CaO-B2O3-SiO2 Glass/Al2O3 Composites for Enhanced Low-Temperature Co-Fired Ceramics in High-Frequency Applications

Significance 

To cope up with the significant and rapid advancement of 5G communication technologies and the proliferation of Internet of Things devices with the increased integration of wearable technology, satellite communication, and automotive radar systems, there is a need for miniaturized, lightweight, and multifunctional electronic devices which makes the development of innovative materials and new manufacturing processes are essential. One promising approach is the use of Low-Temperature Co-Fired Ceramics (LTCC), which allow for the integration of passive components such as capacitors, resistors, and inductors directly into the ceramic substrate and facilitates the creation of compact, multilayered circuits with enhanced performance. Specifically, LTCC technology has several advantages over traditional electronic packaging methods because it operates at lower sintering temperatures which enables the use of highly conductive metals like silver, copper, and gold, and by this reduces energy consumption and enhances conductivity. Additionally, LTCC overcomes the limitations of thick-film printing, such as the need for multiple prints and sintering cycles and by this simplifies manufacturing process and reduces costs. Nevertheless, material compatibility and performance optimization are still a challenge and achieving the desired combination of dielectric, thermal, and mechanical properties. Therefore, developing LTCC materials that meet stringent requirements for low dielectric constant (εr), low dielectric loss (tan δ), high thermal conductivity, low coefficient of thermal expansion (CTE), and structural densification are urgently needed. To address these challenges, new study published in the Journal of the American Ceramic Society and led by Professor Hongyu Yu from the South University of Science and Technology, and conducted by Zilong Xiong, Wenzhuo Xue, Mujun Li, Dr. Feihu Tan, and Yupeng Chen synthesized CBS glass/Al2O3 composites for LTCC applications. The authors explored the effects of different glass compositions within the CaO-B2O3-SiO2 (CBS) system on the densification process, phase compositions, microstructures, and properties of LTCC composites.

The researchers began by designing nine distinct glass compositions within the CBS system, each with varying ratios of CaO, B2O3, and SiO2. The raw materials used included high-purity CaCO3, B2O3, SiO2, and Al2O3. These were processed using conventional glass quenching methods. Specifically, the oxide powders were weighed according to the desired molar percentages, mixed, and then heated in a platinum crucible at 1450°C for 2 hours. The resulting molten glass was quenched in deionized water to form glass blocks, which were then ball-milled to obtain fine glass powders.  The authors mixed the glass powders with nanoalumina in a 3:7 volume ratio and combined with paraffin wax as a binder. The mixture was formed into green sheets using a single-axis compression method and then sintered at 850°C to develop the LTCC composites.

The team investigated the thermal behavior and phase compositions of the synthesized glasses and composites using Differential Thermal Analysis and X-ray Diffraction and the data showed that the softening points of the CBS glasses were all below 650°C which are suitable for LTCC applications. Findings showed that the CBS glasses primarily consisted of crystalline phases such as CaB2O4, CaB4O7, Ca(BO2)2, and SiO2. When co-fired with Al2O3, additional phases like CaAl2B2O7, CaAl2(BO3)2O, and Al4B2O9 were observed. These findings were significant because the presence of these phases influenced the densification behavior and overall properties of the LTCC composites.

