Plating in Acid Sulfate Baths

The chemical composition of acid sulfate baths is simple. Copper sulfate pentahydrate and sulfuric acid are the primary constituents of the copper sulfate electrolyte. The metal ions are furnished by the copper sulfate. Sulfuric acid increases solution conductivity and helps prevent the formation of basic cuprous or cupric crystals on the anodes and the tank, which causes poor anode corrosion and roughness. Low sulfuric acid contents produce more high-current-density burn, poorer leveling, more low-current-density dullness, and more nodular deposits. High sulfuric acid has less effect on the deposit but increases the anode dissolution. With cathode efficiencies of 95 to 100%, the copper sulfate bath is easy to operate and control.

Many copper sulfate plating solutions require the use of additives to produce smooth, fine-grain, bright, leveled, and ductile deposits. Most of the addition agents used in copper sulfate plating solutions are proprietary formulations. These proprietary additives are capable of producing the desired characteristics in the copper deposit, and deposit hardness can be increased where necessary.

In copper sulfate systems that produce bright deposits, a catalyst must be added in addition to the primary constituents to avoid streaky deposits. This catalyst is chloride, which is maintained between 0.02 to 0.1 g/L (0.003 to 0.01 oz/gal), or 20 to 100 ppm. The chloride, usually added as hydrochloric acid, inhibits rough nodular plate from forming. Low chloride can cause dark deposits on the edges and high-current-density areas of the work, loss of leveling, loss of brightness, pitting, and poor anode corrosion. High chloride causes streaks, increased brightener usage, and loss of leveling and brightness in the bright bath formulations. High chloride can be reduced with zinc dust treatments or precipitation with silver.

If solution agitation or work movement is minimal, current densities should not exceed about 4.5 A/dm2 (45 A/ft2), because excessive anode polarization may occur and the deposits can be spongy. Where higher current densities are desired, such as for electrotypes or wire plating, air agitation is used. Air agitation is necessary to accelerate ionic diffusion and produce high-quality, fine-grain deposits where current densities are in excess of 10 A/dm2 (100 A/ft2).

The effect of temperature changes on the grain structure and surface smoothness of deposits plated from the copper sulfate bath is less significant than the effect of changes in cathode current densities. An increase in temperature results in higher conductivity and reduced anode and cathode polarization. Increased temperature also reduces the tensile strength of deposits and increases grain size. Excessive temperatures should be avoided in copper sulfate baths where proprietary brightener formulations are used, because reduced plating ranges, excessive additive use, and solution contamination from additive breakdown result.

Care must be taken to avoid accelerated buildup of copper metal, as in cases where dragout rates are low or improper anode-to-cathode ratios are maintained. An increase in the concentration of the copper sulfate increases the solution resistivity and slightly reduces the anode and cathode polarization. Copper sulfate concentrations in excess of 248 g/L (33 oz/gal) may result in salt crystallization in the plating solution. Normal bath composition is restored by discarding a portion of the bath and adding water and sulfuric acid.

To improve the throwing power of some bright copper sulfate baths used for plating printed circuit boards, a low copper sulfate and high sulfuric acid electrolyte is used. The use of this electrolyte allows a nearly equal deposit distribution when plating the through-holes of the printed circuit board.

In sulfate electrolytes, impurities such as silver, gold, arsenic, and antimony can codeposit with copper. Arsenic and antimony cause copper deposits to be brittle and rough, and silver may cause roughness. Nickel and iron impurities reduce the conductivity of the plating bath. Lead impurities do not codeposit with copper; however, they precipitate in the electrolyte. Soluble silicates may precipitate onto the work. Organic contamination from decomposition products of addition agents, tank linings, and anode bags can cause brittle or discolored deposits. These organics can be removed from the electrolyte by treating it with activated carbon.

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