First-principles calculations of structural and elastic properties of hexagonal Cu2In intermetallic compound

In recent years, Pb-Sn solders have been commonly applied in the microelectronic packaging industry over last decades due to their remarkable properties, such as good wetting, low melting temperature and low cost. Nevertheless, because of the toxic Pb element in the solders, significant environmental and health issues are created. Due to the high environmental awareness, green electronics products are presently advocated and proclaimed through legislation [1]. It has been extensively reported that multi-component Pb-free solders are potential to be the substitute of the Pb-Sn solders for developing green products. In recent years, extensive thermodynamic database of some metal elements, such as Ag, Cu, In, Sn, and Zn, has been established by many studies in replace of Pb to synthesize a novel Pb-free solder alloy [2]. The In-based solders could be one of the favorable candidates due to their good wetting, thermal fatigue durability, high ductility and appropriate melting point. However, there are some significant technical challenges needed to be resolved prior to their full and successful implementation and application. For example, Cu has been widely utilized in the under bump metallurgy (UBM) of chip and substrate metallization for solder bonding in the microelectronics industry. Because of the active chemical diffusion characteristics, it tends to diffuse into Sn-In based solder during assembly process and isothermal aging testing to form several intermetallic compounds (IMCs) at the interfaces between the solder and UBM, such as Cu₂In. The IMCs can induce a great influence on the structural stiffness and material strengths of solder joints, which are essential to the reliability performance of the electronic packaging. In literature, extensive focuses have been placed on the interfacial formation and evolution of the Cu₂In IMC by many researches. It should be noted that the success of application of the Pb-free solder in the advanced inte- connect technology strongly relies on the full comprehension of the mechanical properties of the IMC. Unfortunately, only limited studies have been attempted to explore the essential subject in literature. Thus, the main goal of the study attempts to provide a more complete and comparative investigation of the structural and elastic properties of the Cu₂In IMC through first-principles calculation by density functional theory (DFT) [3] within the generalized gradient approximation (GGA) [4] based on pseudopotential method. It is believed that through the investigation of its elastic properties, one can have a better understanding of its relation to the thermal-mechanical reliability of solder joints.