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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