Strengthening mechanisms of Cu@Sn composite joints formed via transient liquid phase sintering

Herein, Cu@Sn core–shell particles were prepared and used to formulate a novel transient liquid phase sintering composite paste. Composite joints fabricated from this paste achieved low-temperature bonding while offering excellent mechanical strength and high-temperature service capability. Cu@Sn particles with a uniform Sn shell were produced via electroless tin plating, and the effect of the Cu@Sn: Sn mass proportions on the shear strength of the joints was systematically evaluated. The joints prepared by sintering the paste with a Cu@Sn: Sn mass proportion of 6:4 at 260 °C under 10 MPa pressure for 15 min achieved a shear strength of 106.84 MPa at 25 °C and retained 94.26 MPa at 300 °C. Microstructural characterization revealed that appropriate addition of Sn particles effectively regulated the formation and distribution of intermetallic compounds (IMCs) and markedly reduced the porosity of the sintered layer. The formation of fine-grained Cu 3 Sn substantially enhanced the joint strength; meanwhile, the development of subgrains and low-angle grain boundaries on the Cu substrate side of the substrate/sintered layer interface improved the interfacial bonding. The accumulation of dislocations at the soft Cu particles against the hard IMC phases generated back stress and increased strain hardening, promoting hetero-deformation induced strengthening in the heterostructured materials. Twin boundaries observed in some Cu cores also acted as barriers to dislocation motion, promoting dislocation storage while preserving the plasticity of the Cu phase, thereby further increasing the strength of the Cu@Sn composite joints.