First Principles Calculation of the Mechanical and Electrical Properties of η′‐(Cu, Co) ₆ Sn ₅ and η′‐(Cu, Co, Ni) ₆ Sn ₅

The intermetallic compound η′‐Cu 6 Sn 5 plays a critical role in the reliability of SAC/Cu solder joints. This study investigates the crystal structure, mechanical properties, and electronic behavior of Co‐doped and (Co, Ni) co‐doped η′‐Cu 6 Sn 5 using first‐principles calculations. Our results indicate that doping with Co and Ni significantly decreases the formation energy, thereby enhancing the thermodynamic stability of the compound. Mechanically, the doped structures exhibit increased Young's modulus and hardness, alongside reduced elastic anisotropy and improved fracture toughness. Furthermore, the dopants modify the electronic structure by inducing hybridization between the Co‐d/Ni‐d and Sn‐p orbitals. This interaction creates strong chemical bonds and increases the density of states peak near −1.5 eV. Consequently, the incorporation of Co and Ni is predicted to enhance both the mechanical stability and electronic performance of η′‐Cu 6 Sn 5 , making it highly suitable for advanced solder joint applications.