# Re: [SI-LIST] : Trace impedance

Dan Swanson ([email protected])
Thu, 20 Aug 98 16:30:17 -0400

2 cents from a microwave guy:

For single microstrip, Jansen and Kirschning are probably the accepted standard.
These are curve fit formulas, fit to a large data base of infinitely thin
solutions computed using the spectral domain method.

You can add thickness as a "correction" on top of the these equations, thickness
creates a new effective width.

Using Dominics dimensions, I get

Zo=61.67 for t=0
Zo=58.15 for t=1.37

I used LineCalc from HP-EEsof which implements the Jansen and Kirschning
equations with corrections for thickness.

Regarding Fabrizio's post, I agree, you can learn a lot from a 2D cross-section
solver. I would recommend QuickField or FlexPDE. Student (limited number of
nodes) versions are available on the web.

Dan

> Most of these formulae were based on conformal transforms, often based on
> zero thickness conductors. In a lot of modern work, those assumptions are
> not very good, particularly when integrated circuits or multichip modules
> are involved. I compared a lot of these expressions (Wheeler, Hammerstad,
> Jensen, Chang, etc) against real-life measurements on 10u and 25u t-line
> structures of different types in my Master thesis work and got results all
> over the map.
>
> jim
>
> At 12:25 PM 8/20/98 -0400, fabrizio zanella wrote:
> >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.
> >

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