You have some amount of capacitance associated with the sides (and top) of the conductor. This extra C is more or less a constant delta no matter how wide the trace is so it is more visible in narrow traces. To take your example,
If you assume that the fringe capacitance is 5pf, then the we get 5pf for 5 mils and 5 pf for fringe = 10pf.
Then at 20 mils we get 5 pf for fringe and (4 * 5pf (4*5mils = 20)) = 25pf for 20mils.
I think it was a little lucky that the numbers worked out, but you get the idea. You can see this real well if you look at a 2D extraction with field lines.
regards,
jon powell
Viewlogic Consulting Services
Farrokh Mottahedin wrote:
> I hope someone can explain what is happening with the following capacitance
> measurement. A friend asks if capacitance is directly proportional to
> surface area, why doesn't his measurement scale linearly. On a pcb made
> specifically for trace characteristics measurement, he measures
> 1) 10 pf for a 10 cm long trace that is 5 mils (.12 mm) wide
> 2) 25 pf for a 10 cm long trace that is 20 mils (.50 mm) wide
> 3) Both traces are far apart from each other and from other components.
> 4) Both traces are over a solid ground plane.
> 5) Equipment is HP 4285A Precision LCR meter set at 1 MHz
>
> Thanks,
>
> Farrokh Mottahedin
> Quantum Corporation
> (408) 324-7934
> [email protected]
>
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