The inherent brittle behavior and ductile-to-brittle transition (DBT) mechanism of Sn-3.0Ag-0.5Cu (SAC305) solder alloy at the liquid nitrogen temperature (LNT, 77 K) were investigated through uniaxial tensile experiments conducted at different temperatures. Dynamic recovery and recrystallization of SAC305 solder alloy at room temperature (RT, 293 K) activate a softening process. Conversely, intersecting and none-intersecting deformation twins, embedded in body-centered tetragonal Sn, enhance tensile strength and stabilize strain hardening rate, while suppressing the elongation of the alloy at LNT. The irreconcilable velocity difference between twin thickening (~8 μm/s) and dislocation slip (~4 μm/s) results in premature brittle fracture, during the linear hardening and DBT. Moreover, the secondary phases degrade the mechanical property of SAC305 solder alloy, and micro-cracks appear between Cu₆Sn₅ and Ag₃Sn in the eutectic matrix.