<p>Reflow voids increasingly affect the electromigration (EM) reliability of solder joints, particularly as solder joints continue to downsize. This study investigates the effects of reflow voids on failure modes and intermetallic compound (IMC) precipitation in Sn-3.0Ag-0.5Cu ball grid array (BGA) solder joints, using EM tests, finite element simulations, and electron backscatter diffraction (EBSD). In void-free cathodes, EM-induced failure manifests as typical jagged under bump metallization (UBM) dissolution accompanied by microcrack propagation. The presence of a cathode void redistributes the current density, creating a high-current-density region away from the electron entrance. This effect, combined with rapid atomic diffusion along the <i>c</i>‑axis of <i>β</i>-Sn, accelerates localized Cu dissolution and migration. Meanwhile, the void physically blocks Cu transport towards the anode, causing local Cu₆Sn₅ precipitation near the void. By contrast, anode voids primarily alter the local current density distribution at the anode side and produce only limited effects in the observed cathode microstructural degradation patterns. During EM, microcrack propagation at the cathode further intensifies current crowding and localized dissolution, creating a positive feedback loop. To enhance EM resistance, it is essential to minimize the void fraction and, in particular, avoid reflow voids at the cathode interface.</p>