Low Temperature Co-fired Ceramics Tapes Research Articles

Thick‐film strain and temperature sensors on LTCC substrates

PurposeAims to evaluate different thick‐film materials for use in strain sensors and temperature sensors on low‐temperature co‐fired ceramic (LTCC) substrates.Design/methodology/approachLTCC materials are sintered at the low temperatures typically used for thick‐film processing, i.e. around 850°C, The thick‐film resistor materials for use as strain and temperature sensors on LTCC tapes are studied. Thick‐film piezo‐resistors in the form of strain‐gauges are realised with 10 kΩ/sq. 2041 (Du Pont)and 3414‐B (ESL), resistor materials; thick‐film temperature‐dependent resistors were made from PTC 5093 (Du Pont), and NTC‐4993 (EMCA Remex) resistor materials.FindingsThe X‐ray spectra of the 2041 and 3414‐Bb low TCR resistors after drying at 150°C and after firing display more or less the same peaks. The electrical characteristics of 2041 resistors fired on alumina and LTCC substrates are similar indicating that the resistors are compatible with the LTCC material. After firing on LTCC substrates the sheet resistivities and TCRs of the 3414‐B resistors increased. Also, there is a significant increase in the GFs from 13 to over 25.Originality/valueInvestigates the compatibility of thick‐film materials and the characteristics of the force and temperature sensors.

Effect of Friction on Inhomogeneous Shrinkage Behavior of Structured LTCC Tapes

Vias, cavities, and other cutouts are significant inhomogeneities in low temperature co‐fired ceramics (LTCC) tapes and lead to inhomogeneous shrinkage during sintering, which has a negative effect on the quality of the final multilayer device. The influence of such cutouts on the shrinkage behavior of LTCC tapes was investigated by an exact measurement of the geometry before and after sintering and by in situ observations with an optical dilatometer. The investigations show a strong influence of cutouts on the magnitude of shrinkage inhomogeneities. This effect is more pronounced, if the tapes become thinner, the dimensions of the cutouts become larger, or their position becomes less centric. It is shown that the most important factor on the occurrence of shrinkage inhomogeneities in tapes with cutouts is the static friction of the LTCC material on the setter. Severe warpage is caused by interlocking effects, which occur at bumps of the rough setter surface when the inner edges of the cutouts are pulled over the setter. By using a separating agent between the LTCC tape and the setter, the static friction could be minimized, which eliminates the sintering inhomogeneities.

Manufacturing of prestressed piezoelectric unimorphs using a postfired biasing layer

A novel manufacturing method for prestressed piezoelectric unimorphs is introduced and the actuator properties are examined. Prestressed PZT 5A and PZT 5H unimorphs with piezo material thickness of 250 microm and 375 microm were manufactured by using sintering and thermal shrinkage of the prestressing material. The process was carried out by screen printing a layer of AgPd paste on one side of the sintered bulk ceramic. As an alternative method, dielectric low temperature co-fired ceramic (LTCC) tape was used as the prestressing material. Different configurations were tested to obtain high displacements and to make a comparison between materials. After firing, the samples were poled, and the displacement versus load characteristics of the resulting actuators were investigated. A maximum displacement of 118 microm was obtained from a 250 microm thick, prestressed PZT 5H actuator with a diameter of 25 mm, in which LTCC tape was used as the prestressing layer. Similarly, the PZT 5H material provided a maximum displacement of 63 microm with a screen-printed AgPd prestressing layer. The manufacturing method described offers a novel approach for the production of a wide range of integrated active structures on, for instance, an LTCC circuit board. This is especially important because piezoelectric bulk materials with high piezoelectric coefficients can be used to produce high displacements.

