Thick Film Pastes Research Articles

0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 Thick Films for High-Frequency Piezoelectric Transducer Applications

The properties of 0.65Pb(Mg1/3Nb2/3)O3–0.35PbTiO3 (0.65PMN–0.35PT) thick films were studied for high-frequency piezoelectric transducer applications. The films were prepared by screen-printing a thick-film paste on platinized alumina substrates and subsequent sintering at 950 °C. The effective thickness-coupling factor of these films was close to 48%, which is comparable with bulk ceramics having the same compositions. Furthermore, simulations of two configurations representing one element of a high-frequency linear-array transducer (30 MHz) suggests that 0.65PMN–0.35PT thick-films in 50 Ω electrical matching environment improves the performance in comparison with standard Pb(Zr,Ti)O3 (PZT) compositions.

Thick film pastes for nitride ceramics for high power applications

Aluminum and silicon nitride ceramics show high potent as substrate materials for thick film- / hybrid applications in the field of power electronics and microwave technology. A main advantage of AlN is its very high thermal conductivity. Si3N4 has also a thermal conductivity comparable to AlN but higher thermal shock resistance and fracture toughness. Nevertheless, the use of Si3N4 in power electronic packages or heater applications is not achieved due to the lack of suitable connection technology such as thick film pastes. The main challenge for the respective pastes is the adhesion on the substrate. The low thermal expansion (CTE) of about 2.8 ppm/K of Si3N4 and material interactions with the thick film components must be considered in the paste development. Therefore, new glasses and glass-ceramic composites are required. Reason development was performed towards AgPd based thick film heater pastes for Si3N4. The pastes consists of AgPd in a 1:1 ratio, an inorganic filler and a glass phase adapted to the CTE of Si3N4. By variation of the content of inorganic filler and glass content three pastes with a sheet resistivity of 0.1 ohm/sq, 0.8 ohm/sq, and 7.9 ohm/sq and temperature coefficient of the resistance (TCR) between −100 and 100 ppm/K were developed. The performance of heater films prepared from these pastes under electric pulse load was studied. An electrical power of up to 85 W can be applied without a significant change of the resistivity (ΔR/R) of 0.1%. As reference comparable investigations on AlN were performed with adapted pastes. The film structures were analyzed in FESEM studies.

Influence of Emitter Properties on Contact Formation to P+ Silicon

In this study we investigate the influence of the B surface concentration Nsurface on the contact formation of Ag and Ag/Al thick film pastes to p+-type Si. From literature it is known, that for contacting POCl3 emitters a high P surface concentration is necessary for a reasonable contact resistance [1,2]. This work aims to examine if the same holds for B emitters. For this purpose, three emitters with different B surface concentrations are realized and contacted with pure Ag and Al containing Ag (Ag/Al) thick film pastes: a BBr3 emitter (I: Nsurface∼3E19 cm-3, Rsh=50Ω/▭), and two alternative emitters without BBr3 diffusion (IIa: Nsurface∼1E20 cm-3, Rsh=55Ω/▭; IIb: Nsurface∼1E19 cm-3, Rsh=330Ω/▭). The specific contact resistance ec is determined using the Transfer Length Method (TLM). With Ag/Al paste ec=9.95 mΩcm2 on emitter IIa and ec=37.6 mΩcm2 on IIb could be determined. For the same emitter type a higher Nsurface therefore results in a lower ec. This cannot be observed for different types of emitters as ec=4.8 mΩcm2 was measured on emitter I. For a pure Ag paste the same trend can be observed but with ec being at least a factor of four higher, as expected [3–5]. Electron microscopy analysis reveals that for pure Ag pastes no direct contacts between Ag crystals on the Si surface and the Ag bulk exist. In contrast, for the Ag/Al pastes a direct connection between the Ag/Al particle in contact with the silicon surface and the Ag finger can be found. We propose that the low contact resistance for Ag/Al pastes may be due to direct contacts of Ag/Al spots to the Ag bulk.

Influence of the organic vehicle and inorganic additives on the properties of thick film pastes for AlN

New conductor pastes for AlN as substrate material with fired films of a sheet resistance below 25 mOhm/sq were investigated. The pastes consist of AgPd as conductive phase for leach resistance. Other paste ingredients are glasses of matched composition for adhesion and densification properties, little amounts of inorganic additives and organic vehicles for film deposition. The latter has a significant influence on the paste deposition and the resulting film properties as well. In order to shine light into these relationship systematic rheological studies of the organic vehicle and the manufactured pastes were performed. The rheological investigations include measurements in rheological steady-state as well as dynamic experiments. In addition, the solder properties of the fired film were studied. FESEM analyses were performed on the surface of the fired films.

