Constrained Sintering of Low-Temperature Co-Fired Ceramics

This paper discusses the effect of uniaxial compressive stress and pressureless constraint on the microstructure, density, and shrinkage anisotropy during the sintering of two commercial low-temperature co-fired ceramic (LTCC) systems, i.e., Heraeus CT2000 (CT) and DuPont 951Tape (DU). Under uniaxial compression, the ratio of axial to transverse shrinkage of DU is significantly higher than that of CT. A simple linear viscous theory was used to estimate the change in the strain rates produced by the external stress and the stress required to produce zero shrinkage. The theory was found to overestimate the measured stress-induced strain rates. The uniaxial compressive stress required for zero overall shrinkage was estimated to be ∼60 kPa for DU and 80 kPa for CT. The estimate for the DU materials was in good agreement with the experimental data, but there was significant deviation for the CT material. Higher viscosity and higher constraining stresses led to lower densities in pressure-less constrained CT specimens compared with DU.     

Densification and Sintering Viscosity of Low-Temperature Co-Fired Ceramics

In this paper, we present sintering and uniaxial viscosity data of three commercial low-temperature co-sintered ceramic systems, i.e., DuPont 951Tape (DU), Heraeus CT2000 (CT), and Ferro A6M (FE), measured by cyclic loading dilatometry. The viscosity initially decreases with temperature, changes little during the intermediate stage, and increases towards the end of densification. The viscosity increases sharply beyond the onset of crystallization. At slower heating rates, the viscosity increases at lower temperature, because of densification and crystallization. The isothermal viscosity data range from 0.1 to 100 GPa·s between 73% and 95% density. Ceramic particle-filled glasses show a higher isothermal viscosity compared with pure glass system, i.e., FE. From master viscosity curves based on isothermal data, the activation energies for viscous flow were ∼375±30 and 450±10 kJ/mol for DU and FE, respectively. These energies are comparable to values obtained from the master sintering curve approach.     

Co-Firing and Shrinkage Matching in Low- and Middle- Permittivity Dielectric Compositions for a Low-Temperature Co-Fired Ceramics System

Co-firing and shrinkage matching of low- and middle- permittivity dielectric compositions for the application to a low-temperature co-fired ceramics system was investigated. Several glass compositions were initially examined for the low-temperature firing of middle- K ( K ∼20) dielectrics, and their compositions were further modified for shrinkage matching between low- K ( K ∼6) and middle- K layers. By controlling the processing parameters such as the solid loading ratio and stacking sequence, the warpages between hetero-layers could be minimized.     

Novel Microsystem Applications with New Techniques in Low-Temperature Co-Fired Ceramics

Low-temperature co-fired ceramic (LTCC) enables development and testing of critical elements on microsystem boards as well as nonmicroelectronic meso-scale applications. We describe silicon-based microelectromechanical systems packaging and LTCC meso-scale applications. Microfluidic interposers permit rapid testing of varied silicon designs. The application of LTCC to micro-high-performance liquid chromatography (μ-HPLC) demonstrates performance advantages at very high pressures. At intermediate pressures, a ceramic thermal cell lyser has lysed bacteria spores without damaging the proteins. The stability and sensitivity of LTCC/chemiresistor smart channels are comparable to the performance of silicon-based chemiresistors. A variant of the use of sacrificial volume materials has created channels, suspended thick films, cavities, and techniques for pressure and flow sensing. We report on inductors, diaphragms, cantilevers, antennae, switch structures, and thermal sensors suspended in air. The development of "functional-as-released" moving parts has resulted in wheels, impellers, tethered plates, and related new LTCC mechanical roles for actuation and sensing. High-temperature metal-to-LTCC joining has been developed with metal thin films for the strong, hermetic interfaces necessary for pins, leads, and tubes.     

