[SI-LIST] : stackup question

Roland F. Portman (portman@bitmicro.com)
Tue, 14 Apr 1998 17:38:09 -0700

Paul,

I have a simalar stackup for my 10 layer board. Only difference is the
location of Power 2 and Ground 2 .

Put the most heavily used, high frequency, etc. power plane pair in the
middle of the board. Separate this
pair by 2.5 mils of FR4. The results will be capacitive coupling of
these two planes (500 to 1000 pf/ sq. inch). This should help very high
frequency return currents. This doesn't mean you can remove the 0.1uf
0603 bypass caps that you have scattered around the board. I haven't
had much success removing bypass caps.
Make up the board thickness by adding between Signals 2/3 and signals
4/5.

Be sure BOTH grounds are electrically the same (tied together) (each
ground via attaches to both).

That leaves a Ground at layer 2 and Power at layer 9. (See Below)

Be sure to make Signal 1, 2, 3, 4, 5, 6 nearly the same impedance. Be
sure to separate the
traces on Signal 1 and Signal 2 a little more to reduce crosstalk.
Remember, crosstalk drops off
by the square of the distance between the traces, so you don't have to
separate them too much more
to gain significant results. Then concentrate your routing
on the top side to signal 1 and 2, and on the bottom to signal 5 and
6. This will help make sure the
return currents go to the same return plane when you switch layers.
(We are trying to eliminate
signal 3 and 4 and go for a 8 layer board, keeping the rest of the
stackup the same.)

I hope this helps.

Roland

Paul Thompson wrote:
>
> I'd like to know what people think about the following stackup for a 10
> layer board with 2 power planes and 2 ground planes:

> Signal 1
> -4 mil-

Ground

> -5 mil-
> Signal 2
> -8 mil-
> Signal 3
> -5 mil-

Power 1
-2.5 mil-
Ground

> -5 mil-
> Signal 4
> -8 mil-
> Signal 5
> -5 mil-

Power 2

> -4 mil-
> Signal 6
>
> The idea is first to get the impedance of all layers quite close, and
> secondly to get as much of the return currents as possible in the ground
> planes rather than the power planes (this is obviously not the case for
> signal layers 3 and 4, but I can keep my critical signals off them.
>
> Can anyone think of a better stackup, or are my priorities mixed up for a
> high speed (~100 MHz) digital board? Am I sacrificing high speed
> decoupling by not putting power and ground planes right next to each other?
> (One plane is 5V and the other is 3.3V, BTW)
>
> Regards,
>
> Paul
>
> --
> Paul Thompson crash@apple.com
> System Integrator, Macintosh Desktop Systems Apple Computer

Signal 1
-4 mil-
Ground 1
-5 mil-
Signal 2
-8 mil-
Signal 3
-5 mil-
Power 1
-6 mil-
Power 2
-5 mil-
Signal 4
-8 mil-
Signal 5
-5 mil-
Ground 2
-4 mil-
Signal 6

The idea is first to get the impedance of all layers quite close, and
secondly to get as much of the return currents as possible in the ground
planes rather than the power planes (this is obviously not the case for
signal layers 3 and 4, but I can keep my critical signals off them.

Can anyone think of a better stackup, or are my priorities mixed up for
a
high speed (~100 MHz) digital board? Am I sacrificing high speed
decoupling by not putting power and ground planes right next to each
other?
(One plane is 5V and the other is 3.3V, BTW)

Regards,

Paul

--
Paul Thompson                                        crash@apple.com
System Integrator, Macintosh Desktop Systems         Apple Computer