PCB burried capacitance

Larry Smith ([email protected])
Fri, 7 Jun 1996 08:46:08 -0700

> The upshot is that for power
> -ground plane separations in the range 0.5 - 2 mil, and dielectric
> constants around 4 -5, the predicted effective series inductance of
> BC systems are roughly 2-3 orders of magnitude lower than can ever be
> achieved by discrete decoupling capacitors alone - and it therefore provides
> excellent high frequency decoupling.

I couldn't agree more with this statement. I did some finite element modeling
of a decoupling capacitor mounted on solder pads, with vias directly to the
card power planes. The inductance of this structure is a little less than
1 nH. The inductance of closely spaced card power planes is 2 orders of
magnitude less than this.

Decoupling capacitors mounted on PCB's resonate at a fairly low frequency.
For example, a 1 nF cap on 1nH pads resonates at 1/(2*pi(sqrt(LC))) or 160 MHz.
After this frequency, the capacitor is inductive and it's effectiveness
in keeping the power supply low in impedance diminishes with frequency.
The card power planes are all we have to decouple high frequency transients.
With edge rates near 1 nSec, we definitely have energy at .35/tR = 350 MHz.
Decoupling caps mounted on pads and connected with vias are only marginally
effective at these frequencies.

> I won't go into details of the computer model(s) I developed for this, by
> the time you've superposed the behavior of the board decoupling, power
> distribution system and power supply feedback response and output
> filtering it got, well, complex.
> Best regards
> John
> -----------------------------------------------------------------------
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John - Have you written anything up on this? Is it published?

Larry Smith
Sun Microsystems.