# [SI-LIST] Re: Basic question on power plane capacitance

• From: "Mikhail Matusov" <matusov@xxxxxxxxxxxx>
• To: <wolfgang.maichen@xxxxxxxxxxxx>
• Date: Thu, 9 Sep 2010 14:30:07 -0400

```Thanks Wolfgang and all,
Yes, indeed I haven't thought of fringe capacitance. I don't know if there is
an easy analytical way to find out what it will be. The area of the polygon is
1400 sq.mm. The perimeter is 236 mm. The dielectric is FR-4, and the thikness
is about 5 mil (0.127mm). The top layer is not a plane and there is not much
copper in it, mostly pads and vias. The gap around this polygon separating it
from the rest of the plane is 15 mil (0.381mm) wide. The board dimensions are
233x160 mm.

Thanks again,
/Mikhail

----- Original Message -----
From: wolfgang.maichen@xxxxxxxxxxxx
To: Mikhail Matusov
Cc: si-list@xxxxxxxxxxxxx ; si-list-bounce@xxxxxxxxxxxxx
Sent: Thursday, September 09, 2010 2:11 PM
Subject: Re: [SI-LIST] Basic question on power plane capacitance

Mikhail,

If the polygon isn't very large (~100x) compared to its distance from the
ground plane then fringe capacitance will contribute significantly to the total
value - i.e. the actual capacitance will be quite a bit higher than what you'd
expect from the simple formula for a planar capacitor. In addition, if your
dielectric is a low-cost material (e.g. FR-4) its dielctric constant can easily
be off by ~10% from the specified value.

A 2.5D or 3D field solver can tell you how much total capacitance to expect.
One suitable choice would be Sonnet Lite (i.e. the demo version of Sonnet). If
you can tell me the size/geometry of your polygon, distance to the ground
plane, and dielectric constant of the board material I can run a quick
simulation for you.

Just to rule out an obvious mistakes, you did take into account that having
both a plane below and above the polygon will douple the capacitance compared
to just a single pair (polygon and single ground plane)? Forgetting that would
immediately explain a mismatch factor of 2.

Regards,

Wolfgang

"Mikhail Matusov" <matusov@xxxxxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
09/09/2010 10:50 AM
To <si-list@xxxxxxxxxxxxx>
cc
Subject [SI-LIST] Basic question on power plane capacitance

Hi all,

I have a small polygon in a plane layer of a multi-layer PCB. There is a
solid ground plane underneath it and the top signal layer above. I have
calculated from the basic plane capacitance equation that the plane
capacitance for this polygon should be in the range of 420 pF. However, I am
measuring 890 pF with one meter and 1 nF with another. I was wondering what
am I doing wrong?

Thanks,
/Mikhail

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