Fabrication of Microvias for Multilayer LTCC Substrates

Advances in screen printing and photoimageable paste technologies have allowed low-temperature cofired ceramic (LTCC) circuit densities to continue to increase; however, the size of vias for Z-axis interconnections in multilayer LTCC substrates have been a limiting process constraint. In order to effectively exploit the 50-100-/spl mu/m line/spacing capabilities of advanced screen printing and photoimageable techniques, microvia technologies need to achieve 100 /spl mu/m and under in diameter. Three main steps in fabrication of microvias include via formation, via metallization or via fill, and layer-to-layer alignment. The challenges associated with the processing and equipment for the fabrication of microvias are addressed in this paper. Microvias down to 50 /spl mu/m in diameter with spacings as small as 50 /spl mu/m are achieved in 50-254-/spl mu/m-thick LTCC tape layers through the use of a mechanical punching system, whereas the minimum size of 75-/spl mu/m via/spacing is obtained using a pulse laser-drilling system in the LTCC tape layers with the same thicknesses as those for the punching test. The quality of punched microvias and laser-drilled microvias will be presented as well. Layer-to-layer alignment is crucial to the connection of vias in adjacent LTCC tape layers. Through a stack and tack machine with a three-camera vision system and an adjustable precision stage, less than 25-/spl mu/m layer-to-layer misalignment is achieved across a 114.3/spl times/114.3 mm (4.5/spl times/4.5 in) design area. In a six-layer LTCC test substrate (152/spl times/152/spl times/0.762 mm), microvias of 50, 75, and 100 /spl mu/m in diameter are successfully fabricated without the use of via catch pads. The cross section of fired microvias filled with silver conductor pastes at various locations of this substrate demonstrates a minor layer-to-layer misalignment in both X and Y directions across the substrate.

Influence of processing and conduction materials on properties of co-fired resistors in LTCC structures

The motivation of this study is the need for fundamental understanding of the effects of processing conditions on the electrical properties of low-temperature co-fired ceramic (LTCC) tapes, those screen printed with commercial thick-film pastes of electronic components for increased reliability. The method of the study is realized by analyzing the physical and chemical effects of mono/multilayer firing and firing temperatures on the temperature coefficient of resistance (TCR) and sheet resistance (SR) values of the positive temperature coefficient (PTC) resistors screen-printed on LTCC tapes. The results are discussed with respect to the information obtained by the scanning electron microscopy (SEM), electro dispersive X-ray analysis (EDXS), X-ray and dilatometry analysis. It is shown that the content of pastes combined with varying processing conditions result in deviation from expected TCR and SR values due to the chemical and/or mechanical reactions.

Curvature evolution in LTCC tapes and laminates

Low temperature cofired ceramic (LTCC) substrates provide a low-cost packaging approach for radio frequency (RF) integrated circuits. Thermally-driven curvature development in these LTCC tapes and laminates, which can introduce property-limiting defects into a device, was examined quantitatively using an optical profilometer. Contrary to the standard perception of tape cast products, these specimens consistently experienced the largest increases in curvature during burnout rather than during sintering. Sintering actually leads to dramatic decreases in curvature. Slight anisotropic changes in curvature were observed when comparing those directions parallel and perpendicular to the tape casting direction. In the laminates, individual layer orientations had strong effects on overall curvature. Springback after the application of lamination pressure strongly contributes to curvature. "Microslumping" at the ends of the tapes/laminates occurred during sintering and could be accounted for via discrete element models of sintering. The latter phenomenon can be eliminated by controlling relative tape orientation.

Investigation of interactions between co-fired LTCC components

In spite of many advantages it offers for micro-electronic packaging, the low temperature co-fired ceramic (LTCC) technology has yet certain issues to be solved. Physical and chemical reactions, for instance, within and between the passive electronic components (thick film conductors, resistors—TFR) and the LTCC tapes (green sheets) during processing are one of the major challenges of this technology. This study aims to better understand and control the nature of such interactions, which have a direct effect on properties. The work is conducted on PTC (positive temperature coefficient) resistors, which are screen printed on (co-fired) and in (buried) LTCC sheets and fired at various temperatures. The final properties such as temperature coefficient of resistance (TCR) and sheet resistance are evaluated in terms of processing parameters using scanning electron microscopy (SEM), dilatometry and electro dispersive X-ray (EDXS) analysis as characterization tools. The results show that the TFR properties of buried samples deviate more strongly from the expected values compared to those of co-fired ones. It is primarily related to the destroyed conductor and resistor lines, which is closely related to the entrapped gases in buried structures as a result of organic burnout of printed inks.

