Significant inhibition of interfacial Cu6Sn5 IMC and improvement of solder joint strength and reliability by applying (111) nanotwinned Cu substrate and vacuum soldering

In this research, (111) oriented nanotwinned Cu (nt-Cu) substrates, combined with vacuum soldering, were utilized to enhance the strength and reliability of the solder joints. The wettability of the composite solder Sn1.0Ag0.5Cu-Si3N4 (SAC105-Si3N4) was improved on the (111) nt-Cu substrates and the oxidization of the molten solder was avoided by the vacuum environment, ensuring the formation of regular and uniform solder joints. The (111) nt-Cu substrates, characterized by their low surface energy, are demonstrated to be highly efficient in restricting the growth of interfacial IMCs, and this effects becomes particularly pronounced in three-dimensional (3D) solder joints following extended high-temperature aging. In addition, with vacuum soldering, a continuous and uniform interfacial structure was obtained, which is essential for upgrading the reliability of 3D solder joints. Furthermore, the electron back scatter diffraction (EBSD) testing of the solder matrix and interface formed on the nt-Cu substrates revealed that the c-axis of the β-Sn grains was parallel to the (111) nt-Cu substrates, and that the c-axis of the Cu6Sn5 grains was approximated to be perpendicular to the (111) nt-Cu substrates in an air environment. Finally, the shear strength and fracture morphology of the solder joints were evaluated. The macroscopic voids inside the solder joints were suppressed under vacuum, and the solder joints formed on the (111) nt-Cu substrates provided the highest shear strength.