pad-to-pad capacitance

Aug 03, 2022 Last reply: 3 years ago 8 Replies
S

Two part question (my google skills have failed me for both):


  1. Does anyone know approximate pad-to-pad capacitance for the common SMD packages (0201, 0402, 0603, 0805)? Ever measured it? I can't use the parallel plate capacitor formula, since the plates are adjacent, not parallel. (assume there is no copper below the pads)


  1. Given an SMD footprint as above, how close does a ground plane need to be (on the same layer) to decrease the effective pad-to-pad capacitance? Is there a formula which gives a good approximation?


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W

But, regardless of the pads, the component's mating metal surface has that capacitance regardless, doesn't it? You don't need always to make the 'plates' larger than that component feature.

The use of guard rings can make such capacitance negligible, if that's required, but... trying to simply treat the tiny elements as separable capacitances, isn't a good and useful model.

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J

Most surface-mount part capacitance is in the FR4, not in air.

It's often important to know the three capacitances: each pad to the ground plane, and 3-terminal equivalent capacitance pad to pad.

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K

turn PCB has a toolkit for a number of different calculations on PCB structures:

formatting link

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S

Unfortunately Saturn PCB does not cover this... I tried there first.

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J

It's easy to measure attofarads with cheap instruments that a decent home lab should normally have.

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J

Femtofarads, sure, but attofarads? Please explain?

Jeroen Belleman

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J

Femtos, just make a voltage divider from a sig gen, Cx, and the usual

15 pF input of a scope. Signal average a bit or FFT at the low end. A few fF resolution is easy. To get to aF, add a jfet or an opamp.

One could also add an inductor to resonate out most of that 15 pF.

Lots of ways to do this.

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J

The old HP selective (tuned) voltmeters were most cool for things like this.

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