[SI-LIST] : Re: Q:Via Capacitance

Bob Perlman ([email protected])
Tue, 9 Mar 1999 08:10:28 -0800

Hi -

> Would somebody have some info on what the average Via capacitance would
> be ?
> I recall using a average of 0.5pf per Via.
> But now I'm dealing with a 16 Layer PCB and the Vias are 26mils.
> Also, I would expect Blind Vias would have less capacitance than a
> Thru-Hole Via.
> I also recall about the Traces having an average Capacitance of
> 1.5pf/inch.
> Would there be any current statistics on this ?
> My margin on the Bus Loading is at a minimum.
> We're using the MPC8260(Motorola) and the timing is gautanteed at only
> 50pf if we're expected to run at 66Mhz with 0 Waitstates.

Others have addressed the capacitance of vias and traces, so I
won't go into that, other than to point out that the capacitance of
stripline (traces between planes) is even higher than that of
microstrip, on the order of 3-3.5pF/in for FR-4.

Maybe you're looking for the answer to the wrong question. If
you're driving a trace that's long enough to look more like a
transmission line than a lumped load (and the statement that
you're near the 50pF limit leads me to believe that that might be
the case), the line looks principally resistive, not capacitive. If it
does look like a transmission line, meeting the 50pF limit is no
guarantee that the signal will settle in time. A transmission line
simulation is called for. I like Hyperlynx Linesim, but there are a
number of good simulators that will do the job.

Take care,
Bob Perlman

Bob Perlman
Cambrian Design Works
Digital design and signal integrity consulting

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