Dear Dr. Saddam Amir,

as a first analysis, I would do a verification if the black spot on the tin surface is a physical or chemical defect. This can be done quickly, with an optical microscope by evaluating if there are different focal planes between the surface (in the extreme proximity of the defect) and the central area of the spot. Further information could be obtained by morphological and chemical analysis using SEM instruments equipped with an EDX microprobe. Without these information, as a first hypothesis only by looking at the photo, I could think of a detachment of one or more layers from the substrate (due to poor adhesion, corrosive phenomena, mechanical damage, etc.) , which occurred in a very critical area such as the corner of the sample. It would also be useful to analyze other areas of the same sample and / or other samples to check if similar cases are present. Before to try to reproduce this defect, we must, first of all, better understand this problem.

Best regards, Pierluigi Traverso

Dear Pierluigi,

Thanks for your reply. I check on the black spot, there is no detachment of any layer. Moreover, the outermost layer Sn with black spot show bigger thickness than non-black spot. Solderability of that black spot also not affected.

Dear Saddam Amir,

perhaps with the SEM technique with EDX microprobe you could have some more chemical information, especially if the black spot is not too thin, by analyzing, by comparison, some areas nearby.

The black spot could be an impurity created during or after the deposition of tin.

Best regards, Pierluigi Traverso