IPC-SM-840 Rev C (that has just been published) has some additional 
     suggestions and pros/cons regarding plugging vias.  Although I do not 
     agree with all of the conclusions states but it might give you some 
     additional information.
     
     P.S.  You can pick up a copy of this new spec. at your local IPC 
     store.
     
     
     [log in to unmask]
     


______________________________ Reply Separator _________________________________
Subject: Tenting via holes
Author:  [log in to unmask] at corp
Date:    12/7/95 11:06 AM


We have been tenting via holes (dia. .014-.020) with dry film solder 
resist per IPC-SM-840, type B, Class 3, for the following reasons:
     
- increases usable area for silkscreening
     
- eliminates solder fill inspection in the via-hole, thus enabling vias 
to be placed under components
     
On the down side, tenting does introduce the risk of contaminating the 
plated-through via hole if the hole does not get fully tented.
     
     
     
Our PWB fabrication vendors are showing an increasing reluctance to use 
this type of solder mask coating, claiming that it is expensive, wasteful 
panel-wise, and has adhesion problems. In order to get the dry film to 
adhere better, they are forced to do another process (oxide treatment). 
They would rather use a liquid photo-imagable (LPI) coating conforming to 
the same spec, since the process takes less time, with less waste of 
material, better adhesion, and finer pattern definition. Unfortunately, 
LPI coatings don't tent vias reliably under normal processing.
     
     
     
My question:
     
     
     
What material and processing can tent  via holes (dia. .014-.020) in a 
reliable and cost-effective way?