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

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From: Lum Wee Mei (lweemei@dso.org.sg)
Date: Fri Oct 27 2000 - 00:15:52 PDT


Lum Wee Mei wrote:

> JNH wrote:
>
>>
>>
>> Eric,
>>
>> 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:eric@bogent.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 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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>
> 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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