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

Shawn Carpenter (
Fri, 27 Mar 1998 23:21:37 -0500

Good question. I tried it out, and added a twist that I didn't
consider before--the physical thickness of the conductors.

With physically thick metal models, you get a 50 ohm
system with G = S = 12 mils (I had previously reported 13 mils,
but that was with a physically thin metal model applied to the EM

With a large ground plane (microstrip-like configuration) you reach
a 50 ohm system with a signal line width of just under 10 mils
the thickness of the metal). If you used this value for your signal and
finite ground conductor widths, you'd end up with a 59 ohm system -- off
almost 20%.

BTW, I didn't find that the co-planar ground traces affected the
significantly for these dimensions. But maybe they help with the
isolation between the signal traces a bit.

--Shawn wrote:
> Shawn
> It would be interesting to run this again with the bottom layer
> being a large groud plane (much larger than the conductors)
> and no adjacent traces (reducing the problem to a microstrip
> line and compare results.
> wrote:

<---- Some parts deleted ----->
>>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
>>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).

||  Shawn Carpenter
||  Sonnet Software, Inc.    "High-Frequency Electromagnetic Software"
||  1020 Seventh North St., Suite 210
||  Liverpool, NY  13088
||  Phn:  315.453.3096    Fax: 315.451.1694