Aim of Process
To alter the surface of the copper, to improve the adhesion of the bonding process.
The process employed to oxidise the coating is a horizontal process, which then leads to a more controlled uniform oxide.
The use of oxide on Cu in the PCB process is essential in providing a rough surface in preparation for the lamination process. Black oxide coatings create much the same roughness on a smooth copper surface, enabling the PCB layer to laminate more effectively.
As we are following the BGA device, it then looks like the following