Effect of Bimodal Cu Paste on Interfacial Properties and Mechanical Strength of Sintered Joints

SiC power semiconductors are known to exhibit a wide band gap, a high breakdown voltage, and high thermal conductivity. In addition, they are able to operate at temperatures of up to 250°C. Therefore, in contrast to the high-temperature soldering technique that tends to generate unstable structures, Ag and Cu sintering processes have been examined as potential new bonding methods. This technique is of particular interest because Cu possesses thermal and electrical conductivities similar to those of Ag, but is significantly cheaper, thereby rendering it suitable as a potential novel die attach material. In this study, five types of bimodal Cu pastes were manufactured by mixing micro-sized and nano-sized Cu powders at different ratios. The Cu pastes were spread between a direct bond copper substrate and an electroless nickel immersion gold chip. Sintering was then carried out at 280℃ and 10 MPa for 1 min, 5 min and 10 min. The microstructural changes and mechanical strengths of the sintered joints were evaluated for different sintering times and particle ratios, and satisfactory sintered joints were found to form under all conditions examined. Additionally, the interfacial uniformity and microstructure stability improved as the sintering time was increased to 10 min, while the lowest porosity ratio and the highest shear strength were obtained at a particle ratio of 25:75 wt.%. Furthermore, it was found that the shear strength increased with an increase in the sintering time, pressure, or temperature, wherein the sintering pressure had the greatest effect on the shear strength of the sintered joint.