This is the solder-side of a PCBA that failed shorted. The cause of the short was determined to be electrochemical migration (ECM), but there were secondary effects as shown in the next image.
These are vapor deposited copper & tin crystals that most likely formed as a result of the failure where vaporized copper & tin condensed as these feathery structures.
The EDS spectrum suggest the composition of the crystals includes copper & tin.
Kurt Larson says:
Hi Ed… Was the root cause related to pwb specification/design, fabrication, assembly, or operating environment? Best Regards. Kurt
Ed Hare PhD says:
The analysis results suggested that ionic contamination within these board was the most likely root cause of the failure. In SEM Lab’s 20 year history, we had not found chlorine & sulfur levels within PWB laminate like what we documented for these PCBAs. This was likely caused by some type of unusual circumstance.
It is not clear whether the contamination was due to environmental exposure of the laminate stock prior to board fabrication or due to contamination during plating operations. However, it is fairly clear that the exposure occurred prior to final fabrication of the boards.