LTCC material requirements
The requirements for material properties of LTCC devices include electrical properties, thermomechanical properties and process properties.
The dielectric constant is the most critical property of LTCC materials. Since the basic unit of the radio frequency device-the length of the resonator is inversely proportional to the square root of the dielectric constant of the material, when the working frequency of the device is low (such as hundreds of MHz), if a material with a low dielectric constant is used, the device The size will be too large to use. Therefore, it is best to serialize the dielectric constant to suit different operating frequencies.
Dielectric loss is also an important parameter considered in the design of radio frequency devices, and it is directly related to the loss of the device. In theory, the smaller the better. The temperature coefficient of the dielectric constant is an important parameter that determines the temperature stability of the electrical performance of the radio frequency device.
In order to ensure the reliability of LTCC devices, many thermo-mechanical properties must also be considered when selecting materials. The most critical one is the coefficient of thermal expansion, which should match the circuit board to be soldered as much as possible. In addition, considering processing and future applications, LTCC materials should also meet many mechanical performance requirements, such as bending strength σ, hardness Hv, surface flatness, elastic modulus E and fracture toughness KIC and so on.
“Process performance can generally include the following aspects: First, it can be sintered at a temperature below 900°C into a dense, non-porous microstructure. Second, the densification temperature should not be too low, so as not to prevent the discharge of organic matter in the silver paste and the green belt. Third, after adding appropriate organic materials, it can be cast into a uniform, smooth, and strong green tape.
Classification of LTCC materials
At present, LTCC ceramic materials are mainly composed of two systems, namely the “glass-ceramic” system and the “glass + ceramic” system. Doping with low-melting oxide or low-melting glass can reduce the sintering temperature of ceramic materials, but the reduction of sintering temperature is limited, and the performance of the material will be damaged to varying degrees. The search for ceramic materials with low sintering temperature has attracted the attention of researchers. The main varieties of such materials being developed are barium tin borate (BaSn(BO3)2) series, germanate and tellurate series, BiNbO4 series, Bi203-Zn0-Nb205 series, ZnO-TiO2 series and other ceramic materials. In recent years, Zhou Ji’s research group at Tsinghua University has been committed to research in this area.
LTCC material properties
The performance of LTCC products depends entirely on the performance of the materials used. LTCC ceramic materials mainly include LTCC substrate materials, packaging materials and microwave device materials. Dielectric constant is the most critical property of LTCC materials. The dielectric constant is required to be serialized in the range of 2 to 20000 to be suitable for different operating frequencies. For example, a substrate with a relative permittivity of 3.8 is suitable for the design of high-speed digital circuits; a substrate with a relative permittivity of 6 to 80 can well complete the design of high-frequency circuits; a substrate with a relative permittivity of up to 20,000 can make High-capacity devices are integrated into a multilayer structure. High frequency is a relatively obvious trend in the development of digital 3C products. The development of low dielectric constant (ε≤10) LTCC materials to meet the requirements of high frequency and high speed is a challenge for how LTCC materials can adapt to high frequency applications. The dielectric constant of the 901 system of FerroA6 and DuPont is 5.2 to 5.9, the 4110-70C of ESL is 4.3 to 4.7, the dielectric constant of NEC’s LTCC substrate is about 3.9, and the dielectric constant as low as 2.5 is under development.
The size of the resonator is inversely proportional to the square root of the dielectric constant, so when used as a dielectric material, the dielectric constant is required to be large to reduce the device size. At present, the limit of ultra-low loss or ultra-high Q value, relative permittivity (>100) or even >150 dielectric materials are research hotspots. For circuits requiring larger capacitance, materials with high dielectric constant can be used, or a dielectric material layer with a larger dielectric constant can be sandwiched between the LTCC dielectric ceramic substrate material layer, and the dielectric constant can be between 20 and 100. Choose between. Dielectric loss is also an important parameter to consider in the design of radio frequency devices. It is directly related to the loss of the device. In theory, it is hoped that the smaller the better. Currently, LTCC materials used in radio frequency devices are mainly DuPont (951,943), Ferro (A6M, A6S), Heraeus (CT700, CT800 and CT2000) and Electro-science Laboratories. They can not only provide serialized LTCC green ceramic tape with dielectric constant, but also provide matching wiring materials.
Another hot issue in the research of LTCC materials is the compatibility of co-fired materials. When co-firing different dielectric layers (capacitors, resistances, inductances, conductors, etc.), the reaction and interface diffusion between different interfaces should be controlled to make the co-firing matching of each dielectric layer good, and the density rate and sintering shrinkage between the interface layers The rate and thermal expansion rate are as consistent as possible to reduce the occurrence of defects such as spalling, warping and cracking.
Generally speaking, the shrinkage rate of ceramic materials using LTCC technology is about 15-20%. If the sintering of the two cannot be matched or compatible, the interface layer will split after sintering; if the two materials react at a high temperature, the resulting reaction layer will affect the original characteristics of the respective materials. The co-firing compatibility of two materials with different dielectric constants and compositions and how to reduce the mutual reactivity are the focus of research. When LTCC is used in high-performance systems, the key to strict control of the shrinkage behavior is to control the sintering shrinkage of the LTCC co-fired system. The shrinkage of the LTCC co-fired system along the X-Y direction is generally 12% to 16%. With the help of pressureless sintering or pressure-assisted sintering technology, materials with zero shrinkage in the X-Y direction are obtained [17,18]. When sintering, the top and bottom of the LTCC co-fired layer are placed on the top and bottom of the LTCC co-fired layer as a shrinkage control layer. With the help of a certain bonding effect between the control layer and the multilayer and the strict shrinkage rate of the control layer, the shrinkage behavior of the LTCC structure along the X and Y directions is restricted. In order to compensate for the shrinkage loss of the substrate in the X-Y direction, the substrate will be compensated for shrinkage in the Z direction. As a result, the size change of the LTCC structure in the X and Y directions is only about 0.1%, thereby ensuring the position and accuracy of the wiring and holes after sintering, and ensuring the quality of the device.