[SI-LIST] Re: Importance of Package Height
- From: Zhiping Yang <zhiping@xxxxxxxxx>
- To: scott@xxxxxxxxxx
- Date: Mon, 18 Mar 2002 16:42:46 -0800
Larry,
In the measurement method you discribed below, you actually
measured the total loop inductance ( L_cap+L_vias+L_plane),
is it right? There should are some ways to seperate the L_vias
and L_plane from the L_totlal, since L_cap is more meaningful
for some PDS tools. It is always harder to define/extract partial
inductance than loop inductance since loop indutance is always
constant.
Scott, I like your idea about the "effective inductance curve"
related with the location of return current. Did you consider some
special cases, such as on a mutil-layer board the plane just below
the caps may not be the power/ground plane for this cap?
Thanks.
Zhiping
Scott McMorrow wrote:
> Larry,
>
> No, I didn't send it to the list. Since I hadn't thought about this
> much, I wanted to pass
> it through you first. I've now forwarded my comments to the list.
>
> Okay, I see your "effective inductance" approach. It is the self
> inductance of the
> capacitor minus the mutual inductance relative to other structures.
> This effective
> inductance will be different with each mounting and measuring method.
> This most
> dominant factor will be the location of the planes in the z-axis
> underneath the
> capacitor.
>
> It may be possible to come up with an "effective inductance curve" which
> relates
> the partial loop inductance of the capacitor with respect to the
> distance it is from
> the local planes.
>
> regards,
>
> scott
>
> --
> Scott McMorrow
> Principal Engineer
> SiQual Interconnect Engineering
> 18735 SW Boones Ferry Road
> Tualatin, OR 97062-3090
> (503) 239-4400
> http://www.siqual.com
>
> Larry Smith wrote:
>
> >Scott - Your comments are 'right on the money'. When we mount
> >the capacitor directly above and close to a power plane, the
> >loop inductance is much smaller than when it is mounted on 50 ohm
> >traces.
> >
> >Did you mean to send this to SI-list? I think it just came to me.
> >I think it would be good if the whole list saw this note.
> >
> >I like to measure the inductance of the cap mounted on the structure,
> >measure the structure itself, subtract and find the "effective inductance"
> >of the cap by itself.
> >
> >regards,
> >Larry
> >
> >>Delivered-To: fixup-ldsmith@xxxxxxxxxxxxxxxxxx@fixme
> >>Date: Mon, 18 Mar 2002 14:14:33 -0800
> >>From: Scott McMorrow <scott@xxxxxxxxxx>
> >>User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:0.9.8)
> >>
> >Gecko/20020204
> >
> >>X-Accept-Language: en-us
> >>MIME-Version: 1.0
> >>To: ldsmith@xxxxxxxxxxxxxxxxxx
> >>Subject: Re: [SI-LIST] Re: Importance of Package Height
> >>Content-Transfer-Encoding: 7bit
> >>
> >>Larry,
> >>
> >>It seems to me that the inductance of a capacitor would be heavily
> >>influenced by the
> >>placement of a return plane below the capacitor. The mutual inductance
> >>between the
> >>capacitor and the plane would significantly lower the loop inductance.
> >> Is this possibly
> >>what is happening when capacitors are mounted on low impedance power planes?
> >>Here we have mutual inductance cancellation between the planes and
> >>mutual cancelation
> >>between the capacitor body and the closest plane.
> >>
> >>The problem we have is that for a capacitor the inductance is not
> >>defined until the loop
> >>is closed. The partial inductance of the capacitor body is heavily
> >>influenced by the
> >>method of measurement and the insertion into the plane. I am not sure
> >>that there
> >>is one method of measurement which can be used to extract the parameters
> >>for
> >>a capacitor, any more than there is one method which can be used to
> >>extract parameters
> >>for card edge connectors. In both cases, the loop inductance is
> >>dependant upon the
> >>the mutual inductance between the component and the underlying plane and
> >>the
> >>direction of return current flow through the plane.
> >>
> >>For the case of a measurement where a VNA is used with 50ohm launches
> >>into the
> >>capacitor, there is no return current on any plane and therefore, no
> >>mutual cancelation.
> >>
> >>For the case of a measurement in a loaded planar board, there is return
> >>current on the
> >>plane and and associated mutual cancelation which is beneficial to loop
> >>inductance.
