Low-temperature air-fireable glass-free metallic thick-film electrical conductor materials

Low-temperature air-fireable glass-free metallic thick-film electrical conductor materials were developed for interconnections in electronic packaging. The thick film with composition (by weight) 96.60%Ag, 1.38%Cu, 0.28%Al, 0.35%Ti, and 1.39%Sn used Ti−Al as the active binder. After firing in air at 500 C it exhibited low electrical resistivity (6.2×10⁻⁶Ω-cm), good scratch resistance and strong bonding with the alumina substrate, with no pinholes. The firing caused complete melting of the particles in the film. Firing in argon rather than air degraded both electrical and bonding properties, due to the absence of oxygen, which helped to burn out the vehicle. The use of Ti rather than Ti−Al as the active binder resulted in holes in the thick film due to incomplete melting of the Ti-rich particles and also resulted in poor scratch resistance and weaker bonding to the substrate. Tin in the composition was important for promoting melting and protecting the active particles from oxidation during firing.     

Effect of firing atmosphere on air-fireable glass-free electrically conductive thick film

The firing atmosphere (air, oxygen, and argon) was found to affect the electrical and mechanical properties of an air-fireable electrically conductive glass-free silver-based thick film. For the optimum firing temperature of 930°C, air results in the lowest resistivity, but a minor amount of pinholes; oxygen results in the largest thickness, the smoothest surface, and no pinhole; and argon results in the highest resistivity, large pinholes, the smallest thickness, vanishing of macroscopic parts of the film, and the poorest scratch resistance. Argon gives higher resistivity than air or oxygen at essentially all firing temperatures.     

Improving the electrical and mechanical behavior of electrically conductive paint by partial replacement of silver by carbon black

Partial replacement of silver particles by carbon black (low cost) in electrically conductive paint was found to decrease the electrical resistivity and increase the scratch resistance of the resulting thick film, which is for use in electrical interconnections. An effective carbon black content is 0.055 of the total filler volume. By using a total solid volume fraction of 0.1969 and a silane-propanol (1:1 by weight) solution as the vehicle, a paint that gives a thick film with resistivity 2 × 10⁻³ Ω·cm has been attained.     

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.     

Electrically Conductive Metal Polymer Nanocomposites for Electronics Applications

An electrically conductive nanocomposite composed of thermoplastic elastomer and nanosized silver particles was developed. Nanosized silver particles were produced by the liquid flame spraying method. Nanocomposites were produced employing a batch mixing process in the melt state. The percolation curve and the minimum resistivity as a function of silver content were defined. A plasticized styrene block-copolymer was used as the matrix polymer. The results showed that the agglomeration of the silver particles has a major influence on the percolation threshold and the resistivity of the compound. With slightly agglomerated silver particles a percolation threshold with a silver content of 13–16 vol.% was achieved. The corresponding resistivity was 2.0 × 10⁻¹ Ω cm. With heavily agglomerated particles the resistivity is high (2.9 × 10³ Ω cm), even with a silver content of 20 vol.%. With a low primary silver particle size (under 100 nm), the resistivity of the compound was high (5.6 × 10⁵ Ω cm).     

Silver Particle Carbon-Matrix Composites as Thick Films for Electrical Applications

Silver particle (3 μm) carbon-matrix composites in the form of thick films (around 100 μm thick) on alumina, as prepared from pastes comprising silver and mesophase pitch particles (14 μm), have been attained. The films on alumina were fired at 650°C in nitrogen to convert pitch to carbon. The volume electrical resistivity attained ranged from 10⁻⁵ Ω cm to 10⁴ Ω cm, depending on the silver volume fraction. The percolation threshold was 12 vol% silver.     

