Just to kinda add my 1 1/2 cents ...
Yes, the norm seems to be 40C. That comes from the
belief that starting with an ambient of 20C, the
equipment can handle a +20C rise above ambient
without blinking.
That's at the system level. Inside it's quite a
different story. A +20C rise for an individual
chip might just mean an absolute temp of 80C or
even 100C case temp.
I've worked with such beasts used in modules that
could be easily removed by the end user. I never
had any problems NOT labelling them, and I never
had any problems with NRTLs testing them. The whole
crux of the matter was accessibility "during normal
use". Since the modules had to be secured into
the shelf to work, end of issue.
Regarding the Arrhenius thing with every 10C rise
or fall in temp, that's also very true. This is
the basis for the MTBF calculations. But, what's
normally accepted by customers is the system level
temp rise, i.e. only +20C globally. To really get
a peek into the near truth of it all, one has to go
down to the chip/component level, take measurements
for each, then run the predictions.
Regards, Doug McKean
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