Trying to tent just one side of a via hole is certainly tricky. You are right
in that you might end up trapping some corrosive materials inside the via
hole (flux residues, for example) that you can't get out. This could be
especially risky on a life support board design, which you indicated.

Some suggestions might include:

1. Try to convince the customer to leave the via hole relieved on both sides.

2. Completed relieve the mask on the electrical test side of of the via hole
and bring the mask up onto the land area of the via hole on the other side.
This would alllow electrical testing on the one side and should prevent
shorts on the other side. It would allow the via hole to remain open and you
would trap no corrosive materials. You could accomplish this on your solder
mask artwork.

Hope this helps.

Regards,

Larry Fisher
Dexter Electronic Materials
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