Damage Mechanism of Cu(6)Sn(5) Intermetallics Due to Cyclic Polymorphic Transitions

The formation of high-melting-point Cu(6)Sn(5) 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, Cu(6)Sn(5) interconnections will experience cyclic polymorphic transitions in the solid state. The repeated and abrupt changes in the Cu(6)Sn(5) lattice due to the cyclic polymorphic transitions can cause extreme strain oscillations, producing damage at the surface and in the interior of the Cu(6)Sn(5) matrix. Moreover, because of the polymorphic transition-induced grain splitting and superstructure phase formation, the reliability of Cu(6)Sn(5) interconnections will thus face great challenges in 3D packages. In addition, the Cu(6)Sn(5) polymorphic transition is structure-dependent, and the η′↔η polymorphic transition will occur at the surface while the η′↔η(s)↔η polymorphic transition will occur in the deep matrix. This study can provide in-depth understanding of the structural evolution and damage mechanism of Cu(6)Sn(5) interconnections in real 3D package manufacturing.