Interfacial Characteristics and Mechanical Properties of Cu/Ga/Cu Interconnects by Transient Liquid Phase Bonding

The continuous miniaturization and higher power density of components and devices in electronics presents a high challenge for their design and manufacturing process. Transient liquid phase bonding (TLPB) technology has great potential to solve this challenge, where through the metal interdiffusion, the high melting point substrate reacts with low melting point solder to form new intermetallic compounds (IMCs) with high melting point compared to the soldering process. Ga and Ga-based alloys have been explored here for low-temperature bonding in microelectronics packaging. This work has adopted Ga for Cu-to- Cu die-attach interconnection through transient liquid phase bonding. The microstructure of the $\mathrm{Cu} / \mathrm{Ga}$ couples and the properties of the joints are studied. The results show that when the reaction temperature is lower than $200^{\circ} \mathrm{C}$, the interfacial reactions cause the formation of $\gamma_{3}-\mathrm{Cu}_{9} \mathrm{Ga}_{4}$ and $\theta-\mathrm{Cu}Ga_{2}$ where the $\theta-\mathrm{CuGa}_{2}$ phase layer dominates the main body of the joint, and $\gamma_{3}-\mathrm{Cu}_{9}Ga_{4}$ is relatively thin. The $\gamma_{3}$ phase gradually thickened as the bonding temperature increases. When the reaction temperature is higher than $300^{\circ} \mathrm{C}$, the $\gamma_{3}-\mathrm{Cu}_{9} \mathrm{Ga}_{4}$ phase is the only reaction product. In addition, the shear strength of the joint was significantly enhanced as the process temperature increases. This indicates that the $\gamma_{3}$ phase has better mechanical properties compared to the $\theta$ phase.