Let's take the common example of a fully populated .050"
pitch device first:
It is customary in working with BGA devices to put a small
via offset from the actual land, so that soldermask may
provide isolation for the flowing solder from the barrel
of the via hole. Your concern with the draining of the
molten solder is exactly the issue unless you move to a
different fabrication method for making solid copper vias
(pillars).
By keeping this via small, a reasonable web of copper can
be retained on the plane for power distribution and signal
return path.
By polarizing the direction in which this "dog bone" is
routed, i.e. top-left quarter of the device routes toward
10:30, top-right routes toward 1:30 etc. a larger etch path
to the center of the device is created, effectively making
the "swiss cheese" area one quarter the size of the device.
For devices of lower pitch this becomes more difficult.
At a 1mm pitch there may no longer be room for a distinct
via separate from the ball attach pad. Something more
like an "8" is used, again being sure to keep a web of
solder resist (mask) between the edge of the ball attach
pad and the barrel of the via hole. Again, using properly
small via holes the clearance on the plane layer can be
made to allow a web of copper for your return path - if
there is sufficient room between the vias for a trace
there should also be room for a return path (albeit not
very wide). This does intrude on return path but I cannot
quantify how much impact. I can say that we've worked on
product exceeding the signaling speeds you mention with
this technique.
When the pitch decreases to .8mm and under it is usually
not possible to keep the via in the pattern without more
costly fabrication technology. Thankfully most of the
devices of this category that I have seen are not fully
populated, giving the possibility of bringing a surface
wire to an area with more space for vias (as long as that
space doesn't belong to the next device <g>).
Such devices also start stressing other activities such
as bare board test and assembled board test access.
Board thickness makes a big difference in possible via
geometries, the objective being to minimize the aspect
ratio. Also suggest using >= 170oC Tg laminate, to
minimize the rate of thermal expansion at soldering temps
(give those small vias a chance).
I see I've run on a bit. Hope I've helped.
--
Jeff Seeger Applied CAD Knowledge Inc
Chief Technical Officer Tyngsboro, MA 01879
jseeger "at" appliedcad "dot" com 978 649 9800
Printed Board Implementation Services http://appliedcad.com
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