I have seen a couple of references to Black Tar, Not Black Pad. Apparently, it occurs only on the solder side of PTH's after wave solder and primarily on the edges of the land. It is characterized by a black coloration (Not Ni corrosion?) and non-wetting. The component side does not exhibit any problems. Except for a couple of references in the NPL Solder Defects database, I haven't been able to find any info.
Thanks for the input. I read that paper. I'm not convinced the Pb stabilizer and bath MTO's are a cause of the Black Tar defect. If that were the case, given the history of Pb stabilizers and ENIG, I would have thought that there would have been many more examples of the occurrence of this defect. In addition, I don't see that this addresses the occurrence of the defect only on the wave side of the PCB. I say this because of 2 actual examples I have examined plus the 2 examples given by the NPL Solder defects Database.