In many
modern designs using high IO count BGAs or CSPs, vias are more
commonly being placed in the pads to increase routing density. If
you MUST use this layout scheme be aware that there may be assembly
issues associated with this approach. The BESTProto
recommendations are in general "let the PCB fabricator do the
work"!
The
absolute right way to do this is to have your PCB fabricator plug
the via and then plate over the top of it. They can plug with metal
or a thermally /electrically conductive epoxy before the final
plating steps. Done properly this will NOT cause problems in
assembly. Many times due to the rush nature of the prototype
schedule this step is foregone.. This "time saved" is many times
eaten up by the extra time required for rework or fixes when it
comes to board assembly. It is industry best-practice to both fill
the via in this manner and plate over it.
Another
often-used approach is one where a micro-via that only goes through
one layer of the board. This is not the preferred approach but it
can work with some stencil modifications. In this case we would like
to hear from you prior to receiving your materials so we can make
stencil design adjustments. The solder can wick down into the via
chamber if it is not compensated for in the stencil
design.
We have
seen cases where due to time constraints, the underside of the via
is "capped" with solder mask. However, the cap can sometimes open
during processing. The void may be big enough to still starve the
BGA solder joint and pull too much of the solder off of the pad -
even if the mask cap remains intact. We have seen instances where
the solder ball has sucked the solder ball off of the BGA, leaving
an open.
In no
cases does BESTProto recommend open vias on device pads. You
are only asking for delays and rework. | 
