Interfacial reaction and mechanical properties for Cu/Sn/Ag system low temperature transient liquid phase bonding

Low temperature transient liquid phase (LTTLP) bonding is a promising technology to enable in high temperature electronic packaging. In this study, interfacial reaction and mechanical characterizations for Cu/Sn/Ag system LTTLP bonding at temperatures ranging from 260 to 340 °C for various time were investigated. Experimental results showed that Cu and Ag substrate independently reacted with molten Sn, and the growth of IMCs on one side was hindered by the opposite IMCs layer after scalloped Cu6Sn5 contacted with the Ag3Sn, and there was no ternary alloy phase formed all the time. Pores were found and distributed at the Cu6Sn5/Ag3Sn interface or between grain boundaries after the residual Sn was fully consumed, however, they gradually disappeared with continuing reaction of that Cu6Sn5 phase converted into Cu3Sn phase. Shear strength of the LTTLP joints increased with increasing bonding time, and the adhesive strength of Cu6Sn5/Ag3Sn interface was weaker than that of the Cu3Sn/Ag3Sn interface. The rupture behaviors were also discussed with a fracture model. As follow, cracks initiated in the pore and mainly propagated along the Cu6Sn5/Ag3Sn interface for the joint consisted of layered Cu3Sn, Cu6Sn5 and Ag3Sn IMCs, however, failure path only passed through the Cu3Sn layer after Cu6Sn5 islands were completely transformed.