Near-eutectic Sn-Ag-Cu solder bumps formation for flip-chip interconnection by electrodeposition

As one of the most promising lead-free solder candidates for electronics interconnection, eutectic Sn-Ag-Cu alloys are receiving increasing interest due to their merits of low melting temperature, solderability, and reliability. Meanwhile, solder bumping on wafers is of great significance for flip chip interconnection technology which requires fine-pitch, high density bonding for the demands of future interconnection. In the current study, a process of electrodepositing near-eutectic Sn-Ag-Cu alloys was investigated. The electrochemical characteristics of the electrolyte developed were initially measured by cathodic potentiodynamic polarisation. The compositions of electrodeposits were revealed by wave-length dispersive microscopy (WDS), and the surface morphologies of deposits were characterised through scanning electron microscopy (SEM) and atomic force microscopy (AFM). X-ray diffraction (XRD) results indicated that β-Sn, Ag3Sn and Cu6Sn5 were present in the as-electroplated film. In addition, dual-beam focused ion beam (FIB/SEM) and transmission electron microscopy (TEM) further elaborated the microstructure of deposits at a nano-scale. As a result, near-eutectic Sn-Ag-Cu deposits with a fine-grained smooth surface were achieved, and the proposed bath chemistry proved capable of producing fine pitch near-eutectic Sn-Ag-Cu solder bumps through demonstration on a test wafer.