We face the same problem but from the other side of the table. Recently at
IPC EXPO
Gary Brist, et al, presented their paper: "Differential Impedance
Assessment of PCB
Interconnects".  The paper is very good. A major focus was first on
attempting to
quantify and minimize measurement variation and error. Their initial
round-robin
found "NO correlation" (suprised?). I can't and wouldn't try to condense
the entire
paper, but in their particular study  with their variables they did find
that manual probing
varied from 0.8 to 2.0 ohms and automated probing varied from 0.25 - 0.40
ohm.
Obviously when one needs to have +/- 5% on 50 ohms (5 ohms range) you can't
"lose"
2 ohms or more to technique and instrumentation.







Brian Butler <[log in to unmask]>@IPC.ORG> on 04/15/2003 07:10:01
PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
       to Brian Butler <[log in to unmask]>

Sent by:    TechNet <[log in to unmask]>


To:    [log in to unmask]
cc:

Subject:    Re: [TN] gage R&R on TDR measurements


Tom,

Two suggestions regarding impedance test repeatability:

1) Robotics automation of the TDR test process can yield repeatability of
better than 0.05 ohms. This is a significant improvement over traditional
hand probing techniques primarily due to the consistent placement of the
TDR probe. (I have seen hand probing repeatabilities of 0.5-2 ohms).

2) The measurement process (i.e., measurement algorithms, measurement
process) has a big impact on the long term repeatability of the TDR test
process also. There are two groups within IPC which are rewriting the
standards in this area (IPC-2141 section on Impedance Coupon Design,
IPC-TM-650 2.5.5.7 TDR measurement process. Your can contact myself or Nick
Paulter at [log in to unmask]
for interest in either of these ongoing efforts).

Both of these issues address the testing side of tight manufacturing
tolerances but any improvement you can squeeze out of repeatability and
accuracy of the test process you can put right back into the product. And
you also have a tighter control over your process.

Sincerely,
Brian Butler
Introbotics Corporation
[log in to unmask]
www.introbotics.com
(505)345-7785







At 12:18 PM 4/15/03 -0700, you wrote:
Anyone done a study on their TDR lately? Anyone willing to share the
methods, problems or results? Privately, publicly? What I'm looking for is
a "how do we get there from here" for impedance requirements of < 5% (50
ohm).
Thanks in advance!

(signature attached not intended for advertising purposes) :D

Tom Bresnan
Front End Manager
Package Test
Test Products

Kulicke & Soffa
K&S Interconnect, Inc.
10365 Sanden Drive
Dallas, TX 75238-2440



The fellow who does things that count, doesn't usually stop to count them.
--Anon.





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