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 ongoing 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 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.
To ensure that good workmanship has been employed and that the UUT is free of obvious physical defects.
Visually inspect UUT before and after each manufacturing, repair, and test operation.
Workmanship shall meet the applicable standards including T.O. 00-25-234 and shall be free of obvious physical defects. A unit that exhibits any sign that a part is stressed beyond its design limit (cracked circuit boards, loose connectors and/or screws, bent clamps and/or screws, worn parts, etc.) is considered to have failed even if the UUT passes the Functional Testing.
Done before, during, and after ESS testing to verify that the UUT is functioning within design tolerances.
Applying an input signal or stimulus and measuring the output.
Output responses/signals must be within technical data specifications, and the UUT must operate satisfactorily in the next higher assembly.
Testing at the physical environmental conditions (shock, vibration, temperature, altitude, humidity, etc.) that simulate those encountered over the operational life of the component. Random vibration and temperature cycling have proven to be the most successful forms of ESS in terms of effective flaw precipitation.
A stress profile is developed and applied to the UUT. The profile simulates the environmental conditions encountered during transportation, storage, handling, and operational use phases. The UUT is configured to match the phase, e.g. transportation shocks are applied with the UUT in the shipping container, operational use temperature cycles are applied with the UUT operating.
The UUT (Unit Under Test) must pass Functional Testing and Visual Inspection before, during, and after ESS.
The testing of a production-representative unit to demonstrate that the design, manufacturing, assembly, and repair processes have resulted in hardware that conforms to the specification. Satisfactory completion of Qualification Testing denotes readiness for further stages of testing. Limited flight testing may be acceptable before completion of all phases of Qualification Testing.
Formal tests conducted to demonstrate acceptability of the individual unit for delivery. They demonstrate performance to purchase specification requirements and act as quality control screens to detect deficiencies of workmanship and materials. The successful completion of such tests denotes acceptance of the unit by the procurement agency.
This should be part of the Qualification and Acceptance ESS when verification of reliability is required.
This is the UUT for Qualification ESS (typically three UUT are required). The UUT must be representative of the design, production line processes, materials, and workmanship.
Also called First Article. This is the UUT (typically four are required) that demonstrates that the repair source has the capability and processes to perform a satisfactory repair.
Tailoring is the formal engineering task of using existing technical data (requirements, standards, specifications, test plans, etc.) and selecting or modifying applicable areas to meet the requirements unique to the type of unit undergoing test. Non-applicable requirements are deleted. Other requirements may be added due to changes in Federal standards, identification of new hazards, modifications to the item, or changes in the mission/ESS profile. All areas of non-compliance with the technical data shall be identified by the contractor and a Requirements Tailoring Request (RTR) shall be submitted to the Government for each area. The RTR shall include thorough justification. Only the Government Engineering Authority for the component can accept an RTR.
Tailoring generally is to select the applicable areas, best test methods, or for use of an equivalent requirement.
Tailoring generally is to change the test levels and durations, sequence of tests, or reporting requirements. Tailoring shall also identify any test requirements that are to be accomplished through analysis, similarity, or inspection.
Each RTR shall be classified as a MIC, Waiver, or Deviation.