Pulse electrodeposition of Sn-Bi low-temperature solder with phloroglucinol

Advanced semiconductor packaging technology requires higher integration and more precise alignment of multiple chips. Vertical chip stacking improves semiconductor package performance without increasing the chip footprint. Thermocompression (TC) bonding with microbumps consisting of Cu pillars and solder serves as a key process to achieve these goals. However, the relatively high bonding temperature of conventional Sn-Ag solder limits further reduction in interconnection pitch and exacerbates misalignment issues due to wafer warpage. Therefore, low-melting-temperature solder materials offer a solution for achieving denser interconnections in chip bonding. This study investigates the electrodeposition of Sn-Bi as an alternative to Sn-Ag due to its lower melting temperature. The significant difference in the reduction potentials of Sn and Bi causes severe dendritic growth in DC electrodeposition. To address this issue, this study adopts pulse electrodeposition and examines the effect of pulse parameters on Sn-Bi film formation. In addition, organic additives for Sn-Bi electrodeposition are explored to further enhance surface quality. The combination of optimized pulse electrodeposition and phloroglucinol additive results in the formation of smooth and uniform Sn-Bi films. By precisely adjusting the Bi3+ concentration, this study achieves a smooth Sn-61Bi film near the eutectic composition, exhibiting a melting temperature of 139 degrees C, 82 degrees C lower than conventional Sn-Ag solder. The developed Sn-Bi electrodeposition process enables low-temperature chip bonding, contributing to the advancement of high-density semiconductor packaging.