RE: [SI-LIST] : Via Capacitances ...

Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Tue, 13 Jul 1999 12:15:49 -0500

>I think we should consider two cases.
>
>1. A trace encounters a via and then continues on the same plane.
>
>2. A trace encounters a via, goes and goes through its reference plane
>and continues on the other side.

Absolutely!

Anyone can argue that capacitance totally dominates when considering only
case 1.

Many situations fall somewhere between the two, say going from one inner
layer to another.

If modeling vias, some vias have pads not just on the outer layers, but also
on some or all inner layers. I think it's a mechanical issue when building
the boards.

> Years ago at an HF design seminar, I heard that U.S. westerners tended to
> treat vias as inductance, while east-coasters treated them as capacitance.
> I don't know if that bias still prevails.
>
Some of the series inductance may be in the return path, since in many case
the two traces that join at a via do not share the same return plane(s), and
the return current must spread out somewhat around the via.

Regards,
Andy Ingraham

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