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

From: "Istvan NOVAK" <istvan.novak@xxxxxxxxxxxxxxxx>
To: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
Date: Thu, 12 Feb 2004 21:34:53 -0500
Chris,

OK, time to stop the thread.

Regards,
Istvan

----- Original Message -----
From: "Chris Cheng" <Chris.Cheng@xxxxxxxxxxxx>
To: "'Istvan NOVAK'" <istvan.novak@xxxxxxxxxxxxxxxx>; "Chris Cheng"
<Chris.Cheng@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Sent: Thursday, February 12, 2004 4:13 PM
Subject: RE: [SI-LIST] Stack up for EMI reduction, plane resonance and
u-strip radiation etc etc


> No,no,no. We are not in agreement. Especially you haven't answer my
question
> :
> "how could your fancy capacitor or thin core plane help if they are
> electrically further from the reference planes ? "
>
> First of all, you can't get into the power and ground bounce issue with
the
> reference planes without getting into  crosstalk problem. Afterall, it is
> the constructive overlapping of the image current that creates the
> power/ground bounce problem in the first place.
>
> Now let's think about what do you need to do to bring in the thin core
> decoupling plane to relief the problem. You first need to drill vias to
> bring the image current from the reference power plane to the other side
of
> the power plane that has the thin core capacitor. The current then has to
AC
> coupled to the ground plane through the thin core and then come down to
the
> ground reference plane with additional ground vias. This is not a low
> impedance path as compared with direct plane coupling and the vias has to
be
> numerous and very close to the signal traces to be effective. Here comes
the
> problem, if your signal traces are so close that the image current starts
to
> overlap on the reference already, WHERE ON EARTH DO YOU FIND THE SPACE TO
> DRILL THE NECESSARY VIAS ?
> Like I said before these power/gnd bounce problem on signal trace crowding
> is an observable problem on highspeed high density package. I have done
> enough of these packaging analysis to convince myself your trade-off point
> does not exist. i.e. Either your traces will be so spaced out that vias
can
> be drilled near but then their image current don't overlap each other
> significantly on the reference plane to create power/gnd bounce problem OR
> they will be so tight and close that you can't drill your via to bring in
> your thin core relief current anyways.
>
> And for the last time to answer your last question. The spacing between
the
> signal/power/gnd planes are dictated by the impedance control parameters.
It
> is not dependent on the edge rate of the signals.
>
> What I am disappointed and alarmed is all these discussions/arguments has
> been repeated over and over in this forum and if you go back to the
archive
> a few years back, there was a flare up with exactly the same argument and
I
> have pointed out exactly the same problem. Did anything changed since then
?
> I certain have not changed your mind and neither have I so what's the
point
> of continuing this discussion ?
>
>
> -----Original Message-----
> From: Istvan NOVAK [mailto:istvan.novak@xxxxxxxxxxxxxxxx]
> Sent: Wednesday, February 11, 2004 7:59 PM
> To: Chris Cheng; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Stack up for EMI reduction, plane resonance and
> u-strip radiation etc etc
>
>
> Chris,
>
> > a) If your plane reference is so limited and crowed with highspeed
traces
> > that it can not provide the effective capacitance, it will exhibit
itself
> as
> > both xtalk and power/gnd bounce problem. The image current starts to
> overlap
> > each other and either add or subtract from each other. This is an
> observable
> > problem in most signal traces in organic packages. But I will turn the
> table
> > around and ask you, how could your fancy capacitor or thin core plane
help
> > if they are electrically further from the reference planes ? It's like
> > challenging my Covertte saying "hey, I bet you can't drive this car at
> > 300mph" while you are sitting on a pintle.
>
> So I think we are in agreement here that if trace density is increases,
> beyond
> a certain point we will have power/ground bounce issues on the planes.
> You are correct that crosstalk among traces will probably go up at
> a similar rate, but it is a matter of system design, which will pose a
> limitation first.
> If you hit the power/ground bounce limit first, and crosstalk is still not
> harmful,
> a thinner power/ground laminate may help to reduce power/ground bounce.
> If in the new stackup you still reference the same power plane, what has
> changed is that the traces will be 'outside' of the power/ground cavity,
not
> inside as before.  In this case only the ground reference plane for the
> traces is
> what is further away from the power/ground plane pair.  If the components
> on the board force you to have a large number of ground vias anyway, you
> can get the sufficiently tight stitching between the ground planes without
> extra
> expence.
>
>
> > b) At extreme high edge rate, the skin effect is limiting both the
signal
> > trace and the image current that flows on the reference plane, your
> infinity
> > argument doesn't exist. I can't answer an argument that cannot exist.
>
> OK, let me rephrase the question that may be easier to answer.  Say you
> have a working board, and you are satisfied with it.  It has a given
number
> of traces referencing the correct plane.  Say the transition times on
those
> traces are all around 1 nsec.  And lets suppose the power/gnd bounce
> is acceptable: not much lower than your target, but safely below your
> limit.  Suppose the only thing you change next is the silicon, and it puts
> out
> 200psec transition times instead of 1nsec.  There is no other change
> on the board.
> The 200psec edges are 'slow' enough that within an inch radius we cant
> really
> expect any absorption due to skin effect, and the one inch radius
> approximately
> represents the distance the signals can go within 200psec.  So the
> question is: if you want to maintain about the same level of power/ground
> bounce,
> would you change the plane structure; would you put the power/ground
planes
> closer, further apart, or leave them where they are?
>
> Regards,
> Istvan
>

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References:
- [SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-strip radiation etc etc
  - From: Chris Cheng
[SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-strip radiation etc etc

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