You may recall my mentioning having a discussion with a retired EE/chip designer (at the hospital) a while back. I mentioned the surface area thing to him, and he said that the practice where he worked was to fill in any excess available surface area on the top surface of the substrate with low cost capacitors (apparently for some performance improvement). Greek to me, but maybe it helps.... But it seems to me that this practice would make the entire top surface an "active area".

SGE