> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@mitvma.mit.edu] On Behalf Of FalconWireless
> Tech Support - KF4HAZ
> Sent: 12 April 2002 15:48
> To: PICLIST@mitvma.mit.edu
> Subject: [EE]: Trace Capacitance formula
>
>
> Does anyone out there know of a formula for calculating the
> capacitance of circuit traces? I am using "standard 1 oz.
> thickness of copper" 6"X9" boards. (not sure what that comes
> to in actual thickness of copper) If we assume .006" spacing
> between traces how long of a trace would be required to
> create 20pf? There should be a formula for this if the
> thickness of the trace could be converted.
>
> KF4HAZ - Lonnie

- Kevin Blain Wrote -
Capacitance = Eo * Er * A / d
Where Eo is permittivity of free space (8.854 *10^-12 F/m)
if I remember correctly.
Er is relative permittivity if the dielectric (for air it is about 1)
A is the area in question, of the conducting surfaces, see below
D is the distance between the conducting surfaces
For 1oz/sq ft Cu, the thickness is about 35 * 10^-6 m
So based on this, for 20pF
20pF = Eo * Er * A / d
Re arrange
Note: 0.006" = 0.1524 mm
20 * 10^-12 * 0.1524 * 10^-3 / (Eo * Er) = A
Or A = 0.000344 square metres
Divide by thickness to give length
9.83 mm
> Note. This assumes no ground plane or other component near by, which
> would cause parallel capacitance. In this case, you would have to take
> into account the thickness and Er of the board, which does vary with
> frequency!

The other side of the board is open space and so should not pose a problem.

> Hopefully there is enough information which will help you to calculate
> other values. Check the Constant though, because it's just from memory
> from when I was at college! A simple web search should bring up
> something. Note also that you will have to adjust if your copper
> thickness varies.
>
> I would assume this is for a one off project, in which case you can tune
> it manually afterwards. If not, consider employing an EE who could do
> this for you, and there are many other factors to consider when mass
> producing using this type of idea.
>
> Regards, Kevin

The first board revision used a fixed 20pf cap. but I failed to allow for
parasitic capacitance which loaded the circuit down. We have equipment
capable of reading capacitance down to 1pf at any frequency from 1MHz to
1GHz but I did not think to use it.
The second used a trim cap. But we found those susceptible to changes caused
by vibration etc.
I figure if I make the third board revision using the formula you provided,
then trim with an Xacto knife I should be able to go into production by the
fourth board revision with a stable product.

Thanks again
KF4HAZ - Lonnie

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