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.     

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.     

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.     

Intermetallic Reaction of Indium and Silver in an Electroplating Process

The reaction of indium (In) and silver (Ag) during the electroplating process of indium over a thick silver layer was investigated. It was found that the plated In atoms react with Ag to form AgIn₂ intermetallic compounds at room temperature. Indium is commonly used in the electronics industry to bond delicate devices due to its low yield strength and low melting temperature. In this study, copper (Cu) substrates were electroplated with a 60-μm-thick Ag layer, followed by electroplating an In layer with a thickness of 5 μm or 10 μm, at room temperature. To investigate the chemical reaction between In and Ag, the microstructure and composition on the surface and the cross section of samples were observed by scanning electron microscopy (SEM) with energy-dispersive x-ray spectroscopy (EDX). The x-ray diffraction method (XRD) was also employed for phase identification. It was clear that indium atoms reacted with underlying Ag to form AgIn₂ during the plating process. After the sample was stored at room temperature in air for 1 day, AgIn₂ grew to 5 μm in thickness. With longer storage time, AgIn₂ continued to grow until all indium atoms were consumed. The indium layer, thus, disappeared and could barely be detected by XRD. Jong S. Kim now with Applied Materials.     

Performance of Isotropic and Anisotropic Heat Spreaders

Anisotropic heat spreaders (flexible graphite and continuous carbon fiber polymer-matrix composite) and isotropic heat spreaders (copper and aluminum) have been evaluated numerically in terms of thermal resistance. Anisotropic ones are attractive for their through-thickness thermal insulation ability. Flexible graphite is superior to carbon fiber composite in providing lower thermal resistance. Carbon fiber composite is advantageous in its superior through-thickness thermal insulation ability and its smaller critical thickness (the optimal thickness for maximizing heat spreading while minimizing thickness). The isotropic heat spreaders are superior to the anisotropic ones in providing low thermal resistance, provided that the thickness is large, but they do not have the through-thickness thermal insulation ability. A higher value of the in-plane thermal conductivity enhances the effectiveness of flexible graphite. As the heat source area decreases, the thermal resistance increases while the critical thickness decreases. For the same heat source area, a greater in-plane dimension of the heat source perpendicular to the intended heat spreading direction decreases the thermal resistance and critical thickness. Flexible graphite is comparatively more advantageous when the thickness is smaller and when the heat source area is larger. For the same thickness below 2?mm, flexible graphite with in-plane conductivity of 1500?W/(m?K) is superior to copper and that with in-plane conductivity of 600?W/(m?K) is superior to aluminum. The highest thermal conductance obtained is 6.1?×?10⁴?W/(m²?K) when the thermal interfacial resistance is neglected and 5.1?×?10⁴?W/(m²?K) when this resistance is included. The conductance increases with decreasing heat source area and with decreasing heat spreader length.     

Thermal Conductivity of Indium–Graphene and Indium-Gallium–Graphene Composites

Samples of graphene composites with a matrix of indium or indium-gallium alloy were prepared in the form of foils using exfoliated graphene dispersions. The thermal conductivity of the composite samples with different thicknesses was determined using the three-omega method. Indium–graphene composite samples with a thickness of 430 μm exhibited a twofold increase in thermal conductivity, whereas indium-gallium–graphene composite samples with a thickness of 330 μm exhibited a threefold improvement in thermal conductivity over that of the matrix at 300 K. The effective medium approximation (EMA) was used to model the thermal conductivity of the composite samples. The graphene platelet size distribution was used to determine the average thermal conductivity of graphene in the composite samples. The interfacial thermal conductance between graphene and indium or indium-gallium alloy determined from EMA was not the limiting factor in the improvement of the thermal conductivity of the composite samples, although the increase in thermal conductivity was found to be slightly lower than predicted theoretically using acoustic and diffuse mismatch models. The smaller size of the graphene platelets obtained by exfoliation prior to dispersion in the matrix appears to be the limiting factor.     

