[SI-LIST] 0402 capacitors and 1mm pitch BGAs

Hi SI-Listers,

I'm looking for feedback on producibility or other
concerns regarding the use of 0402 capacitors with
a 4 via footprint on a 1.0mm grid.  From measured
data provided in Lee Ritchey's book "Right the
First Time", this footprint yields a mounted ESL
of about 450pH (for a via length of 13.5 mils).
Even better, it fits the pitch of a 1mm BGA.  For
a fully populated BGA, the only alternative
decoupling schemes I can come up with are:

1) blind vias, a good solution electrically but
   not so great economically.

2) via-in-pad, maybe the best solution
   electrically but not so economical.

3) via sharing to open up areas to place
   capacitors.  very economical, but a definite
   compromise electrically.

For a high performance chip in a 1mm BGA package
that requires low-inductance decoupling, dropping
0402 capacitors in between the BGA's dispersion
vias seems to me to be the best engineering
choice.  Even if a higher defect rate means that 1
of the 50 caps on an occasional board has an open
solder joint, I'd claim that board has better
electrical margins than a perfect board designed
with compromised electricals to start with.

What do others think?  To help visualize the cap
and its footprint, I've uploaded a picture to:

http://briefcase.yahoo.com/electronbob2@xxxxxxxxxxxxx?.dir=0402

There are two resolutions: 800x600 (about 85x
magnification assuming a 100dpi screen) and
2032x1524 (about 220x magnification on a 100dpi
screen).

Bob

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