It may be helpful to determine which components are likely to be the failure points. Various materials and components age quite differently with differences in temperature.
For examples, IEC 60950-1 does give a rough ageing test for adhesives used for constructional purposes in section 4.6.5. Adhesives are preconditioned for eight weeks at 82 C, three weeks at 90 C or one week at 100 C. Fixed temperature and humidity cycling is performed after the temperature soak. Somebody seems to think that adhesives age three times as fast for every 10 degree increase in temperature. Lithium-ion batteries can exhibit an even greater decrease in life with increases in temperature. Storing fully charged lithium-ion batteries at elevated temperatures can significantly affect their capacity and operating life. http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries On the other hand, if the failure mode involves stress fractures or is related to the brittleness of materials, increases in temperature may forestall failures. I seem to recall that the first time I heard that increasing temperature by 10 degrees cut product life in half was related to semiconductors. However, even here, the situation isn't quite that simple. The following article from Intersil gives a more rigorous mathematical analysis. http://rel.intersil.com/docs/rel/calculation_of_semiconductor_failure_rates.pdf Ted Eckert Compliance Engineer Microsoft Corporation [email protected]<mailto:[email protected]> The opinions expressed are my own and do not necessarily reflect those of my employer. From: Gary McInturff [mailto:[email protected]] Sent: Monday, October 20, 2014 10:00 AM To: [email protected] Subject: Re: [PSES] Reliability John you're going to be wrong no matter what you do - depending on who you are talking to. The 10 degree rise = twice the failure estimation is based on the Arrhenius model - which is actually a chemical model, but widely applied to electronics So as a rule of thumb you can state you acceleration factor is based on that - assuming that you product is displaying a normal (Gaussian) random mode failure mechanism and not infant or wear-out modes. The more proper way of finding the actual acceleration rate - for temperature or anything else is to run life tests at low, normal and high temperatures and correlating the failure times - but I'm guessing you (and most people) don't have the time or resources to do that so prediction is the only real tool you have - unless you have an ongoing reliability growth mechanism, which might be in-house testing or working with the field data failures and making adjustments. Getting good field failure data is a real problem on its own. Sample size is going to be a real problem to John. If you have one sample does that represent the whole population, or did you get the one unit that was in the 10th percentile or maybe the 90th percentile. Another rule of thumb is that you need 25 - 30 samples to get a better feeling for the full population size not just the test size. I would work with your customer (engineering staff or external customer) and seek agreement that he accepts the acceleration factor and the sample size. This is a much bigger problem than you may realize but if you can't test many samples for many hours then you have to start somewhere and state your assumptions. A good primer of the subject (although it doesn't do into acceleration factors is Reliability Statistics by Robert A. Dovich ISBN 0-87389-086 (at least that's the number on my version - could be newer editions) This is a big and ugly specialty John. But as a starting point, after agreement with your customer, running the unit(s) at a higher temperature and assuming the 10c doubles the failure rate is at least a starting point. Gmac From: [email protected]<mailto:[email protected]> [mailto:[email protected]] Sent: Monday, October 20, 2014 6:38 AM To: [email protected]<mailto:[email protected]> Subject: [PSES] Reliability Hi, Any reliability engineers out there? Based on every 10° rise reduces life by half, and if I call 1 year at 22C my baseline, conditioning at 62C for 3.25 weeks will equal 1 year. Does conditioning at 62C for 6.5 weeks equal 2 years? Thanks, John - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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