Interfacial embrittlement by bismuth segregation in copper/tin–bismuth Pb-free solder interconnect

Microchemistry and mechanical properties of a copper/tin-bismuth Pb-free solder interconnect were examined in the as-reflowed and aged conditions by in situ Auger fracture and interface fracture mechanics techniques. In the as-reflowed condition, the solder-copper interface was highly resistant to fracture, and the fracture mechanism was ductile with the crack path following the interface between the solder alloy and the copper-tin intermetallic phase. Upon thermal aging, bismuth segregation was found to occur on the copper-intermetallic interface. Auger depth profiling indicated that the segregation was confined to about one monolayer from the interface. The segregation was shown to embrittle the interface, resulting in an approximately 5-fold decrease in the interfacial fracture resistance.