Formation of Glass Microcrack Induced by Nonuniform Copper Grain Growth in Through Glass Via During Reflow Soldering

Through-glass via (TGV) technology, as one of the key technical paths for three-dimensional integrated packaging, offers advantages such as shorter interconnection distances, superior high-frequency performance, and higher integration density. However, thermomechanical reliability issues become prominent during 3D stacked packaging processes, significantly hindering the widespread application of TGV technology. This study examines the thermomechanical reliability of TGV substrates during reflow soldering using failure analysis methods. Reflow soldering cycle was subjected to 0, 1, 3, and 5 cycles with a peak temperature of 245°C. Electro-optic terahertz pulse reflectometry (EOTPR) revealed minor waveform fluctuations among the 0-cycle, 3-cycle, 5-cycle samples. For the 3-cycle sample, microcrack with a length of approximately 10 μm and a width of about 100 nm were observed at the top corners of the TGV, although no cracks were found in the copper. Electron backscatter diffraction (EBSD) revealed an increase in copper grain size near the microcrack region, while grain sizes decreased in transitional and waist regions, indicating non-uniform grain growth of copper grains. Finite element analysis (FEA) results indicated that the stress concentration points are located at the top corners of the TGV, with a stress value of 377.91 MPa. The mismatch in the coefficient of thermal expansion (CTE) between copper and glass exacerbates the non-uniform grain growth of copper during reflow soldering, thereby resulting in the formation of glass microcracks. This finding provides a theoretical basis and technical pathway for the process optimization and reliability evaluation of TGV structures.