FreeLists / si-list / [SI-LIST] Re: Parasitic capacitance of vias

[David Kaiser <david.kaiser@xxxxxxxxxx>]

In my experience, on a thick board with many ground layers, the largest
discontinuity effect of the via, by far, was the capacitance between the
barrel wall and the ground planes. Removing the unused pads provided only a
very slight improvement. Pulling back some of the ground planes further away
from the barrel wall reduced the capacitance tremendously. If done with
great care, it can make the via look reasonably close to a 50 Ohm
transmission line.

A word of caution:
If you go too far with relieving the ground from the via, you can lower the
capacitance so much that the via in fact starts to appear inductive. All
these effects are very evident on a TDR. 

Note:
With multi-gigabit signals on very thick boards ( > .2 inches ), lowering
the capacitance of a via only lowers the discontinuity if the signal travels
a significant distance throught he via. If the signal travels only a small
distance throught the via (like to an adjacent layer) then the stubb effect
of the via may still impose a sigificant discontinuity at multi-gigabit
rates.

David Kaiser
4 McDATA Pkwy.
Broomfield, CO 80021
(720) 558-4431
david.kaiser@xxxxxxxxxx




-----Original Message-----
From: chris.mcgrath@xxxxxxxx [mailto:chris.mcgrath@xxxxxxxx]
Sent: Thursday, September 12, 2002 1:29 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Parasitic capacitance of vias




In the Johnson/Graham text (Section 7.2), it discusses parasitic capacitance
being the result of several PCB parameters including the diameter of the via
pad and the diameter of the clearance hole in the ground planes.  However,
there is a caveat that states that the equation "assumes there is a pad on
every layer" and that if the designer omits pads on layers not connected to
traces (as we are doing), this should "slightly reduce the parasitic
capacitance".  Does anybody know by how much?  In my case, the parasitic
capacitance actually makes enough of a difference to warrant asking the
question.

Thanks,
Chris

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