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I've a 0402 footprint and i would like to evaluate the stray (mutual) capacitance between the two pads.

What i know are

  • l*m : pad size

  • w : thickness of the pads

  • d : distance between pad

  • h : distance between pad and GND layer.

Actually i'm a bit confused about how to evaluate this stray capacitance. How can i take into a account the fact that i've both air on one side and FR4 on the other?

The only formula i've found on internet is:

C= e0*eFr4*L*w/d

Can you give me further information about the calculation?

Thanks in advance for the consideration.

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Draw a precise sketch. Fracture the sketch into tiny squares, with Er indicated in each square. Model that as a grid of resistors, implement a Finite Element Model solution, and you will have the potentials and gradients.

Do the same for a simple parallel-plate model of large area, so the fringing adds little area, and use your answer to calibrate the result of your precise sketch of your 0402.

Note the presence of any SMT component, with its conductive ends, upsets any parasitic capacitance modeling. Thus you must perform the modeling with some assumed installed SMT component.

Additionally, a resistor body, insulating, has a dielectric constant, of unknown value, but different from "air".

And a capacitor body, insulating, has the same unknown effect.

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