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.