Cu(In,Ga)Se<Subscript>2</Subscript> Thin Film Preparation from a Cu(In,Ga) Metallic Alloy and Se Nanoparticles by an Intense Pulsed Light Technique

The main contribution of this paper is the development of a novel process for the formation of copper indium gallium diselenide (CIGS) films. CIGS films with a thickness of 4 μm and grain size from 0.3 μm to 1 μm were prepared from a Cu(In_0.7Ga_0.3) (CIG) metallic alloy and Se nanoparticles by the intense pulsed light (IPL) technique. The melting of the CIG and Se nanoparticles and nucleation of CIGS occurred in a very short reaction time of 2 ms. It is believed that the Se diffuses into the CIG lattice to form the CIGS chalcopyrite crystal structure. The tetragonal chalcopyrite crystal structure was confirmed by x-ray powder diffraction (XRD), while the microstructure and composition were determined by field-emission scanning electron microscopy (FESEM), energy-dispersive x-ray spectroscopy (EDAX), and x-ray fluorescence (XRF) spectroscopy.     

Optimization and Fabrication of a Thick Printed Thermoelectric Device

Thermoelectric power generation technology aims to convert thermal energy into electricity. Micromodule design optimization depends directly on the thermal environment. For low thermal energy input, optimized thermoelectric devices require 100 μm to 500 μm element thickness. These dimensions currently present a challenge for standard mass-production manufacturing techniques. In this paper, a unique printing technology for micromodule fabrication is presented. This technology is compared with a traditional bulk thermoelectric manufacturing process to highlight the advantages of the printing process to obtain scalable thermoelectric devices. Initial thermoelectric materials have been integrated in inks and then deposited by a spray technology onto a polymer substrate. A complete micromodule for application on nonplanar surfaces is also presented.     

Absorptivity of semiconductors used in the production of solar cell panels

The dependence of the absorptivity of semiconductors on the thickness of the absorbing layer is studied for crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CuInSe₂, CIS), and copper gallium diselenide (CuGaSe₂, CGS). The calculations are performed with consideration for the spectral distribution of AM1.5 standard solar radiation and the absorption coefficients of the materials. It is shown that, in the region of wavelengths λ = λ _g = hc/E _g , almost total absorption of the photons in AM1.5 solar radiation is attained in c-Si at the thickness d = 7−8 mm, in a-Si at d = 30–60 μm, in CdTe at d = 20−30 μm, and in CIS and CGS at d = 3−4 μm. The results differ from previously reported data for these materials (especially for c-Si). In previous publications, the thickness needed for the semiconductor to absorb solar radiation completely was identified with the effective light penetration depth at a certain wavelength in the region of fundamental absorption for the semiconductor.     

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.     

Numerical Simulation of the Thermomechanical Behavior of Extruded Bismuth Telluride Alloy Module

Our research group developed over the past years a method to produce n- and p-type bismuth telluride alloys by mechanical alloying and powder extrusion. The resulting extruded rods possess a particular crystalline texture, which is advantageous for module fabrication processes but may have an impact on the stress distribution in modules under operating conditions. The reported mechanical strength of the extruded polycrystalline thermoelectric (TE) materials is larger than those of materials produced by directional solidification, allowing the fabrication of thinner TE modules in order to increase power densities. The stress arising from the resulting higher thermal gradients in thinner legs can eventually become greater than the TE material strength, which would limit further module thickness reduction. We present results of numerical simulations of TE modules behavior undertaken to evaluate the effect of leg lengths (1 mm, 500 μm, and 250 μm) on the stress level imposed by a given temperature difference that could cause their fracture. Different boundary conditions were imposed on the outer ceramic surfaces delimiting the module (e.g., both free or one anchored on a flat rigid surface). The boundary conditions and the mechanical strength of the soldering alloys are significant factors influencing the stress distribution in the TE alloy elements. We have also examined the effect of the crystalline texture of the extruded TE materials on the distribution and levels of stress, and found it to be marginal.     

Study of the Au/In Reaction for Transient Liquid-Phase Bonding and 3D Chip Stacking

The latest three-dimensional (3D) chip-stacking technology requires the repeated stacking of additional layers without remelting the joints that have been formed at lower levels of the stack. This can be achieved by transient liquid-phase (TLP) bonding whereby intermetallic joints can be formed at a lower temperature and withstand subsequent higher-temperature processes. In order to develop a robust low-temperature Au/In TLP bonding process during which all solder is transformed into intermetallic compounds, we studied the Au/In reaction at different temperatures. It was shown that the formation kinetics of intermetallic compounds is diffusion controlled, and that the activation energy of Au/In reaction is temperature dependent, being 0.46 eV and 0.23 eV for temperatures above and below 150°C, respectively. Moreover, a thin Ti layer between Au and In was found to be an effective diffusion barrier at low temperature, while it did not inhibit joint formation at elevated temperatures during flip-chip bonding. This allowed us to control the intermetallic formation during the distinct stages of the TLP bonding process. In addition, a minimal indium thickness of 0.5 μm is required in order to enable TLP bonding. Finally, Au/In TLP joints of ∅40 μm to 60 μm were successfully fabricated at 180°C with very small solder volume (1 μm thickness).     

