The paper reports a new autocatalytic electroless gold plating bath that achieves almost neutral, low temperature bath conditions and excellent process stability without cyanide. The bath contains a thiosulfatesulfite mixed complex of Au (I) as the gold source and two reducing agents, thiourea and hydroquinone. The thiourea acts as the primary reductant for the gold complex, while the hydroquinone recycles thiourea by reducing the intermediate thiyl radical derived from thiourea oxidation. Therefore hydroquinone functions as a thiourea regenerator. This unique cooperative reductant system offers superb bath stability. The bath has been applied not only to the most advanced mainframe MLC boards but also to a variety of very fine pitch organic PWBs manufacturing process.
