Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 207 of 707

For circuit board plating, copper is most commonly plated following an electro- less copper metallization of the through holes. Recently, a number of technologies have been developed that replace the electroless copper with other means of impart- ing conductivity to the through holes. They fall into two general categories: carbon- based and palladium-based systems. They use conventional acid copper sulfate elec- trolytes to plate up through the holes. Pulse and periodic reverse plating are being used to improve throwing power, deposit characteristics, and productivity with the development of specialized additives. New additives have been developed with DC current to maintain throw- ing power at higher cathode current densities, improving productivity. Bath Preparation Baths can be made by dissolving copper sulfate in water and then adding sulfuric acid. Carbon treatment is recommended. Cooling to room temperature is recom- mended prior to adding proprietary additives. Baths can also be made using either purified liquid copper sulfate (generally around 36 oz/gal as copper sulfate pen- tahydrate) or commercial premade solutions without the organic additives. Carbon polishing is recommended prior to adding proprietary additives. Maintenance and Control Constituents Copper sulfate is the source of copper ions in solution. Because the anode and cath- ode efficiencies normally are close to 100%, the anodes replenish the copper ions so that the copper concentration remains fairly stable in solution. The sulfuric acid increases the conductivity of the solution and reduces the anode and cathode polarization. It prevents precipitation of basic salts and improves throwing pow- er. In high throw formulations, the weight ratio of copper metal to sulfuric acid should be maintained at less than 1:10. To further enhance the throwing power, the copper sulfate may be reduced to 6.0-8.0 oz/gal (45.0-60.0 g/L) to give metal-to-acid ratios as high as 1:30. This may be useful in plating high aspect ratio printed cir- cuit boards; however, the lower copper content decreases the allowable plating cur- rent densities and increases the plating times. Copper sulfate is reduced in con- centration in high throw formulations to prevent common ion precipitation effects as the sulfuric acid is increased. Sulfuric acid concentrations above 11% by volume begin to reduce cathode efficiency. Chloride ion, in bright and high throw baths, reduces anode polarization and eliminates striated deposits in the high cur- rent density areas. Temperature These baths are operated at room temperature for the majority of applications. If the temperature is too low, cathode efficiency and plating range will be reduced. Baths used where bright deposits in the low current density are not required may be operated at temperatures as high as 50° C (120° F) to increase the plating range in electroforming, printed circuit, or rotogravure applications. Agitation Air, mechanical, solution jet, or rotating work agitation can be used. The more vig- orous the agitation, the broader the allowable plating current density. Contaminants Organic contaminants are the ones most commonly dealt with in acid copper plat- 206

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