Preparation and characterization of Cu and Zn modified nickel manganite NTC powders and thick film thermistors

Abstract A simple ball milling/thermal treatment procedure was applied to obtain fine thermistor powders. Three different powder compositions were analyzed–Cu 0.2 Ni 0.5 Zn 1.0 Mn 1.3 O 4 , Cu 0.25 Ni 0.5 Zn 1.0 Mn 1.25 O 4 and Cu 0.4 Ni 0.5 Mn 2.1 O 4 . XRD analysis showed that all three powder compositions had a cubic spinel structure. Correlation between the sintering temperature, structure and resulting electrical properties was analyzed on bulk samples. Thick film pastes were composed and segmented thick film thermistors were screen printed on alumina, dried and fired. SEM analysis revealed a typical dendrite structure with small grains and a developed surface area. Thick film sheet resistance was measured on a test matrix and the resistance decreased with increasing Cu content. The temperature dependence of sample resistance was measured in a climatic chamber enabling calculation of the material constant and activation energy. Aging of the obtained segmented thermistors was analyzed and the resistivity drift was 0.23% for the Cu 0.2 Ni 0.5 Zn 1.0 Mn 1.3 O 4 NTC thick film thermistor confirming greater stability of thermistors containing Zn and Cu that in combination with the determined good thermistor characteristics make them good candidates for temperature and heat loss sensor applications.

Biocompatibility Evaluation of Human Umbilical Vein Endothelial Cells Directly onto Low-Temperature Co-fired Ceramic Materials for Microfluidic Applications

Expansion of Low-Temperature Co-fired Ceramic materials into microfluidic systems technology has many beneficial applications due to their ability to combine complex three dimensional structures with optical, fluidic, electrical functions. Evaluations of the biocompatibility of these Low-Temperature Co-fired Ceramic materials are vital for expanding into biomedical research. The few biocompatibility studies on Low-Temperature Co-fired Ceramics generally show negative cellular response to thick film pastes used in generating the electronic circuitry patterns. In this study, biocompatibility of Human Umbilical Vein Endothelial Cells was examined on Heraeus's Low-Temperature Co-fired Ceramic tape and two of their conductive pastes. The biocompatibility was assessed by monitoring cellular attachment and viability up to three days. This study examines the idea of leachates being detrimental to cells due to a study that suggests the possibility of harmful leachates. Results indicate difficulty in initial attachment of Human Umbilical Vein Endothelial Cells to sintered Low-Temperature Co-fired Ceramic tapes, but no hindrance of cellular attachment and growth onto the two conductive pastes. Outcomes also demonstrate that possible harmful leachates from Low-Temperature Co-fired Ceramic materials don't thwart cellular attachment and growth for up to three days of cell culturing. These results provide a basis for biological devices using Low-Temperature Co-fired Ceramic materials.

Low Temperature Co-fired Ceramics Processing Parameters Governing the Performance of Miniaturized Force Sensors

For the development of miniaturized force sensors, built up in ceramics technology with piezo-resistive principle, the compatibility of the piezo-resistive thick-film paste with the substrate and termination paste has to be verified. This paper deals with the compatibility of the ESL 3414-A piezo-resistive paste on HTCC (high temperature co-fired ceramics) substrates (alumina as reference and the partially stabilized zirconia tape ESL 42013-A) as well as on LTCC (low temperature co-fired ceramics) substrates (Heraeus AHT01-005, AHT08-047, CT707; and CeramTec GC) under different manufacturing conditions. The sheet resistance at room temperature, the longitudinal gauge factor at room temperature and the temperature coefficient of resistance have been measured. The results are compared with microscope images showing cracks in distinct cases. Finally, the compatibility and thus applicability of the ESL 3414-A on the investigated substrates is evaluated.

