Brandon:

I asked a similar question a couple weeks ago on TechNet and received some 
excellent replies.  My question related to tenting vias on one side, two 
sides or not at all.  I also checked IPC-D-275 and IPC-D-279.  I was worried 
about contamination being trapped in vias tented on the top side only.  
Here's some of the highlights .. see if you can reach a conclusion.

Are you using LPI solder mask?  If so, you can't use it to tent vias.  You 
must use dry film (IPC-D-279 Para N-2.1.3).

Open or untented PTH's and PTV's can allow lliquid flux to be trapped with 
the potential for corrosion ... If solder mask is intended to plug ot tent 
these holes, it must do it consistently (IPC-D-279 Para N-2.0).

Tenting of vias shall be accomplished so that the hole is covered or filled 
from both sides (IPC-D-275 Para 5.5.1).
                                                               
Some responses from others ...

Will the assembly ever see a wave solder machine?  Then tent vias which are 
located under components, where solder balls could be formed.  With the wave, 
you can cap the solder side and leave the top side open, assuming you do not 
top-side flux.  If you do not wave solder, there is no reason to tent vias, 
except to improve vacuum for ICT.

One respondant suggested sizing the top side mask based on the via hole size 
rather than the pad size.  Interesting...

At present we are not tenting vias on either side.  We have some cases where 
we have vias under components, but these designs don't see a wave.  No 
problems that I'm aware of.

good luck

gary ...

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