Sub-Critical Crack Growth Behavior of a Low-Temperature Co-Fired Ceramic

The strength of sintered low-temperature co-fired ceramic (LTCC) disks was measured under dry (<2% relative humidity [RH]) and wet (98% RH) conditions, using ring-on-ring, biaxial-flexure testing in the stressing rate range of 0.002–2 MPa/s. The empirical sub-critical crack growth parameters, n and A , were calculated. The values of the parameters in the wet environment were higher than in the dry environment. It is argued that the high RH n value is more representative of material behavior. Results of fractographic examination are reported and expected lifetime and safe design stresses for LTCC are calculated.     

Fabrication of Low-Temperature Co-Fired Ceramics Micro-Fluidic Devices Using Sacrificial Carbon Layers

Ease of fabrication and design flexibility are two attractive features of low-temperature co-fired ceramics (LTCC) technology for fabrication of complex micro-fluidic devices. Such structures are designed and processed using different shaping methods, the extent and complexity of which depends on the final device specifications (dimensions, and mechanical and functional properties). In this work, we propose a sacrificial layer method based on carbon-black paste, which burns out during the LTCC firing stage. The article will summarize the preparation of the paste, influence of processing conditions on the final dimensions, and demonstrate the mechanically integrated structures obtained using this technique. Some of these are membranes of various diameters (7–12 mm) with a thickness of 40 μm and a variety of internal spacing (15–60 μm), free-hanging thick-film resistor bridges on LTCC for heating micro-volumes. The main methods of the study will be thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and dilatometry in addition to electronic instruments for device characterization.     

电子基板用玻璃/陶瓷复合材料的低温共烧与性能

玻璃/陶瓷低温共烧复合材料具有高导热性、快速电子信号传输性能、热膨胀系数与硅匹配、力学性能良好等优点,被广泛应用作电子基板材料。本文简要阐述了玻璃/陶瓷复合材料的烧结机理和影响因素,综述了主要的制备方法,指出了烧结过程中可能存在的关键问题,并讨论了玻璃/陶瓷复合材料的性能调控方法。最后,展望了玻璃/陶瓷复合材料在电子信息领域的发展方向和应用前景。     

Viscous Flow as the Driving Force for the Densification of Low-Temperature Co-Fired Ceramics

Filled glass–ceramic composites, like low-temperature co-fired ceramics (LTCC), must densify at temperatures <900°C. The densification mechanism of LTCC is often described by liquid-phase sintering. The results of this paper clearly show that densification of ceramic-filled glass–composites with a glass content above 60 wt% can be attributed to viscous sintering, which is decisively controlled by the viscosity of the glass during the heat treatment. This is demonstrated by the experimental determination of the viscosity of a MgO–Al₂O₃–B₂O₃–SiO₂ glass dependent on temperature, by investigation of the wetting behavior of the glass on the ceramic filler mullite, and of the microstructural development. It was found that the glass does not wet the filler material in a temperature range up to 1000°C. Therefore, liquid-phase sintering can be excluded. Independent of any wetting effect and therefore in the absence of capillary forces, densification starts at a temperature of 750°C, which corresponds to a viscosity of 10^9.5 dPa·s. This densification can be attributed to viscous flow of the glass matrix composite.     

Effect of Metallization on the Lifetime Prediction of Mechanically Stressed Low-Temperature Co-Fired Ceramics Multilayers

Multilayer ceramics based on Low-Temperature Co-fired Ceramics (LTCC) are gaining increasing interest in the manufacturing of high-integrated devices for microelectronic and sensor applications. In many applications the parts are exposed to mechanical stresses, which is an important issue regarding the reliability of the device. To predict the lifetime of LTCC multilayer devices, and to extend their application range, basic mechanical data of this material are needed. In this paper metallized LTCC multilayers are investigated concerning their flexural strength, crack growth rate, and lifetime prediction. The results show that the electronic layout concerning the location of vias and metallization has a strong influence on the reliability and lifetime prediction of such co-fired LTCC devices. Mass flow sensors for the measurement of injected fuel quantities, which were fabricated on the basis of LTCC and which are exposed to a stress level of 100 MPa, achieve sufficient lifetimes. Therefore, LTCC is an interesting material to fabricate devices, in which LTCC fulfils the requirements of a functional and structural material.     

