Effects of plating parameters on the Au-Sn co-electrodepositon for flip chip-LED bumps

The Au-Sn alloy was successfully pulse electrodeposited in a new non-cyanide and non-toxic stable electroplating bath which basically was composed of Au compound and tin sulfate (SnSO₄). The composition and morphology of electrodeposited Au-Sn alloy were investigated in terms of current density, temperature and pH value in the developed bath. It was possible to control tin content in the electrodeposits by means of current density. Au-Sn alloy with Sn concentration of about 20 to 45 at.% was deposited at peak current density ranging from 3 mA/cm² to 45 mA/cm². Although the microstructure and surface morphology of the electrodeposits were influenced by the peak current density, dense and smooth Au-Sn films with fine grain had been electroplated at different current density. With increasing pH value, the tin content in the alloy was obviously increased from about 10 to 45 at.%. When the pH value was greater than 8.5, the deposits became porous and incompact mainly owing to the hydrogen evolution reaction during plating. With increasing temperature, the tin content was slightly changed, however, the grain size became bigger and the films had a tendency towards coarser because of the rapid consumption of metallic ions. Therefore, it can be concluded that the optimum condition of pH value for electroplating of Au-Sn alloys was ranging from 7.5 to 8 and that of temperature was 35 to 45°C.