Composition Control of the Eutectic Sn-Based Alloys: Sn-Ag, Sn-Cu, Sn-Ag-Cu, From Simple Plating Baths

The effect of citric acid (CA) as the sole complex agent on the electroplating behavior of binary Sn-Ag, Sn-Cu and tenary Sn-Ag-Cu deposits from a typical bath containing 0.15 M Sn2+, 10 mM Ag+ and 2.5 mM Cu2+ at pH of 3.5 was systematically compared by means of the linear sweep voltammetric (LSV) and voltammetric stripping (VS) analyses. Based on the LSV and VS studies, the composition of these deposits can be precisely controlled through adjusting the concentration of the nobler metallic ions (i.e., [Ag+] or [Cu2+]) in the basic plating bath containing 0.15 M Sn2+ and 0.3 M citric acid (CA). The direct-current (DC) plating from this citrate-based bath provides a reliable route to obtain the nearly eutectic Sn-3.4Ag, Sn-0.7Cu, and Sn-2.9Ag-0.7Cu alloys for lead-free solder bumps. These binary and ternary Sn-based deposits were characterized by scanning electron microscopic (SEM) and electron probe X-ray microanalytic (EPMA), and X-ray diffraction (XRD) analyses.