Re: [SI-LIST] : Focusing on Parasitic Parameters

Mike Jenkins (jenkins@lsil.com)
Fri, 20 Nov 1998 19:05:02 -0800

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Abe,

Here's my two cents on the parasitics of signal vias. Capacitance
can be pretty reasonably upper bounded by the capacitance of a
coaxial structure (C = 2*pi*eps*d/ln(b/a)) where b is the diameter
of the antipad in the ground and power planes and a is the diameter
of the via "barrel" and d is the board thickness. For the example
from Howard Johnson's book, 0.53 pF = 2*3.14*4.2*(8.854 pF/m)*
(0.062"/39.37inch per meter)*ln(3.95). That is, the ratio of
antipad-to-barrel diameter is in the neighborhood of 4. Practically,
it's somewhat less, since the capacitance formula is pessimistic.

The other thing this formula is good for is telling the difference
between 30-mil vias and 10-mil signal vias. For the latter, the
antipad-to-barrel ratio is much larger, so the capacitance is
correspondingly smaller.

Regarding inductance, I think it's fairly pointless to try to
calculate this without knowing the return current path. (Weber
said this, not me.)

Regards,
Mike

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