surface corrosion (of a tin finish)

A localized change to a silver-colored tin surface finish appearing in an optical microscope as nonreflective dark spots ranging in size from about 25 micrometers on the longest dimension to the entire terminal.

NOTE While tin oxide is ubiquitous on tin surface finishes, surface corrosion creates a locally thick layer of tin oxide that may span from the substrate to the surface of the deposit at the dark spot.

References: 

JESD201, 3/06