Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 366 of 707

Table VII. Formaldehyde-Reduced Electroless Copper Bath Copper salt as Cu2+ Rochelle salt Formaldehyde as HCHO Sodium hydroxide 2-Mercaptobenzothiazole (MBT) pH Temperature After post-heat-treatment at 399° F). C (750° 1.8 g/L 25 g/L 10 g/L 5 g/L < 2 g/L 12.0 25° C (77° F) can be produced. The melting point of borohydride-reduced deposits is 1,080° (1,975° bath. F) for 1 hour, values of 1,200 VHN C Table V gives an example of a sodium borohydride-reduced electroless nickel 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 65- 77° C (149-171° of 1,350° C (2,460° derability, 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, polysul- fone, structural foam, epoxy, and ceramics. In such applications, usually a thin deposit (0.127 µm; 0.05 mil) is applied, followed by an additional decorative or protective thickness of copper, nickel, or gold deposited electrolytically or elec- trolessly. 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 Sodium potassium tartrate Dimethylamine borane Sodium cyanide pH (adjusted with NaOH) Temperature 0.01 M 0.014 M 0.013 M 400.0 mg/L 13.0 60° C (140° F) 365 F). DMAB-reduced deposits have a very high melting temperature F). Baths containing less than 1% boron have excellent sol-

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