Abstract
Pulse plating of Sn-Bi alloys was performed in a simple binary Sn 2+ -Bi 3+ bath containing citric acid (CA), ethylenediaminetetraacetic acid (EDTA), and poly- ethylene glycol (PEG400) at pH = 1 under 70 of duty percentage and 10 Hz of pulse frequency on polyimide-micropatterned CuNi substrates (denoted as mPI-CN). From the chronopotentiometric and scanning electron microscopic (SEM) studies, the edge effect of mass transport for Bi 3+ resulted in the composition variation between edge and central regions. The increase in the Sn content of Sn-Bi deposits reduces the dendrite formation in the edge region. Through adjusting the concentration of Bi 3+ ions and the current density of pulse deposition, uniform morphologies and compositions of Sn-Bi deposits could be obtained. From the demonstration shown in this work, the pulse plating method provided a reliable route to obtain uniform Sn-Bi alloys of high Sn contents (e.g., 80Sn-20Bi and 90Sn-10Bi) for the micro-sized lead-free solder bumping. © 2011 The Electrochemical Society.