Copper plating is the process in which a layer of copper is deposited on the item to be plated by using an electric current.
Three basic types of processes are commercially available based upon the complexing system utilized:
- alkaline-(several modifications of cyanide and non-cyanide) complexed bath
- acid-(sulfate and fluoborate) complexed bath
- mildly alkaline-(pyro phosphate) complexed bath
With a higher current, hydrogen bubbles will form on the item to be plated, leaving surface imperfections. Often various other chemicals are added to improve plating uniformity and brightness. Without some form of additive, it is almost impossible to obtain a smooth plated surface. These additives can be anything from dish soap to proprietary compounds.
Copper is an active metal and so difficult to plate onto a passivated surface, making direct plating of iron based metals difficult. Such surfaces often require a nickel strike base coat for the copper to adhere to.
The plating seen in chemistry classes, often obtained with a coin and copper sulphate bath, is in fact deposition as opposed to plating. Subjecting the surface to any wear causes the unstuck deposit to come away. Running such a bath for longer periods, one can often see the grainy like texture of deposition as opposed to the smooth surface of plating.
Commercial platers often use a copper cyanide-based solution to ensure a high level of copper remains in solution. These solutions are inherently dangerous due to the highly toxic nature of cyanide.
Effects of bath chemistry on finish
Cyanide based bath chemistry produces a matte finish favored for its diffusion blocking character. If a bright surface is required, such as one required for decoration or soldering upon, an acid bath chemistry is used.
Chemical changes during copper electroplating
Using copper sulphate as the electrolyte, copper as the anode and the metal to be plated (e.g. iron) as cathode; on passing electricity, copper sulphate (CuSO4) splits into Cu2+ ions and SO42- ions. The Cu2+ ions move to the cathode, receive 2 electrons and become neutral metallic copper and are deposited there. The copper atoms in the anode become Cu ions giving up 2 electrons, enter the solution and react with sulphate ions retaining the electrolyte's concentration