Solid-solution assisted densification and microwave/millimeter-wave dielectric properties of BaF2-SrF2 ceramics for high-reliability LTCC applications

Alkaline earth metal fluorides, BaF2 and SrF2, are promising low-permittivity ceramics with excellent microwave dielectric properties. However, achieving ideal densifications in BaF2 and SrF2 ceramics remains challenging due to their low surface-free energy. In this study, the sintering characteristics of BaF2 and SrF2 ceramics are optimized via forming (1-x)BaF2-xSrF2 (x = 0.2, 0.4, 0.5, 0.6, and 0.8) solid solution ceramics. X-ray diffraction (XRD) patterns and Raman spectra provide compelling evidence for successfully forming (1-x)BaF2-xSrF2 solid solutions across the entire composition range. The solid-solution effect significantly enhances densification at a much lower sintering temperature of around 825 degrees C. Notably, 0.6BaF2-0.4SrF2 ceramics exhibit the optimal microwave dielectric properties with a permittivity (epsilon r) of 7.26, quality factors (Qf) of 61,006 GHz at 10 GHz and 106,000 GHz at 25 GHz, and a temperature coefficient of resonant frequency (tau f) of-97.46 ppm/degrees C. Additionally, the advancement of low-temperature co-fired ceramic (LTCC) technology for high-density electronic packaging presents a significant challenge in achieving compatibility between the coefficient of thermal expansion (CTE) of the packaged components. The 0.6BaF2-0.4SrF2 ceramics exhibit a significantly higher CTE of degrees degrees 21.67 ppm/ C compared to most low-epsilon r ceramics (usually below 10 ppm/ C), resulting in better CTE matching with organic laminates and metal electrodes. Therefore, BaF2-SrF2 solid solution ceramics, addressing the need for low-epsilon r, high-Qf, and high CTE, offer a potential solution for high-reliability LTCC electronics.