Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

COPPER PLATING

Atomic number 29, Atomic weight 63.46, Melting point 136.7K (1983F)
Ionization energy 7.726, Density 8.933Grams/cc , Oxidation states +1 and +2
Electron shell configuration 1S2,2S2,2p6, 3S2,3P6,3D10, and4S1

Why copper plate? Copper plated onto any metal serves as a protective layer, not in the sense of corrosion protection, although it does add to corrosion protection, but as a layer that lowers stress, provides a crystal orientation favorable to the start of subsequent plated layers. The first layer deposited onto a metal substrate tends to follow the crystal structure of the surface material, then changes to the characteristic structure of the metal being deposited, copper in this case. The resulting stress is very high in the transition zone. Since copper is very ductile, the stress induced by the different structure at the basis metal is born by the copper interface. This reduces the overall stress from subsequent plated deposit. Hydrogen embrittlement is also lowered somewhat when cyanide, or pyrophosphate copper plating is used.

History

The name Copper comes from the Latin word cuprum, which means "from the Island of Cyprus."

Archaeological evidence suggests that people have been using copper for at least 11,999 years. 7000 years ago the Romans got their copper from the island of Cyprus. Today copper is found in the United States, Chile, Zambia, Zaire, Peru and Canada. (I read another paper that told about the use of copper 1800 years ago in Mesopotamia. It is possible and likely that immersion (chemical replacement) plating took place very early in history.

The first authentic recorded use of copper plating by electrodepositing is ascribed to Henry Bessemer, better known as a pioneer in steel manufacture. In 1831 Bessemer plated casting of insects, frogs and plants by immersion them in copper sulfate solution on a zinc tray. Electroplating with an external source of electricity followed quickly in 1836.

Early on copper was used for electrotyping and reproducing other metallic shapes and non-metallic shapes. In 1840, J Wright, a Birmingham surgeon, developed the first cyanide plating solutions. For both copper and silver electroplating. Elkington patented them the same year.

Bright acid copper plating began about 1940 by Phillips and Clifton using thiourea and molasses. Later Dayton Bright Copper Co. used acetylthiourea and sulfonated substituted thioureas. Udylite used a hydantoin and mercaptans sulfonated by propane sulfone. Complex amino dyestuff is used in conjunction with a colloidal aromatic amine to produce bright leveling acid copper deposits. Today there are similar compounds used for brightening and leveling.

Characteristics of Popular Copper Plating Solutions

Copper cyanide Plating solutions: Best for throwing power and covering power, as long as the free cyanide is not too high or too low (Range1.5-2 oz/gal). Lower free cyanide results in higher efficiency and faster plating, but less throwing power. To lower the free cyanide, add copper cyanide. One oz/gal of copper cyanide will lower the free sodium cyanide by1.1 oz/gallon. 1 oz of copper cyanide will lower the free potassium cyanide by1.45 oz/gal. Higher free cyanide 2-2.25oz/gal results in better throwing power and less efficiency. Very high free cyanide (3.5-4 oz/gal.) results in slow plating rate or no plating at all. Cyanide copper plating solutions are easy to operate and forgiving of less than perfect cleaning. Cyanide copper strikes are used prior to copper plating to provide a good clean surface for the copper plating solution to do its best job. Strikes are necessary when plating over steel, and zinc die-castings to prevent immersion (non-adherent) plating. Copper strike solutions are formulated to have lower copper higher cyanide and for zinc die-castings a pH less than12. For steel 1-2 oz/gal sodium hydroxide is used in the solution.

Acid Copper

Sulfuric acid copper plating solutions provide rapid deposition and less throwing power compared with cyanide copper and pyrophosphate copper solutions. Acid copper can be made to level and be very bright. Additions agents help micro throwing power such as when used for through hole plating on printed circuit boards, and for plated plastics prior to bright nickel and chrome. The solution is easy to use and maintain. The chloride content is the only critical ingredient that must be controlled between 40-70 ppm. Therefore it is not wise to use hydrochloric acid prior to acid copper plating. Addition agents are available for leveling, brightening and "high throw" formulations that consist of lower copper content and higher sulfuric acid. A cyanide or alkaline non-cyanide copper strike is required when plating onto steel, aluminum or zinc die-castings.

Methansulfonic acid

Another type of acid copper uses methanesulfonic acid. These are similar to sulfuric acid copper but the acid is mild by comparison making it somewhat safer to use. Rapid deposition rate and with additions agents can level and be bright.

Fluoboric acid

This acid copper provides the opportunity to use even higher current density than most other formulations, producing very high deposition rates. Higher rates are reported compared with acid sulfate and methanesulfonic acid copper plating systems. The characteristics are similar to acid-sulfate copper and methanesulfonic acid systems. Fluorides are toxic and dangerous. Extreme care must be used in dealing with fluoboric acid systems even though it is a weak acid. Fluorides can be absorbed through the skin easily.

Pyrophosphate copper

Pyrophosphate copper plating solutions provide good throwing power (not as good as cyanide copper) and is used for printed circuit boards and applications where cyanide copper or acid copper should not be used. There are formulations that can be used as a copper strike, going into the solution with a cathode connection pre made and the current on. There is a risk of an immersion copper coating if the aforementioned step is not used, that will have poor adhesion.

