Here is another example of electro-chemical migration that resulted in shorted signals on a printed circuit board assembly.
These ECM residues can often be hard to see under an optical microscope because there are optically transparent metal oxides present with very small metallic dendrites.
Backscattered electron SEM images show contrast related to atomic number, so for example lead appears very bright (Pb z=82) and carbon appears dark (C z=6). The dendritic structure is a clue that this was ECM related.
The elemental map below shows that the dendrites are mostly lead but there is also some tin. Both lead and tin are likely to be involved for Sn-Pb solder joint assemblies.
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