Wetting of Cu and Cu-Sn IMCs by Sn-Bi Alloys over Wide Composition at 350°C

Wetting of Cu and Sn-Cu intermetallics (IMCs) by molten Sn-Bi alloys with different interfacial reactivity was studied by the sessile drop method. The pre-formed IMC substrate has a better wettability, also it can be a method to control the thickness of the IMC layer as well as preventing Bi segregation, which is a potential way for optimizing mechanical property of joints. The evolution of interfacial structures, both of Sn-Bi/Cu and Sn-Bi/IMCs, can be verified from a Sn-Bi-Cu ternary phase diagram, and Bi segregation at an interface of Sn-Bi/Cu results from the saturated precipitation from solid solution rather than diffusion from liquid. The reaction of Sn with Cu2O (oxide film), both in a Sn/Cu system and a Sn-Bi/IMCs system, is the key for the occurrence of spreading, and the difference of reactivity caused the different spreading characteristics.