Soldering guidelines for freezer applications?
The differential contraction is pretty small; normal SMT works down to at least -40C with no special techniques. Thermal cycling can be a bit more of a problem, if you're going from -40 to +100 and back every few minutes something will snap after a while. BGA parts are more vulnerable to this.

PCB conformal coating (applied after assembly and test) will keep water out of the board.