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44 The PCB Magazine • February 2016 by Jun Nable, Ph.D.; Emely Abel-Tatis; Ernest Long, Ph.D.; John Swanson; and Martin Bunce MACDERMID InC. Introduction Among the most utilized PCB surface finish- es is gold. Gold has been used in conjunction with other metals such as Ni and/or Pd under- neath to provide a solderable surface finish [1,2] . The metal undercoat is commonly applied via an autocatalytic electroless process while the gold top coat can be immersion deposited. Immersion gold plating has long been used in the electronics industry as a surface fin- ish because of its excellent solderability and its ability to resist corrosive substances. Most conventional immersion gold plating solu- tions utilize a cyanide containing gold salt as the gold precursor which poses environmental and health concerns. Besides the health and environmental concerns, most cyanide-con- taining baths are operated at temperatures of about 80°C or above. A cyanide-free gold plat- ing solution that does not consist of any harm- ful and toxic substances would be an attractive alternative. In this paper, we introduce an immersion gold plating process suitable for ENIG surface finishes that utilizes no cyanide-containing re- agent and operates at near neutral pH and 40°C. The performance is evaluated to check its suit- ability as a surface finish. Plating One of the unique features of the cyanide- free immersion gold chemistry is its ease of op- eration. The operating temperature for this gold bath is 40°C while still achieving a plating rate of 0.2 µin/min. The operating pH is close to neutral pH. The plating rate can be maintained consistently throughout the entire bath life with proper pH control. In all the plating shown below, the average electroless nickel thickness is 150 µin while the gold thickness was targeted at a minimum of 2 µin in 10 minutes of gold plating. The plating rate curve from 0–12 mto is presented in Figure 1. The rates are considerably steady throughout the entire bath life staying within the range of 0.2–0.24 µin/min. This plating rate is satisfac- tory for the processing of immersion gold for the ENIG finish which targets about 2 µin of gold within 10 minutes of processing time. Cyanide-Free Immersion Gold Suitable for PCB Surface Finishing feature

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