[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator

  • From: "Vishram Pandit" <vishrampandit@xxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 13 Aug 2003 03:26:43 +0000

 
The example I was referring to had a ground plane. However, with simulations
and with experiments, we determined that the capacitors directly under
(bottom side of the board) the balls of the BGA performed better than on the
top side. If we put the capacitors on the top side instead of at the bottom,
EMI was worse at high frequencies (600 MHz- 800MHz). The harmonics of the
BGAinternal frequencies were stronger. This was observed on multiple designs
with the same BGA. 

I have seen some comments on the SI list that the capacitors are ineffective
say above 500MHz. Is this really true?? I have seen improvement in EMI at
600MHz-800MHz if I tune the 

* value of the capacitors 

* ESR / ESL of the capacitors 

* location of the capacitors 

Any comments/advice will be appreciated. 

Thanks, 

Vishram 

  



  >From: "Lee Ritchey" >Reply-To: si-list@xxxxxxxxxxxxx >To:
si-list@xxxxxxxxxxxxx,"Vishram Pandit" >Subject: [SI-LIST] Re: UltraCAD ESR
and Bypass Capacitor Caculator >Date: Tue, 12 Aug 2003 09:25:16 -0700 >>Ah!
The key here is double sided PCBs. That means no plane capacitor, >which a
part of the UMR test. Problem with double sided PCBs is the >absence of
planes to tie things together. This does make EMI with fast >edge parts very
difficult, if not impossible to control. In this case, >capacitor placement
does matter because there is a great deal of inductance >in the power path
due to the lack of planes. As part of this, it is very >tough to make the
circuit stable at high speeds, as well. >>When we tried to make NICs using
double sided PCBs at 3COM for fast >Ethernet, we had this very problem. Just
barely worked. When we added >encryption, didn't work at all. Too many data
lines switching >simultaneously. >>Lee >>Lee Ritchey
>leeritchey@xxxxxxxxxxxxx >Why Wait? Move to EarthLink. >>>>[Original
Message] >>From: Vishram Pandit >>To: >>Date: 8/11/2003 3:21:05 PM
>>Subject:[SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator >>>>>>I
agree with Ray. I have found in my designs that the EMI is very much
>>dependent on the decoupling capacitor placement. Typically I can place the
>>decoupling capacitors with the PDS analysis. Also, I can find
>>experimentally,the hot-spots and place the capacitors. With simulations
>and >>with measurements, I have found that the decoupling capacitor
placement is >>very critical. It may be in a particular section of the
board,it may be >at >>the bottom (for double sided boards) .... it all
depends on the nature of >>theproblem. >>>>>>Thanks, >>>>Vishram >>>From:
RayAnderson >Reply-To: si-list@xxxxxxxxxxxxx >To: >>si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: UltraCAD ESR and Bypass >>Capacitor Caculator >Date:
Mon, 11 Aug 2003 09:36:25 -0700 >>>>Lee Ritchey >>wrote: >>If the location
ofdecoupling capacitors matters, perhaps some >>technical >>demonstration
would prove that. Short of such a demonstration, >>this is >>speculation and
not the sort of thing that should be used to >make >>design >>choices.
>>>>It's time to do some good engineering on this >subject >>and do away
with>>opinion. The UMR paper is good engineering. Anyone who >>chooses to
>>disagree with it has the burden of showing where it is wrong >by >>using
some >>good science. >>>>Lee >>>>Lee Ritchey
>>>>leeritchey@xxxxxxxxxxxxx>>Why Wait? Move to EarthLink. >>>Or conversely,
>>show that it doesn't matter. >>Take a large system board (say about 24"
>>square) put a bunch of high current >processors and ASICS on the left side
>>ofthe board. Put all your decaps >on the right side of the board (since
>you >>maintain position doesn't matter). >>In this hypothetical case I can
just >>about guarantee the board won't function >properly or pass EMI.
>Admittedly, >>it is a contrived case, but I think it >illustrates the
point.>>As far as >>the technical demonstration goes, we have indeed
demonstrated to >>ourselves >>that position does matter. >>I'm not
attemptingto further an argument, >but >>Ido feel those who have >responded
with viewpoints other than yours >>shouldn'tbe chastised and accused >of
spreading unfounded opinion. Don't >>discount the chance that an opinion
that>differs from yours just might be >>correct. >>-Ray Anderson >Sun
Microsystems Inc.
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