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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: Juan Ignacio Herce Moreno (Juan_Ignacio_Herce_Moreno@msl-vlc.com )
Organization:Manufacturers' Services Limited
Date posted: Wed Jun 20 4:24:57 US/Eastern 2001
Subject: Estimated MTBF from Heat Shock Test
Message:
How can I estimate the MTBF of an integrated digital component that has performed 1000 cycles of heat shock test? (1 cycle = 30 min. at 90C and 30 min at -40C) Thanks.


Reply:

Subject: Thermal Cycle Reliability
Reply Posted by: Bruce Dudley (bdudley@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Thu Jun 21 16:20:59 US/Eastern 2001
Message:
The process to estimate MTBF values from test data such as high temperature shock tests is not an exact process. What I did was to use the life cycle equations from MIL-HDBK-344 Environmental Stress Screening of Electronic Equipment to estimate the percent of life expended in the 130 degree C shock test. The equations used are as follows: Dl = Nl x Sl^Bl and De = Ne x Se^Be Where: Dl = the expected life cycle Nl = number of cycles for the life of the item Sl = temperature range Bl = thermal fatigue exponent De = life under stress Ne = number of test cycles Se = temperature range of the test Be = the thermal fatigue exponent From the information given, the number of test cycle is 1000 and the temperature range is 130 degrees C. I estimated that the normal life cycle would be about 30 years and the temperature range would be about 38 degrees C. The fatigue ductility exponent was estimated based on Coffin/Manson equation which shows a exponent of about 2.26. Using the equations, I found that the number of test cycles equated to approximately 59.9 million operating cycles. Combining this with the normal life cycle estimate of 81.4 million cycles and the 30 year expected life as follows; (De /Dl )(30) = 59.9 x10^6 / 81.4 x 10^6 (30) = 22 years or 194,000 hours My conclusion is that based on the limited information, the MTBF could be approximated to be 194,000 hours


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