ZnO-doped borosilicate glass with a high thermal expansion coefficient solder for Cu/SUS 304 low dielectric sealing

Low-dielectric glass sealing is important for advanced electronic packaging; however, integrating such glasses with high-expansion metals such as Cu and stainless steel (SUS 304) remains challenging because of thermal-expansion mismatch and limited interfacial adhesion. In this work, ZnO was incorporated into a borosilicate glass matrix to tailor the thermophysical properties and promote interfacial bonding. Microstructural analyses show that Zn²⁺ tends to form [ZnO₄] structural units and reduce non-bridging oxygen, thereby restricting alkali-ion mobility and maintaining low dielectric loss. Meanwhile, the partial substitution of SiO₂ by ZnO lowers the average network rigidity, leading to a reduced glass transition temperature and an increased CTE of 11.4 × 10⁻⁶ °C⁻¹. At the hetero-interfaces, ZnO promotes the formation of a ZnCr₂O₄/NaCrO₂ composite layer on SUS 304 and Zn/Cu interdiffusion near the Cu side. Under the selected sealing condition of 810 °C for 10 min, the Cu/glass/SUS 304 joints achieved a shear strength of 20.3 ± 3.8 MPa. These results suggest that ZnO-doped borosilicate glass is a promising low-dielectric sealing material for heterogeneous electronic packaging.