Effects of organic additives and glass on the properties of AlN/glass tape-casting slurries and green tapes

Low-temperature co-fired ceramics (LTCC) technology offers significant benefits over both thick film ceramics and HTCC (high temperature co-fired ceramics) due to high-density, high-radio-frequency, fast digital applications requiring hermetically sealed packaging and good thermal management [1]. LTCC tape has been used in the last 20 years for high-reliability, avionics, and automotive applications, as well as in MCMS for communication and computer applications [2]. In recent years, many LTCC systems were made up of Al2O3 and glass [3]. However, due to the low thermal conductivity of Al2O3, the LTCC systems of Al2O3 and glass have a poor thermal conductivity. AlN, whose thermal conductivity is 10 times greater than that of Al2O3, can be substituted for Al2O3 to improve the thermal conductivity of LTCC [4]. As a convenient and low-cost processing, tape casting has been widely used to produce ceramic substrate [5]. However, LTCC is a multicomponent and complicated system. Each component has a substantial effect on the properties of the slurry and green sheet. The present work was conducted on a new type of LTCC AlN/glass to study the effects of organic additives, including dispersant, binder, and plasticizer, on the properties of the slurries. The effects of glass content on the properties of slurry and green sheet was also investigated. AlN powder and borosilicate glass powder were used as raw materials. The commercial AlN powder (Shenhai Nitride Co. Ltd., China) was synthesized by the self-propagation high-temperature synthesis (SHS) method, with nominal particle size of 0.5 μm. Borosilicate glass powder was milled to 1.7 μm. Borosilicate glass used in this study was found to react with water and, therefore, azeotropic mixtures of 66:34 vol% 2-butanone ethanols were used as solvents. Triethyl phosphate (TEP) and polyvinyl butyral (PVB) were introduced as dispersant and binder, respectively. The plasticizer was a 50:50 wt% mixture of polyethylene glycol (PEG) and phthalate (PHT). The mixed powders with dispersant were first ball-milled for 24 hr in solvent with AlN ball. Binder and plasticizers were added to the slurries and milled for another 24 hr. Then the tape was cast on a glass surface through the action of a blade that leveled the slurry. The tapes were dried in a solvent atmosphere. A rotary viscometer was used (Model NDJ-7, Shanghai Balance Instrument Plant) to measure the apparent viscosity of the suspensions at 350 s−1. Sheardependent behavior was tested with a rheometer (Model SR5, Rheometric Scientific, Inc., Piscataway, NJ). The suspensions, containing 16 ml of solvent and 4 g of the AlN/Glass powder, were ultrasonically mixed with different amounts of dispersant for 10 min. Then the suspensions were settled in 25 ml graded tubes. The density and porosity of the green sheets were measured by mercury porosimetry (Model PoreSizer 9320, Micromeritics Instrument Corp., Norcross, GA). Fig. 1 shows the viscosity of three suspensions (67 wt%) with different dispersants as a function of dispersant content. For the 1.5 wt% dispersant content, the viscosity of the three suspensions simultaneously reached a minimum though the particle sizes were different for AlN and borosilicate glass. This indicates that the adsorption of dispersant onto ceramic particles was not only related to the particle size but also to the chemical nature of the ceramic powder. Fig. 2 describes the sediment volume of three suspensions versus the concentration of dispersant after resting for 4 weeks. As is shown, the sediment volume decreased as the concentration was increased. After a minimum volume was achieved, further increase in the concentration of dispersants produced a slight increase in sediment volume. For the 1.5 wt% dispersant content, the sedimentation volumes of the three suspensions reached a minimum. This indicates 1.5 wt% dispersant content was optimum for three suspensions. These results agreed well with that of the viscosity tests, though the solid loadings of the suspensions were

