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: Johnson Wood
Date posted: Tue Aug 14 3:55:01 US/Eastern 2001
The circuit boards burn-in datas are recorded per month, judy as follows:
Date Sample Size #Failed
2001-2 1494 34
2001-4 3494 144
2001-5 25902 702
2001-6 20082 682
Which method of Weibull is suitable for this type of data?
Subject: Weibull reliability estimates from burn-in data
Reply Posted by: Larry George
Organization: Problem Solving Tools
Date Posted: Tue Aug 21 15:12:39 US/Eastern 2001
The least squares estimates of the Weibull parameters are:
alpha = 9512 hours and beta 0.577 with MTBF of 15,100 hours.
The max likelihood estimates of the Weibull parameters are:
alpha = 9330 hours and beta = 0.582 with MTBF of 14,600 hours. My parameter notation is defined by the Weibull reliability formula R(t) = exp(-(t/alpha)^beta). Contact me for information about the estimators.
The agreement of the lse and mle estimates lends some credibility to them, but extrapolating from 3% failing in 24 hours to ~15,000 hour MTBF stretches credibility, even for MTBF predictions. The ~15,000 hour MTBF is on the low end of PC board MTBF predictions.
Do follow the recommendations to record failures at various times during burn-in. Also, extend burn-in for samples to get longer run times for some. This is known as "on-going reliability" test by some, "rolling reliability" by others. Consider experimental design; do not test all boards the same amount of time or even at the same temperatures!
PC board infant mortality ends after a month or three, so don't use the Weibull distribution to fit reliability after the end of infant mortality.
For more information about field reliability, what really happens, see my web site. Free samples.