[SI-LIST] : thick board & V/gnd microvias

john lipsius (jlipsius@cyras.com)
Fri, 19 Mar 1999 17:02:26 -0800

silisters (sic),

Thanks in advance for any expert opinions..
Summary:
board: very thick board, many V/gnd microvias for BGAs
concern: not enough gnd thru vias and board too thick, anyway?

main features, to give an idea of switching current density:

1) A 9U board with alot of 45mm BGA with microvias (only 1-2 layers
deep) is used, for +V and/or gnd connections, to provide sufficient
routing for traces under BGAs. Board is digital CMOS/GTL/LVPECL
only.

2) 20 high speed xcvrs (622Mbps) in ea. of 4 BGAs near pcb edge, and

3) 300+ single-ended lines in busses between the other 4 BGA, at
155MHz, both with microvias for +V and/or gnd.

4) Board is 0.231 in. thick possibly (ouch).

Q:
a. How important is it to make up for the lack of normal gnd vias around
the BGAs (where most gnds/pwr may use microvia) with a number of
standalone gnd-gnd vias. They won't buy you anything but a costlier pcb
since, for such a thick board, these "standalone" vias will be very
inductive, no? Would the answer depend on how high the ratio
(microvias/normal gnd vias) is?

b. what is the SI benefit of good gnd-gnd plane low imped. path through
vias spread around the pcb (I assume so that there's no accumulation of
return currents too locally on one +V or gnd layer, created noise near
some victim chip).

c. If .231 in. is a problem for all thru vias, is there a good set of rules
for a better via design? I don't think we want blind & double blind vias
everywhere just to getter shorter lengths.

--
Thanks,
John Lipsius

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