Hi Leif,

A few quick thoughts

Conformal coating provides a water resistant barrier, is quick, cost effective 
and easy.   Conformal coating can be applied over entire assembly or applied 
selectively.

Underfill will also help with CTE mismatch and is a slow production process 
(possibly warranting multiple machines to meet thru-put needs).

When considering routes for moisture absorption, humidity migration thru the 
circuit board material (BT Resin or FR4) will be minimized by the solder resist.  
That said, anyone who has baked a board to measure dry weight has seen the 
measurable amount of absorbed moisture present in an assembly.

Both processes are benefited and in many cases require a pre-clean step to 
promote adhesion, flow, and minimize entrapped contaminants (including 
corrosive elements).  

Rework can be a bear with both. 

Cordially,

Shean Dalton
FHP
512.971.2850

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