 100% effective, some defects escape in fielded equipment. A method for estimating the number of defects initially present was developed and is described later. The effectiveness of stress screens is measured by test strength determined from screening strength equations developed in a previous study and updated in this study based on acutal screening results and test detection efficiency. The question of what is a reasonable number of defects to escape is addressed based on equipment complexity and a tradeoff of stress screening cost and resulting field reliability.
ESS is a technique of testing used for minimizing/eliminating hardware production-line workmanship errors. The application of vibration and temperature environments to items requiring screening can minimize/eliminate such errors as loose wires or parts, cold solder joints, loose screws or nuts, plus other assembly or fabrication type deficiencies. ESS is applied at all stages of fabrication or assembly. The Naval Sea Systems requested that the commercial equipment used on the Precise Integrated Navigation System (PINS) be surveyed for compliance with NAVSEANOTE 3900 and NAVMAT P-9492. This investigation sought to determine the extent to which the PINS commercial equipment manufacturers apply ESS to their equipment during the manufacturing process and to compare commercial practices with Navy requirements in order to discover and document the differences. This report concludes that the limited investigation conducted does not provide a definitive basis for decision. Several alternatives for screening programs are discussed.
Environmental Stress Screening (ESS) is a process which involves the application of one or more specific types of environmental stresses for the purpose of precipitating to failure, any latent, intermittent, or incipient defects or flaws which would cause product failure in the use environment. The stress may be applied in combination or in sequence on an accelerated basis but within product design limits. ESS detects manufacturing problems caused by poor workmanship or by faulty and/or marginal parts. It also identifies design problems if the design is inherently marginal and if qualification and engineering tests were too benign. ESS is based on the adjustment of stress screens in response to previously observed screening results to minimize Outgoing Defects. Stress screening is a closed-loop process and relies upon information from monitoring to improve processes and screens; that is, it is an iterative process. Only through this can Adaptive ESS be effective in terms of latent defect removal, and hence be cost effective. Stress screening should be monitored to a depth which ensures that all failure modes cause an 'item failure' indication. In this connection continuous monitoring of items during the screen should be undertaken where possible, since certain failures may only appear under stress and not at ambient conditions With no firm failure mechanism/mode information, Random Vibration followed by Thermal Cycling with few Power On/Off cycles is a good default condition. Screening should not stress the equipment such that fatigue failures are precipitated.
Why read this book on Environmental Stress Screening ? If you are working in the electronic manufacturing industry and you have heard the questions : "Why did it fail in the field and not in the factory ?" The answer to this question is there was not a proper ESS program developed for this product. A good ESS program is develop from the ground up and the fallout data is tracked. The fallout data is used to alter the present ESS program. This new /altered program is tailored to the hardware and usually is less costly and should improve the product reliability. "Why can't we use the same ESS program as the other guy ?" The answer to this is you can, if you are manufacturing the exact same product. If this is the case, you must be working with the other guy. A different product requires a different ESS program. Or If you are working in the electronic manufacturing industry and you have heard the statements : "I won't do ESS, it's going to cost too much." Can you put a price on field returns or lost contracts because of poor quality ? "I don't need to ESS, I'll just be more careful during manufacturing." You can be as careful as you want. If you don't weed out the Latent defects the time bomb is ticking. Or (worst case condition) If you are working in the electronic manufacturing industry and you have heard the front door being locked for the last time because of poor Quality hardware being shipped to the customer: Unfortunately if this happens it is too late to do anything. Hopefully you will develop a strong ESS program that is cost effective, apply dynamic management of the ESS program by analyzing the fallout data and last but not least - use common sense.
