As the transition to eco-friendly vehicles(Electric vehicles, EV/Hybrid electric vehicles, HEV) has accelerated, interest in power conversion modules(power module), a core component, is increasing. The power module generates a lot of heat at the joints during the operation. Therefore, forming a high temperature resistant joint is important. For reliability of joints at high temperature, various technologies such as Ag sintering, transient liquid phase(TLP) bonding are being studied. Ag sintering has good electrical/thermal properties but high processing temperature and pressure, and the electromigration that occurs during long-term use are pointed out as disadvantages. TLP bonding has a low processing temperature and pressure, but it is a time-consuming process to change all joints to intermetallic compounds(IMC). Transient liquid phase sintering(TLPS) bonding using metal powders has been studied to solve the long processing time of the TLP bonding. In TLPS bonding, atoms can diffuse effectively through the large diffusion area to form IMC quickly. In this study, Ni-Sn paste was fabricated using Ni and Sn powders. After printing the paste on DBC(Direct bonded copper) substrates, thermo-compression bonding and ultrasonic bonding were performed. Compared to the thermo-compression bonding, ultrasonic bonding shortened the processing time and minimized the heat and pressure damage of the chip and substrate. Ultrasonic energy applied to the joints causes a cavitation effect and an acoustic streaming effect. Through this, ultrasonic energy provides a continuous reaction site by removing the oxide film on the surface of the metal powder and substrate.
Additionally, the laser surface treatment was performed on the DBC substrate. Optimal laser surface treatment conditions were found by controlling the parameters such as laser power, frequency, and speed. Through surface treatment, a regular pattern of morphology was created on the DBC substrate, which expanded the surface area of the junction between the substrate and bonding material (paste). The expanded surface area accelerated the IMC formation and increased the shear strength of the joints.
In this study, the possibility of using Ni-Sn paste for the TLPS bonding for power module was confirmed. The joint characteristics of the thermo-compression bonding and the ultrasonic bonding were compared, and high-temperature long term reliability test was also conducted. Afterward, the large surface-area substrate effect obtained through laser surface treatment was confirmed.