Soldermask on via-holes with ENIG finish.

Source: Soldermask on via-holes - Eurocircuits Printed circuits blog | Eurocircuits: 

"For closed via-holes we have developed an alternative solution which avoids chemicals getting trapped in the partially closed via-holes during developing of the soldermask or during application of the Ni/Au. 

Before the coating the entire panel with soldermask we selectively print soldermask into the via holes using a stencil. During a second print run we then cover the whole panel. 

This way the via-holes are completely filled with soldermask. An even layer of soldermask now covers the via-holes leaving no pockets to hold residual chemicals. 

We have used this technique for over 6 months, and it has proved successful in dramatically reducing the number of skip pad problems."

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