Ionic contamination, moisture, and electrical bias can combine to create electro-chemical migration (ECM) shorts under an IC device as shown in this example.
This is an optical image showing residue on the bottom surface of the FPGA package.
This is a BSE SEM image of the same corner location. The residue bridges several signals at this corner.
An EDS spectrum of the residue suggest that it contains chlorine and bromine. Part of the bromine signal likely originates from the brominated epoxy molding compound. The chlorine & perhaps some of the bromine is most likely residual solder flux activator.
The analysis results suggest that the most likely cause of failure was external corrosion and ECM due to halide contamination. The corrosion and ECM was likely exacerbated by elevated temperature and humidity conditions.
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