Stable non-cyanide electroless gold plating which is applicable to manufacturing of fine pattern printed wiring boards

Highly stable non-cyanide electroless gold plating baths have been developed, which are applicable to manufacturing of fine pitch printed wiring board. Monovalent gold ion (gold source) is stabilized by thiosulfate and sulfite ligands instead of conventional toxic cyanide ligand. The bath stability of the basic formulation which uses thiourea as a reducing agent has been exceedingly enhanced by introducing the new concept of reducing agent regenerator that recycles oxidized products from the reducing agent back to original thiourea again. The finally improved bath offers good quality soft gold deposit by safe, neutral, low temperature and very stable plating condition.