T

#### Tom Bruhns

- Jan 1, 1970

- 0

the like for calculating the capacitance of a small rectangular

surface against an extensive ground plane? That is, for example, the

capacitance of the pad for a surface-mount part, where there's a

ground plane about 1.5mm behind the pad, through FR4 PC board

material, er about 4.5.

I have a clue that the fringing effects for small pads is significant,

in that I can use a simple-minded calculator that only uses the plate

area for a cap with two equal size plates, and doesn't consider

fringing effects, and get one value -- and then look at the pad as if

it were a short section of microstrip transmission line, calculate the

capacitance per unit length of that geometry line, and multiply by the

length of the pad to get an estimate of the pad capacitance. For one

example get about two times higher capacitance than the "dumb" plate

area/separation applet gives. Given that there is additional fringing

for the pad versus what's considered in the microstrip, I suppose the

actual capacitance is even higher.

I know there are full-blown geometry calculators, some available for

free, but I'm hoping for something simpler to use, along the lines of

typical microstrip applets where you enter length, width, dielectric

thickness, pad thickness, and dielectric relative permittivity, and an

answer with perhaps 10% accuracy pops out.

Cheers,

Tom