[SI-LIST] Effect of Pwr-Gnd plane pairing vs. non-paired planes on signal return currents
- From: Alex Horvath <alexh1@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 16 Feb 2006 11:45:21 -0800 (PST)
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-
Sig - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd---------------
Pwr ---------------
Sig - - - - - - -
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) -
Sig - - - - - - -
Gnd --------------
Pwr --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd --------------
Sig - - - - - - - -
Sig - - - - - - - -
Gnd---------------
Pwr ---------------
Sig - - - - - - -
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?
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.
Since I hate to bet my designs on a gut feel hopefully someone can confirm or
deny my assumptions.
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