Residues on PCBAs are unfortunately common place. SEM/EDS analysis helps to determine the nature of the residue.
The image above shows residue on the surface of the PCBA, but it is not clear what is the nature of the residue.
EDS analysis shows that the residue is likely tin bromide, suggesting corrosive bromide flux activator was left behind after assembly.
This is a BSE SEM image of some of the residue, which shows dendritic growth. The dendrites suggest that electrochemical migration (ECM) was a factor meaning ionic contamination, electric field, and moisture were likely at play. If the dendrites migrate between normally isolated signals (e.g. PWR & GND) then the circuit fails.
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