Carbon-black thixotropic thermal pastes for improving thermal contacts

This paper addresses thermal interface materials for thermal conduction of excess heat for microelectronic applications. Carbon black (30 nm) thixotropic paste based on polyol ethers is comparable to carbon black fluidic paste based on polyethylene glycol (PEG) in its effectiveness as a thermal paste, and in its dependence on pressure history. Prior pressure (up to 0.69 MPa) application is helpful. The optimum carbon black content is 2.4 vol.% for the thixotropic paste. The thermal contact conductance across copper surfaces is 30 × 10⁴ and 11 × 10⁴ W/m²-°C for surface roughness of 0.05 μm and 15 μm, respectively. The volume electrical resistivity is 3 × 10³ Ω-cm. Boron nitride (BN) (5–11 μm) and graphite (5 μm) thixotropic pastes are less effective than carbon black thixotropic paste by up to 70% and 25%, respectively, in thermal contact conductance, due to low conformability.     

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.     

Nanoclay Paste as a Thermal Interface Material for Smooth Surfaces

A paste in the form of a polyol ester vehicle (liquid) containing 0.6 vol.% nanoclay is an effective thermal interface material. Nanoclay with a high conformability and hence a small bond line thickness is preferred, namely montmorillonite containing a quarternary ammonium salt organic modifier (dimethyl dehydrogenated tallow) at 125 meq/100 g clay, after exfoliation by using the vehicle. When it is used between smooth (0.009 μm) copper surfaces at a pressure of 0.69 MPa, the thermal contact conductance reaches 40 × 10⁴ W/m² K, in contrast to the corresponding values of 28 × 10⁴ W/m² K, 28 × 10⁴ W/m² K, 25 × 10⁴ W/m² K, and 24 × 10⁴ W/m² K previously reported for carbon black, fumed alumina, fumed zinc oxide, and graphite nanoplatelet pastes. Between rough copper surfaces (12 μm), the conductance provided by the nanoclay paste is slightly below those of the other pastes. The superiority of the nanoclay paste for smooth surfaces is attributed to the␣submicron bond line thickness; the inferiority for rough surfaces is due to the low thermal conductivity. The conductance provided by the nanoclay paste increases from 31 × 10⁴ W/m² K to 40 × 10⁴ W/m² K when the pressure is increased from 0.46 MPa to 0.92 MPa. This pressure dependence is stronger than that of any of the other pastes studied.     

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.     

Carbon Nanotube Thermal Pastes for Improving Thermal Contacts

The use of 0.6 vol.% single-walled carbon nanotubes in a poly(ethylene glycol)-based dispersion gave a thermal paste that was as effective as solder for improving thermal contacts. A thermal contact conductance of 20 × 10⁴ W m⁻² K⁻¹ was attained. An excessive amount of nanotubes (e.g. 1.8 vol.%) degraded the performance, because of conformability loss. The nanotubes were more effective than hexagonal boron nitride particles but were less effective than carbon black, which gave a thermal contact conductance of 30 × 10⁴ W m⁻² K⁻¹.     

Cement-based controlled electrical resistivity materials

Low-cost controlled electrical resistivity materials based on Portland cement and exhibiting low values of the relative dielectric constant have been attained. The materials are cement paste containing short electrically-conducting fibers. With steel fibers (0.1 vol.%), the resistivity and relative dielectric constant (10 kHz) are 8 × 10⁴ cm and 20, respectively. With carbon fibers (1.0 vol.%) and silica fume, these quantities are 8×10²Ω-cm and 49, respectively.     

Comparative evaluation of thermal interface materials for improving the thermal contact between an operating computer microprocessor and its heat sink

Testing of the relative effectiveness of various thermal interface materials for improving the thermal contact between the well-aligned mating surfaces of an operating computer microprocessor (with an integrated heat spreader) and its heat sink shows that carbon black paste, whether by itself or as a coating on aluminum or flexible graphite, is more effective than silver paste (Arctic Silver), but is comparable in effectiveness to aluminum paste (Shin-Etsu). The carbon black paste by itself is as effective as the Shin-Etsu paste coated aluminum. The Shin-Etsu paste is more effective than Arctic Silver, whether by itself or as a coating. The relative performance is mostly consistent with that assessed by measuring the thermal contact conductance. The correlation is particularly strong for conductance below 3×10⁴ W/m²·°C. The discrepancy is attributed to the difference in surface roughness between computer and guarded hot plate surfaces. In the case in which the mating surfaces of microprocessor and heat sink are not well aligned, Shin-Etsu and Arctic Silver are more effective than carbon black.     

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.     

