Low Temperature Co-fired Ceramic Substrate Research Articles

Morphologies of Reactive Nanolayer Stacks Sputtered on Ceramic LTCC Substrates Having a Micrometer Scale Surface Roughness

The deposition of reactive multilayer systems (RMS) has been investigated on low temperature cofired ceramic (LTCC) substrates having different surface morphologies. This study analyses the morphologies of RMS layers that were deposited on glass ceramic LTCC substrates. Different surface morphologies were prepared through pretreatments of the LTCC surface. The considered surfaces encompass an untreated natural LTCC surface, a modified sintered LTCC surface by laser ablation, a surface with a deposited metallization layer and finally, a surface with an additional solder layer put on the deposited metallization layer. The different pretreatments lead to significant differences in the roughness of the LTCC substrates, resulting in different reaction velocities and peak temperatures on the various surface morphologies after the RMS’s reaction. As a result, different grades of structural integrity (lift‐off, crack formation) between the reacted RMS layer and the LTCC were observed.This article is protected by copyright. All rights reserved.

Weakening the silicate network through six-coordinated magnesium to achieve a high degree of crystallization of β-CaSiO3 in calcium borosilicate glass ceramics for 5G application

β-CaSiO3 with a low dielectric constant and low dielectric loss is one of the decisive components of commercial calcium borosilicate (CBS) low-temperature co-fired ceramic substrate. However, achieving a high degree of crystallization of β-CaSiO3 is difficult in CBS glass ceramics because of the competitive multiphase crystallization during sintering. We have shown that six-coordinated magnesium can be used to weaken the silicate network to achieve a high degree of crystallization of β-CaSiO3 in CBS glass ceramics. The appropriate MgO content to ensure a large amount of MgO6 production gradually destroys the [SiO4] tetrahedral network as the temperature increases, resulting in a β-CaSiO3 content of 76.2 vol%, when the MgO content is 2.71 mol%. The resulting material has the lowest dielectric constant (4.95@15 GHz) and dielectric loss (8 × 10−4@15 GHz), and the best three-point bend strength (77.43 MPa). Our work provides an effective method of changing the crystallization by modifying the glass network in silicate-based glass ceramics with a low dielectric constant.

Development of LTCC IR-Emitter and Its Packaging

In this paper, thick film-based IR source is fabricated using multilayer Low Temperature Co-fired Ceramic (LTCC) Technology. The proposed emitter is developed using screen printing of platinum material on LTCC substrates. The highly uniform meander shaped structure is fabricated with the size of 3.5 mm × 3.5 mm. Electrical, optical and thermal characterization of the fabricated device are carried out. Device temperature reaches 600 °C at 6.5 V. Optical characterization of the developed device shows the spectral range in the mid-IR region with power consumption of ∼3 W. In-house indigenised package is developed using glass-metal seal technique for packaging of developed IR source. Different windows viz., quartz, LiF and CaF2 are used for packaging. Developed IR source demonstrate the potential to meet the performance, size and cost requirements for various applications. The developed LTCC based IR source has planar and simple structure with high temperature stable lead-free interconnects.

Application of Reactive Bonding Methods on LTCC Substrates

This paper discusses the application of reactive bonding for the area of Low Temperature Cofired Ceramics (LTCC) assemblies. The goal is to reduce the thermal-mechanical stresses during soldering by transferring heat only locally to the solder joints without heating the entire component. Such a reactive multilayer system (RMS) consists of alternating nanolayers (10-300 nm) of at least two metal components which produce an exothermal reaction after ignition. Although the deposition of an RMS is established on silicon substrates for the use in micro-electromechanical systems (MEMS), it is very challenging to create them on LTCC substrates. One of the main obstacles is to overcome all issues connected with the significant roughness, becasue it is not an optimum territory to deposit nanolayers. In this paper, different methods like chemical mechanical polishing (CMP) and laser ablation, to modify the surface morphology, are presented. A direct relation between the morphology and the exothermal reaction can be observed. In addition, 3-D Computational Fluid Dynamics (CFD) simulations were conducted to analyze the process in more detail. These simulations make use of a shoebox model with different layers and an adjustable user-defined function for the heat release of the RMS to adapt the reaction front velocity and the combustion temperature to the experimental values.

