# PCB trace caps (a.k.a. finger / interdigital caps) formula

I am trying to calculate a two-fingered, both on the same layer, capacitor. I have run into a few papers on it but they are taking a ground plane on the other side of the PCB into consideration and they seem to be showing a meandered capacitor like this one

which is not exactly what I was looking for. I am after a simpler design:

I know that the capacitance formula is where A = t * L; t being the thickness of the copper pour and L being the length on which the two traces juxtapose, thus forming the plates of our capacitor. Should I apply it as such (no ground plane beneath) ? Am I also right to be using these values ?

• epsilon_r = 1 for air
• t = 35 microns for 1 oz copper
• $\epsilon_R$ is 1 for air, but some of the field will be in the PCB material rather than in air. – The Photon Aug 11 '17 at 22:35
• @ThePhoton I was afraid taht would be the case :S – kellogs Aug 11 '17 at 22:35
• I found one formula for calculating Er in a coplanar waveguide. You could try using that. The formula is kind of involved. You can find it in this PDF: jlab.org/accel/eecad/pdf/050rfdesign.pdf. Make sure you use the formula for the ungrounded coplanar waveguide. – mkeith Aug 12 '17 at 0:58
• Are there any other layers? Do they have copper on them anywhere near where the traces overlap? If you have any copper plane or fill on another layer near the two traces, that will greatly increase the capacitance. – mkeith Aug 12 '17 at 1:04
• Also don't forget about solder-mask. Either remove it or account for its Er of 3 to 5 (depending on the material, there's also outliers of 2 and 7 for speciality purposes) – Asmyldof Aug 12 '17 at 2:50