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?