Residues on PCBAs are unfortunately common place. SEM/EDS analysis helps to determine the nature of the residue.

residue on PCBA 1

The image above shows residue on the surface of the PCBA, but it is not clear what is the nature of the residue.

residue on PCBA 2

EDS analysis shows that the residue is likely tin bromide, suggesting corrosive bromide flux activator was left behind after assembly.

residue on PCBA 3

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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