[SI-LIST] Re: Capacitors UNDER a BGA??
- From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
- To: "'cadpro2k@xxxxxxxxxx'" <cadpro2k@xxxxxxxxxx>,si-list@xxxxxxxxxxxxx
- Date: Tue, 4 Feb 2003 12:55:22 -0700
As the original poster I fell I need to explain
the reason for the question. We have currently
a BGA running SDRAM at 133MHz on a two layer
board. Clearly, the placement of the decoupling
capacitor is of critical importance. We have data
that shows that in the preferred location the
Vcc ripple is below 60mV.
Thanks to all who replied..
Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Email: charles.grasso@xxxxxxxxxxxx;
Email Alternate: chasgrasso@xxxxxxxx
-----Original Message-----
From: cadpro2k@xxxxxxxxxx [mailto:cadpro2k@xxxxxxxxxx]
Sent: Tuesday, February 04, 2003 9:18 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Capacitors UNDER a BGA??
Hi Fred,
You're right and Lee is right... for each of your assumptions.
I've seen BGAs with 4 .01 and 4 .001 caps and a couple bulk caps; and
I've seen BGAs with 10 .01, 10 .001 and 6 bulk caps. Overkill?
Simulation done? Just rule-of-thumb practices... I'm very used to
placing whatever amount of caps I'm giving on the board side opposite
the BGA, as close as possible to the BGA feature.
But... what if you have BGAs mounted opposite one another? My common
practice is still to place the bypass equally around the BGA, and try to
keep it as close to the BGA as possible (keep in mind it's preferred
you keep a .200" clearance around the BGA for rework purposes, if
possible.) And, the high speed teminations again take precedence over
the bypass. When you get into these reaches, plane splits and
terminations have a far greater importance over bypass.
Just my $.02 worth.
Mitch
---------Included Message----------
> Date: Tue, 4 Feb 2003 09:14:57 -0600
> From: "Fred U. Rosenberger" <fred@xxxxxxxxxxxxx>
> Reply-To: <fred@xxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Cc: "Fred U Rosenberger" <fred@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: Capacitors UNDER a BGA??
>
>
> The original post asks for info on a capacitor that would fit
under a BGA
> package. It does not say that only one capacitor will be used.
If it's 20
> or so capacitors with one BGA package, or perhaps 5 BGA packages
with 20
> caps each, or 100 packages with 2 caps each, then it's no longer
true that
> the capacitors can be placed anywhere on the board. The paper by
Hubing et.
> al. (IEEE Transactions on Electromagnetic Compatibility) shows
that with a
> single capacitor the placement is not important. That is far
from showing
> that the placement of 100 or more caps is not important. Any
place on the
> PC board is not fine in this case.
>
> Regards,
>
> Fred
>
> ----- Original Message -----
> From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>; "Michael Khusid"
<mkhusid@xxxxxxxxxx>
> Sent: Thursday, January 30, 2003 10:48 AM
> Subject: [SI-LIST] Re: Capacitors UNDER a BGA??
>
>
> >
> > Mike,
> >
> > It is not necessary to place decoupling capacitors under ICs.
Any place
> on
> > the surface of the PCB is fine. See Todd Hubing's paper "Power
Bus
> > Decoupling on Multilayer PCBs" published on May 1995 in the
IEE
> Proceedings
> > on Electromagnetic Compatibility for more information on this.
You can
> > download this paper from the IEEE web site.
> >
> > Lee
> >
> > Lee Ritchey
> > leeritchey@xxxxxxxxxxxxx
> > Why Wait? Move to EarthLink.
> >
> >
> > > [Original Message]
> > > From: Michael Khusid <mkhusid@xxxxxxxxxx>
> > > To: <si-list@xxxxxxxxxxxxx>
> > > Date: 1/29/2003 7:01:34 PM
> > > Subject: [SI-LIST] Re: Capacitors UNDER a BGA??
> > >
> > >
> > > There are sockets made for BGAs. If you have a socket for
your BGA, you
> > > may get enough height to put caps under it.
> > >
> > > Also, I am not certain whether you can reliably solder caps
under BGA
> > > without shorting the BGA balls to each other and to the cap.
> > >
> > > Best regards,
> > > Mike
> > >
> > >
-----------------------------------------------------------------
> > > Michael Khusid
> > > Ansoft Corporation
> > > SI/HF Application Engineer
> > >
> > > 25 Burlington Mall Road, 6th floor
> > > Burlington, MA 01803-4100
> > >
> > > Tel 781-229-8900 Ext. 34
> > > Fax 781-229-8624
> > > --------------------- http://www.ansoft.com
---------------------
> > >
> > >
> > > -----Original Message-----
> > > From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> > > On Behalf Of Grasso, Charles
> > > Sent: Wednesday, January 29, 2003 7:55 PM
> > > To: si-list@xxxxxxxxxxxxx
> > > Subject: [SI-LIST] Capacitors UNDER a BGA??
> > >
> > > Hi Si'ers
> > >
> > > I am looking for a very thin decoupling cap
> > > that will fit under a BGA. Is there anyone
> > > out there that knows of such a device??
> > >
> > > Best Regards
> > > Charles Grasso
> > > Senior Compliance Engineer
> > > Echostar Communications Corp.
> > > Tel: 303-706-5467
> > > Fax: 303-799-6222
> > > Cell: 303-204-2974
> > > Email: charles.grasso@xxxxxxxxxxxx;
<mailto:charles.grasso@xxxxxxxxxxxx;
> > > >
> > >
> > > Email Alternate: chasgrasso@xxxxxxxx
<mailto:chasgrasso@xxxxxxxx>
> > >
> > >
> > >
> > >
> > >
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