Effects of stirring speed on composition and morphology of non-cyanide co-electroplating Au-Sn thin films

Au-30at.%Sn bumps play a very important role in high-power LED chip packaging. The Au-30at.%Sn bumps are usually fabricated by electron-beam evaporation and sputtering. These techniques are either costly or difficult in operation. Moreover, it is difficult to fabricate solder bumps with the continuously downsizing. Co-electroplating is a promising alternative. In the present work, Au-Sn films were co-electroplated using a non-cyanide solution. The effects of stirring speed on composition and morphology of the co-electroplating Au-Sn films and its mechanism were investigated and the mechanism was also discussed. It was found that when the stirring speed was higher than 300 rpm, few pits formed. With increasing stirring speed, in general the Sn content of the Au-Sn film decreased; when the stirring speed was between 300 rpm and 400 rpm, the Sn content of the Au-Sn film was in the range of 29.51at.%~30.9at.%, which is near the eutectic composition (Au-30at.%Sn). Based on the electrochemical testing and calculation, the diffusion coefficient of Sn was significantly smaller than that of Au, which resulted in the decreasing Sn content in the Au-Sn film with increasing stirring speed.