A Low Firing Temperature Copper Conductor for use on an Aluminum Metal Compatible Dielectric in LED Thermal Substrate Applications

One of the most important contributing factors in the long life of LEDs (Light Emitting Diode) is keeping them cool, i.e., lowering the junction temperature, which can get as high as 150 °C. One way to accomplish this when building a circuit containing many LED packages is to use a thermal substrate. The majority of High Power/High Brightness (HP/HB) LED thermal circuits manufactured today are based on Metal Core Printed Circuit Board (MCPCB) technology. The MCPCB system consists of a copper foil, polymer dielectric layer and either an aluminum or copper base layer that are laminated together. The process of making MCPCB is a subtractive process, where most of the copper foil is removed. The copper foil is etched to create a circuit layer, the polymer dielectric provides the insulation between the copper foil and the metal base, and the aluminum or copper base provides thermal dissipation. The thick film copper paste discussed in this document is processed using an additive process, like screening printing, where the copper conductor is deposited only in the chosen area. The copper thick film paste is screen printed to create a circuit design on top of a thick film dielectric (insulated area) layer which is also printed on a metal base layer. The selective deposition allows for less material usage, and potentially lower overall cost as well as improved thermal performance with inclusion of thermal vias. This paper discusses the results of a newly developed lead (Pb) and cadmium (Cd) free low firing temperature (580–600°C) copper thick film conductor paste on top of a low temperature firing dielectric paste. This study includes evaluations based on SEM photos, solderability, leach resistance, and initial and long term adhesions using SAC 305 solder and RMA flux. There are many different applications for HP/HB LED circuits, from general lighting to street lighting to automotive lighting and more. All of them have varying degrees of performance testing requirement. To try to cover as much of the reliability testing as possible we have included tests such as thermal aging (150°C), thermal cycling from −50 to+150°C, as well as 85°C/85% RH reliability testing under bias.

New conductive thick-film paste based on silver nanopowder for high power and high temperature applications

A new thick-film material for screen-printing technology, based on nanoscale silver powders with the particle size distribution 5–55 nm is presented. Silver nanopowder used for paste preparation was elaborated by the authors. The compatibility of investigated paste was proven with alumina, silicon, Kapton foil and glass. The main advantage of this paste is sinterability at much lower temperatures (around 300 °C) compared to pastes obtained from micro-powders (650–850 °C). The thicknesses of obtained layers are 2–3 μm. The elaborated layers are dense and well sintered, exhibit good adhesion to all above mentioned substrates and low resistivity as well as very good resistance to high power and elevated temperatures. The results of loading the layers deposited on alumina substrates with high current and exposed to high temperature are presented as well.

Influence of the dopant on the contact formation to p+-type silicon

Abstract In this contribution we investigate the influence of the doping element on the contact formation to p+-type Si. Contacting B doped layers with Ag thick film paste leads to very few Ag crystallites at the contact interface and results in poor contact resistances [1] , [2] , [3] . Using Ag/Al thick film paste for contact formation, the contact is not only formed by Ag crystallites, but by diversely shaped Ag/Al contact spots and the contact resistance is reduced by more than one order of magnitude [2] . Al melting at the Si wafer surface forms Al doped rectangles on the Si wafer where the growth of Ag/Al/Pb spikes is enhanced. When contacting Al doped layers with Ag thick film paste a larger number of Ag crystallites is observed than for B doped layers. If the contact is formed with Ag/Al paste the number of Al-rich rectangles is enhanced and we detect higher doped areas under the contact spots. The contacts detected have an ellipsoidal, pyramidal or “L-formed” shape. We conclude that not only the acceptor impurity concentration under the contact area is crucial for the contact formation, but also the properties of the specific acceptor present.

Influence of film thicknesses on the electrical properties of RuO2-thick film resistors on aluminum nitride ceramics (AlN)

Aluminum nitride ceramics are very suitable as substrate material for thick film- / hybrid applications in the field of power electronics and microwave technology. Main features are high thermal conductivity, a thermal expansion coefficient close to silicon, good mechanical stability and dielectric properties. Thick film resistor pastes are usually composed of lead- and bismuth oxide free glass frit and ruthenium dioxide as electrical conductive component. In contrast to oxide ceramics like alumina ceramics, thermo dynamical calculations predict the reaction of ruthenium dioxide with AlN. Thus, resistor pastes for AlN are designed to build defined porosity in the films. In dependence of composition of the paste, substrate roughness and -quality varying interactions of the components and deviating film structures can be observed. On basis of different paste compositions resistors were built with varying film thicknesses. Electrical properties like resistance and its temperature coefficient as well as the power dissipation and short term overload voltage were correlated with geometrical data, chemical composition and substrate properties. Furthermore FESEM was used to characterise film properties like porosity and the interface resistor – ceramics. Users of thick film pastes are shown aspects under which conditions high quality resistors are maintained under maximum materials savings.

