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September 2009

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Subject:
From:
Joyce Koo <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Joyce Koo <[log in to unmask]>
Date:
Thu, 17 Sep 2009 08:10:39 -0400
Content-Type:
text/plain
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text/plain (244 lines)
Heat sink
--------------------------
Sent using BlackBerry


----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Thu Sep 17 04:33:08 2009
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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