Revealing evolutions of intermetallics in a solder joint under electromigration: A quasi-in situ study combining 3D microstructural characterization and numerical simulation

The evolution of primary and interfacial intermetallic compounds in solder joints during electromigration (EM) significantly influences the mechanical properties and reliability of solder joints. In this work, by combining high-resolution x-ray computed tomography and finite element modeling, the 3D evolution of Cu₆Sn₅ in a solder joint containing a single β-Sn grain with representative orientation was revealed. It is found that the growth of primary Cu₆Sn₅ rods and the formations of Cu₆Sn₅ rods/particles within the matrix and on the anode/cathode interfaces are all heavily influenced by local Cu concentrations and Cu diffusion fluxes in β-Sn, which are controlled by the β-Sn orientation and geometry of the solder joint. The unobvious growth of some Cu₆Sn₅ is also attributed to the high angles between [0001]_Cu6Sn5 and [001]_β-Sn (>45°). The orientation relationship between β-Sn and Cu₆Sn₅ also contributes to the growth direction of the newly formed Cu₆Sn₅ rod. This study provides insight into the mechanisms of EM-induced microstructure evolutions in ball-grid-array solder joints.