The interfacial reaction between Cu pad coated with Au/Ni and solder bump of flip chip package, using Sn97.5wt.%-Ag2.5wt%, was studied under thermal shock stress. All joints were subjected to thermal shock test with $-65^{circ}C/+150^{circ}C$ temperature range. For the Sn-2.5Ag solder, a scallop-like (Cu, Ni)$_6Sn_5$ intermetallic compound was formed in the solder matrix after 20 cycles of thermal shock. (Cu, Ni)$_6Sn_5$ was detached from the interface as (Ni, Cu)$_3Sn_4$ grew underneath the (Cu, Ni)$_6Sn_5$ IMC(Intermetallic Compound), whereas the elements of Sn, Ni and Cu were moved by interdiffusion at the interface between solder alloy and Cu pad. The composition of the IMCs in the solder joints and elemental distribution across the joint interfaces were quantitatively measured with EPMA (electron probe micro analysis). Finally, it was found that the crack initiation point and its propagation path could be influenced by the thermal shock conditions, two underfills, and their properties.