Design content of new LTCC composite dielectric materials

Among the numerous packaging technologies, Low Temperature Co-fired Ceramic (LTCC) technology has become the focus of international research, because the products made using LTCC technology not only have high current density and small size, but also have high reliability and excellent electrical performance, transmission characteristics and sealing. LTCC technology is an advanced hybrid circuit packaging technology. It integrates four passive devices, namely transformer (T), capacitor (C), inductor (L) and resistor (R), and configures them in a multi-layer wiring substrate, and integrates them with active devices (such as power MOS, transistors and IC circuit modules, etc.) into a complete circuit system. Therefore, LTCC technology is also called hybrid integration technology, which can effectively improve the packaging density of the circuit and the reliability of the system.

Focusing on the low temperature co-fired ferrite LTCF (Low Temperature Co-fired Ferrite) material in LTCC technology, the author adopted a research model of theory, experiment and application to develop a new type of LTCC composite dielectric material. Not only did he theoretically simulate the composite mechanism of the material, but he also studied its application in LTCC filters.

The author has done some exploratory and innovative work in theoretical models, material preparation and device design. The specific contents are as follows:

(1) An exploratory low-temperature sintering model for LTCC ceramics was established. The model is based on the liquid phase sintering theory, taking the capillary pressure caused by the liquid phase at the grain boundary and the change of chemical potential energy during the dissolution-precipitation process as the sintering driving force, and linking the sintering temperature and time with the final grain size and relative density after sintering, and simulating the change trend of relative density during the low-temperature sintering dynamic process.

(2) The composite theory of ferroelectric-ferromagnetic composite materials was proposed for the first time and a systematic analysis was given. The influence of the chemical structure and electromagnetic properties of the two-phase components in the composite material on the theoretical composite possibility was discussed. A composite model was established based on the microstructure of the material. The model assumes that the ferroelectric phase is evenly distributed on the surface of the ferromagnetic phase grains, and together with the pores, it forms a non-magnetic thin layer that separates the ferromagnetic grains and isolates the ferromagnetic particles. By analyzing the changes in the internal field of the ferromagnetic grains in the composite structure, the relationship equation between the ferroelectric/ferromagnetic component ratio of the composite material and the composite magnetic permeability is derived; in addition, using the equivalent circuit of the current flowing in the microstructure, the relationship expression between the complex dielectric constant and the frequency of the composite material at different ferroelectric/ferromagnetic component ratios is derived.

(3) The influence of process conditions on the electromagnetic properties of materials was studied. According to the process flow, the process parameters such as pre-sintering temperature, secondary ball milling time, heating and cooling rate in the sintering curve, sintering temperature and holding time were changed. The influence of changing process parameters on the microstructure of ferrite materials was understood through SEM, XRD and other analytical methods. The influence of process parameters on the electromagnetic properties of materials was obtained by measuring the dielectric constant spectrum, magnetic permeability spectrum and quality factor of the materials. The optimal ferrite sintering process parameters were obtained based on the experimental data results.

(4) The effects of different doping ions and fluxes on the microstructure and electromagnetic properties of low-temperature sintered ferrite LTCF materials were studied. First, the effects of different MnCO3 and CuO contents on the sintering characteristics, microstructure and electromagnetic properties of NiZn ferrite were studied. For the first time, it was found that the electromagnetic properties of NiZn ferrite doped with Mn ions are sensitive to the sintering temperature. Secondly, the effects of different fluxes Bi2O3, WO3 and Nb2O5 on the sintering characteristics, microstructure and electromagnetic properties of NiCuZn ferrite were studied. The experiment revealed that W6+ improves the microstructure of the material. Finally, the low-temperature NiCuZn ferrite was modified and doped to study the effects of rare earth oxide CeO2 on its microstructure and electromagnetic properties, and the magnetic spectrum and dielectric spectrum of NiCuZn ferrite doped with rare earth elements were given.

(5) A new type of ferroelectric-ferromagnetic composite material with inductance and capacitance based on different low-temperature sintered NiCuZn ferrites and high dielectric constant (BaTiOk+X) perovskites was developed, and the effects of different ferroelectric-ferromagnetic contents on the microstructure and capacitance-inductance duality of each group of composite materials were studied. The effects of different fluxes Bi2O3, WO3 and Nb2O5 on their sintering characteristics, microstructure and capacitance-inductance duality were also studied. Finally, the composite material was modified by rare earth doping, and the effect of rare earth oxide CeO2 on its microstructure and capacitance-inductance duality was studied.

(6) Design and manufacture two bandpass filters for 3G communication equipment using LTCC composite bipolar materials. Use Ansoft HFSS electromagnetic simulation software to simulate the filter model. By adjusting the filter structure parameters, the filter performance indicators meet the requirements and are manufactured. A microstrip bandpass filter with a bandpass center frequency of 3.5 GHz, insertion loss <2.8 dB, bandwidth >400 MHz, and stopband attenuation greater than 35 dB and an LC bandpass filter with a bandpass center frequency of 1.4 GHz, insertion loss <3 dB, bandwidth >160 MHz, and stopband attenuation greater than 30 dB are manufactured.