The intermetallics of Cu6Sn5 that are formed at the Sn-based solder/ Cu substrate interface play a significant role in solder joint reliability. The characterization of the mechanical properties of the interface Cu6Sn5 is essential to understand the mechanical performance and structural integrity of the solder joints. In this study, the interface Cu6Sn5 and (Cu,Ni)6Sn5 formed in Sn-Cu and Sn-Cu-Ni ball grid array (BGA) joints were investigated using nanoindentation. The results demonstrated that the strain rate sensitivity and the activation volume of these intermetallics were affected by the reflow times and load conditions. The strain rate sensitivity of Cu6Sn5 and (Cu,Ni)6Sn5 were estimated from 0.023 to 0.105, and the activation volume of Cu6Sn5 and (Cu,Ni)6Sn5 were estimated from 0.128 b3 to 0.624 b3 (b=4.2062x10-9 m) for 1, 2 and 4-reflowed Sn-Cu (-Ni) samples.