[SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>,"Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>,<Chris.Cheng@xxxxxxxxxxxx>
  • Date: Wed, 11 Feb 2004 20:15:43 -0800

Istvan, if you have multiple plane pairs then yes there should be lots of 
vias breaking-up the space. But for four and six layer boards, that is 
reduced to vias supporting the decoupling capacitors.


At 10:31 PM 2/11/2004 -0500, Istvan NOVAK wrote:
>Something is not clear to me in this argument.  Via fence stitches planes
>on the same DC voltage; mostly people stitch the ground planes.
>Are you concerned about the cavity formed of GND-GND planes?
>This might be a concern on boards where there are very few
>components which would break the cavity with their vias.
>On boards where you have many components, the necessary
>ground vias will create probably sufficiently small sub cavities.
>----- Original Message -----
>From: "steve weir" <weirsp@xxxxxxxxxx>
>To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>;
><Chris.Cheng@xxxxxxxxxxxx>; "'Istvan NOVAK'" <istvan.novak@xxxxxxxxxxxxxxxx>
>Cc: <si-list@xxxxxxxxxxxxx>
>Sent: Wednesday, February 11, 2004 12:13 PM
>Subject: RE: [SI-LIST] Re: Stack up for EMI reduction, plane resonance and
>u-s trip radiation etc etc
> > Charles, sure.  First, because of the ESD issue, I also favor fences.
> >
> > The problem with fences comes back to plane resonance.  Fences do a really
> > great job of setting up big standing waves on the planes as they make near
> > perfect transmission line reflectors.  We can mitigate this somewhat by
> > avoiding evenly spaced vias.  The advocates of 20H who know their stuff,
> > rightly point out that 20H sets up a controlled leak, thereby effectively
> > dissipating energy that would otherwise resonate in the planes.  20H is
> > controversial, because that "dissipation" is release of energy outside the
> > board.
> >
> > As I said before, if the planes were sealed tight, this would not be an
> > issue.  But vias carry energy to the surface and features above the
> > then become radiators.  DET that Istvan has written about is one way to
> > mitigate the problem.  There are some other interesting ideas around, but
> > at least at this time, I don't have the resources to pursue them.
> >
> > Steve.

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