From: Lum Wee Mei (firstname.lastname@example.org)
Date: Fri Oct 27 2000 - 00:13:16 PDT
> For microstrip line measurement, I think we need to consider the
> solder mask, covering the microstrip line with 0.7~1.0 mils thickness.
> So, the microstrip line is an embeded microstrip line not pure
> microstrip. I use the polar tool -- CITS25 to do calculate the
> microstrip and substrate 2~3 ohms to compensate the effect of solder
> mask. The TDR measurment shows bigger deviation for microstrip line
> than that of stripline. I believe it is caused by more processing
> needed for the outer layers of a PCB, such as solder platting and
> solder mask. A 0.5 oz (0.7mils) thickness copper will finally be
> added up to 2.0 mils for the outer layers.
> Best Regards,
> John Lin
> SI Engineer, ARD4
> Quanta Computer Inc.,Taiwan, R.O.C.
> Email: John@quantatw.com
> Tel: 886+3+3979000 ext. 5183
> -----Original Message-----
> From: Eric Bogatin [mailto:email@example.com]
> Sent: Friday, October 27, 2000 5:17 AM
> To: Sun. COM
> Cc: eric
> Subject: [SI-LIST] : Possible TDR microstrip measurement error?
> After a recent talk I gave on TDR measurements, I was approached by a
> from the IPC (I apologize that I did not catch your name, whoever you
> 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
> this problem or is it so obvious that everyone knows to watch out for
> In some shops, a TDR is used to measure the dielectric constant of the
> 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
> calculated as vel = L/TD. The dielectric constant is calculated as
> 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
> be put in a field solver to use to help predict the design rules for
> 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
> dielectric constant, not the board's bulk dielectric constant. Yet, I
> told some board shops use this measurement from microstrips to get a
> for what they think is the bulk dielectric constant of their material
> then use this value in a field solver or approximation. Of course,
> predictions from the field solver- anyone's- would be off by as much
> 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
> their field solvers- that they have had to add their own correction
> to the many approximations that are out there and each shop has their
> 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
> 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
> solver results would probably be much more accurate.
> has anyone else encountered this problem in board shops?
> all comments are welcome.
> Eric Bogatin
> BOGATIN ENTERPRISES
> Training for Signal Integrity and Interconnect Design
> v: 913-393-1305
> f: 913-393-1306
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While I agreed that soldermask has to be considered, whatever plating
added to the base copper should never be taken as part of the thickness
in impedance calculation. I may be wrong, then.
Regards - Wee Mei
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