[SI-LIST] : FPC impedance control

jaydiep@us.ibm.com
Tue, 16 Mar 1999 09:47:23 -0500

John Lin wrote,

>>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 you're seeing is the effects of attenuation in the cable. I can't
explain why it looks different from the two ends, but the ramp is normal.
The TDR is being fooled by the decreased return pulse amplitude caused by
the high freq.
attenuation, which of course increases with length.
>>What causes the impedance ramp up?

>>Any comments on this phenomenon?

Jay Diepenbrock

Senior Engineer
Interconnect Technology & Qualification
IBM Global Procurement, B8UA/061, RTP, NC
Phone: 919-543-8804 Fax: 919-543-3642
Email: jaydiep@us.ibm.com

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