Re: [SI-LIST] : Trace Capacitance

ajmani@us.ibm.com
Mon, 21 Jun 1999 13:03:28 -0600

I agree with Jon's observation. I did some quick calculations on a 2-D field
solver and came out with the following results (I changed other parameters to
get 5 pF capacitance for a 5 mil trace in both cases):

Case 1:

5 mil trace, 1 oz copper, 1 oz plating: capacitance is 5 pF

20 mil trace, 1 oz copper, 1 oz plating: capacitance is 23.1 pF

Case 2:

5 mil trace, 1 oz copper, 0.5 oz plating: capacitance is 5 pF

20 mil trace, 1 oz copper, 0.5 oz plating: capacitance is 23.5 pF

This clearly indicates that the fringing capacitance plays a role in total trace
capacitance, and is inversely proportional to the trace thickness.

Another factor which will affect the measurement, is the loss of copper during
the etching process. This causes more copper to be etched out at the bottom of
the trace than at the top, making the trace cross-section trapezoidal instead of
a rectangle. This loss will be same for both 5 mils and 20 mils traces (say 0.5
mils from either side of the trace bottom). Thus, a 5 mil trace will be only 4
mils at the bottom, whereas a 20 mils trace will be 19 mils at the bottom. This
will reduce the capacitance of 5 mils trace significantly more than for the 20
mils trace.

Regards, Ravinder

Email: ajmani@us.ibm.com
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jonp@pacbell.net on 06/21/99 09:53:31 AM

Please respond to si-list@silab.eng.sun.com

To: si-list@silab.eng.sun.com
cc: (bcc: Ravinder Ajmani/San Jose/IBM)
Subject: Re: [SI-LIST] : Trace Capacitance

The answer to this should be "Fringing capacitance".

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
> farrokh.mottahedin@quantum.com
>
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