Anisotropic phase transformation of Cu₆Sn₅ driven by electromigration in lead-free solder joints

Electromigration (EM) effects on intermetallic compound evolution remain incompletely understood despite their critical role in solder joint reliability. Through correlated transmission electron microscopy (TEM) and ab initio molecular dynamics (AIMD) simulations of Sn-3.0Ag-0.5Cu solder joints under current stressing, we reveal for the first time that EM imposes crystallographic selection during η-to-η' transformation in Cu₆Sn₅—suppressing η' variants via electron-wind-aligned atomic migration while accelerating η' growth. Atomic-scale simulations establish that EM redirects this phase transformation via kinetically impeding atomic shuffling, as indicated by the elevated interfacial shear strain. This newly identified current-steered phase transformation represents a paradigm shift in understanding EM-induced damage—demonstrating that electric fields not only accelerate but also crystallographically constrain solid-state transformations in Cu₆Sn₅ intermetallic. The resulting anisotropic η'-Cu₆Sn₅ microstructures may concentrate degradation pathways, highlighting critical implications for reliability in high-current-density microelectronics where texture-dominated failure may emerge.