The shear force of Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder bumps formed by the stencil printing method was measured and the effects of intermetallic formation on the shear force of solder bumps were investigated. The Sn-Ag(-Cu) solder paste was printed on the Au/Ni/Ti under bump metallurgy (UBM) and then reflowed repeatedly. The shear force of the solder bumps was measured as a function of the reflow times. The intermetallic formation in the Sn-Ag(-Cu) solder/UBM was characterized using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray diffractormeter. Ni3Sn4 phase was formed in the Sn-3.5Ag solder/UBM interface and (Cu,Ni)6Sn5 phase formed at the Sn-3.8Sg-0.7Cu solder/UBM interface. The shear force of solder bumps was sensitive to the depletion of Ni layer and the intermetallic thickness at the solder/Ni interface. The shear forces of Sn-3.5Ag solder bumps decreased rapidly after the fifth reflow due to the depletion of Ni layer in the UBM. The shear forces of Sn-3.8Ag-0.7Cu solder bumps decreased after the tenth reflow due to extensive growth of intermetallic layer in the solder/Ni interface.