Preparation of Sol-Gel-Derived CaO-B2O3-SiO2 Glass/Al2O3 Composites with High Flexural Strength and Low Dielectric Constant for LTCC Application.

Low-temperature co-fired ceramic (LTCC) substrate materials are widely applied in electronic components due to their excellent microwave dielectric properties. However, the absence of LTCC materials with a lower dielectric constant and higher mechanical strength restricts the creation of integrated and minified electronic devices. In this work, sol-gel-derived CaO-B2O3-SiO2 (CBS) glass/Al2O3 composites with high flexural strength and low dielectric constant were successfully prepared using the LTCC technique. Among the composites sintered at different temperatures, the composites sintered at 870 °C for 2 hours possess a dielectric constant of 6.3 (10 GHz), a dielectric loss of 0.2%, a flexural strength of 245 MPa, and a CTE of 5.3 × 10-6 K-1, demonstrating its great potential for applications in the electronic package field. By analyzing the CBS glass' physical characteristics, it was found that the sol-gel-derived glass has an extremely low dielectric constant of 3.6 and does not crystallize or react with Al2O3 at the sintering temperature, which is conducive to improving the flexural strength and reducing the dielectric constant of CBS glass/Al2O3 composites.

Research on Processability and Transmission Performance of Low Temperature Co-Fired Ceramic Ball Grid Array Packaging Based on Electroless Plating Surface Modification for Microwave Transceiver Circuits.

A microwave transmitter/receiver using the low-temperature co-fired ceramic substrate and ball grid array packaging demonstrates superior properties, including high integration, miniaturization, and high electromagnetic shielding. However, it holds limitations of inadequate hermeticity (that is, gas or moist impermeability), high cost, and low reproducibility. In this work, we aim to overcome these difficulties by introducing a new packing technique. The packaging utilizes an electroless plated Ni/Pd/Au surface, resulting in a significant enhancement of the packaging hermeticity by orders of magnitude, approaching the level of <5 × 10-9 Pa·m3/s. Both Sn63Pb37 and Au80Sn20 solder alloys demonstrate exceptional solderability, attributed to Pd atoms diffusing to the Au layer during soldering at 310 °C. A reliability test of the packaging shows that the shear strength of the solder balls drops after thermal shocks but negligibly affects the hermeticity of the packaging. Furthermore, a meticulously designed internal vertical interconnect structure and I/O interconnections were engineered in the ball grid array packaging, showcasing excellent transmission characteristics within the 10-40 GHz frequency range while ensuring effective isolation between ports.

Particle size modulation for effectively optimizing the dielectric and mechanical properties of La2O3–B2O3 based glass-ceramic/ceramic LTCC substrates

Lanthanum borate-based glass-ceramic has recently received considerable attention in the field of high-frequency communications as a sintering aid, due to the low sintering temperature and the precipitation of crystalline phases with excellent dielectric and mechanical properties. Importantly, the densification and crystallization behaviors of glass-ceramic are not only determined by its composition, but also affected by various processing parameters, such as the initial particle size, which is rarely explored for optimizing the dielectric and mechanical properties of lanthanum borate-based glass-ceramic/ceramic composites. Herein, La2O3–B2O3–CaO–P2O5–SiO2 (LBCPS) glass-ceramics with a series of initial particle sizes are achieved by ball milling method through controlling the milling time. Employing LBCPS glass-ceramics with different particle sizes as the sintering aid, various LBCPS/Al2O3 composites are fabricated, and the dielectric and mechanical properties of which are further investigated. Notably, the LBCPS/Al2O3 composite with a moderate glass-ceramic particle size of 2.7 μm exhibits the excellent comprehensive performance with a low dielectric loss of 8.6 × 10−4 at 9.8 GHz and a high flexural strength of 199 MPa. Moreover, the influencing mechanisms of the glass-ceramic particle size on the crystallization, microstructure, and densification of the LBCPS/Al2O3 composite have also been systematically explored and proposed. This research work provides an intriguing strategy for optimizing the dielectric and mechanical properties of the glass-ceramic/ceramic as the high-performance low-temperature co-fired ceramic (LTCC) substrate.

