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Tape casting and characterization of a glass free LTCC for microwave applications
Keywords: Glass-free LTCC, microwave dielectric properties , sintering kinetics
Low-temperature co-fired ceramic (LTCC) technology has been critically important due to their applications in microwave integrated circuits. The microwave dielectric materials used in LTCC applications have a series of dielectric permittivity (r), high quality factor (Qf), near-zero temperature coefficient (τf) and can be cofired with metal electrodes such as silver and copper. As a substrate materials for microwave integrated circuits especially require a low dielectric permittivity to maximize the signal propagation speed. It was recognized that the impact of dielectric loss becomes more dominant than conductor loss with increase in frequency as far as signal attenuation is concerned. So in order to reduce the loss at high frequencies over 1 GHz, it is beneficial to use dielectric material with low dielectric loss. Recently glass-free LTCC material with appropriate microwave dielectric properties is strongly desired for the multilayer structure applications because it simplifies the chemical interaction with the metal electrode and reduces the possibility of cracking caused by the mismatch of the coefficient of thermal expansion (CTE) between the ceramic and glassy phases. Tape casting is a suitable forming technique to produce the thin flat green ceramic tape with large area and widely used in the production of ceramic substrate and multilayer component. Preparation of homogeneous ceramic slurry with good flowing properties is a key step for tape casting process. A glass-free low temperature co-fired ceramic (LTCC) green tapes were prepared from non-aqueous solvent based ceramic slurry by tape casing technique. In present paper we optimized different variables to prepare glass-free LTCC green tape by tape casting process, and characterized the microwave dielectric properties of the sintered sheet. The sintering kinetics of the green tape and cofiring behavior with Ag/Cu electrodes were also investigated. The rheological property of the slurry was characterized using rheometer. The slurry exhibited typical pseudoplastic behavior. The maximum powder loading attainable in this case is 55wt%. The green tape shows a tensile strength of 2 MPa. The sintering kinetics of the green tape was investigated using heating microscope with automatic image analysis. The sintering activation energy of the green tape was determined to be 150 kJ/mol. The green tape could be densified at 900oC/2h with total shrinkage of 18-20% and 96% theoretical density (TD). The sintered body has a coefficient of thermal expansion of 16 ppm/◦C. Microwave dielectric properties of the sintered tape were characterized in a split-post dielectric resonator operating at 10GHz with the aid of a vector network analyzer. The ceramic sheet with thickness of 0.11mm demonstrated good microwave dielectric properties: εr = 10-11, Qf = 6000080000 GHz and τf ~2.0 ppm/oC. The cross sectional microstructure of the cofired multilayer stack was observed by scanning electron microscopy (SEM). The green tape demonstrated good chemical and sintering shrinkage compatibilities with Ag/Cu thick film during sintering process. Thus the material investigated in this paper can be used a promising microwave dielectric material for LTCC applications.
Bian Jianjiang, professor
Shanghai University
Shanghai , Shanghai

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