Anyone encountered black coating like material under plating of heatsink ?

The package is TO 220. Upon desoldering the package or plying off the board 
with a nose plier, it sheared off at plating. A portion of plating stayed on the 
board while remaining on heatsink plate. The surface on parts' heatsink where 
plating peeled off is black in color. On scratching with sharp tool, it appears to 
have copper under.

The surface is tin lead plated. Any guesses what this black surface could be ? 

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------