Quasi-in-situ observation on diffusion anisotropy dominated asymmetrical growth of Cu-Sn IMCs under temperature gradient

Abstract Quasi-in-situ method was carried out to observe the growth behavior of intermetallic compounds (IMCs) in Cu/Sn-3.0Ag-0.5Cu/Cu micro solder joints with single β-Sn grain during aging with and without temperature gradient (TG). The bilayer IMCs of Cu6Sn5 + Cu3Sn showed symmetrical growth at both interfaces without TG, proving that β-Sn grain orientation had no effect on interfacial IMC growth during isothermal aging. Many Kirkendall voids were detected due to the dominated growth of Cu3Sn. However, during aging under TG, the IMCs in the joints with different β-Sn grain orientations showed quite different growth behavior and Cu6Sn5 was always the dominant IMC phase with little Cu3Sn and no Kirkendall voids formation. In the joints with small or medium θ angle β-Sn grains, remarkably asymmetrical IMC growth with thick Cu6Sn5 at the cold end and thin discontinuous Cu6Sn5 at the hot end was clarified. Meanwhile, the cold end Cu6Sn5 in medium θ angle joints also presented step-like morphology. The diffusion anisotropy of Cu atoms in β-Sn was responsible for the asymmetrical growth and step-like morphology. In the joints with large θ angle β-Sn grains, the Cu6Sn5 presented similar symmetrical growth to the isothermal aging cases. The different mechanisms of inhibiting Cu3Sn and Kirkendall voids formation at the cold and hot ends were revealed based on TG-induced Cu flux. Finally, a method was proposed to predict the IMC morphology and thickness considering β-Sn grain orientation, which could be helpful for reliability assessment and may also be suitable for IMC growth analyzes under current stressing.