[SI-LIST] : Possible TDR microstrip measurement error?

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From: Eric Bogatin (eric@bogent.com)
Date: Thu Oct 26 2000 - 14:17:29 PDT


After a recent talk I gave on TDR measurements, I was approached by a fellow
from the IPC (I apologize that I did not catch your name, whoever you were),
with a problem that might be common in the board fab industry. I wanted to
get comments from folks on the SI list as to whether you have encountered
this problem or is it so obvious that everyone knows to watch out for it.

In some shops, a TDR is used to measure the dielectric constant of the board
material using test lines on coupons. Given the physical length, L, and the
time delay, TD, for the one way trip (i.e., 1/2 the time measured by the TDR
for an open terminated line), the speed of light in the material can be
calculated as vel = L/TD. The dielectric constant is calculated as sqrt(2.99
x 10^8 m/sec / vel). This is the straight forward part.

When the trace is a stripline, the dielectric constant extracted is the bulk
dielectric constant of the material surrounding the traces. This value could
be put in a field solver to use to help predict the design rules for traces
made with this material. I have had success in predicting board trace
impedance to better than 2% with some field solvers, limited to how well I
knew the cross section and dielectric constant.

However, when the test line is a microstrip, some of the field lines are in
air, and the dielectric constant calculated in this way is the "effective"
dielectric constant, not the board's bulk dielectric constant. Yet, I am
told some board shops use this measurement from microstrips to get a value
for what they think is the bulk dielectric constant of their material and
then use this value in a field solver or approximation. Of course, their
predictions from the field solver- anyone's- would be off by as much as
10%-20%, for the measured impedance of the test lines. I suspect this is the
basis for the comments I have heard that some fab shops are not happy with
their field solvers- that they have had to add their own correction factors
to the many approximations that are out there and each shop has their own
oracle they consult to design a controlled impedance board.

There is still value in the effective dielectric constant. From the
microstrip test line cross section, a 2D field solver can be used to extract
what bulk dielectric constant the material under the trace must have had to
result in the measured effective dielectric constant. If the board shop used
this extracted value for the bulk dielectric constant, their following field
solver results would probably be much more accurate.

has anyone else encountered this problem in board shops?

all comments are welcome.

--eric

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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