New alkaline Non Cyanide formulations

The newer non-cyanide alkaline baths introduced in 1991 claim to have the ability to plate directly onto steel, zinc, and aluminum and white metal without danger of immersion deposits. The solution can be used as a copper strike, replacing cyanide strikes on steel and zinc die-castings prior to other copper plating solutions or used alone for thick deposits. The bath composition uses 0.8 to 1.8 oz/gal., pH 9-10, temperature 110-140 degrees F., plates at 5-30 amps/sq ft. uses air agitation and OFHC anodes. This non-cyanide solution is sensitive to impurities such as lead, silver and iron. The impurities, lead and silver are removed by dummy plating at ½ amp/sq.ft. Organic contamination is removed by peroxide and carbon treatment. Surface preparation is critical compared with cyanide copper. Cyanide copper solution will clean many oils and other soils whereas acid, pyrophosphate and other non-cyanide solutions will not clean. The waste treatment uses calcium chloride followed by filtration.

Uses for copper plating

Stop off for heat-treating, Electro-forming such as molds records CD’s wave-guides art objects and to make non-conductors made conductive. Semiconductors for trench filling via filling, circuits, printed circuits through hole plating, pattern plating and additive circuits, coating steel wire, continuous strip plating, electrical devices, EMI and RF shielding, plating on steel for buffing, plating zinc die-castings undercoat for nickel plating and pennies. I am sure there are more uses for copper plating. What have I missed?

There is much more to copper plating: Things such as type of anodes, anode to cathode area ratio formulations for the various copper strikes and plating solutions, trouble shooting, equipment and others items.

TROUBLE SHOOTING CYANIDE COPPER PLATING SOLUTIONS

Do’s and don’ts

  1. Use a filter. A filter is essential for good copper plating. Continuous filtering with occasional pump out and filter back
  2. Have a storage tank equal to, or somewhat larger than the size of the plating solution tank.
  3. Use steel hooks for anodes (Never Monel)
  4. Control solution by frequent analysis.
  5. Provide cover for the tank. Keep dust out of the plating solution.
  6. Provide adequate cleaning of parts to be plated. Keep buffing compounds, oils and other soils out of the solution.
  7. Remove fallen parts from the bottom tank every day.
  8. Provide good temperature control.

Don’t

  1. Install polishing or buffing or blasting equipment in the immediate area, preferably not in the same room or building.
  2. Do not use or try to plate anything that has silicone lubricants on it. Don’t use silicone in the same building. They are difficult or impossible to remove in the cleaning process.
  3. Don’t forget that cyanide is poison. And use proper waste treatment methods.

Trouble shooting

Rough deposits -Cause

Remedy

Low free cyanide Adjust to proper amount (1.5oz/gal)
Insufficient cleaning Inspect cleaned parts be sure all compound is removed from recessed areas, threads and holes
Dust or foreign matter Filter the solution
Drag-over of acids or cleaners Improve cleaning
Precipitated salts from hard water Use treated water
Organic contamination Add wetting agent (surfactant)
Treat with hydrogen peroxide
Carbon treat and filter
Polarized anodes
Polarized Anodes

 Remedy

Low Temperature Adjust temperature controller
Low free cyanide Add cyanide
Organic contamination Carbon treat and filter
Zinc Contamination (deposit has brassy look) Dummy plate at low CD 2-3 ASF, Add anodes prefer OFHC
Low copper metal content Add copper cyanide with the appropriate amount of cyanide
Chromium contamination Treat with sodium hydrosulfite, ¼ oz /100 gal. copper plating solution. Repeat if necessary.
Pitting Add surfactant (wetting agent). Chromium contamination, see above
Difficulty buffing copper Free cyanide too low, add cyanide
Organic contamination
See pitted deposits
Roughness- See above
Low or unbalanced additions agents
Burning

 Remedy

High free cyanide Add copper cyanide
Low temperature Raise temperature, increase agitation, Lower CD

adjust anode to cathode distance use 6"

Low copper content Add copper cyanide with additional cyanide
Low agitation Increase agitation
Low caustic content Add sodium or potassium hydroxide
Organic contamination Treat as above
Unbalanced addition agents See manufacturer
Low Cathode efficiency

 Remedy

High free cyanide Add copper cyanide
Copper metal too low Add copper cyanide
Chromium contamination See chromium above, "Polarized anodes"
Plate peeling  
Chromium contamination See chromium contamination above
Insufficient cleaning Improve cleaning cycle
Contaminated rinses Use clean rinse water

                               
 Notes

1 ounce of copper cyanide requires 1.1 oz of sodium cyanide, and 1 oz of copper cyanide requires 1.45 oz of potassium cyanide to form the proper complex without adding any free cyanide.

To lower the free cyanide, add 0.9 oz/gallon copper cyanide by 1 oz/gallon. Or add 0.7 oz/gal copper cyanide to lower the free potassium cyanide by 1 oz/gallon.

Rochelle salt is a great help in keeping the anodes working properly. (Also proprietary additions equivalent to Rochelle salts work well)

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