The authors also studied the densification behavior of the composites using thermal shrinkage curves obtained from a dilatometer and found that the sintering shrinkage varied significantly with the glass composition. This indicated a strong correlation between the sintering behavior and the specific glass composition. Moreover, the sintering process and the resultant microstructures were closely linked. SEM images of the sintered composites revealed that those with higher CaO content, such as CBS-1 and CBS-5, showed dense and well-infiltrated microstructures. In contrast, compositions with higher B2O3 content, like CBS-2 and CBS-7, exhibited more porosity and defects, likely due to insufficient liquid phase flow and higher viscosity of the residual glass phase. These observations highlighted the importance of glass composition in achieving desirable densification and microstructural characteristics. They also measured the coefficient of thermal expansion (CTE) of the LTCC composites    and found the values ranged from 4.5 to 6.5 ppm/°C, which are compatible with materials like silicon and gallium arsenide.  Dielectric properties, including εr and tan δ, were measured at 15 GHz. The researchers found that compositions with higher CaO content exhibited higher dielectric constants and lower dielectric losses. For example, CBS-1 demonstrated a dielectric constant of 4.3 and a dielectric loss of 0.0018, making it highly suitable for high-frequency applications where minimal signal attenuation and thermal losses are desired. The findings indicated that the CBS-5 composition, sintered at 850°C, possessed a dense microstructure and exhibited excellent thermoelectric properties, including a thermal shrinkage of 9.04%, a CTE of 5.23 ppm/°C, a dielectric constant of 4.73, and a dielectric loss of 0.00199 at 15 GHz. These properties make CBS-5 an especially promising candidate for LTCC applications.

In conclusion, the study led by Professor Hongyu Yu and his team provided a detailed understanding of how varying compositions within the CBS glass system affect the densification process, phase compositions, and microstructural properties of LTCC composites. This knowledge is critical for developing materials that can be sintered at low temperatures while maintaining excellent dielectric properties and thermal stability. Moreover, their findings demonstrate that specific compositions, particularly those with higher CaO content, exhibit low dielectric constants and losses, making them ideal for high-frequency applications such as 5G communication, satellite communication, and automotive radar systems. The resulted low dielectric loss is essential for reducing signal attenuation and thermal losses, which are critical for more efficient and reliable high-frequency devices. Furthermore, they showed that the developed CBS glass/Al2O3 composites have CTE compatible with silicon and gallium arsenide and this is significant because it prevents thermal stress and delamination in electronic packaging, and can enhance the durability and performance of integrated circuits and other electronic components.  Additionally, the newly developed  materials are suitable for a wide range of applications, including aerospace, military, MEMS (Micro-Electro-Mechanical Systems), automotive electronics, and consumer electronics.

CaO-B2O3-SiO2 Glass/Al2O3 Composites for Enhanced Low-Temperature Co-Fired Ceramics in High-Frequency Applications - Advances in Engineering

About the author

Prof. Hongyu Yu

School of Microelectronics,
South University of Science and Technology,
Shenzhen, P. R. China

January 2001 – May 2004, PhD, National University of Singapore
July 1999-December 2000, Master’s degree, University of Toronto, Canada
September 1994-July 1999, Bachelor’s degree, Tsinghua University, china

His laboratory focuses on Third generation semiconductor materials and functional ceramic materials, such as GAN-based sensing materials and high-voltage devices. In the aspect of functional ceramics, LTCC packaging materials and high-frequency ceramic devices (ceramic powder with high Q×f , filter, resonant column) are mainly studied. Dr. Yu has published over 450 papers. The total number of separate citation was more than 5900 times, and the H influence factor was 46. He has been invited to write chapters in four professional books and edited two books. He is a Fellow of British Institute of Engineering and Technology (IET), and the associate editor of Science Bulletin.

About the author

Dr. Feihu Tan

School of Microelectronics,
South University of Science and Technology,
Shenzhen, P. R. China

Dr. Tan is currently a Senior research scholar at the South University of Science and Technology. He received his Ph.D. degree in 2017 from General Research Institute for Nonferrous Metals, China. In 2018, he joined the State Key Laboratory of Intelligent Sensing Functional Materials. He then had a postdoc appointment in Shenzhen University working on Inorganic solid electrolyte. His research interest focuses on the Functional ceramic material, including Energy storage film, Solid-state battery and LTCC. He has published more than 30 research publications.

Reference

Xiong Z, Xue W, Li M, Tan F, Chen Y, Yu H. Microwave dielectric characterization and densification mechanism analysis of CaO-B2O3-SiO2 glass–ceramic/Al2O3 composites for LTCC applications. J Am Ceram Soc. 2024; 107: 234–243. https://doi.org/10.1111/jace.19436

Go to J Am Ceram Soc.

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