Ted,

  I was reviewing some text on this subject over the last day and I can
answer with another questions:

We all know that a current in a trace results in a "Mirror Current" in a
plane layer. So, if we place a signal between planes we still get mirror
images of that current in the planes?

So, there is no way that a signal can be buried under a plane as its Mirror
Image rises to the plane!

Did I get that right?

Warning: The Following Might be Hog Wash

I think the solution involves an understanding of near and far fields. At a
distance the net field from the trace and it's Mirror image are zero. There
is an "Antenna Effect where the field between and around the conductors can
radiate but this area is pretty small and the radiation of this "Antenna"
becomes pretty small as the antenna is very short compared to the
wavelengths of trace power. Use a big return loop and you have a lower
frequency antenna.

So I can see that fringe field on a top side trace could induce current into
other nearby conductors. A buried trace would seems to expose less fringe
fields.

At a distance, and where there are no other near by conductors (Antenna
Elements), I don't think Top Side vs Buried makes a big difference. ???

This email is subject to Hog Wash removal.

Bob Kondner



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tontis, Theodore
Sent: Thursday, September 17, 2009 9:39 AM
To: [log in to unmask]
Subject: Re: [TN] Ground Pour Under BGA

Is there actual data that supports the ground plane under the BGA or
just routing all the traces in the mid-layers reducing their ability to
radiate emissions? I always place traces that would radiate/be more
likely to radiate in the mid-layers. This was driven into me years ago
by another design engineer and I still apply it to all my present
designs, when applicable. Also an added benefit of routing all the
traces in the mid layers you make break out easier because you do not
have to route around the BGA pads and vias reducing the over all length
of the trace. I would also think with the solder resist on the ground
plane would reduce the ability to couple the radiated emissions
effectively/efficiently and require an increase in temperature during
the reflow process possibly adding to manufacturing issues or concerns?

Ted T.

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