Acceleration effect of Na₂S₂O₃ on the immersion gold plating on Ni-P surface from a sulfite based electrolyte

Gold electroless plating is widely used in printed circuit board (PCB) manufacturing due its many advantages, however the use of cyanide is an undesirable feature in this method. In this study, the influence of Na₂S₂O₃ on electroless gold plating from sulfite bath was investigated. X-ray fluorescence and electrochemical experiments showed that the thickness of gold film increased faster at higher Na₂S₂O₃ concentrations, in the range of 0 to 8 mM. With the addition of Na₂S₂O₃, electrochemical experiments showed that nickel oxidation and gold reduction were significantly accelerated. The element sulfur was observed in gold deposition film by Energy Dispersive Spectrometer (EDS) and element linear-scanning examination, concentrating on Ni and gold interface. Understanding the influence of Na₂S₂O₃ in this process will be helpful for PCB manufacturers to avoid the sulfur issues and make high quality finished PCBs.