[SI-LIST] Re: Effect of Pwr-Gnd plane pairing vs. non-paired planes on signal return currents
- From: <Aubrey_Sparkman@xxxxxxxx>
- To: <alexh1@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 16 Feb 2006 13:59:12 -0600
Alex,
The thing that caught my attention is that your signal switched from
ground reference on top to power referenced on bottom, not that you
removed a couple of reference planes....=20
But that's just my quick gut reaction....
Aubrey Sparkman=20
Enterprise Engineering Signal Integrity Team
Dell, Inc.=20
Aubrey_Sparkman@xxxxxxxx=20
(512) 723-3592
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Alex Horvath
Sent: Thursday, February 16, 2006 1:45 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Effect of Pwr-Gnd plane pairing vs. non-paired planes
on signal return currents
Normally, when I design a multi layer stackup I always try to alternate
and pair Pwr-Gnd planes to maximize inter-plane capacitance. For
example, a 16 layer stackup might look like-
=20
=20
Sig - - - - - - -=20
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd---------------
Pwr ---------------
Sig - - - - - - -=20
=20
For cost reasons I might be forced into a stackup with the same number
of signal layers but with 2 power planes removed (14 layers, same
dielectric thickness) -
=20
Sig - - - - - - -=20
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd---------------
Pwr ---------------
Sig - - - - - - -=20
=20
=20
Let's assume a signal traverses from the top signal layer to the
bottom layer in both stackups. Let us also assume as a worst case that
there are no nearby vias or decoup caps and the signal edge rate is
approximately 10 times the via length (about 200 psec). Is there a
significant difference in the expected signal quality when comparing
these 2 stackups?
=20
My gut feel is that all else being equal the primary factor
determining signal quality is the spacing of the planes. The DC
assignment of the planes has (again ignoring effect of gnd or pwr vias)
little effect and thus the signal quality in the above cases would be
virtually identical.
=20
Since I hate to bet my designs on a gut feel hopefully someone can
confirm or deny my assumptions.
=20
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=20
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