Phase Evolution and Microwave Dielectric Properties of Lanthanum Borate-Based Low-Temperature Co-Fired Ceramics Materials

A new potential low-temperature co-fired ceramics (LTCC) system based on a simple lanthanum borate (La₂O₃–B₂O₃) glass was investigated with regard to phase development and microwave dielectric properties as functions of alumina filler content less than 50 wt% and firing temperature up to 1050°C. Unexpected crystalline phases, such as La(BO₂)₃, LaAl_2.03(B₄O₁₀)O_0.54, LaBO₃, and Al₂₀B₄O₃₆, developed during the firing process are likely responsible for substantial resultant changes in physical and microwave dielectric properties. As a specific example, a high-quality factor of 785 at 15.8 GHz obtained for a composition containing 30 wt% alumina supports the hypothesis that the phase-dielectric property relation exists in this LTCC system. On a practical basis, two phases of LaAl_2.03(B₄O₁₀)O_0.54 and LaBO₃ must be important in determining the final dielectric performance by manipulating the ratios of glass and filler and by selecting a desirable temperature.     

Influence of Green Formulation and Pyrolyzable Particulates on the Porous Microstructure and Sintering Characteristics of Tape Cast Ceramics

Porous morphology and total porosity produced in sintered ceramic tapes was controlled by the amount and distribution of pyrolyzable graphite particles added to a colloidal suspension during a tape casting operation. A conceptual model of the green tape microstructure was used to explain the influence of graphite and tape formulation on sintering characteristics. The creation of a connected, open porous network in the sintered body was the result of graphite particle percolation within the green body. Additional voidage introduced by particle bridging was the source of excess porosity and also resulted in a bimodal pore size distribution. Sintering shrinkage was determined by the ceramic packing density, which was primarily determined by the tape formulation.     

Development and Simulation of an Embedded Hydrogen Peroxide Catalyst Chamber in Low-Temperature Co-Fired Ceramics

Satellites in the range of 10–50 kg require small propulsion devices to perform station-keeping tasks in orbit. Low-temperature co-fired ceramic structures provide a unique platform to produce a reliable, low-cost micropropulsion system. The design uses microchannels embedded in the ceramic substrate to create a nozzle and embedded catalyst chamber. A hydrogen peroxide monopropellant is injected into a silver-coated catalyst chamber structure. The monopropellant decomposes into hot gas, which is expelled through the nozzle producing thrust. A thermal energy balance and a kinetic model is presented along with performance testing.     

Influences of the Glass Phase on Densification, Microstructure, and Properties of Low-Temperature Co-Fired Ceramics

Multilayer ceramic devices based on low-temperature co-fired ceramics (LTCC) materials provide a very promising technology. Most LTCC tapes available today contain considerable fractions of glass powders to lower the sintering temperature. However, the glassy phases offer more possibilities to set a proper sintering behavior, on the one hand, and to tailor the desired properties of the final LTCC substrate, on the other. The exploitation of demixing and subsequent crystallizing glass compositions was shown on an example of a low-permittivity (4.4)—low-loss (1.5 × 10⁻³) LTCC with a high quartz content. In another LTCC material, undesired demixing could be restricted and the crystal phase anorthite could be triggered by partial dissolution of alumina in the liquid phase during sintering. To estimate the effect of silver diffusion in the latter material, the surroundings of a pure silver via were studied. A silver-contaminated range of 50 μm was detected. Using model glasses containing silver oxide, a strong influence of dissolved silver on viscosity and crystallization behavior of the liquid phase was demonstrated. The dielectric properties of the sintered substrates were not degraded.     

