Decoding the morphological and crystallographic dynamics of Cu₆Sn₅ in solder interconnects under electromigration

Electromigration (EM) has become the primary failure mode of solder joints due to the miniaturisation of electronic devices. In this study, the EM-induced microstructural evolution of Cu₆Sn₅ intermetallic compounds was investigated by combining experimental characterisation and crystal plasticity finite element simulation. It is revealed that the asymmetric growth (anode) and dissolution (cathode) of interfacial Cu₆Sn₅, the directional evolution of primary Cu₆Sn_5, and the selective formation of bulk Cu₆Sn₅ are driven by directional Cu fluxes, governed by current direction and [001]Sn orientation. Bulk Cu₆Sn₅ appears either flat or protuberant relative to the cross-section. Unlike the randomly oriented protuberant Cu₆Sn₅, flat Cu₆Sn₅ exhibits strong textures and three reproducible orientation relationships (ORs) with βSn, two of which show higher lattice mismatches and lower frequencies of occurrence and were revealed for the first time in this study. The reproducible ORs, rather than [0001]Cu₆Sn₅, are the dominant factors controlling the formation of flat Cu₆Sn₅. However, this influence of ORs diminishes as the angle between [0001]Cu₆Sn₅ and the cross-sectional plane increases. The growth of bulk Cu₆Sn₅ introduces localised stress concentrations, evidenced by the accumulation of geometrically necessary dislocations (GND) at phase boundaries. In particular, the protuberant Cu₆Sn₅ increases the risk of short circuits, thus hindering further miniaturisation of electronic components. This work advances the understanding of the EM failure mechanisms associated with Cu₆Sn₅ evolution and contributes to establishing a crystallography-morphology-reliability framework, which is essential for the design of EM-resistant solder joints in hyper-miniaturised electronics.