Some times, it is not the FR4 is in question, the assembly housing  
and mounting would be.  The aluminum housing shrink rapidly may exert  
extra force or bent the PWB (particularly, cyclic, it might cause few  
high resistance vias).  the type of fault you don't get catch if you  
only do PWA (without housing) thermal shock... many thermal shock  
were done at PWA sub-assembly level, due to the thermal mass too high  
at system level that take too long to reach extreme temp due to test  
chamber limitation...
On Jun 1, 2016, at 7:54 PM, Jenkins, Jeffrey A @ CSG - LINKABIT wrote:

> Jack,
>
> As others have indicated, most good FR4s will handle going down  
> there just fine.  In a prior life back in Minneapolis, we took  
> product down to -65°C to stress test it and the board was never the  
> problem (parts on the other hand.....).  If you going for a wide  
> range say -40°C to +80°C on a routine basis, spec a better FR4  
> material.
>
> One other thing you have to watch out for is that clock rates/edge- 
> rates/timing can get a little faster when one goes colder and  
> slower when you get hotter.  So something that works at 0°C or even  
> -20°C may not handle -40°C.  Components can also not handle those  
> temps, so a good review needs to be done.
>
> Best,
>
> -Jeffrey
>
> Jeffrey A. Jenkins
> Sr. PCB Staff Designer, CID+/CIT
> L-3 Communications, Linkabit Division
> Work:  858-552-9832
> Email:  [log in to unmask]
>
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Jack Olson
> Sent: Wednesday, June 01, 2016 10:36 AM
> To: [log in to unmask]
> Subject: [TN] FR4 in COLD environments
>
> 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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