Electrically Conductive Thick Film Made from Silver Alkylcarbamates

A homogeneous electrically conductive silver paste without solid or particle phase was developed using silver alkylcarbamates [(C _n H_2n−1NHCOO)₂Ag, n ≤ 4] as the precursor of the functional phase. The silver alkylcarbamates were light insensitive and had a low decomposition temperature (below 200°C). The paste was a non-Newtonian fluid with viscosity significantly depending on the content of the thickening agent ethyl cellulose. Array patterns with a resolution of 20 μm were obtained using this paste by a micropen direct-writing method. After the paste with about 48 wt.% silver methylcarbamate [(CH₃NHCOO)₂Ag] precursor was sintered at 180°C for 15 min, an electrically conductive network consisting of more than 95 wt.% silver was formed, and was found to have a volume electrical resistivity on the order of 10⁻⁵ Ω cm and a sheet electrical resistivity on the order of 10⁻²–10⁻³ Ω/□. The cohesion strength within the sintered paste and the adhesion strength between the sintered paste layer and the alumina ceramic substrate were tested according to test method B of the American Society for Testing and Materials standard D3359-08. None of the sintered paste layer was detached under the test conditions, and the cohesion and adhesion strengths met the highest grade according to the standard.     

Burnout of the organic vehicle in an electrically conductive thick-film paste

The burnout of the organic vehicle in a silver-particle, glass-free, electrically conductive, thick-film paste during firing in air was studied. For a vehicle consisting of ethyl cellulose dissolved in ether, burnout primarily involves the thermal decomposition of ethyl cellulose. The presence of ether with dissolved ethyl cellulose facilitates the burnout of ethyl cellulose. Excessive ethyl cellulose hinders the burnout. A high heating rate results in more residue after burnout. By interrupting the heating at 160°C for 15 min, the residue after subsequent burnout is diminished probably because of reduced temporal overlap of the processes of organic burnout and silver particle necking. By interrupting the heating at either 300°C or 385°C for 30 min, the temperature required for complete burnout is reduced. The addition of silver particles facilitates drying at room temperature and burnout upon heating.     

Electrically Nonconductive Thermal Pastes with Carbon as the Thermally Conductive Component

Electrically nonconductive thermal pastes have been attained using carbon (carbon black or graphite) as the conductive component and ceramic (fumed alumina or exfoliated clay) as the nonconductive component. For graphite particles (5 μm), both clay and alumina are effective in breaking up the electrical connectivity, resulting in pastes with electrical resistivity up to 10¹³Ω·cm and thermal contact conductance (between copper surfaces of roughness 15 μm) up to 9 × 10⁴ W/m²·°C. For carbon black (30 nm), clay is more effective than alumina, providing a paste with resistivity 10¹¹ Ω·cm and thermal contact conductance 7 × 10⁴ W/m²·°C. Carbon black increases the thermal stability, whereas either graphite or alumina decreases the thermal stability. The antioxidation effect of carbon black is further increased by the presence of clay up to 1.5 vol.%. The addition of clay (up to 0.6 vol.%) or alumina (up to 2.5 vol.%) to graphite paste enhances the thermal stability.     

Electrical Conductivity of Thick Films Made from Silver Methylcarbamate Paste

We have explored the electrical conductivity of thick films made from silver methylcarbamate paste using metallic silver as the electrically conductive phase. The paste was composed of 30 wt.% to 90 wt.% organic vehicle and 10 wt.% to 70 wt.% functional phase precursor (silver methylcarbamate). After the paste was sintered, films with thickness of 4.50 μm to 12.70 μm were obtained, in which the elemental percentage of silver varied from about 5 wt.% to above 99 wt.%. Experiments showed that both the electrical conductivity and the elemental percentage were mainly affected by the initial silver content in the paste and the parameters of the sintering process. For given sintering conditions, higher initial silver content led to higher elemental percentage of silver, improving the electrical conductivity of the thick film. The conditions of the sintering process had a significant influence on the evaporation and decomposition rates of the paste components, the elemental percentage of silver, and the microstructure of the thick film. Higher temperatures, longer times, lower heating rates, and more oxygen-rich sintering atmospheres were found to accelerate the evaporation and decomposition and increase the elemental percentage of silver, both of which served to enhance the electrical conductivity. For initial silver contents less than about 10 wt.%, the lowest electrical resistivity of the thick film only reached the order of 10^?4 Ω cm, irrespective of the sintering conditions. For contents between 10 wt.% and 25 wt.%, it was possible to attain lowest resistivity values on the order of 10^?5 Ω cm. Above 25 wt.%, the lowest resistivity could reach 10^?6 Ω cm, comparable to that of bulk silver.     

