Recently, global awareness of environmental issues has led to higher interest in carbon emission reduction, eco-friendly vehicles, and renewable energy
1). The increased interest in eco-friendly vehicles, such as electric and hybrid vehicles, has also increased the demand for power semiconductors that perform power conversion (DC↔AC), distribution, and control
2,3). Power semiconductors have been widely used as key components in environments that use high-voltage power. Si semiconductors have been mainly used, but research has been actively conducted on silicon carbide (SiC) and gallium nitride (GaN) semiconductors, which are wide band gap (WBG) semiconductor devices, due to the increasing demand for high-level specifications
4-6). Since power semiconductors operate in poor environments (e.g., high temperature, high pressure, and high current density), the development of bonding materials and process technologies for power semiconductors is very important in securing stable characteristics and reliability in such environments
7). Thus far, soldering methods based on Pb-free bonding materials have been mainly used as a bonding process technology for power semiconductors, but most Sn-based Pb-free solders had limitations in being applied to high-temperature environments because their melting point is less than 250°C. The Ag sintering method was presented as technology to secure stable reliability at high temperature, but price competitiveness must be secured for its commercialization due to the high price of the Ag powder material and the high cost of nanopowder production
8-13). Transient liquid phase (TLP) technology was presented as a new method to address problems with the use of Sn-based solders and metal paste. For TLP bonding technology, a material with a relatively low melting point is inserted between materials with high melting points, and only the material with a low melting point is changed into the liquid phase when heated
14). Through the isothermal solidification process, liquid elements diffuse towards the solid base metal, thereby forming intermetallic compounds (IMCs) at the joint
15). The joint with IMCs can secure stable reliability at high temperature due to the high melting point. For some compositions, IMCs are known to have higher electrical resistance than pure metals
16). In particular, it was confirmed that the joint completely converted into IMC after heat treatment has high electromigration (EM) resistance and excellent lifespan characteristics
17,18). Conversion of the entire joint into IMC, however, requires a long period of time. To tackle this problem, transient liquid phase sintering (TLPS) technology, which performs bonding using the paste produced by mixing high-melting point metal powder and low-melting point metal powder, has been presented of late. It can shorten the process time by facilitating atomic diffusion with an increased contact area between atoms compared to TLP bonding
19). In this study, after preparing power semiconductor chip bonding paste for response to high temperature, the joint characteristics were analyzed. After preparing five types of paste by adjusting the contents of Ni powder, Sn powder, and flux, the optimization process was performed. The TLPS bonding process was performed using the selected optimal paste, and the joint microstructure, mechanical strength, and reliability at high temperature were evaluated over time.