Metal Finishing Guide Book

2012-2013

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Table VII. Formaldehyde-Reduced Electroless Copper Bath Copper salt as Cu2+ 1.8 g/L Rochelle salt 25 g/L Formaldehyde as HCHO 10 g/L Sodium hydroxide 2-Mercaptobenzothiazole (MBT) pH Temperature 5 g/L < 2 g/L 12.0 25��C (77��F) After post-heat-treatment at 399��C (750��F) for 1 hour, values of 1,200 VHN can be produced. The melting point of borohydride-reduced deposits is 1,080��C (1,975��F). Table V gives an example of a sodium borohydride-reduced electroless nickel bath. Acid nickel-boron varies from 0.1 to 4% boron by weight depending on the bath formulation. The boron content of electroless nickel is reduced by DMAB. Bath parameters include a pH of 4.8-7.5, with an operating temperature range of 6577��C (149-171��F). DMAB-reduced deposits have a very high melting temperature of 1,350��C (2,460��F). Baths containing less than 1% boron have excellent solderability, brazing, and good ultrasonic (wire) bonding characteristics. A typical DMAB-reduced bath is given in Table VI. ELECTROLESS COPPER Electroless copper deposits are generally applied before electroplating on plastics and other nonconductors, providing a conductive base for subsequent plating. These include acrylonitrile butadiene styrene (ABS), polystyrene, modified polyphenylene oxide, polyvinyl chloride (PVC), Noryl, polyethylene, polysulfone, structural foam, epoxy, and ceramics. In such applications, usually a thin deposit (0.127 ��m; 0.05 mil) is applied, followed by an additional decorative or protective thickness of copper, nickel, or gold deposited electrolytically or electrolessly. The electroless copper in such applications provides good life in corrosive atmospheric and/or environmental exposures. Automotive, appliance, printed wiring boards, molded interconnect devices, plastic composite connectors, multichip modules, and EMI/RFI shielding of other electronic devices represent major markets for electroless copper. In through-hole plating of printed wiring boards, the use of electroless copper has eliminated the need for an electrodeposited flash and provides excellent electrical conductivity in these hard-to-reach areas. In the pretreatment of circuit boards, the most common method involves an acidic aqueous solution of stannous chloride (SnCl2) and palladium chloride Table VIII. Electroless Gold Bath Gold hydrochloride trihydrate 0.01 M Sodium potassium tartrate 0.014 M Dimethylamine borane Sodium cyanide pH (adjusted with NaOH) Temperature 460 0.013 M 400.0 mg/L 13.0 60��C (140��F)

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