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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: Godder Shao (Godder.shao@sg.com.tw )
Organization:System General Corp.
Date posted: Wed Dec 22 22:47:07 US/Eastern 2004
Subject: How to calculate MTTF for Temperature/Humidity bias life
Message:
I have one product(Power IC) need to perform temperature/humidity bias lift test(THB). I follow JEDEC-22 A110 standard to perform it, and test condition as below, 85 degree C/85% R.H./VDD=20V 1000hrs 30pcs. after this test was finished, the test result as below, 22pcs failed at 768hrs, 6pcs failes at 928hrs, 2pcs failed at 1000hrs. could you teach me how to calculate the MTTF at 30 degree C/60% R.H./VDD=20V ? Thanks!


Reply:

Subject: Failure prior to reported failure?
Reply Posted by: Larry George (pstlarry@yahoo.com )
Organization: Problem Solving Tools
Date Posted: Sun Dec 26 13:51:49 US/Eastern 2004
Message:
Doesn't it seem strange that 22 out of 30 failed at 768 hours? That's 32 days or 1 month and 1 day. Doesn't it seem strange that 6 more failed at 928 hours? That's 1 month + 1 week - 1 shift. Could it be possible that the failures occurred before they were reported? If so, the usual statistics will bias MTTF high. I wouldn't pester you about this unless I had the nonparametric maximum likelihood reliability estimator when failures are accumulated before reporting or returning them. See http://www.fieldreliability.com/KMUsrMan.htm for preliminary information.


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