Near Zero Shrinkage of an Low-Temperature Co-Fired Ceramic Package by Constrained Sintering Using Screen Printed Alumina Paste

Conventional free sintering of low-temperature co-firing ceramic (LTCC) technology has several merits such as sintering temperature below 1000°C that enables co-firing with electrode materials of silver or copper metal and multilayer structure formation. But due to the free sintering process, large shrinkage occurs. To fabricate electronic devices and components with near zero shrinkage within x, y directions constrained sintering (CS) technology is required. In this study a constrained sintering paste (CSP) utilizing alumina powder, which has a higher sintering temperature than LTCC powders, was fabricated for CS technology. The effect of CSP formulated using alumina powder on shrinkage was studied according to variation in paste composition. As a result ceramic package structure with a cavity was fabricated with shrinkage control of 0.028%, which is far smaller than the current CS technology shrinkage of approximately 0.1%.     

Lamination and Sintering Shrinkage Behavior in Multilayered Ceramics

Variations in lamination conditions, such as pressure, temperature, and time, changed the laminated density of multilayered alumina but had no effect on the sintered density. The present results showed that sintering shrinkage values differ with lamination conditions and vary inversely with laminated density. When lamination was accomplished using a press die, the difference in shrinkage between the X-Y and the Z directions was <1%. The effect of the press die could be explained by introducing a new factor, the SDF (shear deformation factor), which represents the ratio of area change in the X-Y direction before and after lamination. The lamination of green sheets exhibited almost the same behavior as did the compression of granules. A linear relationship also was found between laminated density and the logarithm of lamination pressure. Results for sintering shrinkage in the overall range of measured laminated densities showed that sintering shrinkage behavior could be divided into three regions; that is, the laminates had three packing structures with different laminated densities. A new factor ( k ), related to packing structure values before and after sintering, was introduced to explain the sintering shrinkage behavior. Each k value was obtained from the relationship between laminated density and sintering shrinkage. Comparing k factors for the laminated densities ( X-Y and Z ) under various lamination conditions made it possible to systematically analyze variations in the sintering shrinkage behavior of laminates with processing conditions. An estimation of sintering shrinkage was possible from that analysis.     

用流延法制备优质陶瓷基片的研究

本文概括了流延成型工艺的现状，介绍了有机基流延成型工艺中溶剂及添加剂的作用和选择原则，同时介绍了几种新的流延成型工艺。     

Low-Temperature Fabrication of Anorthite Ceramics

Fabrication of anorthite ceramics suitable for low temperature, sinterable, multilayered substrates is described. Anorthite ceramics were synthesized from relatively pure kaolin and calcites of varying particle sizes. Mixed powders were uniaxially pressed and fired between 900° and 1200°C. Firing at 1000°C yielded a dense anorthite ceramic when the finest calcite powder was used. The relative density and water absorption of the anorthite ceramic were 94% and almost zero, respectively. Mixtures containing coarser calcite had lower densities and higher water absorption.     

Low-Temperature Aging of Y-TZP Ceramics

The isothermal tetragonal-to-monoclinic transformation of a 3Y-TZP ceramic is investigated from 70° to 130°C in water and in steam by X-ray diffraction and optical interferometer techniques. Aging kinetics followed by X-ray diffraction are fitted by the Mehl-Avrami-Johnson law, suggesting nucleation and growth to be the key mechanisms for transformation. Optical interferometer observations of highly polished samples effectively reveal a nucleation and growth micromechanism for tetragonal-to-monoclinic transformation. A model based on surface change analysis is developed that fits closely to the X-ray diffraction results.     

低温共烧陶瓷技术前景

介绍了低温共烧陶瓷（LTCC）的概念和特点，总结了LTCC的国内发展现状以及新产品开发进展，最后介绍了LTCC产品的广泛应用。     

添加剂对AIN陶瓷高温氧化行为的影响

研究了热压AIN陶瓷在1200－1400℃的氧化行为,分析了不同的烧结助剂对AIN陶瓷氧化行为的影响。结果表明,AIN陶瓷在空气中的氧化符合抛物线规律,以Ni为添加剂的AIN陶瓷的高温抗氧化性优于以Y2O3为添加剂的AIN陶瓷。