This study investigated the microstructure, especially intermetallic compounds (IMCs), formed between a Cu pillar and Cu trace joined by thermal compression bonding with nonconductive paste (NCP). Continuous, uniform layers of Cu3Sn formed on the surface of both the Cu pillar and Cu trace. However, the growth of Cu6Sn5 was suppressed, forming nonuniformly on the Cu trace due to NCP filler entrapment at the Cu–solder interface that hindered Sn diffusion flux. Multireflow induced rapid growth of IMCs within the Cu pillar solder joint. The combination of multireflow and thermal cycle testing gave rise to asymmetric growth of IMCs between the chip side and substrate side as a result of stress migration induced by thermal cycling.