Wire bonding characteristics of gold conductors for low temperature co-fired ceramic applications

Low temperature co-fired ceramic (LTCC) with gold conductors has been used for high reliability applications, such as satellite communications. When the gold metallization is co-fired with the LTCC tape, inorganic adhesion additives in the gold conductor interact with the LTCC glass. This interaction is essential to provide adequate adhesion for the gold metallization. However, this interaction can also reduce the softening point of the LTCC glass. The reaction product can migrate to the surface of the gold conductor and affect the wire bondability of the gold conductor. Effort has been made to develop surface gold conductors with optimized interaction with LTCC. The new gold conductor shows significant reduction of inorganic particles on the surface and improvement of wire bondability. A systematic wire bonding study has been performed on gold conductors under various wire bonding conditions. It is demonstrated that the wire bonding window can be improved significantly by reducing the interaction between LTCC and gold metallization. This paper reports the results of the wire bonding study.

Material Issues of Low Temperature Co-fired Ceramic (LTCC) Fine Pitch Chip Scale Package (CSP) Designs

ABSTRACTLTCC substrates for fine pitch (1.0 mm and 0.8 mm) CSP applications have been designed, fabricated, and assembled. The assembly process, including ball grid array (BGA) solder ball attach, die mount, wire bond, and glob top is described. The material and physical design interaction issues that emerged during development are discussed.The initial CSP design was conventional, with co-fired yellow gold (Au) vias and capture pads and post-fired solderable gold (PtPdAu) pads for solder ball attachment. Because LTCC tape shrinks during co-fire, solder pads were applied post co-fire to ensure proper mating with existing test fixtures and to provide the best alignment relative to the CSP body. Solder pad to capture pad misalignment was visible following solder pad firing. After CSP attachment to a test board, electrical tests revealed opens. Investigation led to the following conclusions. The decreased solder pad diameter necessary to accommodate the fine pitch design was significant relative to the area allocated for the underlying via and capture pad. Misalignment that would have been hidden under larger solder pads was exposed. Even when the capture pad surface was not visibly exposed, the offset solder pad meant less material between the capture pad and the solder ball, less of a barrier to solder leaching. Solder leaching into the yellow gold, observed after CSP removal from the test board, was the cause of the electrical disconnects. In the second design, the capture pad was eliminated in order to discourage leaching by reducing the volume of yellow gold available to alloy with the solder pad during co-fire. Reflow operations still resulted in leached solder pads. A third design replaced the first-layer via yellow gold with a solderable gold. This design proved to be robust.While developing designs and fabricating these prototypes, it was noted that all ball failures consistently occurred between the solder pad and the LTCC substrate. To investigate adhesion using different metallizations, shear tests were performed on LTCC substrates with either post-fired solder pads or co-fired pads. To investigate how the substrate material affects adhesion, alumina CSPs were also sheared. Shear test results are presented.

