re: [SI-LIST] : Controlled impedance in Flex Circuit

fabrizio zanella (fabrizio=zanella%eng%[email protected])
Tue, 24 Mar 98 8:10:53 -0500

Roland, you need to purchase a tool which uses a field solver to calculate
the characteristic impedance. The free tools you mention, available on the
web, most likely use closed form formulas, which are not accurate for a
good percentage of construction types, so I would not trust the results if
you're looking for tight impedance control.
Some good EM solvers I've seen are RLGC from Applied Simulation Technology,
Ansoft's Maxwell EZ2D, and there are several others available.

Fabrizio Zanella
Hardware Engineering
email: [email protected]
(508) 435-1000 Ext. 4645
Fax: (508) 435-8949
EMC Corporation
171 South Street
Hopkinton, MA 01748-9103

Original Text
From: "Roland F. Portman" <[email protected]>, on 3/23/98 5:22 PM:
To: [email protected]@EMCHOP1["SI list" <[email protected]>]

I am designing a two layer Flexible Printed Circuit as shown below.
A ground signal is to the left and right of each signal on the same
and a ground is beneath (above) the signal on the bottom (top) layer.
There are 40 signals on the top and bottom layers.


I am trying to determine the characteristic impedance of this
The main variables will be the width of the trace and the space between
traces. The signal and ground traces do not have to be the same width.
I am hoping to get approximately 50 ohms.

The 2x40 SMD connector pads they will connect to are 0.5mm wide on a
0.8mm pitch.
The bottom traces will be brought to the top side using vias in the
middle of the

The dialectric constant for the Kapton with adhesive is 3.5.
The Kapton is 2mils thick and the adhesive is 1mil thick.
This will lead to a dialectric separation of 5 mils
between the top and bottom traces.
The RA (rolled annealed - soft) copper is 1oz (1.4mils).

All the impedance calculators I have found require a copper plane and
assumes an infinite width for this plane (UltraCAD and Polar).
I have looked through Howard Johnson's Black Magic book and did not
find any applicable examples (ribbon cable came close).

Does someone know of a calculator which will calculate the
impedance of this arrangement?

Thanks in advance,


Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129 (510) 623-2342 Fax

[email protected]