Metal Finishing Guide Book


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Succinimide Potassium citrate pH Temperature Current density 25 g/L 50 g/L 7.5-9.0 20-70ºC 0.54 A/dm2 (3.4 oz/gal) (6.7 oz/gal) (70-160ºF) (5.5 A/ft2) Potassium borate may be used in place of potassium citrate. Tarnish resistance of deposits obtained from these processes is inferior to that of deposits produced from cyanide-based electrolytes unless they are dipped in dilute sulfuric acid immediately after plating is complete. HIGH-SPEED SELECTIVE PLATING Electronic components, such as leadframes, are usually plated with silver at high speed using selective plating methods. Silicon chips can be attached to the silver deposit using conductive, epoxy-based adhesives, and gold or aluminum wires are bonded to the silver by employing ultrasonic or thermosonic wirebonding techniques. The resultant assembly is referred to as an IC package (integrated circuit package). Silver thickness ranges from 1.875 μm (0.000075 in.) to 5.0 μm (0.000200 in.); deposition times are typically between 1 and 4 seconds. The small areas to be plated demand the use of insoluble anodes. Platinumclad niobium mesh and platinum wire are examples of anode materials in common use. Traditional cyanide-silver electrolytes suffer rapid degradation under these conditions, oxidation and polymerization of the cyanide at the inert anodes being the principal cause. Special solutions were developed to overcome this situation; these contain essentially no free cyanide but still depend on potassium silver cyanide as the source of silver. A typical formula is as follows: Silver as KAg(CN)2 Conducting/buffering salts pH Temperature Current density Agitation Anodes 40-75 g/L 60-120 g/L 8.0-9.5 60-70ºC 30-380 A/dm2 Jet plating Pt or Pt/Nb (5-10 oz/gal) (8-16 oz/gal) (140-160ºF) (300-3,500 A/ft2) Conducting salts can be orthophosphates, which are self-buffering, or nitrates, which require additional buffering from borates or similar compounds. Buffering is important in these solutions since there is a significant drop in pH at the inert anode during plating due to destruction of hydroxide ions. Insoluble silver cyanide forms on the anode surface as a result of cyanide depletion in this locally low pH. Plating current drops off rapidly due to polarization. The following equations summarize the reactions involved. (Compare with comments in Noncyanide Processes above where a similar breakdown of the complex occurs.) 4OH— Ag(CN)— 2H2O + O2 + 4e— AgCN + CN— Grain refiners are added to control deposit grain size. Because of the very high rate of deposition, little grain refining occurs without these additives. The type and concentration of grain refiner chosen depends on the desired current density and deposition speed. A typical grain refiner would be a Group VIb element, such as selenium, it would become more effective as current density increases. In other 315

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