[SI-LIST] Re: Is Impedance Enough for Describing the PDS?

Zhangkun,

As I stated, the mounted inductance of IDC's and X2Y's is less than 1/3 the 
mounted inductance of ordinary 0603's using optimized two via mounts.  The 
inductance of the IDC / X2Y capacitors by themselves is even less, but as 
you note the attachment inductance interferes.  The reason that IDCs do 
very well is that it takes four times as many via holes as a regular 
capacitor.  Done correctly, there is an almost proportionate decrease in 
the attachment inductance.  So, an IDC can be viewed as four 0603s, in 
parallel but with only one part to mount.

The physics of the X2Y are a bit different.  In cooperation with X2Y, I 
have developed optimized mounts for X2Ys that use six vias and get results 
essentially identical to the IDC with optimal mounts.  This has been 
verified by both simulation and measured results.  The IDC's require 33% 
more vias, and a much higher component cost to do the same decoupling job 
as X2Ys.

Using either X2Y or IDCs we can cut component count by 3:1 or 
better.  Using X2Ys we reduce the BOM cost as well.  Both Teraspeed and I 
offer services to optimize PDS design to suite needs whether it is density, 
cost or some combination.  There are probably other consultants available 
who do as well.

Regards,


Steve.
At 07:46 PM 7/9/2004 +0800, Zhangkun wrote:
>Dear steve
>
>IDC's, very expensive
>X2Ys, more than cheapy 0603 MLCCs but a lot less than IDCs
>NEC Proadlizer ( consult NEC on price )
>
>How about the value of the inductance of these kind of capacitors? When 
>the cap is soldered on the PCB, there is leading inductance of about 1nH. 
>This could not be eliminated by better caps. When the PCB is dency, the 
>decoupling caps have to be put on the bottom. The leading inductance is 
>larger. When the trace is used for PDS, the decoupling caps have to be put 
>near the IC. How many caps is enough?
>
>What is the paper title which you mentioned, the Chris Cheng's?
>
>Best Regards
>
>Zhangkun
>2004.7.9
>
>----- Original Message -----
>From: "steve weir" <weirsp@xxxxxxxxxx>
>To: "Abhijit Mahajan" <amahajan@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
>Sent: Friday, July 09, 2004 4:40 PM
>Subject: [SI-LIST] Re: Is Impedance Enough for Describing the PDS?
>
>
> > Abhijit, yes localization is an approach that can be very helpful in
> > reducing or completely eliminating cavity resonance issues.  For a long
> > time we have been able to get away with letting planes perform dual
> > duty:  wide low impedance lumped conductors for power, and the return side
> > of signal transmission lines.  But we are long past the point where signal
> > spectra is way beyond board lambda / 4. for common plane size / Er
> > combinations.  Continued dual-use under these circumstances is an
> > invitation to EMC nightmares, and greatly complicates PDS 
> realization.  But
> > it does keep consultants like me busy.  Sigrity, and Ansoft sell tools to
> > try and manage these problems.  The tools are very nice, but I think the
> > tools address the wrong problem.
> >
> > If we "simply" eliminate the cavities ( OK shrink them so that they are 
> not
> > problematic ), as you suggest then I think we are on the right
> > track.  There is documentation out that clearly shows the economic 
> benefits
> > of islanding power.  In one recent paper, the author demonstrated that a
> > board with islanded power and 20mil power / ground separation performed
> > almost identically EMC-wise to the same size assembly using 2mil BC.  The
> > tool and material vendors probably don't like hearing this.
> >
> > Islanding power presents challenges with stack-up planning and 
> routing.  We
> > still have to:
> >
> > 1) Create a local low impedance on the island, and
> > 2) Get the DC current to the island
> >
> > I see the tool opportunities in supporting these efforts, as the physics
> > and economics will really demand it.  On the low-impedance island 
> technique
> > end, I am a big fan of low inductance capacitors and optimized attachment
> > techniques.  The really low inductance capacitors that can be fitted to a
> > PWB today are:
> >
> > IDC's, very expensive
> > X2Ys, more than cheapy 0603 MLCCs but a lot less than IDCs
> > NEC Proadlizer ( consult NEC on price )
> >
> > If done correctly, islanding can save a bundle, and goes directly to 
> one of
> > Chris Cheng's pet mantra's that B/C is a solution looking for a
> > problem.  We can avoid such problems and therefore the cost of B/C.
> >
> > Steve.
> > At 01:06 PM 7/9/2004 +0530, Abhijit Mahajan wrote:
> > >Steve/Others.
> > >
> > >Would it be better then to reduce the area of power planes as much as
> > >possible(by
> > >localizing the power planes to only cover the pins that need it) and pack
> > >the decoupling there?
> > >islands of same voltage can be then connected with wide traces to the
> > >nearest bulk cap
> > >And finally brought together at the regulator.
> > >
> > >This can be vaguely be described as a "star topology" for power
> > >distribution.
> > >
> > >Does anyone think there is any value in this appproach?  Has it been
> > >discussed before?
> > >
> > >
> > >##I am assuming all signal routing is over a common ground plane.
> > >
> > >
> > >Thanks!
> > >
> > >Abhijit.
> > >
> > >
> > >-----Original Message-----
> > >From: si-list-bounce@xxxxxxxxxxxxx 
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> > >Behalf Of steve weir
> > >Sent: Friday, July 09, 2004 6:04 AM
> > >To: zhang_kun@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > >Subject: [SI-LIST] Re: Is Impedance Enough for Describing the PDS?
> > >
> > >
> > >Zhangkun,  the problem is that the PDS overlaps both signal integrity and
> > >EMC compliance.  If the system rules are done right, then the PDS
> > >decoupling problem will eventually reduce to an impedance profile.  But
> > >that profile is both spatial and frequency dependent.  If I shove all the
> > >decoupling capacitors into one corner of the board I get a very different
> > >result than if I distribute them.
> > >
> > >Steve.
> > >At 08:04 AM 7/9/2004 +0800, zhangkun 29902 wrote:
> > > >Dear all
> > > >
> > > >When talking about power delivery system with other engineer, there are
> > > >a
> > > >lot of element to describe the PDS, such as energy storage, decoupling,
> > > >bypassing, and so on. I think all these parameter could be described by
> > > >one element, impedance of PDS.
> > > >
> > > >Is my point right? Is impedance enough for describing the PDS?
> > > >
> > > >Best Regards
> > > >
> > > >Zhangkun
> > > >2004.7.9
> > > >
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