Hi Richard,
Black oxide was the original description for a process adopted by the PCB industry from the metal finishing industry over 30 years ago.
I didnt think it was still being utilized.
At any rate brown oxide is one of the better performing oxides that has evolved to enhance the bonding strength of the Cu innerlayer to the resin bonding material in multilayer boards.
Hope this helps.
C.Cobb
Dynamic & Proto Circuits |