Re: Estimating PC pad capacitance?

On Sat, 24 Mar 2007 10:47:46 -0800, John Larkin wrote:
On Sat, 24 Mar 2007 16:08:07 GMT, joseph2k <quiettechblue@xxxxxxxxx>
Tom Bruhns wrote:

Anyone have any favorite reasonably accurate applets or formulas or
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.

Not speaking for anyone else it sounds like simple paper and pencil from
basic physics is the fastest approach. Get your answer, optionally
including fringing, in less time than firing up and inputting the geometry
into a program.

Suppose we have a 62 mil square pad on a 62 mil thick FR-4 pcb with
ground plane on the far side. How would you go about computing total
pad capacitance (including to the air) with pencil and paper?

I'd measure the capacitance, get out the lab notebook, and write down the
answer. ;-)

Cheers!
Rich

.

Relevant Pages

  • Re: Estimating PC pad capacitance?
    ... the like for calculating the capacitance of a small rectangular ... ground plane about 1.5mm behind the pad, ...
    (sci.electronics.design)
  • Re: Estimating PC pad capacitance?
    ... the like for calculating the capacitance of a small rectangular ... ground plane about 1.5mm behind the pad, ...
    (sci.electronics.design)
  • Re: Estimating PC pad capacitance?
    ... the like for calculating the capacitance of a small rectangular ... ground plane about 1.5mm behind the pad, ...
    (sci.electronics.design)
  • Re: Estimating PC pad capacitance?
    ... the like for calculating the capacitance of a small rectangular ... ground plane about 1.5mm behind the pad, ...
    (sci.electronics.design)
  • Re: Estimating PC pad capacitance?
    ... the like for calculating the capacitance of a small rectangular ... ground plane about 1.5mm behind the pad, ... I have a clue that the fringing effects for small pads is significant, ...
    (sci.electronics.design)
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