[SI-LIST] FW: PCB layer stackup
- From: "Haldor Husby" <Haldor.Husby@xxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 9 Jan 2007 15:05:53 +0100
Do you really need two Power planes? If not, start with stackup2 and make L5
GND. Then make the dielectric thickness between layer 3 and 4, and layer 5 and
6 much ticker than the others (more than 2x). This gives you a symetrical
balanced stack with GND as return for all signals including PWR, and you get
the desireable interplane capacitance between PWR and GND for high-frequency
decoupling. Combined with generously distributed GND vias, this stack will give
you good return without having to think too much.
In this configuration, it is almost benign to cut up the PWR plane in all kinds
of interdigitated shapes so that many PWR domains can be accomodated on a
single plane. If you have so many PWR domains that this is difficult you should
think twice about using PWR as return for anything. Copper can carry a lot of
current, so you normally would not need two planes for that reason, and you can
increase the current carrying capability with higher copper weight on internal
layers. This stack-up has worked very well for me.
________________________________________________
Haldor Husby, Senior Development Engineer
Data Respons Norge AS
Kongsberg Næringspark
P.O. Box 1022
N-3601 Kongsberg, Norway
Tel: +47 32 29 94 00 Fax: +47 32 29 94 40
Dir: +47 32 29 94 18 Mob: +47 48 04 83 68
haldor.husby@xxxxxxxxxxxxxx
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx on behalf of Kindt, Jan
Sent: Tue 1/9/2007 1:26 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] PCB layer stackup
Hi,
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 :
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
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.
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.
Can you comment om which stackup is most preferred ? Or what are the
drawbacks/advantages of one stackup to another.
best regards,
Jan
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