Abstract: The slip-twinning synergistic deformation mechanism in Cu/SAC305/Cu micro solder joints during in-situ tensile test was systematically studied. The joints exhibited an ultimate tensile strength of 91-97 MPa, with fracture surfaces characterized by dimpled zones and fractured brittle intermetallic compounds (IMCs) grains, revealing a ductile-brittle mixed fracture mode strongly correlated with Sn grain orientation. The formation of subgrains and mechanical twins during tensile indicated that the plastic deformation was governed by the concurrent activation of dislocation slip and twinning. Subgrains formation originated from dislocations slipping along the dominant {100}<001>, {110}<001>, and {101}<111>/2 slip systems, as well as the rotation of Sn lattices constrained by the solder/Cu interfaces. Meanwhile, mechanical twins occurred via atomic deformation along the [-101] direction in the {101}<101> twin system. Specifically, one out of four atoms displayed along the slip direction, while the other three atoms rearranged to occupy new lattice positions, with subsequent crystal layers shifting to support mechanical twins formation. In general, under tensile conditions, geometrically necessary dislocations initially generated near grain boundaries, progressively accumulated at subgrain boundaries, and stored within grains, ultimately activated twinning to form mechanical twins upon subsequent dislocation blockage. And the results pointed out that Sn grain with small |∆λ| preferred to form subgrains by slip, while Sn grain with large |∆λ| would generate twin grains by twinning during tensile. Thus, a slip-twinning synergistic deformation mechanism was established, highlighting the synergistic effect of slip and twinning on the formation of subgrains and twins during the plastic deformation of solder joints under tensile conditions.

Key words: Lead-free solder, In-situ tensile test, Plastic deformation, Dislocations slip, Twinning