Estimating PC pad capacitance?

It would be handy to have a graph of c versus size for, say, square pads at a unit distance above a ground plane, namely a fringing graph.

If you google "fringing" it's usually preceded by the word "neglecting."

John

Yes, good idea, John. If I don't find the simple applet I'm looking for, maybe I can make such a graph. I do have applications that can do the calculation accurately, and if I fire one up to make one calc, it should be fairly easy to just change the pad size and run it for a range of sizes. I'd probably do it for not only square but for perhaps 1.2:1, 1.5:1 and 1:2 ratio rectangles too.

Cheers, Tom

If you do that, I'd love to get a copy.

John

Sorry if this spoils the fun with Maxwell's equations:

Scroll down to "Pad Capacitance Extraction for IBIS Models", select and request the paper at the bottom. But check whether they charge for it.

John, if there is a charge maybe they give you access to that for free since you are a PADS customer.

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Regards, Joerg

http://www.analogconsultants.com

equations:

Well, it's not Maxwell's equations I'm using directly, but ADS, to do what the abstract for the paper you reference suggests. I just put an ideal 1uH inductor in series with 50 ohms to a pad whose size I can vary easily, and look for the resonance, indicated by a sharp dip in S11. It's just a bit tedious. It does suggest that even with a pad an inch square on 1/16-inch thick FR4, the fringing is still a moderately significant effect. I got sidetracked and didn't finish the work today. Maybe next week.

Cheers, Tom

Tom Bruhns a écrit :

equations:

Tom,

what I'd try is to adapt the microstrip formula.

With the ustrip you get the fringing correction value along the perimeter of your square pad. I'll be twice the value of the ustrip line of your pad width. Then you'd have to correct for the 'corner effect', which we can assume to be reasonably constant, given your pad width is sufficiently larger than the plane-pad separation. You just have to evaluate this corner effect with a solver, for a few height of interest and maybe add a curve fitting there for fanciness and voilà.

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Thanks,
Fred.

equations:

Possibly. Initially I thought to post a microstrip link but it won't take the corner distortion into effect. Anyhow, for microstrip this can help:

And here is a collection of FEM routines and field solvers:

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Regards, Joerg

http://www.analogconsultants.com

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.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

Hi Joseph,

Thanks for your posting. At the very least, it's gotten me thinking more deeply about the calculation, which is of a sort I seldom need to deal with.

I suppose I could assume the charge is distributed uniformly over a rectangular surface, and get an approximation of the capacitance, and compare that for a few cases against a numerical fields solver to see what error there is. Did you have in mind anything more complicated than the assumption of uniform charge distribution? I'm pretty sure that there are cases where that won't be very accurate.

Once I decided to do it, it wasn't difficult to fire up ADS and get it to run the calcs for me. It's certainly faster than _I_ could do it with paper and pen.

But seriously, I'd be happy to know if there's something I'm missing about doing it with a simple calculation.

Cheers, Tom

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Thanks for the ideas, Fred. I thought some about that, but actually, now that I have ADS set up to do it for me, it's no trouble to put in any particular pad size, thickness, and dielectric constant. And in surface-mount work, I am often dealing with pads whose dimensions are similar to or smaller than the separation to the ground plane.

Cheers, Tom

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?

John

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

Cheers! Rich

Doing this is certainly even not an easy task. I mean measure the capacitance :-)

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Thanks,
Fred.

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The word negligible comes to mind at 1.5mm If your having problems I would look elsewhere.

The cheap little AADE cap-meter is great for 2-wire stuff like this. It resolves 10 fF.

If somebody posts a prediction, I'll confirm it with a measurement.

John

geometry

It's about 0.06 pF without fringing, so may get up towards 0.1 pF with. Sometimes that matters. With thinner dielectrics, like on a multilayer, c will go up but fringing will be somewhat less.

John

In some cases, like yours, the typical basic equation c=e'e(r)A/d fails to give better than correct order of magnitude. It includes the usually unstated assumption that A >>> d^2. For various symmetrical cases for A >=

3*d^2 the fringing can be shown to be equivalent to the base equation. For cases like yours where A~=d^2 obtaining C from the work equation can be made to do but the time required quickly becomes more than using a finite element solver. I am not usually pushing the dimensions quite that hard.

Just the same, i am glad that i had the limitations of my usual methods pointed out to me, and even happier that i got others to step back and re-check the limits of their typical practices as well.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

John Larkin a écrit :

I wasn't speaking of measurement instrument capabilities, I can easily get down to the fF, but rather meant: how do you measure it without introducing perturbations, i.e. field modification with your probe, wire, whatever...?

At this pad size I guess you can't neglect this anymore.

The true test would be a check between a field solver and a careful measurement, but we also need to accurately measure Er.

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Thanks,
Fred.

On Sat, 24 Mar 2007 16:55:39 -0800, John Larkin Gave us:

geometry

We used an embedded pad on our HV circuits (those that needed it) to achieve a feed forward effect on our feedback loop. It was in parallel with our HV feedback resistor voltage divider network.

It gave us about 15 pF for a quarter inch wide pad at about just under an inch length. One "plate" was embedded, and the other was top side. Of course where in the embedding makes a huge difference for, as we all know, plate proximity distance directly affects capacitance achieved.