[SI-LIST] Re: Importance of Package Height
- From: Larry Smith <ldsmith@xxxxxxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx, ARIAZI@xxxxxxxxxxx
- Date: Mon, 18 Mar 2002 11:47:20 -0800 (PST)
Abe - As mentioned earlier, the inductance you get depends greatly on
the assumptions you make, either in measurement or software
extraction.
The thing we really want to know is "what is the series resonant
frequency of a decoupling capacitor mounted on the pads, vias and power
planes of our products?" From the series resonant frequency, we back
calculate the ESL. My definition of ESL is the equivalent inductance
that causes the capacitor to have the low impedance dip at the
frequency that I measure.
We find that the ESL obtained from measurement depends greatly on the
fixture used to make the measurement. Most people are now using a VNA
S21 measurement to characterize capacitors. The purists will make a
fixture that has a 'perfect' 50 ohm through impedance and mount the
capacitor on that fixture. That S21 measurement gives a clear
capacitance, resonance and inductance portion of the curve. Most
people will argue that the "ESL" determined by the resonant frequency
of that measurement is the "ESL" of the capacitor. Valid argument, but
it does not apply to our products.
Ok, now go mount that same capacitor on a set of low inductance pads
and vias that are connected to thin power planes near the surface of a
PCB (my product..). Use the same VNA to make an S21 measurement of the
power planes. You will find that the resonant dip is as much as 25%
higher in frequency than the cap measured on the 50 Ohm fixture. Back
calculate the inductance and you find that the ESL for the capacitor is
50% higher on the 50 Ohm fixture than it is on the power planes.
Hmmmm. Once again, the ESL of the capacitor depends greatly on the
assumptions and fixture that you use when you measure it.
The inductances given on both the Kemet and AVX web sites are too high
for my purposes. They are not useful for calculating or simulating the
series resonant frequency when the capacitor is used for power supply
decoupling in our products. I would encourage any SI'ers that are
truly interested in power distribution to go repeat some of these
measurements on your own products. Find the ESL of a capacitor by
measuring the resonant frequency when mounted on low inductance
products and fixtures.
Sun and Cadence sponsored a capacitor consortium meeting last October
which was attended by about half a dozen major capacitor suppliers. We
described the software tool techniques that we use to design power
distribution systems (Specctra Quest Power Integrity Tool). Very
quickly it becomes obvious that software tools for PDS analysis rely
heavily on the accuracy of capacitor models. Unfortunately, the
standards for measuring capacitor parameters (capacitance, ESR and ESL)
do not apply very well for capacitors used for PDS decoupling above
about 10 MHz. We found that the capacitor suppliers are very receptive
to these ideas and would like to make measurements that are meaningful
for their customers. The problem is that there is not a strong
consensus (much less a standard) in our industry on what ESL is and how
it should be measured.
As you know, Sun has been working in this area for some time. We are
motivated to share this information with the rest of the industry in
hopes that meaningful measurements can be made by the cap vendors, which
lead to accurate simulation models for our PDS tools. But, there must
be agreement on what ESL is and how to measure it. That's why you get
all these loooonng emails from me.. We also publish our data in IEEE
conferences to get peer review on these concepts. Eventually, meaningful
measurement standards and simulation techniques will come out of this.
regards,
Larry Smith,
Sun Microsystems
> From: "Abe Riazi" <ARIAZI@xxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Cc: <ldsmith@xxxxxxxxxxxxxxxxxx>, <jrbarnes@xxxxxxxxx>
> Subject: Re: [SI-LIST] Re: Importance of Package Height
> Date: Sun, 17 Mar 2002 22:52:29 -0800
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
> X-Priority: 3
> X-MSMail-Priority: Normal
> X-Mimeole: Produced By Microsoft MimeOLE V5.50.4133.2400
>
> Larry Smith Wrote:
>
> > >
> > I'm not sure what assumptions Kemet is making for
> > inductance in their calculator. Their inductance seems way high to
> > me. We have measured a lot of caps from a half dozen vendors and Kemet
> > is no different than any of the others. When mounted on a low
> > inductance structure, the Kemet cap will behave as if it is
> > significantly less than 1 nH.
> >
> >
>
> Larry,
>
> Thank you for an instructive reply.
>
> The AVX SpiCap program may output considerably
> smaller ESL for similar capacitor than the kemet Spice Simulator.
> As an example, for the ceramic capacitor we recently discussed
> the AVX answer is 1.0 nH compared to kemet's 1.94 nH.
> Sometimes, use of Kemet software is preferable because it can show
> variations of ESL with capacitance values (for fixed package
> size) while the AVX calculator can not.
>
> I think (please correct me if I am wrong) the 1.0 nH by AVX and
> the Kemet's 1.94 nH (and all other ESL results by these two
> programs) apply to un-mounted capacitors. Therefore, based
> on your expected 600 pH for that 100 nF 0805 X7R cap
> both of these manufacturer calculators are providing ESL values
> which are too large/inaccurate in this case.
>
> Best Regards,
>
> Abe Riazi
> ServerWorks
>
> P.S. I also compared the AVX 1.0 nH value to the following measurement
> result earlier reported by John Barnes:
>
>
> Nominal
> Case Capacitance WV ESL
> Size Dielectric (pF) (V) Manufacturer Partnum (pH)
> ------ ---------- ----------- ---- ---------------------------- --------
>
> 0805 X7R 100000 25 AVX 08053C104JAT2A 900
>
> and found them reasonably close.
>
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