Re: [SI-LIST] : FPC impedance control

Chuck Hill (chuckh@altaeng.com)
Sat, 20 Mar 1999 21:39:48 -0700

OK. I'll try an explanation.

First, there must be two connectors to this assembly, one at each end of
the transmission line. Since the test results are different from the two
connectors, the assembly must not be symmetric, look the same from each
port. There must be something different at one port that cannot be seen
from the other port.

Second, the measured trace impedance increases with time (from one port).
Capacitance in series has this effect since v=(1/C) integral i dt. So my
guess is the circuit is open at or near this port. I don't know if it is
in the ground return or the signal.

Chuck Hill, consultant
Alta Engineering
chuckh@altaeng.com

At 09:23 AM 3/16/99 +0800, johnlin@ccmail.arima.com.tw wrote:
>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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