Thermally conductive polymer composites for electronic packaging

Polyurethane composites filled with alumina or carbon fibers were prepared to study the thermal conductivities and dielectric properties of polymer composites under humid environments. The thermal conductivities of these polymer composites in relation to filler concentrations and filler sizes were investigated and it was found that the thermal conductivities can increase up to 50 times that of pure polyurethane. The results were analyzed using Agari's model to explain the intrinsic reasons to affect the thermal conductivities of composites. The dielectric loss of these polymer composites were also measured to estimate the influence of moisture under various humid environments. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2733–2738, 1997     

AlN/碳掺杂氮化硼纳米管复相陶瓷的制备及性能

将1%～10%(体积分数)碳掺杂氮化硼纳米管(carbon-doped boron nitride nanotubes,BCN-nt)引入到纳米AlN中,采用放电等离子烧结得到致密的AlN/BCN-nt复相陶瓷。结果表明:适当提高烧结温度能使AlN晶粒充分长大,提高AlN晶粒完整性并有效去除结构中的氧杂质,因而显著改善了引入BCN-nt对热导率的劣化。在Kα波段(26.5～40.0GHz),随BCN-nt含量的增加,材料的介电常数实部和虚部都呈现逐渐增大的趋势,损耗因子也逐渐增加。提高烧结温度对介电常数影响不大,而过高的温度使介电常数虚部明显下降。适当的BCN-nt含量和烧结温度能够在提供稳定的介电损耗同时兼顾较高的热导率。     

Role of silica nanoparticle in multi‐component epoxy composites for electrical insulation with high thermal conductivity

Multicomponent epoxy micro/nano‐composites containing micro‐alumina, micro‐quartz, and nano‐silica were fabricated to develop electrical insulation materials with high thermal conductivity. Simply changing the ratio between the alumina and quartz microparticles caused a trade‐off relationship between the thermal conductivity and electrical insulation. Increasing the alumina content in the epoxy‐alumina/quartz micro‐composites enhanced the thermal conductivity but deteriorated the dielectric strength. An increase in the thermal conductivity without incurring a loss in the dielectric strength was achieved by incorporating silica nanoparticles in the epoxy micro‐composites. Adding silica nanoparticles to the epoxy micro‐composites was found to be more efficient in improving the thermal conductivity compared to increasing the alumina ratio, especially at low alumina/quartz ratios. This behavior corresponded to theoretical models. Therefore, we provide a useful insight, both practical and theoretical, into the more advanced optimization of designing multicomponent epoxy composites for electrical insulation with high thermal conductivity.     

Microwave dielectric properties and thermal conductivities of low-temperature sintered (Na1-xKx)2MoO4 (x ≤ 0.2) ceramics

The Mo-based glass-free spinel-type structure of the (Na_1-xK_x)₂MoO₄ (x = 0.0, 0.1, and 0.2) ceramic series was prepared using the traditional solid-state method at the low sintering temperature (<650 °C). The microwave dielectric properties of the (Na_1-xK_x)₂MoO₄ series were determined in terms of phase compositions, crystal structure (via XRD), and microstructure analysis (via FE-SEM and EDS). The results revealed that the double-phase (cubic and orthorhombic) formation plays a significant role in the entire (Na_1-xK_x)₂MoO₄ series. It exhibits excellent dielectric properties: dielectric constant ε_r = 4 (1 GHz)/3.77 (15 GHz), tangent loss tan δ = 8.3 × 10^?2 (1 GHz)/7 × 10^?3 (15 GHz; Q × f = 2143 GHz), temperature coefficient of frequency (TCF) τ_f = ?6.45 ppm/°C, and room temperature thermal conductivity (κ) = 1.76 W/(m.K) for x = 0.1 at a sintering temperature of 575 °C. These make the (Na_1-xK_x)₂MoO₄ ceramic series a potential candidate for low-temperature co-fired ceramic (LTCC) substrate applications (as used in antennas) for high-speed data communications.     

Electric and Dielectric Properties of Some Fluorescent Dye/PMMA Solar Concentrators

The effect of dye concentrations of perylene, RH-6G and K₁ doped in polymethylmethacrylate (PMMA) as matrix material, on the electrical properties of some fluorescent solar concentrator (FSC) have been investigated. The samples were prepared by dissolving grains of both PMMA and dyes in chloroform, which were then left at room temperature to evaporate the solvent. The samples were characterized by differential scanning calorimeter (DSC). The results of both dc conductivity (σ_dc) and ac conductivity (σ_ac) showed that the total conductivity σ_tot(w) is higher than σ_dc and the activation energy of σ_tot is lower than that of (σ_dc) due to the increase of the applied field frequency, which enhances the carrier jumping and subsequently the conductivity value. The dielectric properties (dielectric constant (?′), dielectric loss (?″), and dielectric tangent (tanδ) have been studied. They show that ?′ increases by increasing the concentration of the dyes doped in PMMA. All the dielectric constants, the dielectric loss and loss tangent temperature dependence, show a peak value affected by the dye concentration as well the frequency changes. The temperature dependence of the exponent S shows that at low temperature the conduction obeys a quantum mechanical tunnel model, while at high temperature the conduction obeys the correlated barrier-hopping model.     

