<p>Micro-bump plays a pivotal role in enabling high density interconnect required for three-dimensional integrated-circuits (3D ICs) packaging. However, the growth rate of intermetallic compounds (IMCs) in micro-bumps increases with shrinking bump size due to the emergence of surface diffusion channels, posing significant challenges to the reliability of small-sized micro-bumps. In this study, Fe-Co alloys were introduced as innovative diffusion barrier layers to effectively suppress IMC growth in solder bumps, achieving an exceptionally low IMC growth rate of 0.0118&#xa0;μm/h^0.5. No sidewall IMCs were observed in micro-bumps, demonstrating the Fe-Co alloys’ effectiveness in inhibiting surface diffusion. Notably, an interesting size effect on IMC growth was observed, with larger Cu plate solder joints exhibiting faster IMC growth compared to 12&#xa0;μm micro-bumps during aging. This behavior was attributed to grain size differences in the Fe-Co barriers, where smaller grain sizes in larger joints facilitated grain boundary diffusion, thereby accelerating IMC growth. Finite element analysis (FEA) simulations further demonstrated that variations in current density during electrodeposition led to differences in grain size. These findings propose a powerful candidate for high-performance barrier materials in small-sized micro-bumps and provide critical insights into the role of grain boundary diffusion in IMC growth, offering valuable strategies for enhancing the reliability of electronic packaging.</p> Graphical Abstract <p></p>