A number of people have talked about masking the vias, but I want to give a
caution as well.  I have seen cases where small diameter vias, capped on
one side, had entrapped sulfuric acid or other plating residues trapped in
the vias.  It caused circumferential cracking in the barrels of the holes.
In essense, in capping one side of the via creates a high aspect ratio
cylindar.  As long as the vias have a large diameter, you might be able to
clean down in the via from one side only.  But as the vias get smaller, you
get to an aspect ratio that cannot be cleaned from one side only.  In the
case I referred to, the threshold was 13 mils.  You can envision it this
way:  cleaning a wide mouthed glass you use for drinking water is one
thing.  Try cleaning the bottom of a straw, closed on one end.

Doug Pauls
Rockwell Collins

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