Investigations of Thick-Film-Paste Rheology for Dispensing Applications

Abstract In order to establish dispensing as a promising metallization process in silicon photovoltaics, equipment and metal pastes require further optimization. By conducting several rheological experiments based on rotational tests, shear thinning as well as thixotropic behavior of thick-film pastes were investigated. Both are crucial parameters for continuous dispensing of 60 μm fingers with high aspect ratios. Flow rate fluctuations during dispensing though imply stress peaks that may disturb a continuous paste flow. Thus, a comparison of the flow rate of two pastes was conducted. A comparison of dispensed cells with screen-printed reference cells, on multi-crystalline wafer-material, showed an efficiency increase of 0.3%abs. on average. This is mainly caused by reduced finger widths and higher aspect ratios of dispensed fingers.

Light-Induced Plating of Screen-Printed Multi-Crystalline Silicon Solar Cells

Light-induced plating (LIP) has been used to evaluate solar cells screen-printed on 28 mm × 28 mm multi-crystalline Si (mc-Si) wafers from experimental silver thick-film pastes A and B. Cells were fabricated from paste A using screens with 60, 80, and 100 μm wide line openings. After firing at optimal temperature and measuring electrical performance, the cells were plated using the LIP process. Cell fill factor and efficiency increased noticeably while cell series resistance decreased. The observed efficiency increase ranged from 0.4% to 1.0% and was due to improved contact resistivity and decreased conductor line resistance. The mechanism for improved performance was elucidated through conductivity measurements and microstructural investigation using advanced scanning electron microscopy. Surface morphology of the silver conductor line obtained from the LIP process is reported. Cells from paste B were printed with a 100 μm wide opening screen. The efficiency of cells printed with paste B using the 100 μm screen followed by the LIP process did not show an efficiency improvement. It is shown that an optimally designed, state-of-the-art paste (paste B) can deliver sufficiently low series resistance and achieve high cell efficiency without using the LIP process.

Magnetic Silver-Coated Ferrite Nanoparticles and Their Application in Thick Films

Magnetic silver-coated ferrite nanoparticles with 39.8% weight gain (relative to ferrite nanopowder coated by a silver layer) were synthesized by electroless deposition of silver on ferrite nanopowder. The mechanism of the electroless deposition was explored in terms of pretreatment, sensitization, activation, and the reduction of silver–ammonia complexes. Experiments showed that the optimal deposition conditions were a temperature of 50°C, pH value of 10 to 12, duration of 65 min with ethanol plus polyethylene glycol as additives, and ultrasonic vibration as a method of dispersing the nanoparticles. From transmission electron microscopy (TEM) images, it was observed that as-synthesized nanoparticles had a core–shell structure with a particle size of 35 nm to 90 nm and a shell thickness of 5 nm to 20 nm. X-ray diffraction (XRD) analysis confirmed that only ferrite and metallic silver were present in the product. Electrical resistance and magnetic hysteresis measurements demonstrated that the nanoparticles were both electrically conductive (volume electrical resistivity on the order of 10−4 Ω cm to 10−3 Ω cm when compressed to pressure of 2 × 10 6 Pa) and possessed ferrimagnetic properties. After a thick-film paste, obtained with the nanoparticles as the functional phase, was directly written and sintered, scanning electron microscopy (SEM) analysis and electrical resistance measurements of conductive lines in the acquired array pattern showed that an electrically conductive network with some defects and cavities was formed, with a volume electrical resistivity of 1 × 10−4 Ω cm to 1 × 10−3 Ω cm.

Microstructure for Printed Multilayer LTCC Tape of Different Paste Rheology

Screen printing is the most common method of transferring silver thick film paste to a low temperature co-fired ceramic tape. Silver paste with varying rheology will have a varying print quality. The effect of silver paste rheology on the microstructure is investigated. Although there are many parameters to be considered, this parameter is a critical parameter which will control the line width and roughness on the substrate. To study the effect of paste rheology, a test pattern design was printed on the Heraeus CT2000 LTCC tape using silver paste. Then the LTCC tape was stacked and laminated at pressure of 28 MPa and temperature of 65 oC for 10 minutes. The laminated sample then was fired up to 850 oC. The fired silver conductor printed on the LTCC tape then was analyzed. Prints were evaluated by scanning electron microscopy. It was found that silver paste with high viscosity produce a good microstructure.

