Re: [SI-LIST] : A Question About Power Noise.

Doug Brooks ([email protected])
Wed, 17 Mar 1999 14:39:33 -0800

OK Ray. Here is a loaded question for you. (See below)

> We've found that the MOUNTED inductance of a capacitor is composed
> mainly of three components:
> Capacitor Inductance (a function of capacitor size [primarily height] )
> Pad and Via Inductance (a function of pad geometry and via length)
> PCB Spreading inductance (a function of board stackup and location on board)
> Typically (and this can vary depending the exact situation) these partial
> inductances are about:
> Capacitor Inductance ~ 200pH - 400pH ( approx.300pH for a typical
0603 part)
> Pad and Via Inductance ~ 1 to 5 nH (depending on whether they are
> intelligently or not)
> PCB Spreading inductance ~ 20pH to 500 pH (depending on stackup and

There has been considerable discussion about how to mount/route a bypass
cap. One theory is to run traces from the cap to the device and then vias
from the cap to the plane. The other theory is to run vias to the plane
from both the device and the bypass cap and let the currents flow over the
'low inductance' plane.

Now, if the highest inductance part of the path is the pad/via, would that
favor running (wide) traces from the cap directly to the device, since that
eliminates two vias between the cap and the device? A wide, short path only
has a few nH inductance (I think).

Doug Brooks

Doug Brooks, President [email protected]
UltraCAD Design, Inc.

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