Re: [SI-LIST] : quad offset stripline?

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From: Bradley S Henson (bhenson@notes.west.raytheon.com)
Date: Thu Oct 28 1999 - 06:54:38 PDT


I see you're using 'W' element t-line models, so I guess your using HSPICE for
generating the eye diagrams too???

To answer your question, I've done a similer stackup under duress, and it wasn't
a production board. I think it can be done with the type of rigorous engineering
you appear to be doing. I'll suggest adding extra margin for inter-layer
effects: When we did inter-layer SSO we saw more interaction than
predicted...the z-axis and diagonal coupling of lots of bus lines to a victim on
another layer appeared to have more of a cumulative impact than we expected.
I've bounced around jobs a couple times in the last 2 years so I no longer have
the writeup, but I think I've hit the high/low points.

Luck,
Brad Henson, Raytheon

"Pat Zabinski" <zabinski.patrick@mayo.edu> on 10/28/99 05:25:48 AM

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 Subject: [SI-LIST] : quad offset stripline?
                                                              

On a rather route-intense design we're working on, we are trying
to squeeze in as many signal layers as we possibly can in
a given overall board thickness. We've been playing around
with different scenarios with different board vendors for
the past month, and what we've come up with is a layer stackup
based on "quad offset stripline", meaning:

---------------------- plane
     ---- signal-T
         ---- signal-S
   ---- signal-Y
          ---- signal-X
---------------------- plane

X is horizontal, Y is vertical, S is 45, and T is 135. We have
buried vias between S & T and between X & Y. For a particular
signal, we only route on orthogonal layer-pairs.

We've been analyzing this for a short time now, and it looks like
it might work out for our application. But before we take it too
far, I'd like to get input from folks on potential gotchas that
I should be concerned with.

As background, we have:

     * designed a line width for the respective layers to obtain
       our target impedance (50 ohms).

     * ran SSN eye diagram simulations of multiple signals
       on one layer at a time to determine the minimum
       trace-pitch for that layer.

     * using the minimum-pitch per layer, mutual capacitance
       and inductance of the crossovers (taking into account
       the relative angle of the traces), and a W-element
       representation of lines on each of the four
       layers, we ran an SSN eye diagram simulation of random
       signals on all four layers to determine the effects
       of the mutual parasitics from the other layers.

So far, if we keep the trace pitch wide enough, this seems to
work just fine. However, I'd like input of other areas we
should look at.

Any ideas? Has anyone used this sort of thing in the multi-100's
of MHz (<500 psec Tr) regime? Am I missing something?

Thanks,
Pat

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