First-principles calculations to investigate the structural stability, electronic and mechanical properties of CuGa2 and Cu9Ga4

Abstract

This paper aims to evaluate the service reliability of intermetallic compounds (IMCs) in Cu-Ga joints. It accomplished this by calculating the thermodynamic and mechanical properties of CuGa2 and Cu9Ga4, which are common IMCs in Cu-Ga low temperature interface reaction, based on the first-principles calculation. According to the calculation results of enthalpy of formation and binding energy, both CuGa2 and Cu9Ga4 are thermodynamically stable, and Cu9Ga4 exhibits superior thermodynamic stability compared to CuGa2. The mechanical properties of Cu9Ga4 are better than CuGa2, with larger bulk modulus, shear modulus and Young's modulus. By comparing the elastic modulus with that of IMCs of Cu-Sn system, Cu9Ga4 shows larger elastic modulus than η′-Cu6Sn5 and smaller than Cu3Sn. In addition, Cu9Ga4 exhibits excellent toughness. By fitting the thermal expansion coefficients of Cu, CuGa2 and Cu9Ga4, the result shows that the thermal expansion coefficients of Cu and Cu9Ga4 are better matched. Additionally, the rapid volume expansion of CuGa2 weakens the interaction force between its atoms, resulting in a faster decay of its bulk modulus. The prediction of the mechanical properties of Cu-Ga compounds provides a deeper understanding of their properties and is important for the understanding of the feasibility of low-temperature brazing of Cu-Ga soldering systems.