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 (ε_r), quality factor (Q×f) and temperature coefficient of the resonant frequency (τ_f). According to the microwave transmission theory, the larger the ε_r 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.

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.

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.

Microwave dielectric ceramics are vital for radar systems, wireless communication and so on, due to their optimizable dielectric constant, high temperature stability and ultra-low loss tangent. These properties make precise electromagnetic wave transmission available by virtue of microstrip antenna ceramic substrates. Their performance heavily depends on fabrication methods, which govern microstructural uniformity, dimensional precision and efficiency of the final products. However, there exist persistent challenges in balancing defect minimization, scalability and cost-effectiveness.

Gel casting can be divided into water-based and non-water-based systems. Water-based gel casting requires polymer additives, such as acrylamide, gelatin, sodium alginate and isobam, to tailor rheology and curing behaviors of slurries. Non-water-based systems involve alcohol or organic solvents. Tape casting is classified into two primary modalities, one is the aqueous-based technique with water as the dispersion medium, while the other is non-aqueous-based approach utilizing organic solutions. The aqueous-based tape casting can be further categorized into singular-layer formation and multi-layer co-casting. The aqueous methodology offers substantial benefits, including superior economic viability and reduced ecological pollution, in addition to sintering enhancement properties. Press forming has two principal methodologies. One is dry pressing and the other is isostatic pressing. The isostatic pressing technique can be further categorized into two distinct variants, one is cold isostatic pressing and the other is hot isostatic pressing. The paramount advantage of employing such advanced technology lies in its exceptional capability to precisely regulate the dimensional accuracy of the ceramic substrate, while simultaneously ensuring superior density of the ceramic components. Non-aqueous systems are classified into ester-based and non-ester-based solvent formulations, which typically improve slurry dispersion and sintering performance, respectively. Extrusion forming adopts two ways, including screw extrusion and hydraulic extrusion. Clearly, this method excels in continuous production of elongated or tubular ceramic components. Actually, it is also applicable to the production and manufacturing of substrates. Additive manufacturing is an emerging approach. It integrates SLA (stereolithography apparatus), SLM (selective laser melting) and FDM (fused deposition modeling). These technologies are facilitative for rapid prototyping and customized fabrication of geometrically complex structures.

This paper was aimed to systematically analyze five key forming methods, including gel casting, cast film molding, compression molding, extrusion molding and additive manufacturing. Focuses are mainly on their fundamental technical mechanisms, comparative advantages and applicable scenarios, thus offering references for the fabrication and optimization of microstrip antenna substrates. In future, software for simulation experiments is needed, which is helpful to optimize the parameters to reduce times of experiments. It is possible to combine different methods to minimize their respective limitations. Meanwhile, eco-friendly forming is an important consideration.