Effect of CoSn₃ nanocrystals on Sn3Ag plating for electronic packaging

Plating Sn3Ag on copper substrates represents a crucial electronic packaging technique. In this study, we propose a novel composite plating approach, wherein CoSn₃ nanocrystals are deposited within the Sn3Ag coating. The resulting reflowed Sn3Ag joints exhibit a range of distinctive properties. Notably, CoSn₃ nanocrystals dissolve in Sn during the reflow process, thereby lowering the supercooling required for Sn nucleation. Consequently, Sn crystals grow in six-fold cyclic twins. Additionally, the dissolution of Co atoms in Sn leads to a reduced solubility of Cu atoms in Sn, consequently lowering the supercooling required for the nucleation of Cu₆Sn₅. Simultaneously, this phenomenon promotes the nucleation of Cu₆Sn₅, resulting in a considerable precipitation of Cu₆Sn₅ nanoparticles within the joints. Therefore, the mechanical properties of the joints are significantly enhanced, leading to a notable 20% increase in shear strength. Furthermore, the presence and distribution of Co elements within Sn induce changes in the growth pattern of interfacial Cu₆Sn₅. The growth process of Cu₆Sn₅ is dominated by the interfacial reaction, leading to its growth in a faceted shape. During the aging process, the dissolution of Co elements in Sn impedes the continuous growth of Cu₆Sn₅ at the interface, causing Cu₆Sn₅ to be distributed in the form of islands inside the joint. Remarkably, elemental Co acts as an inhibitor for the development of Cu₃Sn and reduces the occurrence of Kirkendall voids.