Heatsinks on BGAs can radiate EMC

While at DesignCon last month I saw a neat presentation from some Cisco chaps that showed how high frequency signals can couple into the heat sink mounted on the top of a BGA (ball grid array) and then radiate the energy out and maybe cause you to fail FCC or Cispr. They are talking about 3 GHz and up so maybe you don’t care, but what was so cool about this is that the shape and number of fins make a big difference. A Flomerics guy was there too—they made the simulation software that CISCO used to predict how bad the radiation would get. They got pretty good correlation, but I am one of those fellows that thinks an ounce of trial is worth a pound of opinion and any computer simulation is just an opinion, nothing more.

Here is a diagram of the heat sink.

Here is just one chart that shows how much difference you can get in energy radiation based on the size and number of fins. The fins form a phased array antenna—pretty neat stuff.