Electrodeposition and characterisation of Sn-Ag-Cu solder alloys for flip-chip interconnection

A pyrophosphate and iodide based bath was investigated for the electrodeposition of near-eutectic Sn-Ag-Cu alloys, which are promising lead-free solder candidates for electronics interconnection. Near-eutectic Sn-Ag-Cu electrodeposits (2.5-4.2 wt.% Ag and 0.7-1.5 wt.% Cu) were achieved from the system as measured by wavelength dispersive X-ray spectroscopy (WDS). Electroplating such near-eutectic ternary alloys at higher deposition rates was possible with the application of electrolyte agitation. Different morphologies of deposited Sn-Ag-Cu films were analysed using scanning electron microscopy (SEM). X-ray diffraction (XRD) data indicated that Sn, Ag3Sn and Cu6Sn5 were present in the 'as-electrodeposited' Sn-Ag-Cu film. The microstructure of the deposits and the morphology of Ag3Sn and Cu6Sn5 intermetallics were characterised from cross-sectional images produced from a focused ion beam scanning electron microscopy and then imaged from transmission electron microscopy (TEM) micrographs. The proposed bath proved capable of producing fine pitch near-eutectic Sn-Ag-Cu solder bumps as demonstrated on a glass test wafer.