Comprehensive review
Research Progress of Microwave Ceramic Filter Molding Methods

GUO Baolin, WU Yiwen, LI Xiaokang, YAO Qing

(School of Mechanical Engineering, Nantong University, Nantong 226109, Jiangsu, China)

Extended Abstract:[Significance] Microwave dielectric ceramics refer to the ceramics that are applied as dielectric materials in circuits operating, at the microwave frequency band (300 MHz–300 GHz), with one or more functions. Generally, dielectric properties are evaluated by three main parameters, including relative permittivity (εr), quality factor (Q×f) and temperature coefficient of the resonant frequency (τf). According to the microwave transmission theory, the larger the εr value, the smaller the size of the dielectric resonator will be. This is conducive to the miniaturization and quality improvement of devices. The Q×f value should be high to ensure reduce the insertion loss of the devices. The temperature coefficient is a parameter used to measure stability of the resonant frequency. The smaller the temperature coefficient, the less the shift in resonant frequency of the device will be. With the continuous innovation of science and technology and the modernization of communication technologies, there are demands for filters with the requirements of miniaturization, lightweight and high reliability. Microwave dielectric ceramics have been widely used in microwave communication devices. Among them, ceramic filters are indispensable electronic components in 5G communication technology.[Progress] Firstly, the application of the dry pressing to prepare ceramic filters is introduced. As a mature process, the dry pressing is widely used in the fields of forming ceramic substrates for microstrip filters, dielectric resonators and waveguide filters. It has the advantages of high precision and fast forming speed. Combination of the dry pressing method with CNC (Computer Numerical Control) technology enabled the preparation of filters with excellent accuracy. However, the high cost of molds and the uneven density of green compacts are the two main limits. Secondly, the preparation of circuit substrates by using tape casting method combined with low-temperature/high-temperature co-firing processes is introduced. This method is especially useful to realize the miniaturization and integration of microwave devices. However, there is a certain difference between the simulation and experimental results, indicating that there is room for optimization. Then, the current research status of additive manufacturing in the fabrication of ceramic filters is presented. The forming principles and application fields of three technologies, namely SLA (Stereolithography Apparatus), DLP (Digital Light Processing), and DIW (Direct Ink Writing), are discussed in detail. The advantages and disadvantages of additive manufacturing technology in the preparation of microwave devices with complex structures are explored. After development for thirty years , the gel-casting method has been employed to develop a number of mature systems. Ceramic slurries with high solid content and low viscosity are required to ensure the high precision and consistency of the green bodies. With this method, net-shape forming of complex structures can be achieved, while the green bodies have a certain mechanical strength after drying, which guarantees the feasibility of machining. Finally, a composite forming method that combines the dry pressing method and the tape casting method is introduced. A Ka-band pass filter is prepared by using a four-layer tape-cast ceramic substrate and an embedded dielectric resonator. The actual measurement results of the filter are in good agreement with the simulation results, exhibiting advantages such as high selectivity, low profile and small volume.[Conclusions and Prospects] This paper is aimed to briefly review the forming methods of ceramic filters, including dry pressing forming method, tape casting forming method, additive manufacturing method, gel casting method and others. The application scenarios, as well as the respective advantages and disadvantages of different methods, are compared. Additionally, the opportunities and challenges faced in the preparation of ceramic microwave filters are put forward, providing a reference for the research and development of the next generation of ceramic filters. In the future, in-depth research should be carried out in the preparation of ceramic slurries with high solid content and low viscosity, the optimization and improvement of various forming technologies and composite forming technologies, so as to promote the development of microwave devices.

Key words: microwave dielectric ceramics; filter; 5G


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