When vacuum fixture testing is required we encourage our customers to use 
the following fix.

Soldermask phototools are created that leave an open pad area on the 
solder side but tent the component side of the pcb.  Our curtain coat LPI 
process of course does not tent or plug the vias.  However the subsequent 
wave solder process will!

This has the advantage that we have no risk of chemical residues from 
HASL being trapped in the via holes.  We can also mass produce plugged 
vias effectively without the use of dry film solder mask etc.

Our customers really like this cost effective fix and have not seen 
problems with solder balls on the top side.


Richard Wood-Roe
Artetch Circuits Ltd.
Littlehampton - UK