Our experience with this issues has not been totally positive.  Although one
may successfully protect the actual bare board (assuming certain minimum
thicknesses of soldermask), any exposed copper on components still are
subject to corrosion.

A second issue specifically on SMT boards is the height of the  soldermask.
 If the mask is too high, it creates problems at assembly.  If it is not
thick enough over the corners of isolated circuits, it does not afford
adequate protection.

A third issue is handling vias under components.  LPI does not tent well
enough to protect the via.  HASL prior to plugging, especially on mixed
technology boards, creates assembly issues.  Successfully 100% plugging vias
with mask is an artform.

I would not recommend soldermask as an alternative to conformal coating.
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Subject: [TN] ASSY:  Soldermask vs conformal coat