Re: [SI-LIST] : effect of trace width on the performance

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Wed Nov 01 2000 - 12:18:48 PST


Quite a stackup! You are not working for NASA, are you? :-)))) I wish I
were allowed to do something like that!

Mike.

"Muhammad S. Sagarwala" wrote:

> Here is the board stackup... --------------- Top layer with all the
> active componenets--------------- Ground layer--------------- Vdd1
> layer--------------- Ground layer--------------- Vdd2
> layer--------------- Ground layer--------------- Signal
> Layer--------------- Ground layer--------------- Signal
> layer--------------- Ground layer--------------- Signal
> layer--------------- Ground layer--------------- Signal
> layer--------------- Ground layer--------------- Bottom layer (with
> decoupling caps and other discretes) Just to sum it up.. there are two
> power layers pretty close to the device.All the signals are routed as
> strip lines. The decoupling caps were placed on the bottomside of the
> board because of space limitations on the top side. All the
> decouplingcaps are x7r (dielectric). The layer stackup (arrangement)
> is exactly the same for both boards. The differenceis the thickness
> of the boards. Since traces on both boards are 50 ohms
> characteristicimpedence but since one board has 9 mil wide traces
> while the other has 7.5 milwide traces therefore the board with 9 mil
> wide traces is thicker. Also the board with9 mil wide traces has 1 oz
> copper on all power/ground planes. The reason I suspect ground bounce
> is the culprit in this case (1) since we followed the same routing
> rules on both boards so cross talk could not be an issue (because
> if it were, we would see it in the first board i.e. the one with 9 mil
> wide traces)(2) since the boards are made of Cynate Ester (loss
> tangent very low)...I do not think dielectric losses are a major
> problem.(3) one of the tests that we performed was to operate the
> devices near the Vil threshold. We found that the board with 9 mil
> wide traces and 1oz. power planes had no problems while the board
> with 7.5 mil wide traces did not work. I hope this clarifies
> everything.... Muhammad *
>
> -----Original Message-----
> From: Scott McMorrow <scott@vasthorizons.com>
> To: Muhammad S. Sagarwala <msagarwa@san-jose.tt.slb.com>
> Date: Wednesday, November 01, 2000 9:12 AM
> Subject: Re: [SI-LIST] : effect of trace width on the
> performance
> Muhammad,
>
> It would be helpful to the group if you were to provide a
> picture of
> the before and after of the stackups. It seems that we are
> all guessing
> about what your configuration really is at this time. I
> would also include
> the location of all ground and power planes.
>
> Also, a list of all things which have changed from the
> previous fab
> to this one would be helpful. This way we can eliminate
> possible
> causes of your problem.
>
> Finally, why do you think it is a ground bounce problem?
> Do you have root cause for your failures?
>
> regards,
>
> scott
>
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual, Signal Quality Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 885-1231
> http://www.siqual.com
>
>
>
> "Muhammad S. Sagarwala" wrote:
>
> > Thanks a lot for your input Mike... The stackup is the
> > same but the dielectric thickness is different...I mean
> > the arrangement of the planes is the same but the
> > thickness of thedielectric (and hence the boards is
> > different).. The lengths of the traces on these boards is
> > between 7-10 inches. One more question...I am suspecting
> > ground bounce....do you think that copper weight and trace
> > widths could play a major role... Muhammad
> >
> > -----Original Message-----
> > From: Michael Nudelman <mnudelman@tellium.com>
> > To: Muhammad S. Sagarwala
> > <msagarwa@san-jose.tt.slb.com>
> > Cc: si-list@silab.eng.sun.com
> > <si-list@silab.eng.sun.com>
> > Date: Wednesday, November 01, 2000 6:18 AM
> > Subject: Re: [SI-LIST] : effect of trace width
> > on the performance
> > Muhammad:
> >
> > 1. If your stackup is the same (dielectr.
> > thicknesses etc) - then how does the impedance
> > stays the same with traces' widths changed? (if
> > it is different, than it is possible.)
> > 2. How long are the traces? In case of 7.5 mils
> > your skin effect losses increase and at certain
> > lengths they may impaire your signal quality.
> > 3. The pwr plane copper oz are not that
> > important. Or at least I think so :-)))
> >
> > Mike.
> >
> > "Muhammad S. Sagarwala" wrote:
> >
> > > Hi SI Gurus, I am in a big problem. I
> > > designed two boards which are pretty much the
> > > same.The changes between them are: (1) One has
> > > traces that are 7.5 mils wide and the other has
> > > 9 mils wide traces (both have a characteristic
> > > impedence of 50 ohms)(2) The one that has 9 mil
> > > wide traces has 1 oz copper for the pwr planes
> > > and the one with 7.5 mil wide traces has 0.5
> > > oz. copper on pwr planes. The stackup is
> > > pretty much the same.The Frequency of signals
> > > on the boards is 400 Mhz.The decoupling scheme
> > > on both boards is pretty much the same. The
> > > problems is the board with 9 mil wide traces
> > > and 1oz. copper is performing very good and the
> > > other board with 7.5 mil widetraces and 0.5 oz.
> > > copper is behaving very very bad. My question
> > > is "do you think these changes make a big
> > > difference or is there another variable(s) that
> > > I am missing?????" Comments suggestions are
> > > most welcome....(I need to be sure before I
> > > make a decision to respin the board )
> > > Muhammad Muhammad S. Sagarwala
> > > Schlumberger SABER
> > > Ph. (408) 586 7065
> > > Fax (408) 586 4668
> >
>
>
>

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