Nucleation and growth of Cu6Sn5 during the aging process of Cu/Sn interface in electronic packaging-by in-situ TEM

The rapid nucleation of Cu6Sn5 during the soldering process, as well as its growth at the solid-state interface system (Cu/Sn) during chip service, present significant challenges. To study this, we introduced a small amount of Co element into the Cu-Sn system in order to reduce the formation energy of Cu6Sn5. Through transmission electron microscopy, we performed in situ observations of the nucleation and growth of Cu6Sn5 at the Cu/Sn solid phase interface. Our experimental findings confirm that the diffusion of Cu element at the interface plays a dominant role in the nucleation and growth process of Cu6Sn5. During this diffusion process, once the local concentration of Cu surpasses a threshold, Cu6Sn5 begins to nucleate around the Cu-rich region. The driving force behind this phase transition arises from two factors: non-uniformity of atomic flux at grain boundaries leading to stress accumulation, and localized bonding between a pair of valence electrons on Cu and adjacent Sn atoms, resulting in the formation of a shared orbital. Based on the curvature of the nucleation solid interface, three distinct growth modes have been observed: spiral growth, layer-layer growth, and island growth. These variations stem from changes in the dynamic mechanism associated with the morphology coefficient linked to the interface arc length. The nucleation and growth of Cu6Sn5 are governed by anisotropic attachment kinetics, with the growth process transitioning to diffusion-controlled continuous growth.