Silver sulfide. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ashok Dhawan
Sent: Tuesday, August 31, 2010 6:18 PM
To: [log in to unmask]
Subject: [TN] Black material under plating

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 ? 

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