In-situ EBSD investigation on shear deformation mechanism and fracture behavior of Ni@CF composite solder joints

This study employs in-situ electron backscatter diffraction (EBSD) and miniature shear testing to investigate microstructural evolution and fracture in 1 wt% Ni-coated carbon fiber (Ni@CF)-reinforced SAC305 solder joints. Real-time observations reveal that unreinforced SAC305 joints exhibit orientation-dependent shear deformation. β-Sn grains with a smaller angle between their c-axis and the shear direction undergo preferential plasticity and rotation, while grains with a larger c-axis/shear direction angle remain rigid. Strain localization at the brittle Cu6Sn5 IMC/Sn interface triggers rapid crack propagation. In contrast, Ni@CF additions impart dual strengthening mechanisms: grain refinement via heterogeneous nucleation and crack blunting/deflection at Ni@CF/matrix interfaces, redirecting cracks into the solder matrix and promoting energy dissipation. Consequently, the 1 wt% Ni@CF composite solder joint achieves a 304 % higher fracture strain (1.112 vs. 0.275) and sustained plastic deformation. The approach directly probes microstructural responses (e.g., grain rotation, crack propagation) under shear loading, advancing understanding of composite solder mechanics.