Sunday, June 15, 2008

Bath Function on Acid Copper Electroplating

Copper(II) sulfate (CuSO4.5 H2O) and sulfuric acid, or copper (II) flouborate (Cu(BF4)2) and fluoboric acid, are the primary constituents of the metal ions in solutions such as those can look on the table. Copper can be deposited at very low cathode current densities from the acid-free aqueous solutions of the salt, but at higher current densities the deposits from the sulfate bath are spongy and contain occluded salts. Plate characteristics are improved, solution conductivity is increased, and anode and cathode polarization are greatly reduced when free acid is added to either solution. The acid also prevents the preparation of basic salts.

The concentration of copper sulfate is not particularly critical, although the resistivity of the solution is greater when the concentration is increased. Cathode polarization increases slightly at copper sulfate concentrations above 1 M (250 g/l). A concentration of less than 60 g/l copper sulfate results in a decreased cathode efficiency. When very high cathode current densities are used, a high concentration of copper sulfate, within the limits such on table is recommended. The solubility of copper sulfate is decreased when the sulfuric acid concentration is increased.

Formulation of Commercial Acid Copper Baths
Copper Sulfate
Copper Sulfate, CuSO4. 5H2O, g/l
Sulfuric Acid, H2SO4, g/l
Specific gravity, at 25 oC
Resistivity, at 21oC, ohm-cm

To improve the throwing power of some bright copper sulfate baths used for plating printed circuit boards, a low copper sulfate concentration such as 60 g/l of CuSO4 5H2O is combined with 80 g/l of sulfate acid, 30 mg/l of hydrochloric acid, and appropriate brighteners. Analogously, copper flouborate solutions containing 56 g/l of Cu(BF4)2, 160 g/l of fluoboric acid and brighteners are also regarded as thigh throw baths for through hole plating. Air agitation is used for both types of solutions.