Au-Sn Co-electroplating solution for Flip Chip-LED Bumps

A new non-cyanide and non-toxic stable electroplating solution for co-deposition Au-Sn alloys was investigated in this study. A stable sulfite-based electroplating solution for gold and a stable solution for tin were prepared, respectively. L-ascorbic acid was added into the Au solution and a reducing agent was added into the Sn solution, and then the two solutions were mixed to generate Au-Sn solution. The effects of additives on solution stability at room temperature and high temperatures were studied. Polarization tests of Au, Sn and Au-Sn solutions were carried out to reveal the effect of additives on Au, Sn and Au-Sn plating. Then a series of electroplating tests at different current densities were performed in order to demonstrate plating capability of Au-Sn solutions. It was shown that the species of the main salt for gold affected the stability of solution prominently. EDTA could improve the stability of Au solution at high temperature and the complexing agent for Sn was also benefit to the stability of Sn solution. Two waves in Sn solutions with the complexing agent indicated two cathode reactions occurred during Sn plating. The reducing agent was added to prevent the oxidation of Sn (II), but it turned out to affect the polarization of Sn solutions and the compositions of Au-Sn alloys during co-electroplating. Electroplating experiment results showed that co-electroplating of uniform Au-Sn deposits was feasible.