[SI-LIST] Ground vias and the land of ID ("It Depends")

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: "si-list (si-list@xxxxxxxxxxxxx)" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 22 Oct 2014 12:44:52 +0000

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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