Temperature acceleration factor for ALT planning (question posted to Linkedin Society of Reliability engineers group, 5/7/12
Hello, can anyone advise me how to calculate temperature acceleration factor for a complex system including cards, RF elements, cables, motors and moving parts? Is the Arrhenius model valid for such systems, or there are more precise models? Thank you!
…and my response…
The acceleration factor equations are commonly tied very closely to a specific failure mechanism. For example, for SAC solder joint fatigue uses the modified Norris Landzberg model. And, for metal migration/corrosion within plastic encapsulated packages Peck’s equation is useful.
Each failure mechanism reacts to stresses differently, some more so than others. Making testing at a system level with accelerated stresses, … , more difficult.
So be careful, unless you know which failure mechanisms are most likely to occur during use and you accelerate appropriately.
If you know that temperature is the stress of interest related to the dominate failure mechanisms, again be careful, as you will quickly find, the activation energy is important and is generally associated with a specific failure mechanism.
And just temperature may not be sufficient – for moving parts, load and frequency of motion may be more useful for acceleration. For connectors and cables, maybe thermal cycling is more important.
If I don’t run a motor with a high temperature and humid environment it may fail due to corrosion. If I simply run it with an unbalanced load, it may wear out the bearing quickly – both lead to failure, yet have completely different acceleration factors (failure mechanisms) and one test or one AF is just not sufficient.
In short, there are more precise models – depending on the failure mechanisms involved.
hope that helps.