# [SI-LIST] : Thank you for suggestions of FPC impedance control

[email protected]
Fri, 19 Mar 1999 08:59:19 +0800

Dear Martyn and all others=2C

Thank you and all others for providing many insight explanations to me=2E=
=20

Appreciate=2E

Regards=2C

John Lin
=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=A6^=ABH=A4=
=C0=B9j=BDu=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=5F=

=A5D=A6=AE=A1G Re=3A [SI-LIST=5D : FPC impedance control=20
=A7=40=AA=CC=A1G Martyn Gaudion <mail=40polar=2Eco=2Euk=3E
=A4=E9=B4=C1=A1G 1999=2F3=2F18 =A4U=A4=C8 02=3A32

Dear John

I have seen this effect on tapered=20
traces=2E If the trace is both lossy and
tapered the impedance will appear to rise
from on end to the other=2E

A tapered trace without loss shows
rise measured from one end and fall
from the other=2E=20

If there is series loss which matches the
change from the tapered trace the trace can
=28if the series loss equals the change in Zo per
unit length=29 appear flat from one end and steeply
rise when measured from the other=2E=20

I first saw this on some fine line test coupons
from Japan around 18 months ago
hope this helps=2E

Kind regards

Martyn Gaudion=20
Marketing Manager
Polar Instruments
Tel=3A + 44 1481 253081
Fax=3A + 44 1481 252476

www=2Epolar=2Eco=2Euk
email=3A <mail=40polar=2Eco=2Euk=3E
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=3EDate=3A Tue=2C 16 Mar 1999 09=3A23=3A14 +0800
=3ETo=3A <si-list=40silab=2Eeng=2Esun=2Ecom=3E
=3ESubject=3A [SI-LIST=5D : FPC impedance control
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=3EDear all SI gurus=2C
=3E
=3EOne question=2E
=3E
=3ERecently=2C I design a stackup structure for a FPC=2C flexible print=
ed circuit
=3Eboard=2C to get right controlled impedance=2E
=3EThe FPC is an embedded microstrip structure with a thin silver epoxy=
layer as
=3Ethe ground layer and 20cm trace length=2E
=3E
=3EThen I measure the trace impedance of the prototype of the cable fro=
m one
=3Eend of
=3Ethe trace with TDR=2E
=3E
=3EI find that its impedance smoothly rises up from 50 to 70 ohms=2E=20=
=0A=3EHowever=2C measuring from the other end of the same trace=2C I fi=
nd that the
=3Eimpedance curve looks flat ,around 60 ohms=2E
=3E=28 The FPC cable has a U turn at its tail=29=2E
=3E
=3EWhy I got two different results by measuring the two ends of the sa=
me trace=3F
=3EWhat causes the impedance ramp up=3F=20
=3E
=3EAny comments on this phenomenon=3F=20
=3E
=3EThank you for your helps in advance=2E
=3E
=3EJohn Lin
=3ECAE Engineer @ Arima
=3E
=3E
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