Cyanide-free and controllable AuSn eutectic electroplating using MPS and TGA: Synergistic coordination and co-deposition mechanism

MPS-TGA synergy enables stable, controllable cyanide-free Au Sn co-deposition. • MPS selectively coordinates with Au + , while TGA coordinates with both Au + and Sn 2+ . • Au Sn co-deposition follows an irreversible, diffusion-controlled mechanism. • Baths remain stable for six months, consistently yielding reproducible Au Sn coatings. • Au Sn eutectic coating is mirror-bright, flat ( R a  < 30 nm) and homogeneous after reflow. To replace toxic cyanide-based and unstable complex systems, a cyanide-free Au Sn electroplating bath using sodium 3-mercapto-1-propanesulfonate (MPS) and thioglycolic acid (TGA) as coordinating ligands, and the underlying co-deposition mechanism is systematically elucidated. The bath enables green processing, high bath stability (stable for six months) and superior coating quality. Electrochemical analyses reveal that MPS selectively coordinates with Au + , whereas TGA coordinates with both Au + and Sn 2+ , forming stable Au-MPS-TGA and Sn-TGA complexes. This synergistic coordination negatively shifts the deposition onset potentials of Au and Sn to −0.99 V and − 1.09 V (vs. SCE), respectively, narrowing their gap to 0.1 V and establishing the Au Sn co-deposition onset at −1.03 V (vs. SCE). Optimal concentration ratios are c (MPS): c (Au) = 8:1–12:1, c (SnCl 2 ): c (Au) = 3.0:1–4.0:1 and c (TGA): c (Au) = 8:1–10:1, under which Sn deposition is accelerated and the operating current density range is broadened to 2–6 ASD, yielding controllable and reproducible coating compositions (16.7–50.0 at.%Sn). The Au Sn co-deposition follows an irreversible, diffusion-controlled mechanism with a low activation energy ( E a ) of 9.50–27.36 kJ/mol, driven by sufficient overpotential to overcome the initial energy barrier for complex dissociation, thereby promoting fine-grained nucleation and growth. Consequently, the mirror-bright, flat Au Sn eutectic coating electroplated at 3 A/dm 2 (ASD) exhibits dense, uniform and residue-free deposits, which transform into a homogeneous eutectic microstructure after reflow. Download: Download high-res image (499KB) Download: Download full-size image