Environmental stress screening

Environmental stress screening (ESS) refers to the process of exposing a newly manufactured or repaired product or component (typically electronic) to stresses such as thermal cycling and vibration in order to force latent defects to manifest themselves by permanent or catastrophic failure during the screening process. The surviving population, upon completion of screening, can be assumed to have a higher reliability than a similar unscreened population.

Developed to help electronics manufacturers detect product defects and production flaws, ESS is widely used in military and aerospace applications, less so for commercial products. The tests need not be elaborate, for example, switching an electronic or electrical system on and off a few times may be enough to catch some simple defects that would otherwise be encountered by the end user very soon after the product was first used. Tests typically include the following:

ESS can be performed as part of the manufacturing process or it can be used in new product qualification testing.

An ESS system usually consists of a test chamber, controller, fixturing, interconnect and wiring, and a functional tester. These systems can be purchased from a variety of companies in the environmental test industry.

The stress screening from this process will help find infant mortality in the product. Finding these failures before the product reaches the customer yields better quality and lower warranty expenses. Associated military terminology includes an operational requirements document (ORD) and on-going reliability testing (ORT).

'The following is extracted from a paper on ESS testing prepared by the U.S. Air Force to provide standardized definitions and methods. The paper is available for unrestricted distribution by writing to OO-ALC/ENR, Hill AFB, Ut. 84056. Ask for OO-ALC Technical Note 01-2002, Environmental Stress Screening of Replacement and Repaired Components, Standardized Definitions and Process, by David Franz.'

The purpose of this paper is to provide standardized definitions and a roadmap of test processes for the Environmental Stress Screening (ESS) of replacement and repaired components used on Air Force systems. The term “component” is used interchangeably with the term “unit” and includes Line-replaceable unit (LRU) and sub-units (SRU). A component selected for testing is a Unit Under Test (UUT). Operational Safety, Suitability, and Effectiveness (OSS&E) policy and instructions require consistency in the disciplined engineering process used to ensure that activities such as maintenance repairs and part substitutions do not degrade system or end-item baselined characteristics over their operational life. Baselined characteristics are highly dependent on reliability, which is verified and maintained by ESS testing. OSS&E policy and instructions also require consistent engineering processes to ensure manufacturing and repair entities are accountable for delivering quality products, and to provide selection and qualification criteria for new sources of supply. Determinations of product quality and source capabilities usually require ESS testing. While considerable information concerning ESS methods and procedures is available including United States Military Standards, handbooks, guides, and the original equipment manufacturer’s test plans, often these publications use differing and confusing definitions for the testing phases where ESS is applied. Lengthy explanations were needed to clarify contract clauses citing these publications. This paper ensures testing requirements are uniformly applied and clearly understood in writing source qualification requirements and contracts.

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