Effect of the interface glass on electrical performance of screen printed Ag thick-film contacts of Si solar cells

Glasses with compositions (40 − x)PbO-- x ZnO--60(B 2O 3--SiO 2) ( x = 5, 10, 15, 20) have been prepared. Substitution of ZnO for PbO increased glass bandgap ( E g ) and crystallization ability greatly. Crystalline phases of bulk glasses after rapid thermal processing (RTP) were identified by X-ray diffraction (XRD). Transmission line model (TLM) was employed to measure the electrical performance of Ag electrodes screen printed on polycrystalline Si substrates using Ag thick-film pastes and by RTP. The conductivity ( σ) decreased while specific contact resistance ( ρ c ) was not monotonic varied with increasing ZnO content. The correlation between electrical performance and glass barrier formed on the Ag gridline and Si emitter interface has been investigated.

Glass Binder in Thick Film Metallization Paste for AlN

In this paper, the sol-gel process was used to synthesize a glass binder in thick-film paste for aluminum nitride (AlN) metallization. Some major factors that influenced the sol-gel process including PH value, quantity of water were discussed in detail. The xerogel was analyzed by means of TG-DSC in order to determine its optimum heat treatment process. Meanwhile, the sintering behavior of silver/glass composite thick film had been studied by varying the glass content. The results show that a glass powder of a low melting point and chemically compatible with AlN substrate can be obtained by sol-gel method when the composition (wt %) is CaO (40), B2O3 (15), SiO2 (35), BaO (10). During sintering, the glass becomes liquid and largely affects the microstructure development and square resistance of the thick film. When the glass content is optimum (10wt %), the film adhesion strength achieves maximum and the square resistance has a relatively low value.

Aerosol Jet® Material Deposition for High Resolution Printed Electronic Applications

Aerosol Jet printing, is finding wide use in a number of electronic manufacturing applications. The Aerosol Jet systems deposit a wide variety of functional materials onto a wide variety of substrates without conventional masks or thin-film equipment. The process is non-contact, enabling traces to be printed over steps or curved surfaces. Printed features can be from less than 10 microns to several centimeters, with thicknesses from 10's of nanometers to 10's of microns. The Aerosol Jet process utilizes both low viscosity inks and diluted pastes for an operating range of approximately 1-1000cP. Typical materials that can be printed include nanoparticle metal suspensions, polymers and adhesives. Conductor traces can be printed using gold, silver or other nanoparticle inks. Conductors can also be formed by printing a seed layer, followed by electroless plating. Polymer thick film pastes can be printed to form embedded resistors. Polyimide and various epoxies can be printed for adhesives, overcoat dielectrics, etc. The system also supports deposition of semiconducting inks, carbon nanotubes (CNTs) and biomaterials. The Aerosol Jet process is compatible with a wide variety of substrates, including silicon, polyimide, glass, FR-4 and aluminum oxide. In principle, virtually any substrate can be used provided that the ink is compatible with the substrate. Applications for the Aerosol Jet technology include flexible displays, touchscreens, fuel cells, high efficiency solar cells, 3-dimensional interconnects, and embedded components including sensors, resistors, and antennae. Aerosol Jet has distinct functional and cost advantages over traditional methods such as screenprinting, lithography and wirebonding. The solution also compares well versus InkJet printing, especially in meeting high-volume manufacturing requirements. The Aerosol Jet process is compatible with a variety of substrates, including silicon, polyimide, glass, FR-4 and aluminum oxide. In principle, virtually any substrate can be used provided that the ink is compatible with the substrate. Applications for the Aerosol Jet technology include flexible displays, EMI shielding, solder-free electronics, high efficiency solar cells, 3-dimensional interconnects, and embedded components including sensors, resistors, and antennae.

Processing and qualification of polymer based pastes in electronics production

This paper presents results from experiments to integrate printable polymer thick-film pastes into the process chain of Surface Mount Technology (SMT). With a novel paste system under investigation the focus was to examine technological limitations resulting from processing the paste in screen printing. Furthermore tests were performed to qualify bonding alternatives between printed paste and SMT-components and conductor-tracks, respectively. Thereby excellent printing results of structures sized down to approximately 125 μm on flexible and rigid substrate material emerged, as well as reliable interconnections.

In-Band Frequency-Tunable Ceramic Planar Inverted-F Antenna (PIFA) Utilizing an Integrated BST-Based Variable Capacitor

The dielectric properties of low-sintering-temperature (LTCC) Barium Strontium Titanate (BST) thick film paste at high frequencies (0.8–5 GHz) are briefly presented. The utility of the BST paste was demonstrated by tuning the resonance frequency of a ceramic PIFA for mobile terminals operating at 2 GHz. The frequency tuning achieved can be used to compensate frequency detuning caused by the proximity of the user to the antenna. The impact of tuning on the antenna's total efficiency and radiation pattern was experimentally studied through a comparison with the performance of a reference antenna not incorporating the varactor.