**From:** Lum Wee Mei (*lweemei@dso.org.sg*)

**Date:** Fri Oct 27 2000 - 00:13:16 PDT

**Next message:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"**Previous message:**Scott McMorrow: "Re: [SI-LIST] : Search for ADSP-21160 IBIS model"**In reply to:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Next in thread:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"**Reply:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"

JNH wrote:

*>
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*>
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*> Eric,
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*>
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*> For microstrip line measurement, I think we need to consider the
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*> solder mask, covering the microstrip line with 0.7~1.0 mils thickness.
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*> So, the microstrip line is an embeded microstrip line not pure
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*> microstrip. I use the polar tool -- CITS25 to do calculate the
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*> microstrip and substrate 2~3 ohms to compensate the effect of solder
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*> mask. The TDR measurment shows bigger deviation for microstrip line
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*> than that of stripline. I believe it is caused by more processing
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*> needed for the outer layers of a PCB, such as solder platting and
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*> solder mask. A 0.5 oz (0.7mils) thickness copper will finally be
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*> added up to 2.0 mils for the outer layers.
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*>
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*>
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*>
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*> Best Regards,
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*>
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*> John Lin
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*> SI Engineer, ARD4
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*> Quanta Computer Inc.,Taiwan, R.O.C.
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*> Email: John@quantatw.com
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*> Tel: 886+3+3979000 ext. 5183
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*>
*

*> -----Original Message-----
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*> From: Eric Bogatin [mailto:eric@bogent.com]
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*> Sent: Friday, October 27, 2000 5:17 AM
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*> To: Sun. COM
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*> Cc: eric
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*> Subject: [SI-LIST] : Possible TDR microstrip measurement error?
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*>
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*> After a recent talk I gave on TDR measurements, I was approached by a
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*> fellow
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*> from the IPC (I apologize that I did not catch your name, whoever you
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*> were),
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*> with a problem that might be common in the board fab industry. I
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*> wanted to
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*> get comments from folks on the SI list as to whether you have
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*> encountered
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*> this problem or is it so obvious that everyone knows to watch out for
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*> it.
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*>
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*> In some shops, a TDR is used to measure the dielectric constant of the
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*> board
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*> material using test lines on coupons. Given the physical length, L,
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*> and the
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*> time delay, TD, for the one way trip (i.e., 1/2 the time measured by
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*> the TDR
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*> for an open terminated line), the speed of light in the material can
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*> be
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*> calculated as vel = L/TD. The dielectric constant is calculated as
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*> sqrt(2.99
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*> x 10^8 m/sec / vel). This is the straight forward part.
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*>
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*> When the trace is a stripline, the dielectric constant extracted is
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*> the bulk
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*> dielectric constant of the material surrounding the traces. This value
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*> could
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*> be put in a field solver to use to help predict the design rules for
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*> traces
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*> made with this material. I have had success in predicting board trace
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*> impedance to better than 2% with some field solvers, limited to how
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*> well I
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*> knew the cross section and dielectric constant.
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*>
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*> However, when the test line is a microstrip, some of the field lines
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*> are in
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*> air, and the dielectric constant calculated in this way is the
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*> "effective"
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*> dielectric constant, not the board's bulk dielectric constant. Yet, I
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*> am
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*> told some board shops use this measurement from microstrips to get a
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*> value
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*> for what they think is the bulk dielectric constant of their material
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*> and
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*> then use this value in a field solver or approximation. Of course,
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*> their
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*> predictions from the field solver- anyone's- would be off by as much
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*> as
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*> 10%-20%, for the measured impedance of the test lines. I suspect this
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*> is the
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*> basis for the comments I have heard that some fab shops are not happy
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*> with
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*> their field solvers- that they have had to add their own correction
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*> factors
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*> to the many approximations that are out there and each shop has their
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*> own
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*> oracle they consult to design a controlled impedance board.
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*>
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*> There is still value in the effective dielectric constant. From the
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*> microstrip test line cross section, a 2D field solver can be used to
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*> extract
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*> what bulk dielectric constant the material under the trace must have
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*> had to
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*> result in the measured effective dielectric constant. If the board
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*> shop used
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*> this extracted value for the bulk dielectric constant, their following
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*> field
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*> solver results would probably be much more accurate.
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*>
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*> has anyone else encountered this problem in board shops?
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*>
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*> all comments are welcome.
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*>
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*> --eric
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*>
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*>
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*> Eric Bogatin
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*> BOGATIN ENTERPRISES
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*> Training for Signal Integrity and Interconnect Design
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*> v: 913-393-1305
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*> f: 913-393-1306
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*> e: eric@bogent.com
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*> web: <http://www.bogatinenterprises.com/>
*

*> ftp: ftp://ftp.BogatinEnterprises.com
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*>
*

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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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**Next message:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"**Previous message:**Scott McMorrow: "Re: [SI-LIST] : Search for ADSP-21160 IBIS model"**In reply to:**JNH: "RE: [SI-LIST] : Possible TDR microstrip measurement error?"**Next in thread:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"**Reply:**Lum Wee Mei: "Re: [SI-LIST] : Possible TDR microstrip measurement error?"

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