As Doug would say "it depends" are you referring to the PCB or the PCBA?

If it's the PCBA, If it's switched on and taken down to -40 there should not be too much of a problem. Switching on power to a cold board at -40 depends on the circuit. Some components are only rated to -10 and will not fire up from colder temps. 

If the circuit is capable of powering up from -40 be sure to use automotive grade components as some overmold and other materials used in chips have very different properties at low temperatures and this can be a problem when a circuit powers up from -40 and the silicon hits +80 degrees when encapsulated in a low thermal transfer coefficient material whose properties have changed due to the low temp. If you research that is why automotive components tend to use different molding compounds.

Best regards,

John Burke
 

> On Jun 1, 2016, at 10:35 AM, Jack Olson <[log in to unmask]> wrote:
> 
> Has anyone ever heard anything about the reliability of circuit boards in
> cold environments?
> (continuous -40C)
> I couldn't find anything on TechNet or in the Printed Circuits Handbook...
> 
> thanks,
> Jack
> 
> 
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