Wednesday, March 18, 2009

Electroless Nickel/Immersion Gold

Nickel-gold finishes may cover an entire circuit or be selectively plated onto certain areas of a circuit. Nickel-gold formulations can produce hard gold, with the addition of cobalt or another metal being co-deposited in small amounts, or soft gold by utilizing pure gold.

Hard Gold. Hard gold is electrolytically plated. The most common application of hard gold is edge connectors, but hard gold may also be plated over circuit areas as well. Automated edge plating machines are common since manual plating is quite labor-intensive. Typically a plater's tape is applied to the board masking off all of the circuit above the edge connector. The panel is then processed through a nickel-gold plating line, with just the edge connectors immersed in the plating fluid. Nickel is plated first and Watts or sulfamate nickel is common. Cyanide gold is the most common gold electroplating chemistry.

Soft Electrolytic Gold. Soft gold is a pure gold coating over a nickel deposit. It may be electroplated over the entire circuit or selectively over certain portions of a circuit (excluding edge connectors, which require hard gold). Selective electroplating requires a combination of masking and bussing (providing current to the portion of the circuit being electroplated). Selective gold applications include contact points (which may require hard gold), press pads, wire bond sites, or portions of a board that may reside in a corrosive environment. Selective gold plating can be labor-intensive and is not frequently specified for production lots (all gold plating is often substituted; the labor savings offset the extra gold required).

Electroless Nickel/Immersion Gold. The electroless nickel/immersion gold process is another method of applying soft gold. Electroless plating can be conveniently performed after etching because no bussing is required. Therefore, these all-gold boards can be processed with a standard tin etch-resist and processed identically as SMOBC, except the gold plating step replaces the HASL step. This process has advantages over SMOBC/HASL and electrolytic gold plating. When compared to SMOBC/HASL, electroless all-gold circuits have a much longer shelf life. The flat surface profile of the electrolessly plated surface-mount pad and overall excellent solderability make electroless nickel/gold ideal for surface-mount technology. Cost, however, is an obvious disadvantage when compared to HASL. When compared to electrolytic gold, electroless has the advantage of full copper encapsulation because plating is performed after etching, not before, as with electrolytic gold plating. Selective gold plating is made somewhat easier by the electroless plating method since no electrical bussing is required. Cost is the main disadvantage. Immersion gold and electroless nickel process baths are short-lived compared to electrolytic formulations and maintenance and control of these baths is more difficult. Immersion gold plating is a self-limiting process which, for common baths, cannot produce thicknesses of much more than 10 micro-inches. The main application of electroless nickel-gold coatings is chip-on-board technology, where component leads are ultrasonically or thermosonically bonded to gold pads rather than soldered.

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