| Summary: | Nanotwinned copper (nt-Cu) is considered to be very desirable choice for a pad material in system-in-package (SiP) devices due to its excellent anti-electromigration and anti-element migration properties. However, complex diffusion reactions exist between nt-Cu and Sn-based solder. Especially, morphology and grain orientation transition of intermetallic compounds (IMCs) appeared after multiple reflow processes. There is a lack of deep understanding of the mechanisms of the above processes. In this paper, nt-Cu was obtained by direct-current electroplating as a pad material with (111) preferred orientation, and a two-step soldering process including dip-soldering and reflow was designed to investigate the influence of temperature on IMCs morphology. The thermodynamic and molecular dynamic simulations were used to analyze the evolution mechanism of the above process. During dip-soldering process, the size of the atomic clusters was influenced by temperature, resulting in the formation of scalloped and roof-like Cu6Sn5 grains at the solidification stage. The roof-like Cu6Sn5 solder joints had a 10.7% higher shear strength and a 15% slower growth rate compared with the scalloped Cu6Sn5 solder joints. Furthermore, roof-like Cu6Sn5 had a (-12-10) preferred orientation. However, the orientation of Cu6Sn5 grains transitioned to (0001) during reflow process. Molecular dynamics simulations showed that (0001)-oriented Cu6Sn5 grains had the lowest surface energy. The decrease in surface energy was the driving force for the grain orientation transition. This work shows that the influence of temperature on interfacial IMCs could be used to form nt-Cu/Sn/nt-Cu micro-solder joints with controlled Cu6Sn5 morphology and orientation, which can significantly enhance their shear strength and reliability.
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