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

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From: Muhammad S. Sagarwala (msagarwa@san-jose.tt.slb.com)
Date: Tue Oct 31 2000 - 18:27:27 PST


I am sorry Patrick,

I forgot to mention ... the thickness of the boards is different. The one
with 9 mil wide traces is thicker than the one
with 7.5 mil wide traces.

As regards other possibilities, the boards use the same dielectric material
(Cynate Ester), the power planes on them
are exactly the same, and the stackup is the same too and the placement of
the decoupling caps is pretty much the same too
(both the physical location and quantity)..

Muhammad

-----Original Message-----
From: Zabinski, Patrick J. <zabinski.patrick@mayo.edu>
To: msagarwa@san-jose.tt.slb.com <msagarwa@san-jose.tt.slb.com>;
si-list@silab.eng.sun.com <si-list@silab.eng.sun.com>
Date: Tuesday, October 31, 2000 6:13 PM
Subject: RE: [SI-LIST] : effect of trace width on the performance

>Muhammad,
>
>You state below that the stackups of the two boards are "pretty much the
>same", but to maintain 50 ohm impedance with the two different widths,
>you must have changed the dielectric thickness. Taking a guess on
>my part, I'm assuming you kept the overall board thickness the same.
>So, for the 7.5 mil wide trace board, you have thinner dielectrics
>around the signals, which means you must have thicker dielectrics between
>any power and ground plane layers. This in turn has reduced the
>effectiveness of any decoupling you've added. As such, the board with
>wider traces (i.e., thinner dielectrics between ground/power) would
>perform better.
>
>In terms of the copper weights, I don't believe this would play much of
>a factor.
>
>Outside of this, there are 100's of possibilities (different dielectrics,
>routing, split planes, details on decoup caps, voltages, etc.), and it
>would take consider more info to get a better idea.
>
>Pat
>
>--------------------
>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 wide
>traces 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
>
>

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