AIN陶瓷烧结技术研究进展

氮化铝（AIN）因其具有高热导率，作为基片材料在电子元器件中得到日益重视。本文主要论述了氮化铝陶瓷制备过程中各种烧结参数，包括烧结助剂、烧结气氛、保温时间、常压烧结、热压烧结、微波烧结和等离子烧结等对氮化铝陶瓷性能的影响。并指出可通过合适的AIN粉体制备技术，结合快速烧结方法可得到具有晶粒细小、结构均匀、高致密度和高导热率的AIN陶瓷。     

AlN陶瓷的烧结致密化与导热性能

氮化铝陶瓷具有高热导率、低介电常数、与硅相匹配的热膨胀系数等优良特性,应用领域非常广泛。对A l N粉体的合成、烧结工艺、助烧结剂及其应用等方面进行了介绍,并对A l N未来的发展趋势进行了展望。     

Transverse Thermal Conductivity of Thin C/SiC Composites Fabricated by Slurry Infiltration and Pyrolysis

Thin C/SiC composites were fabricated by infiltrating a woven carbon fiber fabric with a slurry of SiC powder and polymer precursor for SiC, followed by heat treatment for pyrolysis. The effects of heat treatment parameters on the crystallization of the polymer-derived SiC, the composite microstructure, and the transverse thermal properties were assessed. Whereas composites heat-treated at 1000°C were crack-free and nearly fully dense, composites that were subjected to further multiple reinfiltration and heat treatment cycles at 1600°C developed porosity and cracking. However, the transverse thermal conductivity was increased significantly by the higher-temperature heat treatment, to values higher than that of a composite with a chemical-vapor-infiltration SiC matrix and the same fiber reinforcement.     

Cold sintering process: A new era for ceramic packaging and microwave device development

Cold sintering process (CSP) is an extremely low‐temperature sintering process (room temperature to ~200°C) that uses aqueous‐based solutions as transient solvents to aid densification by a nonequilibrium dissolution‐precipitation process. In this work, CSP is introduced to fabricate microwave and packaging dielectric substrates, including ceramics (bulk monolithic substrates and multilayers) and ceramic‐polymer composites. Some dielectric materials, namely Li₂MoO₄, Na₂Mo₂O₇, K₂Mo₂O₇, and (LiBi)_0.5MoO₄ ceramics, and also (1?x)Li₂MoO₄?xPTFE and (1?x)(LiBi)_0.5MoO₄?xPTFE composites, are selected to demonstrate the feasibility of CSP in microwave and packaging substrate applications. Selected dielectric ceramics and composites with high densities (88%‐95%) and good microwave dielectric properties (permittivity, 5.6‐37.1; Q × f, 1700‐30 500 GHz) were obtained by CSP at 120°C. CSP can be also used to potentially develop a new co‐fired ceramic technology, namely CSCC. Li₂MoO₄?Ag multilayer co‐fired ceramic structures were successfully fabricated without obvious delamination, warping, or interdiffusion. Numerous materials with different dielectric properties can be densified by CSP, indicating that CSP provides a simple, effective, and energy‐saving strategy for the ceramic packaging and microwave device development.     

Low-Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics

The sintering behavior and dielectric properties of perovskite Ag(Nb_{1− x} Ta _x )O₃ (0 < x < 1) solid solutions and two-phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid-phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid-solution series and by creating composite microstructures. Two-phase assemblages consisting of Ag(Nb_3/4Ta_1/4)O₃ and Ag(Nb_1/4Ta_3/4)O₃ were synthesized to achieve a temperature-stable dielectric material for high-frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q × f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high-frequency ring-resonator measurements and microstructural characterization. The Ag(Nb_{1− x} Ta _x )O₃ solid solutions and composites are promising candidates as embedded capacitors for radio-frequency/microwave applications.     

Cold sintering co-firing of (Ca,Bi)(Mo,V)O4–PTFE composites in a single step

Because of large differences in the processing temperature windows between ceramics and polymers, the single-step co-sintering of microwave dielectric ceramic–polymer substrates remains challenging. In this work, a dense (Ca_0.65Bi_0.35)(Mo_0.65V_0.35)O₄ (CBMVO) ceramic was first prepared through cold sintering at 150°C, under a uniaxial pressure of 300 MPa for 60 min with Li₂MoO₄ (LMO) as a transient low-temperature solvent. Cold-sintered CBMVO–5 wt% LMO ceramic shows excellent microwave dielectric properties: ε_r ∼ 11.4, Q × f ∼ 7070 GHz, τ_f ∼ −7.4 ppm/°C. Moreover, the optimized cold sintering process enabled the preparation of a layered co-sintered (2–2 type) CBMVO–polytetrafluoroethylene composite, which maintained excellent microwave dielectric properties and showed a good heterogeneous interface bonding. The proposed cold sintering co-firing of ceramic–polymer composites in a single step shows great potential for application in the seamless integration between ceramics and polymer substrates.