[SI-LIST] Re: Ground vias and the land of ID ("It Depends")
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 22 Oct 2014 06:08:53 -0700
Jeff, I don't think (hope?) that there is any dispute as to what Vss-Vss
vias do electromagnetically. Your simulations reaffirm what I hope at
least most of us understand about: metal, dielectric, E/M energy, and time.
I think the polarized opinions have to do with whether once the vias
that are necessary for the PDN have been placed whether a design will
exhibit problems that adding more Vss-Vss vias will fix. The debate may
not settle until someone offers up a case history with detailed analysis.
Best Regards,
Steve.
On 10/22/2014 5:44 AM, Loyer, Jeff wrote:
> At the risk of finding myself embroiled in a furball, I thought I'd see if I
> could clear up (in my own mind anyway) some of the effects of ground vias. I
> ran 3-D simulations on a pair of single-ended signal vias with various
> configurations of ground vias around them and compared impedance, NEXT, and
> FEXT. The results strongly suggest that nearby ground vias are a good
> investment to reduce via crosstalk. Of course, if you can absorb the extra
> crosstalk, it's a moot point. And ground vias are often hard to put where we
> want them, since they impede routing on every layer, so we seldom have the
> luxury of all the vias we might like (the "no xtalk" configuration in the
> study).
> Here's a link to the study.
> https://www.filesanywhere.com/fs/v.aspx?v‹6e658761616dab72a2
>
> Note: I'm leaving the presentation a bit vague (leaving out any conclusions,
> many exact dimensions, and not stipulating simulation assumptions) on purpose
> - this should not be regarded as any formal "report-out". It's only a
> "quick-and-dirty" peek at the issue. Others may duplicate the simulation
> with different results, though that seems improbable - I don't see anything
> leading me to believe they are fundamentally flawed. They are not overly
> complex topologies, so I invite others to replicate the simulations if the
> results seem questionable.
>
> And, of course, exact results will depend on design specifics (stackups, via
> dimensions, etc.). Your mileage may vary...
>
> Cheers,
> Jeff Loyer
>
>
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