[SI-LIST] : FPC impedance control

johnlin@ccmail.arima.com.tw
Tue, 16 Mar 1999 09:23:14 +0800

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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