Low-Permittivity and Low-Temperature Cofired BaSO₄-BaF₂ Microwave Dielectric Ceramics for High-Reliability Packaged...

In developing low-temperature cofired ceramic (LTCC) technology for high-density packaging or advanced packaged electronics, matching the coefficient of thermal expansion (CTE) among the packaged components is a critical challenge to improve reliability. The CTEs of solders and organic laminates are usually larger than 16.0 ppm of °C^1-, while most low-permittivity (ε_r) dielectric ceramics have CTEs of less than 10.0 ppm °C^1-. Therefore, a good CTE match between organic laminates and dielectric ceramics is required for further LTCC applications. In this paper, we propose a high-CTE BaSO₄-BaF₂ LTCC as a potential solution for high-reliability packaged electronics. The BaSO₄-BaF₂ ceramics have the advantages of a wide low-temperature sintering range (650-850 °C), low loss, temperature stability, and Ag compatibility, ensuring excellent performance in LTCC technology. The 95 wt %BaSO₄-5 wt %BaF₂ ceramic has a ε_r of 9.1, a Q × f of 40,100 GHz @11.03 GHz (Q = 1/tan δ), a temperature coefficient of the resonant frequency of -11.2 ppm °C^1-, a CTE of +21.8 ppm °C^1-, and a thermal conductivity of 1.3 W mK^-1 when sintered at 750 °C. Furthermore, a dielectric resonant antenna using BaSO₄-BaF₂ ceramics, a typically packaged component of LTCC and laminate, was designed and used to verify the excellent performance by a gain of 6.0 dBi at a central frequency of 8.97 GHz and a high radiation efficiency of 90% over a bandwidth of 760 MHz. Good match and low thermal stress were found in the packaged components of BaSO₄-BaF₂ ceramics, organic laminates, and Sn-based solders by finite element analysis, proving the potential of this LTCC for high-reliability packaged electronics.