Pressure electrical contact improved by carbon black paste

Pressure and pressureless electrical contacts were evaluated by measuring the contact electrical resistivity between copper mating surfaces. Pressure electrical contacts with a contact resistivity of 2×10⁻⁵ Ω·cm² have been attained using a carbon black paste of a thickness of less than 25 μm as the interface material. In contrast, a pressureless contact with silver paint as the interface material exhibits a higher resistivity of 3×10⁻⁵ Ω·cm² or above. A pressureless contact with colloidal graphite as the interface material exhibits the same high contact resistivity (1×10⁻⁴ Ω·cm²) as a pressure contact without any interface material. On the other hand, pressureless contacts involving solder and silver epoxy exhibit lower contact resistivity than carbon black pressure contacts.     

Neutralization of electrically active aluminum in recrystallized silicon-on-sapphire films

We report the effect of steam oxidation at 875° C on the electrical resistivity, crystalline quality (measured by ion channeling), and Al concentration (measured by secondary ion mass spectrometry) in 0.25 μm thick, Si-implanted and recrystallized, Si-on-sapphire films. After a deep Si implantation (180 keV, 1.4×l0¹⁵ Si/cm²) at room temperature, and solid-phase epitaxial regrowth from the non-amorphized, 0.03 μm thick surface region, the initially undoped SOS films become doped p-type, and their resistivity decreases from (1−5)xl0¹⁴ ficm to 0.5 Ωcm. The doping is due to electrically active Al, released from the A1₂O₃ by the Si implantation, and present in the recrystallized films at a concentration of ≃2×l0¹⁶ Al/cm³ . After a 75 min steam oxidation at 875 °C, which consumes 0.06 Μm of Si, the resistivity of the recrystallized films increases to over 40 Ωcm, but the Al concentration is unchanged. The oxidation also uncovers higher quality material below the non-recrystallized surface layer. A semi-quantitative model is proposed to explain the electrical data, based on the diffusion of oxygen from the Si/SiO₂ interface into the SOS film during oxidation, and the formation of Al-O-Si neutral complexes. Data on the stability of the high-resistivity films against high-temperature annealing or re-amorphization and annealing is given.     

High conductivity copper-boron alloys obtained by low temperature annealing

The electrical behavior during annealing of copper films with a nominal concentration of 2 at.% boron has been investigated. The evolution of the resistivity of the film was monitored using an in situ technique, in which the film was rampannealed at constant ramp rates. At temperature of 150–200°C, the resistivity of the Cu(B) undergoes a first drop. This is followed by one or two such drops in resistivity, so that after completion of a ramp-anneal from 50°C to 750°C, the room temperature resistivity decreases from the initial value of 13 μΩ cm to 2.1 μΩcm, close to that of bulk copper. Isothermal annealing of the film also leads to substantial decreases in resistivity, from 13 μΩcm to 3 μΩ cm after annealing at 350°C for 8 h and to 2.5 μΩ cm at 400°C for 4 h. These results show that a dramatic reduction in resistivity of Cu(B) alloys takes place at temperatures below 400°C, suggesting possible applications for silicon device interconnections.     

Active brazing alloy paste as a totally metal thick film conductor material

A silver-based active (titanium-containing) brazing alloy, namely 63Ag-34.25Cu-1.75Ti-1.OSn, was found to serve as a totally metal (no glass) thick film conductor which exhibited lower electrical resistivity, much greater film/substrate adhesion, much lower porosity, similar solderability, and lower scratch resistance compared to the conventional silver-glass thick film. The brazing alloy film was formed by screen printing a paste containing the alloy particles and then firing at 880°C in vacuum.     

Studies on Inkjet-Printed Conducting Lines for Electronic Devices

Inkjet printing is considered one of the most promising methods for patterning and materials deposition. The feasibility of employing inkjet technology for the creation of conductive pathways on printed circuit boards is addressed herein. Prediction of the width, length, and thickness of printed lines as a function of the dot diameter, resolution, and volume fraction of the particles in the ink is presented. Surface treatment of the substrate to promote desirable adhesion and wetting properties as well as the adjustment of the curing process to reduce the surface roughness of the printed traces were studied. In a sintering study, samples sintered at 250°C for 20 min showed a resistivity of 4.2 μΩ cm, which is approximately 2.6 times that of bulk silver. A low-temperature sintering method through the reduction of a metal salt is presented. The resistivity of printed samples sintered at 140°C for 30 min in the presence of silver nitrate with N,N-dimethylformamide showed a resistivity of 22.5 μΩ cm.     

