<Abstrate>

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|SnAg_3.0Cu_0.5|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.
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