A short follow on to my earlier comments on capacitor
inductance.
We've actually evolved our pad geometries and stackups far
enough along that we are at the point where the partial inductance
of the capacitor is about 50% or more of the total loop inductance.
To achieve this you need to be very aggressive with the board stackup
and pad escape and via design.
This means that very high performance layouts can now have
cap/spreading/pad inductance proportions of around 50%/5%/45%
or thereabouts. This makes it worthwhile to pursue a low inductance
cap if you are trying to achieve the highest SRF possible.
All 0603 and 0402 caps are not created equal as far as
parasitic inductance is concerned.
Our group has run a bunch of finite element simulations
on capacitor geometries and has come up with the following
#'s. So if you know the physical height of your capacitor you
estimate its inductance:
Thickness (mils) Inductance (pH)
20 200
30 300
40 350
50 400
So if you have multiple sources of 0603 caps and one vendor's
parts are 30 mils thick and the others is 40 mils thick,
for minimum inductance select the thinner part.
(those who attended the Power Distribution Workshop at the
EPEP conference last October should find these results somewhat
familiar)
-Ray Anderson
Sun Microsystems Inc.
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