Interfacial intermetallic phases and nanoeutectic in rapidly quenched Sn-Ag-Cu on Au under bump metallization

The microstructure of Sn96.4Ag2.8Cu0.8 (at.%) solder alloy when rapidly quenched onto an Au surface finish has been found to range from nanometer to micrometer-sized features. This striking microstructure contained a spectrum of phases from the Au-Sn phase diagram, while Ag and Cu did not appear to participate in the solidification process. Within the solder contact, a eutectic with 40 nm interlamellar spacing was observed, along with the presence of layered intermetallics. Observations of these phases and their orientation offer better understanding of the solidification of Pb-free solders onto Au.