Rheological Behavior of Thermal Interface Pastes

Polyol-ester-based thermal pastes containing carbon black, fumed alumina or nanoclay exhibit Bingham plastic behavior with shear thinning. Carbon black gives double yielding, but fumed alumina and nanoclay give single yielding. The plastic viscosity increases with the solid content. Antioxidants increase the plastic viscosity and yield stresses. Nanoclay (1.0 vol.%) gives low shear moduli, high critical shear strain, and high loss tangent, thus resulting in low bond-line thickness and high thermal contact conductance for smooth (0.009 μm) proximate surfaces. Carbon black (Tokai, 8.0 vol.%) gives high moduli, low critical strain, and low loss tangent, thus resulting in high bond-line thickness, though the high thermal conductivity due to the high solid content results in high thermal contact conductance for rough (15 μm) proximate surfaces. Antioxidants enhance the solid-like character, increase the yield stress, plastic viscosity, and bond-line thickness, and decrease the thermal contact conductance.     

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.     

Comparative study of electrically conductive thick films with and without glass

An air-fireable, glass-free, electrically conductive thick-film material (96.6% Ag, 1.38% Cu, 0.28% Al, 0.35% Ti, and 1.39% Sn by weight) and a conventional glass-containing, electrically conductive thick-film materials (96.6% Ag and 3.4% glass frit by weight), both on alumina substrates, were studied by electrical, mechanical, thermal, and microscopic methods. The volume electrical resistivity of the glass-free thick film (2.5×10⁻⁶ Ω·cm, 30-μm thick) is lower than that of the glass-containing thick film (3.9×10⁻⁶ Ω·cm, 19-μm thick), with each film processed at its optimum firing temperature. The optimum firing temperature is 930°C and 850°C for glass-free and glass-containing thick films, respectively, as indicated by the criteria of low resistivity and high scratch resistance. The glass-free thick film has a higher scratch resistance than the glass-containing thick film, both fired at their respective optimum temperatures, suggesting that the former has higher bond strength to the alumina substrate. The formation process of the glass-free and glass-containing thick films is similar. The process involves solid-state diffusion of silver, which results in a silver network and grain boundaries. However, the sintering of silver particulates in the glass-containing thick film is enhanced by the viscous flow of glass.     

High Performance Nonconductive Film with <Emphasis Type="Italic">π</Emphasis>-Conjugated Self-Assembled Molecular Wires for Fine Pitch Interconnect Applications

In this paper, we introduce a novel approach of using π-conjugated molecular wires to improve the electrical properties of nonconductive films (NCFs) for the electronic interconnects. Two thiol (-SH) terminated conjugated molecular wires, 1,4-benezenedithiol (BDT) and biphenyldithiol (BPD), are incorporated into an NCF formulation and investigated. The molecular wires can be well assembled on the gold (Au) electrodes and improve the interfacial properties of the NCF joints. The BPD exhibits higher thermal stability after curing at 150°C than BDT due to the higher aromaticity (more aromatic rings) and thus more rigid structure. Formation of a self-assembled monolayer on Au electrodes can tune the effective work function (Φ) of Au electrodes and reduce the tunnel resistivity. Therefore, the joint resistance of NCF joints, which is the sum of tunnel resistance and constriction resistance, can be significantly reduced from 0.15 × 10⁻³ Ω to 0.1 × 10⁻³ Ω and 0.05 × 10⁻³ Ω with BDT and BPD, respectively. The improved electrical conduction and current carrying capability enables the application of the NCF in fine pitch and high performance electronic interconnects in microelectronics.     

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).     

Thermoelectric Properties of Half-Heusler Bismuthides ZrCo<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi (<Emphasis Type="Italic">x</Emphasis> = 0.0 to 0.1)

The usefulness of half-Heusler (HH) alloys as thermoelectrics has been mainly limited by their relatively large thermal conductivity, which is a key issue despite their high thermoelectric power factors. In this regard, Bi-containing half-Heusler alloys are particularly appealing, because they are, potentially, of low thermal conductivity. One such a material is ZrCoBi. We prepared pure and Ni-doped ZrCoBi by a solid-state reaction. To evaluate thermoelectric potential we measured electrical resistivity (ρ = 1/σ) and thermopower (σ) up to 1000 K and thermal conductivity (κ) up to 300 K. Our measurements indicate that for these alloys resistivity of approximately a few mΩ cm and thermopower larger than a hundred μV K⁻¹ are possible. Low κ values are also possible. On the basis of these data we conclude that this system has a potential to be optimized further, despite the low power factors (α ² σT) we have currently measured.     

