Assuming the following:
1 ) a PCB is built/tested and assembled in to unit.
2 ) the unit is then stored un-powered for a period of time.
3 ) what is the probability of the unit working for variable amounts of time ?
What variables come into play ? - I'm guessing temperature - but any others ?
One of our apps guys may be able to provide more information but here is a link to a white paper I found which may answer some of your questions.
Foe - Texas Instruments WW Community Program Manager
If you are worried about this, don't be! The military & aerospace folks have studied these questions for years and they have produced a prodigous body of work on the subject. The lifetime of semiconductors depends promarily on temperature and voltage stress. Unpowered, a semiconductor Mean Time To Failure (MTTF) follows an Arrhenius curve, dependent of temperature and activation energy. Other circuit components also have similar characteristics and their activation energies may be quite different.
Unpowered lifetime can also depend on other unusual storage conditions, such as vibration, humidity, radiation, corrosive or hydrogen atmospheres, etc. Not a likely scenario!
Back in the vacuum tube days this was something to worry about but, under ordinary conditions, it is not worth worrying about these days.
Regards, Neil P. Albaugh ex-Burr-Brown
I would still like to be able to find a source that I can quote to the customer.
They are stipulating a shelf life of 30 years - mainly I believe to mitigate issues of obsolescence.
A Google search of semiconductor lifetime, reliability, or MTTF will turn up a number of useful sources. One typical paper is: http://rel.intersil.com/docs/rel/calculation_of_semiconductor_failure_rates.pdf
Remenber that this paper addresses only the semiconductor portion of a circuit; in practice, components such as electrolytic capacitors, tantalum capacitors, etc, will be far more likely to fail during long-term unpowered storage.
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