Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 368 of 707

many conventional waste treatment procedures. ELECTROLESS GOLD There is a growing need in the electronics industry for selective plating to con- serve plating costs and to allow the electronics engineer freedom for circuit design improvement. Many electronic components today are difficult to gold plate by electrolytic means. Thus, electroless gold is currently being used in the fabrication of semi- conductor devices, connector tabs, chips, and other metallized ceramics. Most commercially available electroless gold deposits are produced first by plating a thin deposit of immersion gold, followed by electroless gold plating. There are a few true autocatalytic gold processes available with 99.99% purity. Table VIII gives an example of an electroless gold bath. Electroless gold can successfully be applied to Kovar, nickel, nickel alloys, elec- troless nickel, copper, copper alloys, electroless copper, and metallized ceramics. Electroless gold can be deposited onto already present thin electrodeposited gold to give added strength. ELECTROLESS PALLADIUM Electroless palladium deposits are ductile and ideal for contacts undergoing flexing (i.e., printed circuit board end connectors and electronic switch con- tacts). The deposit has also been used as a less expensive replacement for gold, pro- viding tarnish resistance and solderability. Electroless palladium has been used to replace rhodium for wear applications. Using specific bath components, the deposit can be hard and bond to elec- troless nickel with a bond strength greater than the tensile strength of the pal- ladium plate itself. Metals such as stainless steel and nickel can be plated direct- ly. Copper, brass, and other copper alloys require an electroless nickel preplate. The electroless nickel preplate can be either from a hypophosphite- or boron- reduced bath. Table IX gives an example of an electroless palladium (hypophosphite-reduced) bath. ELECTROLESS COBALT Thin electroless cobalt deposits have use in the electronics industry on magnetic memory discs and storage devices primarily for their magnetic properties. Table X gives an example of an electroless cobalt bath. COMPOSITES AND POLYALLOYS The uniform dispersion of micron or submicron particles in an electroless com- posite deposit will enhance the lubricity and the wear and/or abrasion resistance over base substrates and conventional electroless deposits. Composites containing fluorinated carbon (CFx ), fluoropolymers (PTFE), natural and synthetic (polycrystalline) diamonds, ceramics, chromium carbide, silicon carbide, and aluminum oxide have been codeposited. Most commercial deposition occurs with an acid electroless nickel bath owing to the unique physical characteristics available to the final code- posit. The reducing agent used may be either a hypophosphite or boron complex. For Lamellar solids, starting materials are naturally occurring elemental forms like coke or graphite. Fluorinated carbon (CFx ) is produced by reacting coke with 367

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