If it's connected to ground and has a pwr layer directly beneath then
there will be a distributed capacitance benefit for bypassing.
At 05:11 AM 9/17/2009, Eric CHRISTISON wrote:
>Inge,
>
>If the big slab of metal in the device isn't connected to the PWB ground
>then it can make things worse, not better. If it is, then I agree with
>you that it will have a shielding effect.
>
>Regards,
>
>
>
>Terry Kozlyk wrote:
> > Hi Inge;
> >
> > Disturb away ! I find your 2 questions very good. Very good points !!!
> >
> > TDKtalks (Terry)
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Terry D.Kozlyk ACS, CL, C.E.T.
> > TDKtalksTM PodCast
> > TDKtalksTM Blog
> > Silver Springs Toastmasters - Club #3987
> > [log in to unmask]
> > [log in to unmask]
> > home (403) 239-7939
> > work (403) 295-6773
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
> > Sent: Thursday, September 17, 2009 2:33 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] Ground Pour Under BGA
> >
> > Hi,
> > CADing boards is not my job, but I became curious when I read
> your pingpong.
> > We have a board with a number of SuperBGAs, number of balls about 660.Vias
> > in pad. I have two questions for you professionals. 1)what's the benefit
> > with a groundplane (that looks more like a copper net)under chip, when the
> > whole BGA has a thick metal plate all over the BGA. That metal sheet is
> > mainly for cooling and stabilizing the body mechanically, but isn't this
> > cover also a effective ground plane and a electrostatic shield?
> 2)will such
> > an extra groundplane under chip make 3D X-raying even harder to read, e.g.
> > bond wires, which are already hard to inspect due to the massive top metal
> > plate.
> > Sorry for disturbing you with such trivia.
> > /Inge
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Dwight Mattix
> > Sent: onsdag 16 september 2009 17:26
> > To: [log in to unmask]
> > Subject: Re: [TN] Ground Pour Under BGA
> >
> > yes. If BGA pitch is large enough then it makes nice little pwr/gnd island
> > with some distributed capacitance directly under the device (available with
> > very low inductance).
> > When picking and choosing which of several device voltages to
> locate thusly,
> > for greatest benefit use it for the heavy fast currents (e.g.
> > core voltages).
> >
> > At 07:16 AM 9/16/2009, Kenneth Wood wrote:
> >
> >> Keep in mind that on HDI designs you are not JUST adding a few micro
> >> vias and pouring GND on the top layer...you are using via-in-pad on all
> >> devices so to try and minimize top side pour cuts. Also, you are
> >> routing as many nets on internal layers as possible.
> >> The top layer GND is not usually the only GND plane too, there is
> >> usually an internal plane to back up the top layer.
> >> This does reduce EMI significantly, shorter runs and external shielding
> >> works.
> >> Ken
> >> Saturn PCB Design, Inc.
> >> www.saturnpcb.com
> >>
> >>
> >>
> >> -----Original Message-----
> >> From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
> >> Sent: Wednesday, September 16, 2009 10:07 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] Ground Pour Under BGA
> >>
> >> Ken,
> >>
> >> With all the BGA pads and possible vias I don't see how a top side
> >> can have a better mesh of copper paths than an internal layer. I would
> >> think a poor copper mesh plane would be a generator of large GND return
> >> loops. Aka antennas.
> >>
> >> As for a GND plane being a few mils closer to the BGA and this
> >> reducing EMI, that sounds like hog wash. Consider that conductor
> >> lengths from a ball to chip is probably about 1/2 inch at times. How is
> >> adding 10 mils going to affect anything?
> >>
> >> I am open to Hog Wash ideas but they need to be well defined.
> >>
> >> Thanks,
> >> Bob Kondner
> >>
> >>
> >> -----Original Message-----
> >> From: TechNet [mailto:[log in to unmask]] On Behalf Of Kenneth Wood
> >> Sent: Wednesday, September 16, 2009 9:54 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] Ground Pour Under BGA
> >>
> >> It is an technique used in HDI designs that utilize micro vias.
> >> Top layer is GND and layer 2 can be a power layer. It also helps with EMI.
> >> Ken
> >> Saturn PCB Design, Inc.
> >> www.saturnpcb.com
> >>
> >>
> >> -----Original Message-----
> >> From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
> >> Sent: Wednesday, September 16, 2009 8:42 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] Ground Pour Under BGA
> >>
> >> Hi,
> >>
> >> What would be the goal of placing atop side GND plane under the BGA?
> >>
> >> Low Z Gnd return? Electrostatic shield?
> >>
> >> Having a plane under the BGA vs another layer is not going to make a
> >> difference in plane Z.
> >>
> >> Sounds like a good chance of creating shorts if the solder mask chips
> >> or is damaged during rework.
> >>
> >> Bob Kondner
> >>
> >> -----Original Message-----
> >> From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
> >> Sent: Wednesday, September 16, 2009 7:43 AM
> >> To: [log in to unmask]
> >> Subject: Re: [TN] Ground Pour Under BGA
> >>
> >> IPC 7095, BGA Technology by Lau.
> >>
> >> -----Original Message-----
> >> From: TechNet [mailto:[log in to unmask]] On Behalf Of Toby Carrier
> >> Sent: Tuesday, September 15, 2009 8:45 PM
> >> To: [log in to unmask]
> >> Subject: [TN] Ground Pour Under BGA
> >>
> >> Hello all,
> >>
> >> Is it a good idea to create a top layer ground flood under a BGA
> >> package? I am guessing that the fanout will not allow much area for the
> >> ground pour, am I correct in thinking so? Does anyone have any good
> >> reference info on this topic?
> >>
> >> Also, if you don't have a ground pour under the BGA, how will that
> >> affect the impedance control of the traces going to the BGA ball?
> >>
> >> Thanks for the help,
> >>
> >> Toby
> >>
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> >
>
>--
>Eric Christison Msc
>Mechanical Engineer
>Consumer & Micro group
>Imaging Division
>
>STMicroelectronics (R&D) Ltd
>33 Pinkhill
>Edinburgh EH12 7BF
>United Kingdom
>
>Tel: +44 (0)131 336 6165
>Fax: + 44 (0)131 336 6001
>
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