Rapid thermal diffusion of zinc into GaAs

Rapid thermal diffusion of zinc into semi-insulating GaAs from spin-on Zn doped silica film was performed. Spin-on films act both as Zn diffusion sources and GaAs surface encapsulant layer against decomposition during the rapid thermal diffusion. The very shallowp ⁺ layers were obtained at a diffusion temperature of 900° C for 5 sec. Non-alloyed ohmic contacts to thesep ⁺ layers were achieved with an average contact resistivity of 2.4 × 10⁻⁶ Ω cm². The interface is very smooth. The zinc diffusion coefficient for rapid thermal diffusion with effective diffusion time of 6 sec at 900° C was numerically calculated from SIMS profiles. In contrast to the common Longini-Weisberg-Blanc model, the rapid thermal diffusion is under nonequilibrium condition. Complications due to interstitial-substitutional nonequilibrium, vacancy supply resulted from the interface stress field, and zinc precipitation are briefly discussed.     

Hygrothermal Stability of Electrical Contacts Made from Silver and Graphite Electrically Conductive Pastes

The hygrothermal stability of electrical contacts made from silver and graphite electrically conductive pastes is comparatively evaluated by measurement of the increase in contact electrical resistance during immersion in water at 15°C and 40°C. The pastes are silver paint, silver paint with a nonconductive epoxy overcoat, silver epoxy, and graphite colloid. Each electrical contact is made between a seven-strand tin-coated copper wire and the surface of a carbon fiber epoxy-matrix composite. Silver paint and graphite colloid penetrate the spaces among the 130-μm-diameter strands, but silver epoxy does not. Partly due to its low penetrability and the silver flake (15 μm) preferred orientation, silver epoxy gives contacts of significantly higher resistance than silver paint. Graphite colloid is comparable to silver epoxy in the resistance. Among the four pastes, silver paint with an epoxy overcoat is most durable, though it gives slightly higher resistance than silver paint without epoxy. Silver epoxy is less durable than silver paint without an epoxy overcoat, particularly at 40°C, due to the low hygrothermal stability of epoxy. Graphite colloid is even less durable than silver epoxy, due to its being water based.     

Numerical Modeling of the Performance of Thermal Interface Materials in the Form of Paste-Coated Sheets

The performance of thermal interface materials in the form of core sheets coated on both sides with a thermal paste is numerically modeled by finite-element analysis. The paste is polyol-ester-based carbon black paste and serves to improve the conformability. Good agreement is found between modeling and experimental results that involve copper proximate surfaces sandwiching the thermal interface material. The core sheets are copper, aluminum, indium, and flexible graphite. Flexible graphite (made from exfoliated graphite) is advantageous in its low elastic modulus, whereas copper and aluminum foils are advantageous in their high thermal conductivity. Indium is advantageous in its low elastic modulus compared with copper or aluminum and in its high thermal conductivity compared with flexible graphite. Among the four types of core sheet with identical thickness, coated indium foil gives the best performance for the range of foil thickness of 6 μm to 112 μm for the case of smooth (0.01 μm roughness) proximate surfaces and 117 μm to 320 μm for the case of rough (15 μm roughness) proximate surfaces. Aluminum foil gives the best performance for the thickness range of 112 μm to 2000 μm in the case of smooth proximate surfaces. For thicknesses below these ranges, flexible graphite performs the best. For thicknesses above these ranges, copper foil performs the best.     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.     

A study on the key factors affecting the electronic properties of monocrystalline silicon solar cells

The model of monocrystalline silicon solar cells is established,and the effects of wafer parameters,such as the p-Si（100） substrate thickness,the defect density,and the doping concentration,on the electronic properties of monocrystalline silicon solar cells are analyzed.The results indicate that the solar cells with an Al back-surface-field will have good electronic properties when the wafers meet the following three conditions：（i） the defect density is less than 1.0×1011 cm^-3;（ii） the doping concentration is from 5×10^15 cm^-3 to 1×10^17 cm^-3,i.e.the bulk resistivity is from 0.5 Ω·cm to 10 Ω·cm;（iii） the cells substrate thickness is in the range of 100 μm to 200 μm.     

Thermomechanical effects in embedded passive materials

Thermomechanical fatigue was measured using electron-beam moiré (EBM) and infrared (IR) microscopy. A specimen was made using a FR-4 printed wiring board (PWB), a silver-filled conductive adhesive, and a carbon-filled conductive paste. Both filled polymeric materials are used for embedded resistor applications. We studied the behavior of these materials and, particularly, the interfaces between these materials as a function of thermal cycling. The EBM gives quantitative information on localized strains, whereas the IR microscopy gives quantitative information on changes in heat flow. The filled polymeric materials showed strains of −0.6% at −55°C and 1.4% at 125°C. The interface between the silver and carbon-filled materials increased in thermal resistance on the order of 10⁻⁶ m²·K·W⁻¹ per thermal cycle.