Black Barrel - Wave solder defects

We are experiencing a high incidence of what we call "black barrel" (to differentiate from black pad) with multiple contract manufacturers and PCB vendors. The PCBs are ENIG and during wave solder (SACX-0307) multiple thru-hole components are seeing random solderability.

I have seen the issue referred to as "Black Tarr" on the defects database with out suggested resolution. There are good pictures of the issue, just search "black tarr":

http://defectsdatabase.npl.co.uk/defectsdb/defects_query.php

Does anyone have any insights on this one?

reply »