Sn-P Alloys for Rapid and Stable Production of Tin Solution for Tin Plating

For fast preparation of tin solution for electrodeposition, the dissolution characteristics controlled by oxygen reduction were investigated for Sn metal and Sn-P alloys containing cathodic inclusions using electrochemical measurement in p- phenolsulfonic acid. The dissolution rate of tin increased with phosphorus content of the alloys due to faster oxygen reduction and showed the maximum at about 1 at % phosphorus. The cathodic inclusions were dispersed in the Sn matrix by water quenching of molten mixture of Sn and Sn-50at%P, and were identified as the Sn4P3 phase by X-ray diffraction. The Sn4P3 compound has the higher activity for oxygen reduction than Sn metal and platinized platinum. The mechanism for the cathodic reduction of dissolved oxygen on Sn metal and Sn-P alloys was investigated in p-phenolsulfonic acid. Increasing phosphorus content of Sn-P alloys resulted in an increase in the limiting oxygen reduction current density which was proportional to the partial pressure of oxygen, but independent of pH. It was concluded that the rate-determining process of oxygen reduction was adsorption of oxygen molecule on the Sn metal and Sn-P alloys.