[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

  • From: Kim Flint <si@xxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 13 Aug 2003 17:53:12 -0700


I just treated the cap as a lumped element with .1uF capacitance, 1nH 
series inductance, .1 ohm series resistance. I used 13 mil vias, spaced 130 
mils as if it were about a usual 0603 type placement with no special 
techniques to reduce loop area of the mounting. Properly, I realize it 
would be more accurate to do this more carefully by including pads and 
such, but I didn't spend the time to do that for this exercise. (there's 
only so many hours in the day.) Certainly all of those parameters of the 
cap can be varied. I just wanted to see the basic effect of the location, 
then thought I would share what I had here. If you or anyone else wants to 
take my simulation files as a starting point and add more details, by all 
means please do! and please share! I would be interested to see the result. 
Like I say, more simulation monkeys banging on more computers might give us 
some more useful knowledge. Or suggest some idea and maybe I'll give it a 
try later.


At 05:10 PM 8/13/2003, Chris Cheng wrote:
>Just curious, what kind of via and pad structure did you used to connect the
>bypass caps to the planes (in particular to the 50 mil planes) ?
>  -----Original Message-----
>From:   Kim Flint [mailto:si@xxxxxxxxxxxx]
>Sent:   Wednesday, August 13, 2003 3:59 PM
>To:     si-list@xxxxxxxxxxxxx
>Subject:        [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator
>At 08:26 AM 8/11/2003, Lee Ritchey wrote:
> >If the location of decoupling capacitors matters, perhaps some technical
> >demonstration would prove that.  Short of such a demonstration, this is
> >speculation and not the sort of thing that should be used to make design
> >choices.
>In light of Lee's request for more analysis and less opinion, I thought I
>might try doing a few simulations to see what effect due to capacitor
>location could be shown. I mainly looked into layer switching and the
>effect of cap placement on resulting power fluctuations. I used Sigrity's
>Speed2000 demo version, so other people can take my files and easily
>experiment with them as they like even if they don't own a copy. The demo
>version is free and limited in functionality, but useful for small test
>cases like this. You can get it from Sigrity's web site,
>http://www.sigrity.com. I put the simulation files, some explanation about
>them, and some interesting plots on my website, where you are welcome to
>grab them:
>I think there are at least two aspects to this issue of capacitor placement
>to consider, and they both appear to be mixed together in the discussion so
>far. One is the impedance of the PDS with frequency and whether capacitor
>placement affects it, especially considering I/O behavior on the die. This
>is important in determining whether voltage margins will be maintained when
>current is switched by a transistor on our die at a given frequency. We
>want to analyze whether we get SSO problems and such due to supply collapse
>and where we might place capacitors usefully to help. That much I think has
>been covered already.
>A second aspect might be called "return current control", which I would
>like to discuss some more. A few people noted that they believe capacitor
>placement affects EMI, or loop currents. Probably we should include
>crosstalk in there as well, since a large loop current on an aggressor
>return path can result in crosstalk to victim nets. Capacitors are often
>placed in strategic locations on a board to control such problems, by
>providing a path for the return current to switch layers. For example, we
>might place caps at locations where traces switch across layers, cross
>plane splits, or escape packages. So does that work? Or more usefully, when
>does the cap location matter and when does it not matter? How much effect
>does it have?
>In an ideal case, all of our signals would have a perfectly continuous
>transmission line environment from start to finish. Practically however,
>this is rarely the case in real board designs. For example, as a signal
>transitions from the package to the board, it may end up with different
>reference planes on the PCB than it had on the package. It may be necessary
>to do this briefly for a BGA escape, or because we simply don't have any
>information about the package routing of another vendor's BGA and we don't
>know what reference planes to use. Or, in a dense board we may be forced to
>switch layers on some signals due to routing congestion. Of course we all
>try to minimize this, but it's gonna happen. We know that return currents
>travel on the reference plane following our trace, and wherever we switch
>layers the return current has to find a way to switch reference planes
>along with it. If there is no convenient place for the return current to
>transition to the new reference planes, we may end up with a large current
>loop. This radiates and causes crosstalk, EMI, noise, etc. We use
>capacitors to make an ac path across the planes and hopefully limit the
>loop area.
>It is really interesting to run the simulations and see the 3d animation of
>the planes with the wave propagating as the signal switches layers. It
>looks like a pebble dropping in a pond, with the waves rippling out from
>the via where the layer switch happens. It gives you an intuitive idea of
>what happens on your planes when you switch layers, so I encourage you to
>take my files and try the simulation yourself if you haven't done this
>The simulations I did tend to lend credence to the idea that you shouldn't
>rely on rules of thumb or absolute statements. You need to consider your
>application! And simulate! Because from what I interpret from the results,
>sometimes the cap locations matter and sometimes they don't. It is no more
>correct to say "location never matters" then it is to say "location always
>In my simulations I compared closely spaced planes vs. widely spaced
>planes, different cap positions, and different plane sizes. I used a signal
>pulse with 100ps edge rates, so the frequency content is fairly high. In
>cases where the plane spacing is close (3mil) and the interplane
>capacitance relatively high, the cap location did not matter too much.
>However, when the plane spacing was larger (50mil) and therefore less
>natural interplane capacitance, the bypass cap location did make a
>difference even with 100ps edge rate signals. Clearly though, improving
>interplane capacitance had a much better result, reducing peak voltage
>fluctuations on the planes by an order of magnitude in this case. So, in
>cases where you can't get much interplane capacitance due to stackup
>limitations, small area power islands, or switching across multiple layers,
>cap location might make a difference for you even at fairly high
>frequencies. In other cases where you are switching across closely spaced
>planes, it probably doesn't matter.
>Again, I would urge you to try it yourself and use your imagination to come
>up with your own conditions. Feel free to use my files as a starting point.
>I'm just one simulation monkey banging on one computer, and there are many
>variables I didn't try. (different edge rates, different capacitor
>parasitics and mounting techniques, more caps, etc.) A few more monkeys
>providing their results could give us all a lot more knowledge. also,
>comments welcome! If you spot some error in what I did, let me know.
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