Characterization of PZT thick films fired on LTCC substrates

Ferroelectric ceramic materials based on solid solutions of Pb(Zr,Ti)O3 (PZT) are used in the electronics industry for sensors and actuators and for electromechanical transducers, to name just a few examples. Thick-film technology, i.e., the deposition of thick-film pastes by screen printing, primarily on alumina substrates, is a relatively simple and convenient method to produce layers with a thickness up to 100 μm. The characteristics of thick-film ferroelectrics are similar to those of bulk materials [1–4]. Low-temperature co-fired ceramics (LTCC) materials, which are sintered at the low temperatures typically used for thick-film processing, i.e., around 850 ◦C, are based either on crystallizable glass [5, 6] or a mixture of glass and ceramics, for example, alumina, silica or cordierite (Mg2Al4Si5O18) [7, 8]. Jones et al. have presented a comparison of the mechanical and chemical characteristics of both green and fired LTCC tapes from different suppliers in [9]. Ceramic multi-chip modules (MCM-C) are multilayer substrates with buried conductor lines. An additional contribution to the smaller size and the higher density of MCM-C is the ability to integrate screenprinted resistors, or sometimes capacitors and inductors. These screen-printed components can be placed either beneath the discrete components on the surface of the multilayer dielectric or buried within the multilayer structure. For an overview of passive integrated components in MCM see, for example [10]. For some applications, for example integrated sensors or micro-actuators, PZT thick-films on LTCC that are sintered at relatively low temperatures (around 850 ◦C) comparable with LTCC’s firing temperatures, would be of interest [11, 12]. The aim of this work was to study the compatibility between LTCC and screenprinted PZT as well as the electrical characteristics of the PZT layer. PZT 53/47 powder (PbZr0.53Ti0.47O3) with an excess 6 mol% of PbO was prepared by mixed-oxide synthesis at 900 ◦C for 1 h from high-purity PbO (litharge) 99.9% (Fluka), ZrO2 99% (Tosoh), and TiO2 99% (Fluka). To this was added 2 wt% of lead germanate, with the composition Pb5Ge3O11 (melting point 738 ◦C) as a sintering aid. Lead germanate (PGO) was also prepared by mixed-oxide synthesis from PbO and GeO2 99% (Ventron) at 700 ◦C. After synthesis, both compositions were ball milled in acetone for 1 h and dried. A thick-film paste was prepared from the PZT (2% PGO) and an organic vehicle (ethyl cellulose, alpha-terpineol and butil carbitol acetate) by mixing on a three roll mill. The green LTCC 951 tape (Du Pont) and alumina ceramics were used for substrates. The thick-film structure was prepared by first printing gold film (Remex 3243) and then the PZT film. The PZT film was printed 6 times with intermediate drying. The gold and PZT layers were cofired at 850 ◦C for 8 h in a closed alumina crucible. The thickness of the PZT films after the thermal treatment was around 50 μm. The green and fired Du Pont LTCC 951 tapes were analyzed by X-ray diffraction (XRD) analysis with a Philips PW 1710 X-ray diffractometer using Cu Kα radiation. X-ray spectra were measured from 2 = 20 ◦ to 2 = 70 ◦ in steps of 0.04 ◦. X-ray spectra are shown in Fig. 1. The unfired material is a mixture of alumina and glass. After firing at 850 ◦C peaks of anorthite ((Na,Ca)(Al,Si)4O8) phase appear. The peaks of alumina and anorthite are denoted by “A” and asterisk, respectively. For the electrical measurements gold electrodes were sputtered onto the PZT films. The values of the remanent polarization and the coercive field were determined from ferroelectric hysteresis curves measured with an Aixact TF Analyzer 2000 at 50 Hz. The real and imaginary parts of the complex dielectric constant were measured with an HP 4284 A Precision LCR Meter at 1 kHz. In Table I the electrical parameters, i.e., remanent polarization Pr, coercive field Ec, dielectric constant e′ and dielectric loss tan δ, of the co-fired LTCC/Au/PZT structure are presented. The electrical characteristics of this structure are compared to the characteristics of a similar structure printed on alumina substrates [13]. Hysteresis loops of the PZT films on the alumina and LTCC substrates are shown in Fig. 2.

Three-dimensional low-temperature co-fired ceramic shells for miniature systems applications

Low-temperature co-fired ceramic (LTCC) tapes (DuPont, 951 series), originally developed for monolithic packaging of interconnects and hybrid microelectronic circuitry, have been used in the past five years to develop a meso-scale integrated fluidic technology. The LTCC fluidic technology is shown to be versatile, inexpensive, fast, and free of packaging problems. The manufacturing basis for this technology is the patterning of individual tapes, which are laminated and co-fired to create a layered three-dimensional (3D) structure. In this paper, we present another attractive facet of this technology by creating 3D shell structures with a single layer of LTCC tape. The processing technique is illustrated with actuated hemispherical and cylindrical shells that have internal 3D conduits. In order to demonstrate the application of an active device using magnetostatically actuated curved LTCC shells we also present a novel three degrees-of-freedom spherical stepper motor.

