For high demand of large bump density, fine pitch micro Cu bumps are usually connected to trace on the substrate by flip chip technology. In this study, TCNCP (Thermal Compression Bonding with Non-Conductive Paste) method was used to attach die Cu pillar onto substrate trace with surface finish of thin Ni and thick Au in a flip chip package. The micro Cu bump used in the device under test had a dome shaped Sn-based solder cap. SEM/EDX study on HTS samples (150degC, 0hr, 500hr, 1000hr) showed that the joint has a large amount of IMC mainly consisting of (CuxAu1−x)6Sn5 phase after reflow, followed by Cu3Sn phase after 1000hr on the die side. Phase segregation was seen here when HTS time went up to 1000hrs. From this study, we found that Cu bumps on the die contributed to IMC formation at beginning and then Cu trace on the substrate did so with further aging. Though Ni was not detected, its presence was displayed by retarded Cu3Sn formation on the substrate side with thin Ni/thick Au surface finish. Cu-rich IMC phase formed at this location after HTS 1000hr suggests complete dissolution of Ni after such a long time aging. Also, voided microstructure of the HTS 500hr sample appeared relatively dense with further formation of Cu-Sn phases at a later stage. No joint failures were found up to HTS 1000hr.