John,
As Doug says "it depends" (OK, I owe Mr. Pauls a Mtn Dew)
If your final finish does not add chemical entrapment concerns or if you
really need one side sealed for vacuum or if you are concerned with
solder bleed through during wave or if you are probably starting to see
there are multiple factors to consider.
I would start with the why do you want to mask 1 side and follow up with
a conversation with your fabricator.
Some houses will say no worries, while others will shriek with concern.
If you use one or two fabricators and they agree, then go ahead, but if
you don't have say in who builds your boards (it's a purchasing
decision), then you may want to stay on the cautionary side of not
masking.
ENIG, IAg, ISn and OSP all need to wash through the hole for good
coverage and protection of the copper, so if you call out SMOBC there is
a real possibility of chemical entrapment.
If you allow final finish to be applied prior to soldermask then there
should be minimal concerns.
Figure out your reasoning why you want it and talk to your supplier to
decide if it really makes sense (both in performance and cost).
Hope it helps.
FNK 

Frank N Kimmey CID+
Principal PCB Designer
PCB Programs
Powerwave Technologies Inc
Office 916-941-3159
Fax 916-941-3195
Mobile 916-670-0645

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Foster
Sent: Tuesday, May 01, 2007 8:48 AM
To: [log in to unmask]
Subject: [TN] unsymetrically masked vias

Is it problematic for a fabricator if a via

.062 board   10 mil finished via


is masked on one side and not the other?

Any input will be appreciated

Thank You
John Foster

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