Abstract Nano-sized, nonreacting, noncoarsening ZrO 2 particles reinforced Sn–3.0 wt%Ag–0.5 wt%Cu composite solders were prepared by mechanically dispersing ZrO 2 nano-particles into Sn–Ag–Cu solder. The interfacial morphology of unreinforced Sn–Ag–Cu solder and solder joints containing ZrO 2 nano-particles with Au/Ni metallized Cu pads on ball grid array (BGA) substrates and the distribution of reinforcing particles were characterized metallographically. At their interfaces, a Sn–Ni–Cu intermetallic compound (IMC) layer was found in both unreinforced Sn–Ag–Cu and Sn–Ag–Cu solder joints containing ZrO 2 nano-particles and the IMC layer thickness increased with the number of reflow cycles. In the solder ball region, AuSn 4, Ag 3Sn, Cu 6Sn 5 IMC particles and ZrO 2 nano-particles were found to be uniformly distributed in the β-Sn matrix of Sn–Ag–Cu solder joints containing ZrO 2 nano-particles, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism. The fracture surface of unreinforced Sn–Ag–Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn–Ag–Cu solder joints containing ZrO 2 nano-particles ductile failure characteristics with rough dimpled surfaces.