Re: [SI-LIST] : Trace impedance

Bill Dempsey (bdempsey@dnaent.com)
Thu, 20 Aug 1998 10:05:19 -0700

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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