In terms of tonnage, probbly more copper is plated in general plating than any other metal except nickel. The reasons that made copper so good a plating metal are:
The metal is easily plated, from several types of sloutions which are relatively easy to control.
Copper is the only metal that can be practically deposited directly on zinc die castings in preparation for further processing.
Copper plating baths have good coverage and in general good throwing power.
Copper is among the les expensive metals available to the metal finisher, only zinc is cheaper than it.
The supply of copper is generally reliable.
Copper is an excellent undercoat for further deposits, it is easily buffed to a high luster, providing a good surface for subsequent coatings.
The physical and chemical properties of copper make it useful in many applications.
Copper is relatively inert to most plating solutions, thus basis metals that would be attacked bymany plating solutions if used directly are protected from such action by a preliminary copper plate.
Three types of copper baths are in general use; cyanide, acid, and the pyrophosphate complex bath.
Copper cyanide baths
the only practical way to deposit adherent copper plates on active metals such as zinc and steel is to use acyanide bath, containing the copper(I) cyanide complex Cu(CN)43-.
Excess of free cyanide such as NaCN or KCN is essential. It is required to prevent the immersion deposition of copper, and prevents the formation of insoluble CuCN films on the copper anodes.
Carbonate is added to some types of cyanide baths, primarily to facilitate control of pH, it also reduces polarization.
Acid Copper Plating
Acid copper baths are based on copper(II) salts, they yield only half as much copper pr unit quantity of electricity as the cyanide baths, they exhibit sufficient advantage over the cyanide to make their use not only practical ut preferred for many applications.
Acid copper baths are used for all the occasions requiring a copper plare, exceptions are: they cannot be used for direct application to steel and zinc die catings and steel in which cyanide is essential. Acid copper baths find use not only in plating but also in electroforming and electro fining. Their advantage over cyanide baths are:
the chemical cost is low
effluent control is simplified
the baths are simple and easy to control
their compositions are not critical,
anode and cathode efficiencies are high, and
they can tolerat much higher current densities than the alkaline cyanide baths.
Composition of acid copper plating
The acid copper solutions are very simple in composition, consisting of only a copper (II) salt and the corresponding acid, ie, copper sulfate plus sulphuric acid, or copper fluborate plus fluboric acid. such solutions produce a dense mat deposit, for some of the newer baths, chloride ions are added to the sulphate baths.
Copper(II) Sulphate is available as the pentahydrate, Cu(SO)4·5H2O. Copper(II) fluborate Cu(BF4)2, is available in the form of a solution concentrate.
Excess acid is needed in the acid copper baths for producing satisfactory deposits. These acid account for the high conductivity of the baths, anode and cathode polarization are nealy negligible at low current densities.
Pyrophosphate Copper Baths
The pyrophosphate ion P2O74-, ofrms many complexes with cations. It has come into general use fairly recently, especially in the printed circuit (PC) industry, where it has many uses in the plating of through-holes in PC boards.
In addition to its use in the electronics industry, the copper pyrophosphate bath may be considered for almost any application calling for a copper deposit.
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