RE: [SI-LIST] : Board Stackup Question

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From: Patel, Bhavesh (bpatel@cyras.com)
Date: Wed Mar 21 2001 - 09:46:42 PST


Hi! In both the stackups after S8 it is the bottom layer so you have power
plane depends upon the thickness of the board say approx. 10 mils away from
outside world(air) so the noise from the board will be radiating outisde and
will cause EMI problems.
I would try to keep the power and gnd layer close to each other somewhere in
the middle so that you can get a reasonable decoupling effect(not very
effective unless you go to a thinner dielectric).
Also, you need to mention what is your target impedance for each signal
layer and do you have a lot of high speed traces that fall under critical
routing?
If you had the liberty to increase planes at the cost of routing layers then
I would keep the two power planes but add more gnd planes.
Based on that my stackup for 12 layer board would look like:
Stackup: S1 G S2 S3 P G S4 S5 P S6 G S7
With this stackup you have only S2 S3 S4 S5 S6 as routing layers because S1
and S7 I assume will be used for the components. Is S1 and S8 your top and
bottom layer?
Bhavesh

-----Original Message-----
From: subramanya C K [mailto:subramanya.murthy@wipro.com]
Sent: Wednesday, March 21, 2001 2:20 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : Board Stackup Question

Hello all ,
I have the following query regarding the Board stackup:

If U were supposed to design a 12 layer board with superior EMC, which
stackup among the following would U prefer and why ?

STACKUP-1: S1 G S2 S3 P S4 S5 G S6 S7 P S8
STACKUP-2: S1 P S2 S3 G S4 S5 G S6 S7 P S8
*S = Circuit (routing) Layer ; P = Power Plane ; G = Ground Plane

Next, is there any hazzard due to the power noise in 3rd and 4th circuit
layers (S3 and S4) of the first topology ? if yes, what's the reason
behind that and how do U overcome it ?
(same for S7 and S8 of second stackup)

Finally, if I am given the liberty of increasing the number of power
planes on the cost of circuit layers (say 4:4), will it create any
crosstalk among the tarces on the four circuit layers ? (because now
there are more number of high speed tracks bunched nearer than before)
If yes, is there any go for that ?

Thanks and Regards,

Subramanya C K
Tel: +91-80-8520408 Ext: 4179

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