Pressureless Cu–Cu bonding using hybrid Cu–epoxy paste and its reliability

Cu sintering bonding has been considered a promising interconnection method for high-temperature applications like power electronics because of the low cost, low electrical resistivity, and high heat endurance of copper. However, Cu sintering bonding requires high bonding temperature and pressure as well as Cu oxidation prevention. To overcome these problems, we fabricated a hybrid Cu paste consisting of sub-micron and flake fillers, mechanically mixed with various epoxy contents (0, 5, 10, or 15 wt%). Cu–Cu bonding with printed hybrid Cu–epoxy paste was performed in N 2 at 250 °C for 30 min without pressure. The process temperature and pressure could be lowered to these levels because the mechanical reinforcing effect of the epoxy supplemented the weak bonding between Cu fillers. When 10 wt% epoxy was added, the shear strength of joint increased remarkably to 12.1 MPa, whereas electrical resistance was maintained at 1.8 Ω. The joint with 15 wt% epoxy was further strengthened, but electrical resistance increased drastically. A high-temperature storage test was then performed at 175 °C for 72 h to investigate the bonding reliability. Since Cu fillers were encapsulated in epoxy to prevent oxidation, the bonding shear strength of the Cu–Cu joint with hybrid Cu–epoxy remained robust even after thermal aging.