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

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 degrees C1-, while most low-permittivity (epsilon(r)) dielectric ceramics have CTEs of less than 10.0 ppm degrees C1-. 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 degrees C), low loss, temperature stability, and Ag compatibility, ensuring excellent performance in LTCC technology. The 95 wt %BaSO4-5 wt %BaF2 ceramic has a epsilon(r) of 9.1, a Q x f of 40,100 GHz @11.03 GHz (Q = 1/tan delta), a temperature coefficient of the resonant frequency of -11.2 ppm degrees C1-, a CTE of +21.8 ppm degrees C1-, and a thermal conductivity of 1.3 W mK(-1) when sintered at 750 degrees 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.