LAUSR

Abstract The rapid accumulation of the Cu 6 Sn 5 phase at the anode is one of the major electromigration-induced phenomena characterizing solder interconnections; however, the outcome of the growth mode has always been conflated with that of thermomigration. In this work, after the effects of the non-uniform thermal distribution of the Cu|SnAg 3.0 Cu 0.5 |Cu lap-type joints are decoupled from the influence of the current stress, the microstructural evolution of the anodic Cu 6 Sn 5 grains is studied under an average current density of 7.12 × 10 7  A m −2 for 0–300 h. The results show that, due to the anisotropy of the Cu 6 Sn 5 in the absorption factor and the action of the electron wind, the [0001] directions of the Cu 6 Sn 5 grains at the anodic Sn|Cu 6 Sn 5 interface gradually reorient toward the current density vectors with the stress time, and certain original Cu 6 Sn 5 grains (termed normal grains) with unfavorable surface orientations are replaced by newly generated grains (termed abnormal grains) with favorable surface orientations. Consequently, the growth mode of the anodic Cu 6 Sn 5 grains is not invariable, and the corresponding transition process is conjectured to transition from the reaction-controlled mode to the diffusion-controlled mode in three stages. Finally, the anodic Cu 6 Sn 5 grains, both normal and abnormal, grow to assume elongated rod-type shapes and may further form a reliable interconnection layer to improve the joint reliability.