Evaluation of thermal anisotropic evolution in the sinter structure of direct sinter joining to silicon via coupled microstructural characterizations

Sinter joining is vital for developing next-generation power devices, and therefore, the joining properties of suitable materials such as fine metal particles have drawn considerable research attention. The evolution of the sinter structure and its relationship to the joint strength has previously been studied in terms of pore size, porosity, and grain size; however, its link to fracture morphology remains ambiguous, especially for dissimilar interfaces. In this study, the effect of the anisotropic microstructural evolution of an Ag/Si sintered joint on fracture behavior and joint strength during thermal storage at 250 °C was investigated. A coupled approach involving two-dimensional (2D) characterization based on cross-sectional observation and three-dimensional (3D) characterization rooted in X-ray nano-computed tomography was devised for clarifying the relationship between the thermal storage and macroscale/microscale fracture behaviors of a joint. A steep decrease in joint strength occurred at the initial stage of thermal storage; however, high durability with a joint strength greater than 40 MPa was maintained even after thermal storage for 1000 h. The 2D and 3D analyses reveal identical microstructural evolutions with respect to the anisotropic movement of Ag, oxygen-containing Ag, and pores in the sintered structure. Furthermore, microscale tensile tests show a marked decrease in local strength in the sintered layer because of the formation of large voids. Pore aggregation is found to dominate the anisotropic microstructural evolution and play a significant role in determining the fracture behavior and macroscale properties of Ag/Si sintered joints.