[PCB_FORUM] Re: Power plane -vs- gnd path signal return path
- From: "Ritter, Alan" <Alan.Ritter@xxxxxxxxxx>
- To: icu-pcb-forum@xxxxxxxxxxxxx
- Date: Thu, 13 Mar 2008 09:16:41 -0500
Power supply planes are in effect an AC (HF) ground. Your stackup using
solid planes next to your HF signals is preferable. Running HF signals over
splits in planes is asking for trouble. One thing to watch for is via
placement in your planes so you don't end up creating splits because of
overlap of the vias' antipads in the planes. You may have to space your
vias out a bit if you end up with a group of them lined up
shoulder-to-shoulder.
/s/jar (Alan Ritter, alan.ritter@xxxxxxxxxx)
http://www.mtritter.org <http://www.mtritter.org>
_____
From: Jim Wages [mailto:jwages@xxxxxxxxx]
Sent: Thursday, March 13, 2008 8:20 AM
To: icu-pcb-forum@xxxxxxxxxxxxx
Subject: [PCB_FORUM] Power plane -vs- gnd path signal return path
Ok, folks. Let me know if my thinking is wrong here.
I have a 14 layer stack up. External etch layers are adjacent to power
planes, which are adjacent to ground planes. Lyr 2 is a split power plane.
Lyr13 is a solid 3.3V plane.
Do you see any signal "return path" or integrity issues with routing a
critical signal trace on the bottom layer as long as the driver of the
signal was powered by 3.3V. What about a differential pair?
I'm assuming that the power and gnd planes would be perceived as a short to
hi-frequency signals, so that the return path would not have to get to a
ground plane, but would just stay on the 3.3V plane directly beneath the
trace until it returned to the driver.
Thoughts?
Jim S. Wages
SR. PCB Layout Designer
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