Additive manufacturing of low-temperature co-fired ceramic substrates and surface conductors based on material jetting

Integrated manufacturing of ceramic substrates and surface conductors using 3D material jetting (MJ) technology is a novel additive manufacturing route for low-temperature co-fired ceramics (LTCC). This study evaluates the rheological and jetting properties of LTCC nanoceramic inks with varying solids contents. Ceramic inks with 35 % solid content were found to be the optimal choice with high particle concentration and Ohnesorge numbers (Oh) below 1. The fabrication process used two piezoelectric printheads to inject nanoceramic ink and nanosilver ink to create a ceramic substrate and a surface interdigital electrode (IDE). The substrate sintering shrinkage was 21.5 % and the surface IDE co-fired at the same shrinkage rate as the substrate thus ensuring all electrode pattern details. The electrode conductors demonstrated effective line widths of up to 0.21 mm. Microscopic characterisation of the interface between silver and substrate using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) confirmed the absence of significant elemental diffusion, and X-ray diffraction spectroscopy (XRD) results demonstrate that there is no second-phase generation, thus ensuring the effectiveness of the silver electrode. The ceramic substrates exhibit good thermal stability below 400 ℃ and have a coefficient of thermal expansion (∼ 4.1 ppm/℃) similar to that of silicon, indicating reliability in packaging. A microstrip patch antenna with the design frequency at 9–11 GHz was made through the same MJ procedure, which was experimentally measured to have a resonant frequency of 10.1 GHz, an S11 measurement of −24.6 dB, a simulated radiation efficiency of 88.01 %, and a high gain of up to 7.52 dB. Finally, a curved LTCC substrate with a radius of curvature of 50 mm and curved circuits was successfully fabricated using MJ technology. Interestingly, the curvature of the substrate maintained its original shape both before and after the sintering process, and the serpentine conductor pattern exhibited synchronous contraction, which implies the potential for developing a conformal LTCC utilizing MJ technology.

Hydrothermal performance analysis of microchannel heat sink with embedded module with ribs and pin-fins

A new microchannel heat sink with embedded modules with ribs and pin-fins is proposed as an effective solution to realize microchannel heat dissipation within low-temperature co-fired ceramic substrates. Enhance heat dissipation with the following strategies: 1. microchannel cooling within the low-temperature co-fired ceramic substrate to shorten the heat transfer path; 2. embedding embedded modules in the substrate to reduce the overall thermal resistance; 3. creating ribs and pin-fins on the embedded modules to enhance convective heat transfer. The impacts of relative rib height, relative pin-fin height, and relative number of auxiliary channels on the heat transfer performance, temperature uniformity, and pressure drop were investigated. The performance of the proposed design has been compared to the performance of similar designs like straight rectangular microchannels, microchannels with pin-fins embedded modules, and microchannels with ribs embedded modules. Under the condition of Reynolds number 458, the heat transfer performance is improved by 63.91%, the temperature uniformity is improved by 67.65% and the pressure drop is increased by only 10.24%. The heat transfer performance of the microchannel heat sink with embedded modules with ribs and pin-fins can be effectively improved by adjusting the height of the pin-fins and the ribs height on the embedded module.

5G polarization converter based on low-temperature co-fired ceramic (LTCC) substrate

5G polarization converter based on low-temperature co-fired ceramic (LTCC) substrate

Synthesis and Characterization of Single-Phase α-Cordierite Glass-Ceramics for LTCC Substrates from Tuff.

