[SI-LIST] Re: PCB layer stackup
- From: "Curt McNamara" <CurtM@xxxxxxxxxxx>
- To: <jan.kindt@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Jan 2007 08:34:28 -0600
I like stack-up 2 with the suggestion of turning one of the internal
power planes to a ground plane. Here is why:
Many times you will get lower emissions from a product by routing your
clock traces on the outer layers (assuming they are over a ground
plane). I first saw this in an IEEE EMC conference paper and experience
bears it out.=20
The second reason is that your devices (also sources of noise) are
mounted on the outside of the board. In stack-up 2, this means they are
directly over the ground layer, also increasing coupling to ground.
So from an EMC perspective stack-up two (with an internal ground layer
next to the power layer) is best.
Curt=20
Curt McNamara // senior electrical engineer=20
Logic Product Development
411 Washington Ave. N. Suite 400=20
Minneapolis, MN 55401
T // 612.436.5178
F // 612.672.9489=20
www.logicpd.com
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Kindt, Jan
Sent: Tuesday, January 09, 2007 6:26 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] PCB layer stackup
Hi,
=20
I need to specify a 8layer board. Fastest edges on the board are on the
600Mbit LVDS channels : about 500ps. I have two stackups in mind :
=20
stackup 1 stackup 2
---------------------------------------------------------
L1 routing routing
Substrate
L2 power plane GND
Substrate
L3 power plane routing
Substrate
L4 GND power plane
Substrate
L5 routing power plane
Substrate
L6 routing routing
Substrate
L7 GND GND
Substrate
L8 routing routing
=20
Stackup 1 has the advantage that all routing on L5, L6 and L8 is
referenced to GND. We need to push the LVDS channels over a board to
board connector. On the board to board connector, there are GND's
between separate pairs. So we keep the same reference to GND over the
connector.
=20
Stackup 2 is the stackup I would normally use due to it's symmetry.
Signals on L3 and L6 are referenced to both GND and a power plane. But I
don't know why I would need to favour stackup 2 over stackup 1.
=20
Can you comment om which stackup is most preferred ? Or what are the
drawbacks/advantages of one stackup to another.
=20
best regards,
Jan
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