Composition control for lead-free alloy electroplating on flip chip bumping

In lead-free alloy electroplating for Sn-Ag, Sn-Cu or Sn-Ag-Cu solder bumping, composition control is an important subject since the composition ratio is influential to the alloy structures and properties of the solders. The general assumption is that it´s very difficult to control the compositions of plated lead-free alloys in plating production lines because the composition ratio of the alloy element Ag or Cu is small and sensitive to the plating conditions in Sn base eutectic alloys. Also, when comparing with Sn-Pb solder, the larger electric potential difference between the components of lead-free alloys will make the composition ratio of plated alloys unstable in electroplating. Good bump height uniformity is not the only requirement for the integration process. It is also important to attain well-controlled compositional uniformity of lead-free solder. Therefore, it is important to control the electric potential distribution, metal ion concentrations, and their concentration ratios in the plating solution near the plating surface. In bump plating production lines, there are also other factors influencing the plated solder compositions including plating rate, plating solution flow, anode, and substrate pattern conditions. This paper describes our study of the factors affecting the control of the compositions in the lead-free alloy plating processes. The composition control of lead-free solder bumping has been primarily experienced on 300mm wafers for flip chip or wafer level packaging. The results of this study show that an advanced electroplating process can be successfully applied to the fabrication of the lead-free solder bumps, and further, that it can be carried out with the present electroplating and reflow tools.