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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Roman (rkatchmar@med-eng.com )
Organization:Med-Eng
Date posted: Wed Feb 16 16:29:55 US/Eastern 2005
Subject: ESS Strategies
Message:
I am trying to identify the most current or relevant miltary requirements or guidelines regarding ESS. I frequently see much reference to Mil Hdbk 344A, but also find Mil Hdbk 2164A a bit more prescriptive than the former for what has generally been a non-prescriptive burn-in test problem. Nowhere can I find reference to use of HAST/HASS as feeder info to ESS as often done in the commercial world. All thoughts welcome.


Reply:

Subject: ESS
Reply Posted by: bwd (bdudley@alionscience.com )
Organization: RAC
Date Posted: Mon Feb 21 10:44:31 US/Eastern 2005
Message:
The most current military document for ESS (Environmental Stress Screening) is MIL-HDBK-344, “Environmental Stress Screening of Electrical Equipment”. MIL-STD-2164, “Environmental Stress Screening Process for Electronic Equipment”, has been obsolete for over ten years and since it was not a performance specification, it has been cancelled. HAST testing is stress testing on microcircuits to determine the life characteristics. The descriptive definition is: Highly Accelerated Stress Testing (HAST) - This is a very high multi environment test usually performed on individual components such as microcircuits. The test stress levels approach and sometime exceed the design limits for the components. The multi stress conditions are usually temperature cycling or constant temperature combined with humidity and possibly altitude. This test is performed in an autoclave test chamber. This testing is done to identify failure modes and mechanisms within the individual devices in order to improve manufacturing processes. Component level testing that occurs during the design phase. HASS testing is subset of the HALT program defined as follows: Highly Accelerated Stress Screening (HASS) - This is the subset of the HALT testing. In this test, screens using the stress parameters developed in the HALT are applied to the units. The parameters are usually less then the destruct limits. This test is run for a fixed time or number of cycles to eliminate early defects from the population and is performed on 100% of the units. The problem with this test is the possibility of causing destruction in the unit from the high levels of stress. This testing also removes a substantial part of the life of the unit and should be considered life limiting test and not used on 100% of the units. Highly Accelerated Life Testing (HALT) - This is a destructive test process that steps the stress being applied until the unit fails. The stresses can be temperature, electrical, mechanical or electrostatic. The test results are then examined to determine root cause and correction are then applied to fix the problem. The results of this testing can be misleading as many of the failures do not represent the same failure modes in nominal conditions. HALT/HASS can be used in the development cycle to improve design weaknesses, but are not recommended as screen tests as they tend to reduce the life of the product due to overstress so should not be a feed test to ESS. HAST is a screen test for parts and could be used as a lead-in to the higher level ESS program.


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