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

Matching thermal expansion rates among electronic components remains a challenge in advanced packaging. Organic laminates and solders typically expand more than ceramic substrates during heating. This mismatch can lead to mechanical stress and reduced reliability in packaged electronics. Low-temperature cofired ceramics (LTCCs) are often used for their compatibility with silver and low losses. However, their low coefficient of thermal expansion (CTE) does not align well with other materials. This gap motivated the search for an LTCC with a higher CTE. Researchers have explored various ceramic compositions to address this issue. Prior work has shown that CTE differences can cause delamination or cracking. No prior work had resolved the need for a low-permittivity ceramic with a CTE closer to solders and laminates. This paper introduces a new ceramic formulation to bridge that gap.

The goal of this research was to develop an LTCC material with a higher CTE to better match solders and laminates. A mismatch in thermal expansion can reduce the reliability of packaged electronics. The authors aimed to find a ceramic that maintains low permittivity while aligning with the CTE of other components. They focused on a BaSO4-BaF2 system due to its sintering flexibility and low losses. The study sought to confirm the material’s performance in real-world applications. They designed a resonant antenna to test the material’s behavior. The researchers also evaluated thermal stress using finite element analysis. Their objective was to demonstrate the feasibility of this ceramic for high-reliability packaging.

The researchers synthesized BaSO4-BaF2 ceramics using a low-temperature sintering process. They varied the composition to optimize the CTE and permittivity. The sintering range was set between 650 and 850 °C to ensure compatibility with silver. They measured the dielectric properties, including permittivity and quality factor. The team also assessed the thermal expansion using dilatometry. A resonant antenna was fabricated to test the material’s microwave performance. Finite element analysis was used to simulate thermal stress in packaged components. The researchers compared the results to conventional LTCC materials to evaluate performance differences.

The 95 wt %BaSO4-5 wt %BaF2 ceramic achieved a permittivity of 9.1 at 750 °C. It exhibited a quality factor of 40,100 GHz at 11.03 GHz. The material had a CTE of +21.8 ppm °C-1, matching that of solders and laminates. The temperature coefficient of the resonant frequency was -11.2 ppm °C-1. The thermal conductivity was measured at 1.3 W mK-1. A resonant antenna using this ceramic showed a gain of 6.0 dBi at 8.97 GHz. The radiation efficiency reached 90% over a 760 MHz bandwidth. The finite element analysis confirmed low thermal stress in packaged components.

The BaSO4-BaF2 ceramic demonstrated a CTE that aligns with solders and laminates. This alignment reduces thermal stress in packaged electronics. The material’s low permittivity and high quality factor support microwave applications. The researchers propose that this ceramic is suitable for high-reliability packaging. The resonant antenna test confirmed its performance in real-world conditions. The finite element analysis supported the material’s compatibility with other components. The authors suggest that this ceramic could replace conventional LTCCs in certain applications. They emphasize the need for further testing in commercial packaging systems.