Abstract
Based on the experimental strategies including fractional factorial design (FFD), path of the steepest decent (PSD), and central composite design (CCD) coupled with response surface methodology, the composition of lead-free Sn-Bi deposits plated under dc mode can be precisely controlled and predicted. As a result, deposits of the eutectic point, 42Sn-58Bi, as well as tin-rich deposits (80Sn-20Bi and 90Sn-10Bi in wt %) for lead-free solders can be obtained, although the Sn-Bi chloride-based solutions were recognized as a complicated electroplating system. In the FFD study, pH, current density, and citric acid concentration were identified to be the key factors affecting the composition of Sn-Bi deposits. The effects of current density and citric acid concentration on the composition of Sn-Bi deposits were examined in the PSD and CCD studies to construct a reliable model for composition control. A robust electroplating setting derived from this model is applicable to other solutions with variable compositions. A linear relationship between Sn (Sn+Bi) in the deposit and [Sn4+] [Sn4+ + Bi3+] in the electroplating bath is obtained under the robust setting. From the morphologies and crystalline information, all binary Sn-Bi deposits prepared in this work belong to heterogeneous alloys. © 2008 The Electrochemical Society.