[SI-LIST] Re: Half OT: large PCB at -40C

Augusto Einsfeldt wrote:

> I am involved in a design where a large board (7"x11") may work at -40C most
> of the time while, for 1 to 2 hours, few times a day, goes to +40 and stay
> there for this time.
> My concern is the care I have to take while drawing the board to cope with
> contractions and expansions, and also if there are other concerns like 
> snip <

I'd expect your primary issues to be solder joint fatigue and PTH failures.
In general for Hi-Rel product you should be specifying properly both bare
board and assembly, IPC &/or JSTD Class 3.  You should probably be talking
with your manufacturing and physical design resources, for their opinions
on specifics.

For the solder, if you have BGA components you should be using an underfill.
Gullwing or Through-Hole are not an issue so long as the joints are formed
properly (driven by specifications above).  I've seen a couple of tricks to
help discrete components but these will again depend on those who assemble
and their capabilities.

For the vias, consider building your boards with additional copper plate in
the holes, and be gentle on the aspect ratio with respect to your 
fabricators'
capabilities.  With these measures FR4 should be fine.

-- 
 
      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

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