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

Shawn Carpenter (scarp@sonnetusa.com)
Thu, 26 Mar 1998 14:13:33 -0500

Roland:

I tried out your problem on the Sonnet em Suite simulation tools.

<Sales pitch filter on>
We use an FFT Method of Moments high-frequency electromagnetic analysis
technique for 3D planar circuits, and structures like this map into our
software quite well (with or without vias).
<Sales pitch filter off>

I isolated on a single signal trace (S) and it's nearby ground traces
(G) with a cross-section configuration like this:

G S G
_____ _____ _____
-----------------------------------------------
///////////////////////////////////////////////
/////////////kapton dielectric/////////////////
-----------------------------------------------
------
G

Then, assuming a 5-mil dielectric between them (and no large nearby
ground planes) with the kapton dielectric constant of 3.5 I ran a few
simulations at 1.5 GHz (a steady-state stimulus is necessary since we
do the analysis in the frequency domain with a single-tone signal).
I kept the pitch between G and S at 0.8mm, and found a pretty good 50
ohm
system to exist with the following dimensions:

S Trace Width G Trace Width Zo
------------- ------------- --
12 mil (0.3mm) 16 mil (0.4mm) 50 ohms

Other combinations that get close, when you keep S and G the
same width (you can extrapolate to the "perfect value, but
there's probably 2-3% error in the simulation results--I didn't
use a real tight mesh):

S Trace Width G Trace Width Zo
------------- ------------- --
12 mil 12 mil 52 ohms
13 mil 13 mil 49 ohms
14 mil 14 mil 47 ohms

Most of the interaction is between the signal trace and the ground
trace located across from it (through the dielectric--the one located
broadside to it), so changing the combination of these two provides
the greatest change in characteristic impedance (as would shifting
the dielectric constant or the thickness of the kapton).

However, I wonder if your adhesive has the same dielectric constant
as the kapton? If not, it will have an effect on the impedance of
the system as well.

By the way, with the computed effective dielectric constant (~2.7)
for this configuration, the wavelength is ~180mm at 1 GHz, and
~18mm at 10 GHz. Your FPC (2 inches) is about 50.8mm, which may be
"electrically significant" for some harmonics of your high rise time
signals?

Hope this helps.

--Shawn
Sonnet Software, Inc.

Roland F. Portman wrote:
>
> 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
> layer,
> and a ground is beneath (above) the signal on the bottom (top) layer.
> There are 40 signals on the top and bottom layers.
>
> G-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G
> G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-S-G-G
>
> I am trying to determine the characteristic impedance of this
> construction.
> The main variables will be the width of the trace and the space between
> the
> 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
> connector.
>
> 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
> characteristic
> impedance of this arrangement?
>
> Thanks in advance,
>
> Roland
>
> --
> Roland F. Portman
> BiT Microsystems, Inc.
> 48499 Milmont Drive
> Fremont, CA 94538
>
> (510) 623-2341 Ext 129
> (510) 623-2342 Fax
>
> portman@bitmicro.com
> http://www.bitmicro.com

-- 
||  Shawn Carpenter
||  
||  Sonnet Software, Inc.    "High-Frequency Electromagnetic Software"
||  1020 Seventh North St., Suite 210
||  Liverpool, NY  13088
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