Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 245 of 707

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 tech- niques. 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. Platinum-clad niobium mesh and platinum wire are examples of anode materials in common use. Traditional cyanide-silver electrolytes suffer rapid degradation under these con- ditions, 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 Temperature Current density Agitation Anodes Conducting/buffering salts 60-120 g/L pH 40-75 g/L 8.0-9.5 60-70° C 30-380 A/dm2 Jet plating Pt or Pt/Nb (5-10 oz/gal) (8-16 oz/gal) (140-160° F) (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 local- ly low pH. Plating current drops off rapidly due to polarization. The following equa- tions 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 densi- ty 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 oth- er words, the concentration of grain refiner that gives a smooth, satin finish at 100 A/dm2 will produce full bright deposits under similar conditions at 200 A/dm2 Pre- and Posttreatments Since silver is being plated selectively the electrolyte is exposed to cleaned and acti- vated substrate surfaces (typically copper-rich alloys or nickel-iron alloys) that are prone to forming immersion silver deposits. Any silver detected outside the pack- age profile (i.e., on the outer leads) is cause for rejection of the leadframe due to fears that silver outside the package will migrate across the dielectric and cause short circuits. Hence, all traces of silver must be eliminated outside of the selec- tively plated spot itself. A typical process flow is degrease and rinse, electroclean and rinse, acid dip and rinse, anti-immersion predip, selective silver plate, silver 244 .

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