Electroplating Approaches for Low-Melting Point In–Sn Alloys

In pursuit of solder alloys enabling reflow at significantly lower peak temperatures than those required for the widely used SnAgCu solder variants, the most viable options are expected to be based on combinations of Sn, In, and Bi. For fine-pitch connections, electroplating emerges as the most practical fabrication method. Various approaches have been explored, including the codeposition of In, Sn, and Bi, as well as sequential plating of these metals in different combinations followed by reflow. The present work introduces two different electrochemical methods developed and optimized for the synthesis of In–Sn alloys in aqueous media. The first method involves sequential layer-by-layer deposition of plain Sn and In layers, followed by a reflow. In the second approach, an electroplating technique for codepositing In–Sn alloys was developed by merely adjusting the pH of the plating bath containing SnSO₄ and In₂(SO₄)₃. Five different pH values ranging from 2.2 to 3.5 were applied while keeping other bath-composition and plating parameters constant. This led to the formation of In–Sn alloys with distinctly different, pH-dependent compositions and served as a foundation of the pursued pathway for In–Sn codeposition. Lastly, SEM, EDS, and DSC were used to examine the alloys’ morphologies and determine their compositions and homogeneities.