Several Sn-Ag-Cu lead-free solders and two kinds of metallic substrates, Cu and Cu/Ni/Au, were selected to explore the effect of microelements (Ni and Ge) on the interfacial reaction between solder and substrates. When solders reacted with Cu substrate, the thickness of the interfacial intermetallics of the Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge (in wt%) solder is several times as that of the Sn-3.5Ag-0.7Cu solder. Besides, the addition of microelements would transfer the feature of interfacial intermetallics from pebble shape to worm shape. However, the negligible difference in XRD data of these two alloys suggests that both interfacial intermetallics have the same crystal structure. The major interfacial intermetallic formed with Sn-3.5Ag-0.7Cu solder is Cu6Sn5, while it is (Cux, Ni1-x)6Sn5 with the solder containing Ni and Ge. The results of Electron Probe Microanalyzer (EPMA) investigation show that the aggregation of Ni in the interfacial intermetallics affects the interfacial reaction rate and the morphology of interfacial intermetallics. On the other hand, the features of the interfacial layer formed with a Cu/Ni/Au substrate are similar for the Sn-3.2Ag-0.5Cu and Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge solders. However, Ni and Ge enhance the shear strength of BGA solder ball attachment. The results of the cross section investigation indicate that the interfacial intermetallics were composed of coarse (Cuy, Ni1-y)6Sn5 and fine uniformly dispersed Ni3Sn4.