First-Principles Study of Cu Addition on Mechanical Properties of Ni3Sn4-Based Intermetallic Compounds

Ni–Cu under-bump metallisation (UBM) can reduce stress and improve wetting ability in technology for electronic packaging technology advances with three-dimensional integrated circuit (3D IC) devices. The bond between the Sn-based solder and Ni–Cu UBM is affected by the formation of intermetallic compounds (IMCs), specifically Ni3Sn4 and (Ni,Cu)3Sn4. This paper investigates the mechanical properties of IMCs, which are critical in assessing the longevity of solder joints. First-principles calculations were carried out to investigate the phase stability, mechanical properties and electronic structures of Ni3Sn4, Ni2.5Cu0.5Sn4, Ni2.0Cu1.0Sn4, and Ni1.5Cu1.5Sn4 IMCs. The calculated formation enthalpies show that the doping of Cu atoms leads to a decrease in the stability of the phases and a reduction in the mechanical properties of the Ni3Sn4 crystal structure. As the concentration of Cu atoms in the Ni3Sn4 cells increases, the bulk modulus values of (Ni,Cu)3Sn4 formed with different compositions decrease from 107.78 GPa to 87.84 GPa, the shear modulus decreases from 56.64 GPa to 45.08 GPa, and the elastic modulus decreases from 144.59 GPa to 115.48 GPa, indicating that the doping of Cu atoms into the Ni3Sn4 cells may adversely affect their mechanical properties and increase the possibility of microcracking at the interface during actual service. The anisotropy of (Ni,Cu)3Sn4 is more significant than that of Ni3Sn4, with Ni2.0Cu1.0Sn4 showing the highest anisotropy. After evaluating the electronic structures, the metallic properties of Ni3Sn4 and the Ni2.5Cu0.5Sn4, Ni2.0Cu1.0Sn4, and Ni1.5Cu1.5Sn4 phases are revealed by electronic structure analysis. The total density of states (TDOS) for (Ni,Cu)3Sn4 structures is mainly influenced by Ni-d and Cu-d states. The addition of Cu atoms can increase the brittleness of Ni3Sn4. In addition, the region where d and p hybridisation occurs gradually increases with increasing Cu content. The electronic properties suggest that the binding energy between Ni and Sn atoms weakens with the addition of Cu atoms, resulting in a decrease in the elastic modulus. This research can serve as a valuable reference and theoretical guide for future applications of these materials.