**From:** Eric Bogatin (*eric@bogent.com*)

**Date:** Thu Oct 26 2000 - 14:17:29 PDT

**Next message:**Eric Bogatin: "[SI-LIST] : Differential TDR probing"**Previous message:**Mark Apton: "[SI-LIST] : S. I. Position @ nVidia, location Santa Clara, CA.;"**Next in thread:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Lusk, John B: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Dave Hoover: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Willis, Ken: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"

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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**Next message:**Eric Bogatin: "[SI-LIST] : Differential TDR probing"**Previous message:**Mark Apton: "[SI-LIST] : S. I. Position @ nVidia, location Santa Clara, CA.;"**Next in thread:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Lusk, John B: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Dave Hoover: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Maybe reply:**Willis, Ken: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"

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