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 - 12:09:34 PST


Well...for a GND plane the copper density is important if you are
concerned with DC current; for fast AC current the skin effect makes the
current concentrate in the outer layer of the plane; the other
dimensions being the same the inductance will vary little with
thickness, as long as your thickness is thicker than skin-layer and it
probably is.
The main culprits of the GND bounce - (with solid GND plane) - are pins,
vias and GND traces to de-coup caps.

7-10 inches is not bad for 400MHz; still you'll have somewhat higher
losses with narrower traces.

If you can keep wider traces and have no crosstalk penalty (due to
increased proximity of the traces) - keep them wide.

Also - did you check (with TDR) that your impedance INDEED is kept the
same, and not varied, say, 10-15% ?

Mike.

"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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