SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Godder Shao
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
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!
Subject: Failure prior to reported failure?
Reply Posted by: Larry George
Organization: Problem Solving Tools
Date Posted: Sun Dec 26 13:51:49 US/Eastern 2004
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.
Subject: 85/85 Accelerated Testing
Reply Posted by: BWD
Date Posted: Wed Jan 5 9:15:52 US/Eastern 2005
Accelerated testing of integrated circuits is a process that has been used for a long time to identify failure problems in the manufacturing of integrated circuits. The test that you describe (85/85) is a standard test for integrated circuits. The combination of humidity and temperature accelerated testing is based on the Hallberg-Peck relationship. This relationship combines the ratio of high humidity to use humidity to a power factor (most tests use a factor of 2) with the Arrhenius temperature test. Given your use data of 30 degrees C and 60% humidity and the stress test parameters of 85 degrees C and 85 % humidity, I calculated an acceleration factor of 68 with 0.6 activation energy (Arrhenius test) and 221 with 0.8 activation energy. With the test times shown, this results in a MTTF of 1 year or 2 years depending on the correct activation energy and humidity acceleration. To determine the correct activation energies, you need to perform several (at least two more) tests with different stress parameters and plot the results on log log paper for life vs. stress.