# Re: [SI-LIST] : Trace impedance

fabrizio zanella (fabrizio=zanella%eng%[email protected])
Thu, 20 Aug 98 12:25:35 -0400

Gentlemen, formulae for calculating impedances of stripline/microstrip
traces are only somewhat accurate for certain geometries. The formulae
were developed many years ago, when line widths were .010"+, dielectric
thicknesses differed from today's designs, and most designs used 50 ohm
transmission lines. For high impedance tlines, (80+ ohms), the accuracy of
the formulae gets terrible.
If you want to match your TDR measured impedances to calculations, you need
to use a field solver tool. There are many good tools available that use
boundary element or finite element analysis. Try Apsim's 'RLGC' tool, or
Ansoft's Maxwell EZ2D.

Regards,
Fabrizio Zanella
EMC Corporation
[email protected]
-------------
Original Text
From: "Bill Dempsey" <[email protected]>, on 8/20/98 1:05 PM:
To: [email protected]@EMCHOP1["casperdd" <[email protected]>]
Cc: [email protected]@EMCHOP1["[email protected]"
<[email protected]>]

casperdd wrote:

> Hi
>
> I was hoping that someone might be able to shed a little light on a
> problem that I have encountered regarding the impedance of stripline,
> and microstrip PCB tracks.
> I require a formula that is capable of predicting the impedance's of
> stripline, and microstrip tracks. I have already found several from
> different source, but none of them seem to agree with each other, my
> TDR
> results, or the TDR results from "MECL system design handbook" by
> Motorola. All the formulas that I have come across claim to have an
> accuracy to within about 5%, but as far as I can tell the discrepancy
> can reach up to 20%, depending on the dimensions involved.
> The region of interest to me is:
>
> Track width = 10 mils
> Track thickness = 1.37 mils
> Ground plane/Track separation (dielectric height) = 8 mils
> Relative permittivity = 4.7
> TDR edge rise time = 28ps
>
> The most alarming problem that I have encountered is the natural
> logarithm that most formula seem to have, this then returns a negative
>
> number when its argument is smaller than one, even though most formula
>
> claim to still be accurate at this limit.
>
> I would appreciate it if anyone could show me where I have been going
> wrong, or point me in the direction of an accurate formula.

Plenty of research has gone into most of the formulas. Assuming a
"fixed" value of er= 4.7 somewhat troubles me as this value is not a
constant over frequency. er = 4.7 *seems* high to me. I suspect you'll
be able to empirically determine the er at the frequency of interest if
you build some test boards. Most good board shops know how their
material acts at these higher frequencies and back their numbers up with
real-world measurements. Check around and see what they think your
materials' er is in the gigahertz region.

Also check with IPC -- they have a technical paper IPC-TP-949 that
covers measuring impedance of stripline/microstrip traces. It's written
for the layman so it's easy to read.

Hope this helps,
Bill Dempsey

>
>
> Many Thanks
>
> Dominic Casperson
>
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