Electromigration-induced growth mode transition of anodic Cu6Sn5 grains in Cu|SnAg3.0Cu0.5|Cu lap-type interconnects

The rapid accumulation of the Cu6Sn5 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|SnAg3.0Cu0.5|Cu lap-type joints are decoupled from the influence of the current stress, the microstructural evolution of the anodic Cu6Sn5 grains is studied under an average current density of 7.12 × 107 A m−2 for 0–300 h. The results show that, due to the anisotropy of the Cu6Sn5 in the absorption factor and the action of the electron wind, the [0001] directions of the Cu6Sn5 grains at the anodic Sn|Cu6Sn5 interface gradually reorient toward the current density vectors with the stress time, and certain original Cu6Sn5 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 Cu6Sn5 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 Cu6Sn5 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.