[SI-LIST] : effective dielectric constant as bulk dielectric constant error

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From: Eric Bogatin (eric@bogent.com)
Date: Mon Oct 30 2000 - 06:36:39 PST


Thank you all for your comments on my posting related to possible
measurement error in TDR. The thread was moving toward predictability of Zo
for microstrips, which, though an interesting topic, wasn't where I had
intended to go.

I was actual interested in the specific case of a board shop using a
measurement of the time delay (TD) of a microstrip test trace to get the
dielectric constant they would use for later Zo simulation.

What they would measure from a microstrip is the effective dielectric
constant, which could be as low as 3.8 for FR4, compared to the bulk value
of 4.5. If they used 3.8 for their field solver predictions of Zo, even
including trace thickness and solder mask effects, they would still be off
by about 8-10% because they have the wrong value of the bulk dielectric
constant. Not knowing this, it might cause them to question their tool or
add some in-house fudge factors to get better agreement between what they
simulate with their tool or favorite approximation and what they measure.

In fact, their problem was interpreting the dielectric constant measured
from the microstrip as the bulk value rather than as the effective value.
Does anyone know of any board shops that might be making this effective
dielectric constant as bulk dielectric constant error, or is it an obvious
problem that is always avoided?

thanks for your comments.

--eric

(ps- for those wondering about how well you can predict the Zo of test lines
with a field solver, I have some papers on my web site that discuss this)

Eric Bogatin
BOGATIN ENTERPRISES
Training for Signal Integrity and Interconnect Design
v: 913-393-1305
f: 913-393-1306
e: eric@bogent.com
web: <http://www.bogatinenterprises.com/>
ftp: ftp://ftp.BogatinEnterprises.com

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