Copper electroplating is the process of electroplating a layer of copper onto the surface of a metal object. Copper is used both as a standalone coating and as an undercoat onto which other metals are subsequently plated. The copper layer can be decorative, provide corrosion resistance, increase electrical and thermal conductivity, or improve the adhesion of additional deposits to the substrate.
Overview
Copper electroplating takes place in an electrolytic cell using electrolysis. As with all plating processes, the part to be plated must be cleaned before depositing metal to remove soils, grease, oxides, and defects. After precleaning, the part is immersed in the cell's aqueous electrolyte solution and functions as the cathode. A copper anode is also immersed in the solution. During plating, a direct electric current is applied to the cell which causes the copper in the anode to dissolve into the electrolyte through oxidation, losing electrons and ionizing into copper cations. The copper cations form a coordination complex with salts present in the electrolyte, after which they are transported from the anode to the cathode. At the cathode, the copper ions are reduced to metallic copper by gaining electrons. This causes a thin, solid, metallic copper film to deposit onto the surface of the part.
The anodes can be either simple copper slabs or titanium or steel baskets filled with copper nuggets or balls. The anodes may be placed in anode bags, which are typically made of polypropylene or another fabric and are used to contain insoluble particles that flake off the anode and prevent them from contaminating the plating bath.
Copper electroplating baths can be used to plate either a strike or flash coating, which is a thin highly-adherent initial layer that is plated with additional layers of metal and that serves to improve adhesion of the subsequent layers to the underlying substrate, or a thicker coating of copper that may serve as the finish layer or as a standalone coating.
Types of plating chemistries
There are a variety of different electrolyte chemistries that can be used for copper electroplating, but most can be broadly characterized into five general categories based on the complexing agent: Alkaline cyanide Alkaline non-cyanide Acid sulfate Acid fluoroborate Pyrophosphate
Alkaline cyanide
Alkaline cyanide baths have historically been one of the most commonly-used plating chemistries for copper electrodeposition. Cyanide copper baths typically provide high covering and throwing power, allowing uniform and complete coverage of the substrate, but often plate at lower current efficiency. They produce a metal finish favored for its diffusion blocking character. Diffusion blocking is used to improve the long term adherence of different metals, e.g. chromium and steel. It is also used to prevent the second material from diffusing into the substrate.
Cyanide baths contain cuprous cyanide as the source of copper(I) ions, sodium or potassium cyanide as a source of free cyanide that complexes with cuprous cyanide to render it soluble, and sodium or potassium hydroxide for increased conductivity and pH control. Baths may also contain Rochelle salts and sodium or potassium carbonate, as well as a variety of proprietary additives. Cyanide copper baths can be used as low-efficiency strike-only baths, medium-efficiency strike-plate baths, and high efficiency plating baths.
Bath composition
Operating conditions
Temperature: 24-66 °C (strike); 40-55 °C (strike-plate); 60-71 °C (high-efficiency) Cathode current density: 0.5-4.0 A/dm2 (strike); 1.0-1.5 A/dm2 (strike-plate); 8.6 A/dm2 (high-efficiency) Current efficiency: 30-60% (strike); 30-50% (strike-plate); 90-99% (high-efficiency); pH: >11.0
Toxicity
Commercial platers typically use a copper cyanide solution, which retains a high concentration of copper. However, the presence of free cyanide in the baths makes them dangerous due to the highly toxic nature of cyanide. This creates both health hazards as well as issues with waste disposal.
Alkaline non-cyanide
Due to safety concerns surrounding the use of cyanide-based plating chemistry, alkaline copper plating baths that do not contain cyanide have been developed. However, they generally see only limited use compared with the more common cyanide-based alkaline chemistry.