Dave - Tenting of vias is a wise precaution to reduce the potential for
intermetallic growth formations between exposed conductors.  The more exposed
metal you have under part bodies and in hard to clean areas, then the  greater
the risk that some small amount of process residue, coupled with moisture and
operating current, will become a problem.  Vias themselves can also represent
a cleaning problem, simply from their small size.

Even though solder mask is not to be used as a conformal coating, you will
note that its dielectric values and process durability are quite respectable
for this purpose.

A word of caution: some of the less durable solder masks are not necessarry
acceptable to tent vias.  The heat of soldering can generate considerable
pressure from the atmosphere in  a via, causing the mask tent to rupture.
Recommend you test this thoroughly, as ruptured via tents tend to inhale
process fluids as they cool following soldering.

As with any solder mask application, IPC-SM-840 indicates that the user must
confirm compatibility with his own processes and applications.  We have
encountered a number of masks which are susceptible to some of the non-CFC
cleaners, as well as a few which reacted to various fluxes.

Regards - Kelly

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