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

Ray Anderson (raymonda@radium.eng.sun.com)
Wed, 17 Mar 1999 11:11:45 -0800 (PST)

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 designed
intelligently or not)

PCB Spreading inductance ~ 20pH to 500 pH (depending on stackup and location)

By proper selection of pad geometry and board stackup the total loop inductance
can be reduced dramatically.


Back when people didn't care/understand about the implications of this (not that
long ago) the cap/spreading/pad inductance broke out something like 10%/10%/80%

Today, high performance systems have this total inductance divided up something
like 15%/5%/80% however the total inductance has been reduced from maybe 4.5nH
(or more) down to 2 nH (or less).

The inductance of the capacitor hasn't changed very much, but the pad/via/spreading
components have been reduced by a fact of 2 or so by paying attention detail.



The MOUNTED inductance along with the capacitors capacitance determines
the series resonant frequency above which the part starts looking inductive
to the system. A resonant frequency based on the partial inductance of
the capacitor only isn't what the system will see in use.


-Ray Anderson

Sun Microsystems inc.

>
>
> Mark,
>
> Is a low value of capacitance really important? At 1GHz+, the package inductance dominates the
characteristics and so the impedance of a 0.1uF and a 100pF capacitor in an 0603 package will not be
very different. As you point out,
> for better performance at higher frequencies one would need a lower inductance 0402.
>
> Thanks,
> Vinu
>
> Mark Randol wrote:
>
> > SMT 0603 ceramic caps are still pretty good at 1GHz+ as long as you keep
> > the value under a couple 100pF. 0402's go up through a couple GHz.
>

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