The drop shock reliability of an Sn-Ag-Cu solder system in ball grid array (BGA) interconnects were improved by selecting a lower Ag chemical content and the addition of a small amount of Ni. The drop shock reliability of Sn-Ag-Cu solder in BGA interconnects was enhanced by the addition of small amount of Ni, and LF35(Sn-1.2Ag-0.5Cu-Ni) had twice better drop shock reliability than LF45(Sn-3.0Ag0.5Cu). One of the main reasons is the solder with lower Ag contents in the Sn-Ag-Cu solder system demonstrated softer properties in terms of hardness. Different intermetallic compound (IMC) layer morphologies were found on the Cu electrodes after reflow between Sn-1.2Ag-0.5Cu-Ni and Sn-3.0Ag-0.5Cu, in which the former is smooth IMC and the latter is like a peninsula IMC. Different cracks modes were also detected. Cracks in Sn-1.2Ag-0.5Cu-Ni were mainly inside solder and cracks in Sn-3.0Ag-0.5Cu were near the solder/electrode interface for all drop shocks tested. From SEM, EPMA mapping and TEM analysis, the small amount of doped Ni was mostly segregated at the interface. The doped Ni mainly existed in the Cu6Sn5 IMC layer region as formed (Cu,Ni)6Sn5, and the Cu 3Sn IMC region contains less Ni. Both different IMC morphologies and crack locations were discussed on the basis of the lattice distortion relaxation. The Cu6Sn5 peninsula IMC growth was discussed that was caused by the compression stress of itself, and the Sn-3.0Ag-0.5Cu crack inside the IMC was also discussed that was caused by the difference stress between Cu6Sn5 (compression stress) and Cu3Sn (tensile stress). Namely, the doped Ni was substituted for the Cu site of Cu6Sn5 and their lattice distortions were relaxed due to the smaller atomic radius of Ni compared with Cu. The stress difference between (Cu,Ni)6Sn5 and Cu3Sn was relieved by the Ni substitutions, which improved the drop shock reliability in BGA of Sn1.2Ag-0.5Cu-Ni.