**From:** Willis, Ken (*[email protected]*)

**Date:** Fri Nov 10 2000 - 15:34:19 PST

**Next message:**Phares, Charles C: "RE: [SI-LIST] : Question about Thin Flexible Coaxial cable"**Previous message:**George Rasko: "[SI-LIST] : One Differential To Another"**Maybe in reply to:**Eric Bogatin: "[SI-LIST] : Possible TDR microstrip measurement error?"

Hi Eric,

I was just starting to write some of the same comments when I read this

message from John. I started out in PC fab way back when, and built and

TDR'd more impedance controlled boards than I want to remember (still

recovering from the fumes coming off the cupric chloride etch line).

Meeting microstrip impedance specs was usually a bit more

challenging, for a couple of reasons.

- We were doing mostly foil construction externally, so that first

dielectric was usually prepreg instead of pre-cured core material, so

you'd get some variation there in thickness.

- Boards electroplate from the "outside in", so you get variation in

trace thickness across the panel. The impedance coupons are always on

the edge of the board, so they plate up pretty high. If you start with

1/2 oz copper (.7 mils), it would generally plate up to 2.1 or so.

- If you get variation in trace height, you will also get variation

in finished trace width due to the etching process. Taller traces will

see less line width reduction but more trapezoidal shape than the lower

ones.

- Most of our boards then were SMOBC (solder mask over bare copper),

so in theory you should be designing them as buried microstrips, with a

couple mils of soldermask over the traces, then air. But try getting Er

values from a soldermask vendor. I didn't have any luck, so empirically

you TDR the boards after external etching, and make sure

you are 3 ohms or so over nominal. Then applying the mask generally

dropped the measurement back into mid-range.

So you add all these variables up, plus the mistake some may make of

using Er of FR4 above and below the trace in the model, and you have a

recipe for confusion. Hence the empirically-derived fudge factors by

every fab shop.

My recommendation to those on the SI list is that if you really have

traces where the Zo is that critical, do NOT put them on external

layers. There is too much process variation there.

You will get a lot more consistency on non-plated 1/2 oz. inner layers.

And don't kid yourself into thinking that because you spec'd external

layers +/- 10% that you got it. You have probably just bought a batch of

impedance-controlled coupons. There will be significantly

more variation in thickness, line width, and Zo across that 18x24 panel.

Some board shops will do better than others, but in general impedance

controlled microstrips are tougher to do well.

Ken Willis

Sycamore Networks

-----Original Message-----

From: JNH [mailto:[email protected]]

Sent: Thursday, October 26, 2000 10:30 PM

To: Eric Bogatin

Cc: [email protected]

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

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: [email protected]

Tel: 886+3+3979000 ext. 5183

-----Original Message-----

From: Eric Bogatin [ mailto:[email protected] <mailto:[email protected]> ]

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: [email protected]

web: < http://www.bogatinenterprises.com/

<http://www.bogatinenterprises.com/> >

ftp: ftp://ftp.BogatinEnterprises.com

<ftp://ftp.BogatinEnterprises.com>

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**Next message:**Phares, Charles C: "RE: [SI-LIST] : Question about Thin Flexible Coaxial cable"**Previous message:**George Rasko: "[SI-LIST] : One Differential To Another"**Maybe in reply to:**Eric Bogatin: "[SI-LIST] : Possible TDR microstrip measurement error?"

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