Microstructure and Long-Term Reliability of Ultrasonic-Assisted Solder Ball Joints

Due to the implementation of various environmental regulations, vehicles using internal combustion engines are being replaced by electric vehicles. Consequently, the demand for electronic components that drive electric vehicles is increasing. Power semiconductors, which control and distribute electric energy, are being replaced from Si-based to SiC-based semiconductors. SiC-based semiconductors have high power conversion efficiency and are stable in high temperature and high voltage environments. However, since SiC operates at higher temperatures than Si, there is increasing attention on high melting point solders (Au, Zn-based), metal nanoparticle sintering, and transient liquid phase (TLP) bonding technologies.<br/><br/>With the advancement of power semiconductors, securing the reliability of electronic components has become a significant issue. Particularly, the ultrasonic-assisted soldering process has the potential to contribute to reducing soldering time and enhancing the reliability of solder joints. Ultrasonic energy accelerates the interaction between Cu and Sn through the cavitation effect, promoting the formation of a complete intermetallic compound (IMC). The objective of this study is to analyze the microstructure of joints formed through the ultrasonic-assisted soldering process and to evaluate the mechanical properties and reliability of the joints through long-term reliability tests.<br/><br/>In this study, solder paste was prepared using Sn-58Bi solder (Type 4), Cu powder (4 μm), and flux. The prepared paste was printed on a Cu substrate using a stencil printing method, and a Cu chip was mounted on the printed paste. The ultrasonic-assisted soldering process was performed under various soldering conditions, and high-temperature storage (HST) tests and thermal cycling (TC) tests were conducted for long-term reliability evaluation. Additionally, the microstructure of the soldered joints was observed using a scanning electron microscope (SEM). As a result, the soldering process utilizing ultrasonic energy significantly reduced the soldering time to approximately 6 minutes due to the cavitation effect induced by the ultrasonic waves. These results are expected to contribute to improving the reliability of power semiconductors and electronic components.