Single-phase α-cordierite glass-ceramics for a low-temperature co-fired ceramic (LTCC) substrate were fabricated from tuff as the main raw material, using the non-stoichiometric formula of α-cordierite with excess MgO without adding any sintering additives. The sintering/crystallization behavior and the various performances of dielectric properties, thermal expansion, and flexural strength of the glass-ceramics were detected. The results indicated that only single-phase α-cordierite crystal was precipitated from the basic glass sintered at the range 875-950 °C, and μ-cordierite crystal was not observed during the whole sintering-crystallization process. The properties of glass-ceramics were first improved and then deteriorated with the increase in tuff content and sintering temperature. Fortunately, the glass-ceramics sintered at 900 °C with 45 wt.% tuff content possessed excellent properties: high densify (2.62 g∙cm-3), applicable flexural strength (136 MPa), low dielectric loss (0.010, at 10 MHz), low dielectric constant (5.12, at 10 MHz, close to α-cordierite), and suitable coefficients of thermal expansion (CTE, 3.89 × 10-6 K-1).

Experimental application of a laser‐based manufacturing process to develop a free customizable, scalable thermoelectric generator demonstrated on a hot shaft

AbstractGeometry, design, and processing in addition to the thermoelectric material properties have a significant influence on the economic efficiency and performance of thermoelectric generators (TEGs). While conventional BULK TEGs are elaborate to manufacture and allow only limited variations in geometry, printed TEGs are often restricted in their application and processing temperature due to the use of organic materials. In this work, a proof‐of‐concept for fabricating modular, customizable, and temperature‐stable TEGs is demonstrated by applying an alternative laser process. For this purpose, low temperature cofired ceramics substrates were coated over a large area, freely structured and cut without masks by a laser and sintered to a solid structure in a single optimized thermal post‐processing. A scalable design with complex geometry and large cooling surface for application on a hot shaft was realized to prove feasibility. Investigations on sintering characteristics up to a peak temperature of 1173 K, thermoelectric material properties and temperature distribution were carried out for a Ca3Co4O9/Ag‐based prototype and evaluated using profilometer, XRD, and IR measurements. For a combined post‐processing, an optimal sintering profile could be determined at 1073 K peak temperature with a 20 min holding time. Temperature gradients of up to 100 K could be achieved along a thermocouple. A single TEG module consisting of 12 thermocouples achieved a maximum power of 0.224 μW and open‐circuit voltage of 134.41 mV at an average hot‐side temperature of 413.6 K and temperature difference of 106.7 K. Three of these modules combined into a common TEG with a total of 36 thermocouples reached a maximum power of 0.58 K and open‐circuit voltage of 319.28 mV with a lesser average hot‐side temperature of 387.8 K and temperature difference of 83.4 K.

A Wideband Doherty Combiner with Phase Variation Compensation Using LTCC Applicable for High Power Transmission

In this paper, we propose a small-sized Doherty combiner with phase variation compensation using low temperature co-fired ceramic (LTCC) substrate. The proposed design theory for the Doherty combiner is derived using the phase calculation of the &lt;i&gt;S&lt;/i&gt;-parameter based on the relation between the input and output ports. The proposed circuit is designed after determining the band edge frequency and the targeted degree of the phase balance. The proposed circuit is verified using the microstrip line and the LTCC substrate. The implemented structure, using LTCC as the substrate, is operated under a high-power test of continuous wave 50 W, the results of which also show that the amplitude and phase balance have variations within 0.2 dB and ±1°, respectively. The high-power test shows that the implemented structure is applicable for high power Doherty amplifiers or combiners.