> >>
> >>It might be possible to perform some theoretical modeling of a
> >>simplified capacitor
> >>structure with different measurment configurations using a 3D field
> >>solver to extract
> >>partial inductances and come up with some more general conclusions. My
> >>guess is that
> >>there will not be one inductance number that can be used for a specific
> >>capacitor, since
> >>it is such a poorly referenced structure in the first place. But, there
> >>is probably a range
> >>of inductances which are dependent upon the placement of planes in a
> >>board and whether
> >>or not currents flow between the capacitor and the plane.
> >>
> >>Since inductance is only defined around a loop, the only valid
> >>measurement is within
> >>the same structure where the device will be applied, so that all mutuals
> >>are correctly
> >>accounted for.
> >>
> >>regards,
> >>
> >>scott
> >>
> >>
> >>--
> >>Scott McMorrow
> >>Principal Engineer
> >>SiQual Interconnect Engineering
> >>18735 SW Boones Ferry Road
> >>Tualatin, OR 97062-3090
> >>(503) 239-4400
> >>http://www.siqual.com
> >>
> >>
> >>
> >>>It turns out that the resonant frequency of a capacitor mounted on low
> >>>impedance (50 mOhm or so) power planes is higher than the same
> >>>capacitor mounted on a 50 Ohm transmission line. Therefor, the ESL is
> >>>lower for the cap on power planes than it is for a cap on 50 Ohm
> >>>traces. The physical reason has a lot to do with the resonant
> >>>structure for each measurement. The most meaningful measurments of
> >>>capacitors intended for decoupling will be made on low impedance (low
> >>>inductance) mounting structures.
> >>>
> >>>regards,
> >>>Larry Smith,
> >>>Sun Microsystems
> >>>
> >>>>Date: Mon, 18 Mar 2002 12:44:35 -0800
> >>>>From: Zhiping Yang <zhiping@xxxxxxxxx>
> >>>>X-Accept-Language: en
> >>>>MIME-Version: 1.0
> >>>>To: ldsmith@xxxxxxxxxxxxxxxxxx
> >>>>CC: si-list@xxxxxxxxxxxxx, ARIAZI@xxxxxxxxxxx
> >>>>Subject: Re: [SI-LIST] Re: Importance of Package Height
> >>>>Content-Transfer-Encoding: 7bit
> >>>>
> >>>>Larry,
> >>>>
> >>>>Thanks for sharing your ideas.
> >>>>
> >>>>Can you explain why the measurement from direct VNA with 50Ohm fixture
> >>>>is greater than on PCB?
> >>>>
> >>>>Here are my understandings:
> >>>>1. VNA with 50Ohm fixtures (Calibrate VNA to the cap pads, can we do this
> >>>> in lab? I am not positive.)
> >>>> L_measure=L_cap_self_inductance
> >>>>2. VNA with 50Ohm fixtures (VNA is not calibrated)
> >>>> L_meausre=L_cap_self_indutnace+L_fixtures+M_mutual
> >>>>3. Measurement with cap on PCB structures
> >>>> L_meaure=L_cap_self_inductance+L_vias+L_plane+M_mutuals
> >>>>
> >>>>For my understanding, case 1 gives the smallest inductance. Maybe you
> >>>>are refering the VNA measurement to case 2.
> >>>>
> >>>>For the case 3, the results are also heavily depended on cap layout
> >>>>and PCB stackup. How can you build the universal cap component
> >>>>library for different PCBs?
> >>>>
> >>>>Here is my thought. When you build the cap libraries, the ESL should
> >>>>only be self inductance of the cap. The mounting inductance (vias, plane
> >>>>and traces) should be compensated in simulation tools. In this way, the
> >>>>cap library can be applied to any PCBs. Anyone disagree?
> >>>>
> >>>>Thanks.
> >>>>
> >>>>Zhiping
> >>>>
> >>>>
> >>>>
> >>>>--
> >>>> Zhiping Yang, Ph. D.
> >>>> Hardware Engineer
> >>>> Cisco Systems
> >>>> 270 West Tasman Drive
> >>>> Mail Stop:SJCG/2/2
> >>>> San Jose, CA 95134 | |
> >>>> email: zhiping@xxxxxxxxx :|: :|:
> >>>> Tel : 408 525 5690 :|||: :|||:
> >>>> Fax : 408 526 5504 .:|||||||:..:|||||||:.
> >>>>*****************************************************
> >>>>
> >>>>Larry Smith wrote:
> >>>>
> >>>>>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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