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

Shawn Carpenter (scarp@sonnetusa.com)
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
simulator).

With a large ground plane (microstrip-like configuration) you reach
a 50 ohm system with a signal line width of just under 10 mils
(considering
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
by
almost 20%.

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

--Shawn

rbmccammon@mmm.com 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.
> rbmccammon@mmm.com 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
>>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).

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