Re: [SI-LIST] : different 4-layer board Stack up (S-P-G-S) ?

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From: Nirmal Jain (nirmalj@rambus.com)
Date: Tue Mar 07 2000 - 13:58:46 PST


Hi Jeff,

Your gnd/pwr via inductance for P-S-G-S stackup would definitely be
smaller but you would now have a smaller integrated decap between gnd
and pwr planes and also you may have higher L for the discrete deccap
connections to the gnd and pwr planes. So this stack up may need further
analysis if SSN is an issue.

Best Regards

Nirmal

jwalden wrote:
>
> Adding to the original question; what are the opinions about a P-S-G-S
> stackup?
>
> Keep the spacing between the ground & the signal layers the same and as
> close as possible. This will keep a more uniform impedance (whatever it is)
> and shorten the via barrel length (and resulting L) for the signal traces.
> The signal lines are tightly coupled to the same reference plane ... just
> different sides. Might this reduce high frequency mode conversion that
> occurs with fast edge rates on longer vias between loosely coupled (or at
> least seldom tight enough) layers such as power & ground?
>
> If there are a large amount of outer layer signal lines, then more careful
> attention might have to made to ensure that the inner signal lines'
> characteristics more closely match the outer. Such as moving the power
> plane further away to reduce its effect on the inner signal layer. And,
> adjusting the inner layer signal line widths to compensate for being
> embedded in the dielectric as well as some effect of the power plane.
>
> Thanks,
> Jeff Walden
>
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