Microstructural investigation on the interfacial evolution of SnBi/Cu interconnect during reflow and solid-state aging

The microstructures of SnBi/Cu interconnect after reflow and its evolution during solid-state aging at 393K were investigated using transmission electron microscopy (TEM). It was found that after reflow there were two kinds of intermetallic compounds (IMCs) - Cu
6
Sn
⁵
and Cu
3
Sn in solder joint. Above these IMC layers there was a Bi-rich layer which consisted of discontinuous Bi particles. During solid state aging, the Cu
3
Sn phase grew faster into Cu substrate than into Cu
6
Sn
5
, and the Bi-rich layer did not change much. However, new Bi particles were observed at the Cu
3
Sn/Cu interface, which introduced interfacial voids around Bi particles. First principles calculation revealed that Bi segregation reduced the adhesion energy of Cu/Cu
3
Sn interface and caused serious reliability problem.