Maybe from an embedded cellular module or something? It was sitting as a loose chip in my microscope-food bin and I have no record of its origin.
It's very sticky on one side, some kind of adhesive or thermal compound residue that didn't come off with acetone. I have no special interest in the part so didn't bother trying to clean it further (plus it wasn't obscuring any markings)
The underside is... weird. It's 28 balls square with three concentric rings, each consisting of two rows of balls then a void space, then a bunch of power/ground balls in the middle. This much is normal.
But there's some strange gaps in the soldermask between the rings, and almost all of the balls seem to have connections going to these void areas. I wonder if there is some kind of test point or plating ground or something in this area that is removed late in the packaging process? Never seen anything like this before.
