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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.

Damage Mechanism of Cu6Sn5 Intermetallics Due to Cyclic Polymorphic Transitions