Synthesis of the combination solder of 80Au-20Sn/42Sn-58Bi and thermodynamic interpretation of the microstructural evolution

Abstract

A Pb-free combination solder structure was successfully attained from a high-melting 80Au-20Sn solder alloy completely wrapped in a low-melting 42Sn-58Bi solder paste (the numbers of which are all in mass% unless specified). The phases of (Au,Ni)(3)Sn-4 and AuSn4 were observed at the interface between Sn-Bi solder and Cu/Ni/Au UBM, whereas the (Au,Ni)(3)Sn-2 phase was observed at the interface between Au-Sn solder and Cu/Ni/Au UBM. The interfacial reaction between the Au-Sn solder and the Sn-Bi solder resulted in the formation of AuSn2 on the side of the Sn-Bi solder and AuSn on the side of the Au-Sn solder. All of these interfacial reactions were explained with relevant equilibrium phase diagrams. Moreover. we found that there is a limit in the optimum solder volume for a well-defined combination solder structure that can be applied to organic substrates with an increased remelting temperature.