Re: [SI-LIST] : Modeling Package parasitics

D.C. Sessions ([email protected])
Thu, 02 Sep 1999 14:24:41 -0700

Howard Johnson wrote:
> Hello D.C.,
> What about the ESR of a chip's parasitic input capacitance? We had
> a big discussion about that in the gigabit ethernet group about
> a year ago, and nobody really ever came up with a
> satisfactory answer. Is it .1, 1, 10, or 100 ohms?

Hope I'm not shocking anyone with this answer, but:
It depends.

There are several components to Cchip.

First there's the pad itself. Bonding pads are generally pretty
large chunks of metal (80 microns square for 500nm processes is
common). This is over field oxide and P-bulk material, so it
tends to have a fairly high ESR -- without calculating, at least
hundreds of ohms.

Then there's the drain capacitance of outputs and bidirectional
cells. This one tends to be somewhat variable, depending on the
Rdson of the predriver since the dominant capacitance is the
gate-to-drain capacitance of the OFF transistors. This is commonly
5x or so of the Rdson of the driver itself, maybe 60-120 ohms each
for pullup and pulldown. In parallel is the drain-to-well capacitance,
which tends to have hundreds of ohms of ESR.

If the signal is input-only then it needs a separate ESD device
(on outputs the driver transistors do the job). Some designers
use hard grounds on the gates of these parts, giving very low ESRs
on the order of ohms. Others use resistors or turned-on transistors
and can have kohms.

At this point the input path runs into a protection resistor
(a hundred ohms or so) and can generally be treated as complete.

Bottom line: I'd SWAG the Cchip as two parts: about half with an
ESR of a kilohm, and half with an ESR of about 50 ohms. Those
who want a better answer than a guess from an I/O designer who's
spent the day on lawyerese should have a ago at lab measurements.

> At 04:13 PM 2/16/98 -0700, you wrote:
> >Mark Nass wrote:
> >>
> >> Does anybody have an accurate way of modeling package parasitics,
> >> in particular in a QUAD simulation environment. I have been using
> >> a lumped inductance for a BGA & SQFP package but find this to be very
> >> inaccurate with fast edge rates and small voltage level swings. The
> >> from the inductor causes the signal to cross switching thresholds in the
> >> simulation environment, but this ringing is not seen on the lab bench.
> >> My feeling is that a specified ZO, TPD & and Length should be used
> >> for the bonding wire, trace on the BGA package and the pin.
> >> Does anybody have any thoughts on what values I could use for
> >> the bonding wire?
> >
> >Bondwires are actually very good inductors. The package itself
> >may be a PWB in implementation (consider BGAs). A good figure
> >for bondwire inductance is 40uH/in.
> >
> >One of the reasons that many simulations show excessive ringing
> >on ICs is that the capacitors are too high-Q. Most semiconductor
> >capacitances (such as the ESD structures on I/O circuits) have
> >quite large ESRs. Your best bet would be to correlate the ESR
> >to the lab data.

D. C. Sessions
[email protected]

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