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

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Tue Oct 31 2000 - 18:07:27 PST


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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