Study on micro failure mechanism of flip-chip bump interface by in-situ transmission electron microscope

The mechanical reliability of microbumps is critical for advanced flip-chip packaging. Conventional macroscopic tests, such as bump pull and shear tests, provide overall joint reliability but cannot directly probe interfacial behavior at the nanoscale. In this work, quantitative in-situ transmission electron microscopy (TEM) mechanical tests are conducted to evaluate the intrinsic bonding and fracture behavior of heterogeneous microbump interfaces. Using microbump specimens with electroplated Ni under-bump metallization (UBM), the tensile strength of the Cu/Ni interface (∼1775 MPa) is found to be significantly higher than that of the Ni/SnAg interface (∼335 MPa). The Cu/Ni interface fails within the Cu layer, where nanotwins effectively hinder microcrack propagation. In contrast, the Ni/SnAg interface shows distinct fracture mechanisms: under tension, void growth and coalescence at the IMC/SnAg boundary lead to interface cracking, while under shear, cracks propagate along IMC grain boundaries. These findings highlight the effectiveness of the proposed in-situ TEM approach for quantitatively assessing interfacial reliability and provide guidance for minimizing void formation and strengthening IMC grain boundaries in practical microbump applications.