Re: [SI-LIST] : 18 layer stackup question

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From: Dave Hoover (dave_hoover@yahoo.com)
Date: Wed Apr 12 2000 - 09:51:37 PDT


Ellis,
From a fabrication stand point, the stack-up is
leading
edge. 3 pairs of ZBC-2000-TM, the ZBC locations forces
one to use core construction as opposed to foil
lamination (on the outer material sets). There would
be
a bit of slip sliding to optimize during manufacturing
(dimensional stability). Are the linewidths at .004"?
If so, the manufacturing yields would increase using
1/2 oz on the signals along with a better chance to
target and achieve 50 Ohms.

Dave
--- e <evillaf@home.com> wrote:
> > I am currently in layout with an 18 layer board
> stacked up as follows:
>
> 1 - TOP
> 3 mil
> 2 - GND
> 5 mil
> 3 - IN1
> 4 mil
> 4 - IN2
> 5 mil
> 5 - GND
> 2 mil (buried cap)
> 6 - 3.3V
> 5 mil
> 7 - IN3
> 4 mil
> 8 - IN4
> 5 mil
> 9 - GND
> 2 mil (buried cap)
> 10 - 5V
> 5 mil
> 11 - IN5
> 4 mil
> 12 - IN6
> 5 mil
> 13 - GND
> 2 mil (buried cap)
> 14 - 1.8V
> 5 mil
> 15 - IN7
> 4 mil
> 16 - IN8
> 5 mil
> 17 - GND
> 3 mil
> 18 - BOTTOM
>
> All layers use 1 oz Cu except TOP and BOTTOM use 0.5
> oz. I have specified minimum of 20 mil
> center-to-center distance between signal layers to
> minimize cross talk. This give a D/H ratio of 4,
> per
> Howard Johnson's book. Is this overkill? Most
> signals are running between 66-100 MHz, with rise
> times of 1-3 ns. I believe Dr. Johnson provides an
> example where he set the D/H ratio to 5 minimum.
>
> Does anyone have any comments on the stackup? Is
> there anything I should watch out for?
>
> All inner signal layers are closer to each other (4
> mil) than to their respective reference planes (5
> mil). Will this cause any problems?
>
>
>
> Thanks in advance.
> Ellis
>
>
>
>
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