Jack -

PWR and "Ground" are DC values. Signals are switching therefore they are
AC (even digital bus signals.) Any DC Plane can serve as an AC return
path (a much better term than ground.) The real issue to watch for when
routing is switching between reference planes, i.e. when changing from
vertical to horizontal routing directions. Changes from one reference to
another should only be done at points where decoupling or other
filtering will allow the signal return energy to be transferred between
planes quickly and with minimum loop area. 

If you are working with a 6-layer PWB keeping 1-3 and 4-6 as routing
pairs would be the most efficient, even though one pair uses PWR as a
signal reference. Watch out for layer stack-up's that create copper
imbalances or you will end up with potato-chip shaped boards, warp bow
and twist all working against you.

If placement density keeps you from using external layers for routing it
may be better to treat the board as an HDI 4-layer +2 cap layers and
place your planes as layers 3 and 4. 

Onward thru the ...um... fog, yeah that's it its fog ;-D.... 

Jeffrey McGlaughlin, CID 
Engineering Designer 
Battelle 
505 King Avenue 
Columbus Ohio 43201-2693 
(614)424-7582 Phone 
(614)458-7582 Fax 



-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
Olson
Sent: Thursday, January 04, 2007 10:51 AM
To: [log in to unmask]
Subject: [DC] Straddling the Return Path

I have a question about layer stackup and routing.

Most of our designs have a pair of routing layers
sandwiched between planes, one for vertical and
the other for horizontal traces.
Maybe I haven't had enough coffee yet today, but
I started wondering how the return path energy
gets from one plane to the other (unless there
happens to be a decoupling cap nearby?) If both
planes were GND there wouldn't be a problem,
since they would be stitched together in many
places, but one is GND and the other is PWR.

So I started wondering.... Wouldn't it be better to
have one routing layer ABOVE the GND plane and
one BENEATH? In this scheme all the routing would
be straddling the SAME return path and it seems
logical the it would be less noisy. There would never
be a discontiuity from the energy trying to get from
one plane to the other, right?

maybe the gains are not worth the loss in the
overall scheme, though?

onward thru the fog,
Jack

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