An evaluation of materials and processes for integrated microwave circuits

This paper presents an evaluation of materials and processes applicable to the fabrication of hybrid microstrip microwave circuits. Substrate materials evaluated included aluminas, beryllias, quartz, and glass of varying purities and surface finishes. Conductor materials evaluated included silver, copper, gold, and aluminum. Fabrication processes studied included vacuum deposition sputtering electroless and electroplating, thick-film screening and firing, and photoetching. Sapphire and high-purity alumina (99.5 percent pure or better) substrates were found superior as substrates for microstrip circuits. Conductor materials and processing methods found best were 1) vacuum deposited chromium-gold thin film which was gold electroplated and photoetched; 2) thick-film silver which was photoetched to delineate the microwave pattern.     

Sintering of Inkjet-Printed Silver Nanoparticles at Room Temperature Using Intense Pulsed Light

Intense pulsed light (IPL) was used to sinter printed silver nanoink patterns consisting of 20-nm to 40-nm silver nanoparticles dispersed in diethylene glycol (DEG). Three consecutive pulses at 50 J/cm² in less than 30 ms was sufficient to adequately sinter silver nanoink patterns for printed electronics without degradation of the substrates. This is an exceptionally short time compared with that of the conventional thermal sintering process. On the sintered conductive silver patterns, neck-like junctions between nanoparticles were observed using scanning electron microscopy (SEM). The melting temperature, 194.1°C, of silver nanoparticles was found using differential scanning calorimetry (DSC). Also, x-ray diffraction (XRD) was used to find the grain size of the printed silver nanoink patterns. The IPL-sintered silver pattern had a grain size of 86.3 ± 7.2 nm. From this work, it was found that the IPL-sintered silver pattern had a low resistivity of 49 ± 3 nΩ m, which is low enough to be used for printed electronics.     

Conductivity enhancement of nano silver-filled conductive adhesives by particle surface functionalization

Five different types of surfactants were employed for nanoparticle functionalization and the effects of the surfactants on electrical properties of nano silver (Ag)-filled conductive adhesives were investigated. The Ag nanoparticles pretreated with the surfactants were incorporated into isotropic conductive adhesives (ICA) formulations as conductive fillers. By using the surfactants (S3, S4, and S5), the reduced resistivity of the nano Ag-filled adhesives could be achieved with 2×10⁻⁴ Θ-cm. The morphology studies showed that the low resistivity resulted from the sintering of nanoparticles.     

An Evaluation of Materials and Processes for Integrated Microwave Circuits

This paper presents an evaluation of materials and processes applicable to the fabrication of hybrid microstrip microwave circuits. Substrate materials evaluated included aluminas, beryllias, quartz, and glass of varying purities and surface finishes. Conductor materials evaluated included silver, copper, gold, and aluminum. Fabrication processes studied included vacuum deposition, sputtering, electroless and electroplating, thick-film screening and firing, and photoetching. Sapphire and high-purity alumina (99.5 percent pure or better) substrates were found superior as substrates for microstrip circuits. Conductor materials and processing methods found best were 1) vacuum deposited chromium-gold thin film which was gold electroplated and photoetched; 2) thick-film silver which was photoetched to delineate the microwave pattern.     

An Evaluation of Materials and Processes for Integrated Microwave Circuits

This paper presents an evaluation of materials and processes applicable to the fabrication of hybrid microstrip microwave circuits. Substrate materials evaluated included aluminas, beryllias, quartz, and glass of varying purities and surface finishes. Conductor materials evaluated included silver, copper, gold, and aluminum. Fabrication processes studied included vacuum deposition, sputtering, electroless and electroplating, thick-film screening and firing, and photoetching. Sapphire and high-purity alumina (99.5 percent pure or better) substrates were found superior as substrates for microstrip circuits. Conductor materials and processing methods found best were 1) vacuum deposited chromium-gold thin film which was gold electroplated and photoetched; 2) thick-film silver which was photoetched to delineate the microwave pattern.