After developing, the copper thickness of the exposed circuits needs to be increased by a secondary copper plating, namely pattern plating. The purpose is to make the hole copper and surface copper of the conductive circuits satisfy customers' requirements and at the same time meet the rated current load of each circuit to ensure its conductivity and other physical properties.
Before pattern plating, a series of treatments are applied to the surface of the board, including degreasing, micro-etching, and acid pickling. Oxides, inks, and residual films are first removed from the copper surface of the circuits to ensure good adhesion between the panel plating and the pattern plating. Micro-etching is then carried out to remove the oxide layer on the surface and in the holes to further improve the bonding force. The board needs to receive acid pickling treatment before being submerged into the plating tank to further remove oxidation, reduce contamination of the tank by poor pre-treatment cleaning, and also maintain the stability of acid concentration in the tank. Next, under the action of voltage, the copper at the anode is oxidized to copper ions, and the copper ions in the copper tank get electrons and are reduced to copper at the cathode, thus increasing the copper thickness of holes and the circuits.
After copper plating, a layer of tin is deposited as an etching resist on all the exposed copper, so as to protect the conductive circuits from being etched away. During alkaline etching, the etching solution has no significant effect on the tin. The tin protects the parts of the board where the copper is to remain, while the unwanted copper is removed from the board. The subsequent process is going to remove the unwanted copper.
