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

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From: Bill Dempsey (bdempsey@dnaent.com)
Date: Thu Oct 28 1999 - 08:20:40 PDT


Pat,

  I have to ask a basic question. Why did you choose to do this vs. inserting a 2 mil power core between layers S & Y? Realistically you can get better route densities and you *don't* have to change the total stack height. And
you've reduced a lot of problems out. Have you considered this?

Regards,

Bill D.

Pat Zabinski wrote:

> 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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