RE: [SI-LIST] : FPC impedance control

Kowal, Keith (kowal@cabletron.com)
Tue, 16 Mar 1999 08:28:57 -0500

I've seen this type of thing before -
First check calibration of you instrument - for ours, we has to
calibrate it many times
Before we had been conformable with the result....
Second, the connection to the trace itself and an appropriate ground
connection may
Change your result - I think that this may be the problem.
I tried this using a flex cable - with both hatched 1/2oz cu as a
ground ref plane
And 2nd using solid cu ground.... I did notice that the ground
plane made all
The difference when trying to measure Zo of a trace....
Third - are there any traces around the trace you are measuring?
If so they may
Effect your result...
For you application - check the consistency of your ground ref
plane - minor variations
Will compromise your result...
With our instrument we never measured a flat wave - we had to
average the result
Using about four points will put you in the right Zo....

Good luck - I hope this helped some.

Keith

<<...>>

* Outside 978-684-1574
* Internet: Kowal@ctron.com <mailto:Keith.Kowal@digital.com>
* Fax: 978-684-1334

-----Original Message-----
From: johnlin@ccmail.arima.com.tw
[SMTP:johnlin@ccmail.arima.com.tw]
Sent: Monday, March 15, 1999 8:23 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : FPC impedance control

Dear all SI gurus,

One question.

Recently, I design a stackup structure for a FPC, flexible printed
circuit
board, to get right controlled impedance.
The FPC is an embedded microstrip structure with a thin silver epoxy
layer as
the ground layer and 20cm trace length.

Then I measure the trace impedance of the prototype of the cable
from one end of
the trace with TDR.

I find that its impedance smoothly rises up from 50 to 70 ohms.
However, measuring from the other end of the same trace, I find that
the
impedance curve looks flat ,around 60 ohms.
( The FPC cable has a U turn at its tail).

Why I got two different results by measuring the two ends of the
same trace?
What causes the impedance ramp up?

Any comments on this phenomenon?

Thank you for your helps in advance.

John Lin
CAE Engineer @ Arima

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