Formation of Sn-Bi alloys through sequential electrodeposition

The fabrication of near eutectic Sn-Bi alloy can be developed by reflowing a metal stack containing layers of Sn and Bi films. The Sn and Bi metal films are sequentially electroplated using two separate plating baths. Composition control of Sn-Bi alloy can be achieved by manipulating the thickness of constituent metal films. The thickness ratio of Sn to Bi is targeted to be approximately 0.97 to obtain the eutectic composition. Hence, the effect of thickness as a function of time is investigated. The thickness of electroplated layers increased with plating time but a more uniform thickness can be obtained from shorter plating periods. Methane sulphonate acid (MSA) based baths were fabricated for both Sn and Bi plating and the current density used is 20 mA/cm² and 5 mA/cm² respectively. Hydroquione (HQ) and gelatin were added into Sn plating solution bath as additives to improve the bath stability. The dual layer metal stack was reflowed at 170°C to enhance the interdiffusion between Sn and Bi layers. Field Emission Scanning Electron Microscope (FESEM) analyses coupled with Energy Disperse X-ray (EDX) analyses test have been performed to investigate the microstructure and composition of Sn-Bi alloy. Homogeneous microstructure of Sn-Bi alloy is obtained after reflow and near eutectic composition Sn- 54.6 wt.%Bi is achieved.