Overview of low temperature co-fired ceramics tape technology for meso-system technology (MsST)

For certain applications low temperature co-fired ceramic (LTCC) tape materials used in multi-layer packages offers the potential of emulating a great deal of silicon sensor/actuator technology at the meso scale level. The goal of this review is to describe meso-system technology (MsST) using LTCC, thick film and silicon technologies. A mayor MST application being addressed today is fluid handling for miniaturized chemical analytical systems. For larger MST-3D applications, in the meso-size (from 10 to several hundred microns), it would be desirable to have a material compatible with hybrid micro-electronics, with suitable thermal, mechanical and electrical properties, easy to fabricate and inexpensive to process. Such a material is the LTCC tape multilayer system. One of the important features of LTCC technology is the possibility of fabricating 3D structures using multiple layers. In this review, we want to emphasize sensors and actuators for meso-systems exploring LTCC Tape possibilities in the following ways: • Sensors for proximity measurement; • Fluid media realization of vias, holes, cavities, channels and manifolds; • Sensors for flow measurement; • Actuators for hybrid microvalves & micropumps.

Meso-Scale Pressure Transducers Utilizing Low Temperature Co-Fired Ceramic Tapes

AbstractPressure transducers with promising characteristics at high pressure and temperatures have been developed using low temperature co-fired ceramic (LTCC) tape technology. All parts for the transducer were machined from DuPont 951 series LTCC tapes utilizing either a numerically controlled milling machine, or an isotropic etching technique involving the removal of the glassy binder of a partially sintered LTCC tape. Device dimensions are in the meso (intermediate) scale range, with the smallest device size of 8mm in diameter, and the cavity of 2 mm. Utilizing the anisotropy induced during the casting process a chemical exfoliation technique was developed. This technique allowed us to separate the original tape in three layers, the middle one being highly elastic, isotropic and homogeneous. The middle layer can be chemically thinned to achieve membrane like behavior, with thickness of the order of 50 µm. The pressure is measured as a function of the membrane deformation where two piezo-resistors are screen printed. Two piezo-resistors were used to achieve temperature compensation. Using shrinkage matched paste, nominal thick film technology was used in the screen printing of the piezoresistors. The rest of the transducer was fabricated using several layers of LTCC tapes, which were laminated and fired. Devices of different sizes were fabricated and compared. Devices were characterized by comparing its dynamic response with a micro-fabricated silicon pressure transducer.

Meso (Intermediate)-Scale Electromechanical Systems for the Measurement and Control of Sagging in LtcC Structures

AbstractSagging of suspended or laminated structures is a common problem in the processing of Low Temperature Co-Fired Ceramics (LTCC). These glass-ceramic composites are susceptible to plastic deformation upon lamination, or under the stress of body forces once the glass transition temperature of the glass binder is reached during processing. We have designed and fabricated, using the conventional methods of LTCC fabrication, meso-scale structures (ranging in size from 100 mm to 1 cm) to quantify and seek control strategies for this problem. We have implemented bridge structures and membranes to emulate most of the conventional structures encountered during packaging or sensor (actuator) device fabrication.We have observed that when an LTCC tape with holes larger than 400 µm in diameter is laminated, the tapes above and below deform into the cavity. For smaller diameters, deformation is negligible. Bridging structures can be compensated for the potential effect of body forces by screen-printing a thick film over-layer which exert internal tensile stresses on sintering. This can often yield straight bridges. The use of fugitive phase materials, which disappear or flow during firing, is another way of supporting bridging structures. Several of these strategies have been explored, and results are presented.