Analysis and Design of a DC-12-GHz Distribution Power Amplifier for Quantum Key Distribution Application

A CMOS distributed power amplifier (DPA) for quantum key distribution (QKD) system to drive an electro-optical phase modulator is developed. Due to the requirement of amplifying high-speed non-return-to-zero (NRZ) signal, our design is supposed to provide enough bandwidth, including dc. To achieve sharp roll-off performance, the m-derived filter section is applied to realize the artificial transmission line rather than the constant-k section. Combining the theory analysis and simulation, the bridges between gain ripple, group delay fluctuation, or maximum linear output power and the parameters of time domain waveform are established. Furthermore, through analyzing the attenuation and delay of the transmission line, approaches to improving the frequency performance, such as gain variation and group delay fluctuation, are provided. The DPA is implemented in the 0.25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> CMOS process and achieves a bandwidth of dc-12 GHz, a gain of 11.54 dB with variation of ±1.34 dB, a group delay of 400 ps with fluctuation of about 100 ps, and a maximum output 1-dB compression point of 20.6 dBm. When a 1-Gb/s NRZ signal is applied to the DPA, the rise and fall times are both about 90 ps. The amplifier occupies an area of 2.8 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times0.94$ </tex-math></inline-formula> mm including pads and adopts the flip-chip package with low-temperature co-fired ceramic (LTCC) substrate.

Modification of CaO–B2O3–La2O3 glass powder and its application in low-temperature co-fired ceramic substrate

CaO–B2O3–La2O3 (CBL) glass is widely used in high-frequency packaging and co-firing with highly conductive metal electrodes as a new lead-free glass/ceramic system in low-temperature co-fired ceramic (LTCC) materials due to its multiple advantages, such as low softening point, controllable crystallisation properties, and chemical stability. The silane coupling agent, KH-570, was used as a surface modifier for preparing modified CBL glass powder to obtain a tape-casting slurry with low viscosity and excellent stability in a polymethyl methacrylate binder system; this system achieved better co-firing compatibility between the composite and Ag electrodes. In this study, the modification mechanism of the silane coupling agent with the powder is elucidated. The effects of the modified powder on the kinetic stability and rheological properties of the slurries were investigated. A slurry with a solid content of 39.6 vol% and lowest viscosity of 1500 mPa s could be obtained with the addition of 2 wt% KH-570. In particular, the uniformity of tape-casting slurry was observed during tape casting, leading to a difference in the density of green tapes, which increased from 2.45 (without KH-570) to 2.61 g/cm3 (with 2 wt% KH-570), and the tensile strength increased to 5.11 MPa. Simultaneously, the micromorphology and surface roughness of the green tapes and substrate were analysed by scanning electron microscopy and atomic force microscopy. The results showed an improvement in densification and a decrease in Ra after modification, resulting in the sintered substrate obtaining a higher density of 3.06 g/cm3 and a lower dielectric loss of 9.8 × 10−4 (12 GHz). The excellent co-firing compatibility between green tapes and silver paste at 850 °C was confirmed by energy-dispersive X-ray spectroscopy. In this study, high-quality LTCC materials were manufactured by coupling modification, with their stable performance ensuring their wide application.

Analysis of selective bonding processes using reactive multi-layers for system integration on LTCC based SiPs

This paper discusses the use of reactive multi-layers for selective assembly of ICs (Integrated Circuits) in an LTCC (Low Temperature Co-fired Ceramics) based SiP (System-in-Package). To understand the requirements for the use of self-propagating reactive multilayers in die bonding, CFD (Computational Fluid Dynamics) simulations have been carried out to simulate the die bonding process of a silicon chip onto a ceramic LTCC substrate. Reactive foils of 40 and 80 µm thicknesses and a simulated reaction propagation speed of 1 m/s were studied and used to melt a solder preform underneath a silicon chip. The results of the CFD simulations were analysed, particularly with respect to temperature and liquid fraction contours, as well as time–temperature histories obtained from temperature probes which were included in the model, such as to approximate the real behaviour of Pt-100 temperature probes, when a real bonding process is being tracked. The CFD method, in this instance realised with ANSYS Fluent software, can track the melting and solidification of the solder as well as model the influence of latent heat, which is crucial to ascertaining the true evolution of the bonding process.

