Abstract: Sn-3.0Ag-0.5Cu solder balls with diameters of 200, 300,400 μm were reflowed on electroless nickel electroless palladium immersion gold (ENEPIG) pads for 1, 3, 5 and 7 times to form Sn-3.0Ag-0.5Cu/ENEPIG solder joints. The solder joints after 3-time reflow were subjected to aging treatment at different temperatures (75,100,125 ℃). The effect of the solder ball size on the interface microstructure and shear strength of the solder joints after reflow and aging was investigated. The results show that after reflow and aging treatment, the thickness of the interfacial intermetallic compound and the shear strength of the solder joints decreased with increasing solder ball diameter, showing obvious size effect. When the size of solder ball was the same, the increase of reflow time and aging temperature led to the increase of interfacial intermetallic compound thickness and the decrease of the shear strength of the solder joint.