Copper electroplating


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:
  1. Alkaline cyanide
  2. Alkaline non-cyanide
  3. Acid sulfate
  4. Acid fluoroborate
  5. 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 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 ; 40-55 °C ; 60-71 °C
  • Cathode current density: 0.5-4.0 A/dm2 ; 1.0-1.5 A/dm2 ; 8.6 A/dm2
  • Current efficiency: 30-60% ; 30-50% ; 90-99% ;
  • 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.

Acid sulfate

Acid copper sulfate electrolytes are relatively simple solutions of copper sulfate and sulfuric acid that are cheaper and easier to maintain and control than cyanide copper electrolytes. Compared to cyanide baths, they provide higher current efficiency and allow for higher current density and thus faster plating rates, but they usually have less throwing power, although high-throw variations exist. Additionally, they cannot be used to plate directly onto less-noble metals such as steel or zinc without first applying a cyanide-based strike or other barrier layer, otherwise the acid in the bath will cause an immersion coating to form that will compromise adhesion. Due to this phenomenon as well as the lower throwing power, acid sulfate baths are not usually used as strike baths.
Along with alkaline cyanide, acid copper baths are among the most commonly used copper plating electrolytes, with industrial applications that include decorative plating, electroforming, rotogravure, and printed circuit board and semiconductor fabrication.
Acid sulfate baths contain cupric sulfate as the source of copper ions; sulfuric acid to increase bath conductivity, ensure copper salt solubility, decrease anode and cathode polarization, and increase throwing power; and a source of chloride ions such as hydrochloric acid or sodium chloride, which helps reduce anode polarization and prevents striated deposits from forming. Most baths also contain a variety of organic additives to help refine the grain structure, improve ductility, and brighten the deposit. Variations of the acid copper electrolyte include general-purpose baths, high-throw baths, and high-speed baths. The high-throw and high-speed baths are used when greater throwing power and faster plating rates are required, including for printed circuit board fabrication where high throw is required to plate the low-current-density areas in the through holes.

Bath composition

Operating conditions

  • Temperature: Usually ambient, although some baths may operate as high as 43 °C
  • Cathode current density: 2–20 A/dm2 ; 1.5–5 A/dm2 ; 5–20 A/dm2
  • Current efficiency: 100%

    Additives

Various common and proprietary additives have been developed for acid copper electrolytes to help improve throwing and leveling power, brighten the finish, control hardness and ductility, and impart other desired properties to the deposit. Historical formulations dating to the mid-20th century often used thiourea and molasses, while other formulations used various gums, carbohydrates, and sulfonic acids.
For semiconductor and printed circuit board applications, acid copper baths use additives that facilitate plating in high-aspect-ratio vias and through holes. Such additives can be grouped into three categories:
  • Suppressors
  • Accelerators
  • Levelers
Without these additives, copper will preferentially deposit on the surface near the top of the vias instead of inside the vias due to the lower local current density inside the vias, leading to top-down via filling and undesirable voids. The suppressor inhibits plating near the top of the via and the surface, while the brightener accelerates plating near the bottom of the via. The leveler helps prevent buildup at the via opening and creates a smoother surface finish.

Acid fluoroborate

Copper fluoroborate baths are similar to acid sulfate baths, but they use fluoroborate as the anion rather than sulfate. Copper fluoroborate is much more soluble than copper sulfate, which allows one to dissolve larger quantities of copper salt into the bath, enabling much higher current densities than what is possible in copper sulfate baths. Their main use is for high-speed plating where high current densities are required. Drawbacks to the fluoroborate chemistry include lower throwing power than acid sulfate baths, higher cost to operate, and greater safety hazards and waste treatment concerns.
Acid fluoroborate baths contain cupric tetrafluoroborate and fluoroboric acid. Boric acid is typically added to the bath to prevent hydrolysis of the fluoroborate ions, which generates free fluoride in the bath. Unlike acid sulfate baths, fluoroborate baths usually do not contain organic additives.

Bath composition

Operating conditions

  • Temperature: 18-66 °C
  • Cathode current density: 13-38 A/dm2 ; 8-13 A/dm2
  • pH: 0.2-0.6 ; 1.0-1.7

    Pyrophosphate

Pyrophosphate copper plating baths possess gentler chemistry compared to the toxic alkaline cyanide baths and the corrosive acid copper baths, operating at mildly alkaline pH and utilizing relatively non-toxic pyrophosphate compounds. While pyrophosphate electrolytes are easier to waste treat than alkaline cyanide and acid plating baths, they are more difficult to maintain and control. Pyrophosphate baths offer high throwing power and produce bright, ductile deposits, making them particularly useful for printed circuit board fabrication where high throw is required for plating high-aspect-ratio through holes.
Pyrophosphate baths contain cupric pyrophosphate as a source of copper ions, potassium pyrophosphate as a source of free pyrophosphate that increases bath conductivity and helps with anode dissolution, ammonia for increased anode dissolution and deposit grain refinement, and a source of nitrate ions such as potassium or ammonium nitrate to decrease cathode polarization and increase the maximum allowed current density. When the bath is made up, the copper pyrophosphate and potassium pyrophosphate react to form a complex, , which dissociates to form the Cu26− anion from which copper deposits. Variations of the pyrophosphate electrolyte include general-purpose baths, strike baths, and printed circuit baths. Printed circuit baths typically contain organic additives to improve ductility and throwing power.
In pyrophosphate baths, orthophosphate ions are formed from the hydrolysis of pyrophosphate and tend to build up in the electrolyte over time, which presents maintenance challenges. Orthophosphate ions decrease bath throwing power and deposit ductility at concentrations above 40–60 g/L, and they lead to lower solution conductivity, banded deposits, and lower bright current density range at concentrations beyond 100 g/L. Orthophosphate is removed from the bath by either doing partial bails and dilutions or by completely dumping and remaking the bath.