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

Alkaline earth metal fluorides, BaF 2 and SrF 2 , are promising low-permittivity ceramics with excellent microwave dielectric properties. However, achieving ideal densifications in BaF 2 and SrF 2 ceramics remains challenging due to their low surface-free energy. In this study, the sintering characteristics of BaF 2 and SrF 2 ceramics are optimized via forming (1– x)BaF 2 - x SrF 2 (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)BaF 2 - x SrF 2 solid solutions across the entire composition range. The solid-solution effect significantly enhances densification at a much lower sintering temperature of around 825 °C. Notably, 0.6BaF 2 -0.4SrF 2 ceramics exhibit the optimal microwave dielectric properties with a permittivity (ε 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 (τ f) of −97.46 ppm/°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 0.6BaF 2 -0.4SrF 2 ceramics exhibit a significantly higher CTE of 21.67 ppm/°C compared to most low- ε r ceramics (usually below 10 ppm/°C), resulting in better CTE matching with organic laminates and metal electrodes. Therefore, BaF 2 -SrF 2 solid solution ceramics, addressing the need for low- ε r , high- Qf , and high CTE, offer a potential solution for high-reliability LTCC electronics. [ABSTRACT FROM AUTHOR]