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.
|