Switching and Heat-dissipation Performance Analysis of an LTCC-based Leadless Surface Mount Package

A leadless surface mount package was developed to enhance the switching and heat-dissipation properties of a power semiconductor. The package was implemented through a low-temperature co-fired ceramic (LTCC)-based multilayer circuit substrate that could form embedded cavities. A silicon carbide (SiC) Schottky barrier diode (SBD) bare die was attached to the cavity in the LTCC substrate. Chip interconnection was realized using a wide and thick copper (Cu) clip with a low parasitic inductance and electrical resistance compared to those of a conventional wire. Silver-filled multiple vias and wide metal planes were used to reduce the electrical parasitic effects and enhance the heat-dissipation of the package. The DC and dynamic characteristics of the 600 V/10 A-class SiC SBD package involving the proposed technologies were evaluated. The dynamic test results indicated that the reverse recovery charge (Qrr) was 18.7% lower than that of a traditional TO-220 packaged product with the same bare die. Furthermore, two leadless commercial products and he proposed package prototype were applied to a power factor correction (PFC) converter, and the power loss and heat-dissipation performances were compared. The proposed package exhibited a lower loss and higher heat dissipation than those of the commercial products.

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 (Na1-xKx)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 (Na1-xKx)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 (Na1-xKx)2MoO4 series. It exhibits excellent dielectric properties: dielectric constant εr = 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) τf = −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 (Na1-xKx)2MoO4 ceramic series a potential candidate for low-temperature co-fired ceramic (LTCC) substrate applications (as used in antennas) for high-speed data communications.

Mm-Wave Complex Permittivity Extraction of LTCC Substrate Under the Influence of Surface Roughness

As many emerging technologies require the use of high-speed signals, the understanding of dielectric properties of materials used in manufacturing printed circuit boards (PCBs) is an essential aspect for accurate high-speed circuit designs, especially at millimeter-wave (mm-wave) frequencies. This work demonstrates a methodology for extracting complex relative permittivity of dielectric substrates covering mm-wave frequencies. For this purpose, low-temperature cofired ceramic (LTCC) substrate was measured up to 85 GHz and its complex relative permittivity was extracted. The approach used in this work is based on multiline thru–reflect–line (TRL) calibration for measuring the propagation constant and electromagnetic (EM) simulations to estimate the losses contributed by the conductor while accounting for surface roughness. An estimate of complex relative effective permittivity is obtained, from which the actual relative dielectric constant and the loss tangent of LTCC substrate are extracted. The estimated values for the relative dielectric constant and the loss tangent show an excellent agreement compared with the results obtained via split cavity resonator measurements.

Microwave dielectric properties of a low firing and temperature stable lithium magnesium tungstate (Li4MgWO6) ceramic with a rock-salt variant structure

The Li4MgWO6 ceramic was synthesized by the conventional solid-state reaction method, and its sintering behavior and microwave dielectric properties were studied for the first time. The XRD patterns and structural refinements indicated that Li4MgWO6 formed an orthorhombic phase at temperatures above 900 °C, and TEM analyses confirmed the ordered and layered rock-salt superstructures for the orthorhombic Li4MgWO6. The SEM photographs and relative density curve showed that the sample sintered at 950 °C had the highest densification. When sintered at 950 °C in a WO3-rich atmosphere for 4 h, Li4MgWO6 had the following optimum properties: εr = 15.06±0.33, Q×f = 28,300±600 GHz (f = 10.1 GHz), and τf = 0.9±3.4 ppm/°C. Compared with other reported low permittivity compounds, Li4MgWO6 shows more potential for the Low-Temperature Cofired Ceramics (LTCC) substrate materials due to its intrinsic low sintering temperature, novel temperature stability, and relatively low dielectric loss.