<p>The Cu₂Ni/Sn/Cu₂Ni structure is proposed as a promising candidate for solder micro-joints in chip-stacking applications. The evolution of its microstructure and mechanical properties was systematically investigated under various bonding durations. A single type of interfacial (Cu, Ni)₆Sn₅ intermetallic forms within the solder micro-joint, with no formation of Cu₃Sn throughout the entire bonding process. The (Cu, Ni)₆Sn₅ grains exhibit an elongated prismatic morphology and grow randomly into the solder matrix. These distinctive phenomena can be strongly attributed to the influence of Ni in the Cu₂Ni pad. With increasing bonding time, the elongated prismatic (Cu, Ni)₆Sn₅ grains undergo significant coarsening at both ends and eventually coalesce to form (Cu, Ni)₆Sn₅ micro-joints at the expense of the entire Sn solder. Subsequent tensile testing revealed clear correlations between microstructural evolution, tensile strength, and fracture behavior. Specifically, the thickness ratio of the interfacial (Cu, Ni)₆Sn₅ governs the fracture mode, while the evolved grain morphology primarily influences the fracture path and mechanical strength. Fracture mechanisms were comprehensively analyzed to elucidate the anomalous variations in tensile strength associated with (Cu, Ni)₆Sn₅ microstructural evolution. These findings provide valuable insights into the design and implementation of reliable interconnections in 3D chip stacking, owing to the suppression of Cu₃Sn formation and the stabilization of the Cu₆Sn₅crystal structure.</p>