Abstract: A mainstream lead-free Sn3.0Ag0.5Cu solder with cross-scale ball diameters in the range of 600 µm to 60 µm was used to attach on Cu pads to form micro-bump structure Sn3.0Ag0.5Cu/Cu(SAC305/Cu) joints by reflow at 260 ℃ for different dwelling times of 10 to 300 s. The microstructure evolution of interfacial intermetallic compound(IMC), shear strength and fracture behavior of SAC305/Cu cross-scale joints were investigated. Results show that for solder balls with diameter larger than 200 µm the growth rate of interfacial IMC layer of the joints increases with the decreasing solder joint size, while decreasing with the decrease of the solder joint size for solder balls with diameter less than 200 µm. Meanwhile, the coarsening growth of interfacial IMC grains exhibits two featured stages: Ostwald ripening growth and grain boundary absorption/migration growth, and the transition between two growth stages occurs earlier as the size of solder joints decreases. Under quasi-static shear loading, the shear strength of Sn3.0Ag0.5Cu/Cu joints increases with the decrease of the solder joint size. In the range of reflow time from 10 s to 300 s, the reflow time has little influence on shear strength of the joints, while showing an obvious effect on the fractural path of the joints.