I have a design that uses many of these opamps as high impedance buffers. After several months in service, I've found contamination on the backside of some parts and below them on the soldermask. Laser Induced Breakdown Spectroscopy of the contamination shows it is an organic compound with a high percentage of copper. I'm seeing leakage currents as high as microamps. The contamination seems to spread out from solder balls eventually bridging them causing leakage.
While there are many other parts on the board, the contamination seems to be present only at these parts. This is the only chip scale package used on the board. All other ICs are plastic encapsulated.
Is there anything about the construction of DSBGA chip scale package that would make it more susceptible to contamination issues?