Process description: Plating copper and copper alloy
Back to Plating copper and copper alloy in metal industry
- Copper and copper alloy plating
Copper plating is common for items in daily use, such as coins, and buttons or zip fasteners with a patina for haberdashery. These types of workpieces can be plated on jigs or in barrels. Copper plating is also essential in printed circuit board manufacture.
- Cyanide copper
Cyanide copper low temperature electrolytes are necessary for strike plating on steel and zinc die casts to prevent spontaneous cementation of copper and poor adhesion of the subsequent metal deposit. This type of solution is based on copper cyanide and sodium cyanide, with a copper concentration of 15 – 30 g/l. Copper strike layers are usually no thicker than 2 – 3 μm.
Thicker layers (6-8 μm) are achieved with potassium cyanide and potassium hydroxide based high performance electrolytes at a metal content of 25-50 g/l, mainly for barrel plating and others.
Another high performance electrolyte is based on copper cyanide and sodium cyanide with potassium sodium tartrate providing a higher current densities system, enhanced brightness of layers and reduced the tendency to anode passivation. The metal content is 40-60 g/l.
Potassium carbonate is generated in potassium-based systems during processing. This disrupts bright copper deposition and causes roughness at concentrations beyond 90 g/l.
- Acid copper
The solutions are now the usual choice for copper plating. Due to their excellent levelling capability, acid copper electrolytes based on copper sulphate and sulphuric acid are used to make polishing and buffing redundant prior to decorative bright nickel and copper plating on furniture frames, bathroom fittings, wire work, etc. The normal copper content is 50 – 60 g/l and sulphuric acid is 60 – 90 g/l.
An important technical application of acid copper electrolytes is for through-hole, panel and pattern plating of printed circuit boards and multilayers. Sulphuric acid enhances the conductivity, the macro-throwing power and provides fine crystalline and ductile layers. In such electrolytes, the concentration of sulphuric acid is in the range of 180 – 200 g/l and the copper concentration is 20 g/l.
The solutions are also cyanide free, do not suffer from carbonate build-up and are more electrolytically efficient.
- Pyrophosphate copper
Pyrophosphate copper electrolytes no longer play an important role. They are based on copper pyrophosphate (110 g/l) and potassium pyrophosphate (400 g/l). Additives are citric acid (1o g/l) and ammonia (3 g/l).
They are used for special technical applications such as shielding on heat treated parts, as a drawing aid for wires, and for other thick bright layers that need little or no polishing, to prevent hydrogen embrittlement and as an intermediate layer below nickel and silver.
Pyrophosphate electrolytes can also be used for through-hole and panel plating of printed circuit boards and multi layers, but are currently substituted mainly by acid copper electrolytes.
Pyrophosphate is continuously decomposed by hydrolysis if pH is not maintained sufficiently, shortening the lifetime of the process solution. Appropriate regeneration means are not currently available. However, properly maintained baths can last more than 10 years.
- Brass
Brass is predominantly an alloy of copper and zinc although nickel, tin, or lead may be added. A mixture of copper and zinc cyanides in solution are widely used to deposit copper and zinc alloys for decorative purposes. They are alkaline electrolytes containing between 8 – 15 g/l copper and 5 – 30 g/l zinc (depending on the formulation used). The total sodium cyanide content can vary from 70 – 90 g/l and working pH around 10. The deposited alloy contains 65 to 80% of copper and the colour is light yellow. It can be used as a flash over a bright substrate or if heavier deposits are used then different finishes can be obtained through a subsequent chemical colouring of the deposit.
- Bronze
Bronze is copper alloyed with tin and zinc. Cyanide bronze alloy is used as a decorative plating process. It is used as a substitute for nickel in jewellery as a “nickel free” coating to avoid skin allergy effects. Acid bronze is under development.
The metal concentration is on this stannate and cyanide-based electrolyte is 4 – 10 g/l of stannate, 4-20 g/l copper, and 1 – 4 g/l zinc, with 6 – 10 g/l potassium cyanide. It is used in two different colours: white or yellow bronze.
Lead is used in low concentration as a brightener in some electrolytes. Its future use in many products is banned by new directives.
Cyanide oxidises with the passage of current to carbonate. Solutions have to be discarded as soon as the carbonate level exceeds 40 g/l.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
