Damage Mechanism of Cu₆Sn₅ Intermetallics Due to Cyclic Polymorphic Transitions

The formation of high-melting-point Cu₆Sn₅ interconnections is crucial to overcome the collapse of Sn-based micro-bumps and to produce reliable intermetallic interconnections in three-dimensional (3D) packages. However, because of multiple reflows in 3D package manufacturing, Cu₆Sn₅ interconnections will experience cyclic polymorphic transitions in the solid state. The repeated and abrupt changes in the Cu₆Sn₅ lattice due to the cyclic polymorphic transitions can cause extreme strain oscillations, producing damage at the surface and in the interior of the Cu₆Sn₅ matrix. Moreover, because of the polymorphic transition-induced grain splitting and superstructure phase formation, the reliability of Cu₆Sn₅ interconnections will thus face great challenges in 3D packages. In addition, the Cu₆Sn₅ polymorphic transition is structure-dependent, and the η'↔η polymorphic transition will occur at the surface while the η'↔η↔η polymorphic transition will occur in the deep matrix. This study can provide in-depth understanding of the structural evolution and damage mechanism of Cu₆Sn₅ interconnections in real 3D package manufacturing.