Electrical and Mechanical Properties of Through-Silicon Vias and Bonding Layers in Stacked Wafers for 3D Integrated Circuits

Thermal stress issues in a three-dimensional (3D) stacked wafer system were examined using finite-element analysis of the stacked wafers. This paper elucidates the effects of the bonding dimensions on mechanical failure and the keep-away zone, where devices cannot be located because of the stress in the Si. The key factors in decreasing the thermal strain were the bonding diameter and thickness. When the bonding diameter decreased from 40 μm to 12 μm, the equivalent strain decreased by 83%. It is noteworthy that the keep-away zone also decreased from 17 μm to zero when the bonding diameter decreased from 40 μm to 12 μm. When the bonding thickness doubled, the equivalent strain decreased by 44%. The effects of the dimensions and arrangement of through-silicon vias (TSV) were also analyzed. Small TSV diameter and pitch are important to decrease the equivalent strain, especially when the amount of Cu per unit volume is fixed. When the TSV diameter and pitch decreased fourfold, the equivalent strain decreased by 70%. The effects of TSV height and the number of die stacks were not significant, because the underfill acted as a buffer against thermal strain.     

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.     

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.     

Development of (Bi,Sb)<Subscript>2</Subscript>(Te,Se)<Subscript>3</Subscript>-Based Thermoelectric Modules by a Screen-Printing Process

In major applications, optimal power will be achieved when thermoelectric films are at least 100 μm thick. In this paper we demonstrate that screen-printing is an ideal method to deposit around 100 μm of (Bi,Sb)₂(Te,Se)₃-based films on a rigid or flexible substrate with high Seebeck coefficient value (90 μV K⁻¹ to 160 μV K⁻¹) using a low-temperature process. Conductive films have been obtained after laser annealing and led to acceptable thermoelectric performance with a power factor of 0.06 μW K⁻² cm⁻¹. While these initial material properties are not at the level of bulk materials, the complete manufacturing process is cost-effective, compatible with large surfaces, and affords a mass-production technique.     

Study of Spectroscopy and Microstructure in Nanocrystalline Cr-Doped Fibers Grown by the Drawing-Tower Technique

The spectroscopy and microstructure of Cr-doped fibers (CDFs) fabricated by the drawing-tower technique are investigated. The spontaneous emission spectrum exhibited broadband emission of 1.2 μm to 1.55 μm. High-resolution transmission electron microscope images showed that there were nanocrystalline structures in the core, surrounded by an amorphous matrix of SiO₂; they also revealed that the nanoparticle density was about 3.2 × 10³ μm⁻² near the core/clad interface, and 1.5 × 10⁴ μm⁻² near the core center. The results indicate preliminary success in fabricating nanocrystalline CDF with low transmission loss and crystal-like active properties.     

Effect of Pd Thickness on the Interfacial Reaction and Shear Strength in Solder Joints Between Sn-3.0Ag-0.5Cu Solder and Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG) Surface Finish

Intermetallic compound formation at the interface between Sn-3.0Ag-0.5Cu (SAC) solders and electroless nickel/electroless palladium/immersion gold (ENEPIG) surface finish and the mechanical strength of the solder joints were investigated at various Pd thicknesses (0 μm to 0.5 μm). The solder joints were fabricated on the ENEPIG surface finish with SAC solder via reflow soldering under various conditions. The (Cu,Ni)₆Sn₅ phase formed at the SAC/ENEPIG interface after reflow in all samples. When samples were reflowed at 260°C for 5 s, only (Cu,Ni)₆Sn₅ was observed at the solder interfaces in samples with Pd thicknesses of 0.05 μm or less. However, the (Pd,Ni)Sn₄ phase formed on (Cu,Ni)₆Sn₅ when the Pd thickness increased to 0.1 μm or greater. A thick and continuous (Pd,Ni)Sn₄ layer formed over the (Cu,Ni)₆Sn₅ layer, especially when the Pd thickness was 0.3 μm or greater. High-speed ball shear test results showed that the interfacial strengths of the SAC/ENEPIG solder joints decreased under high strain rate due to weak interfacial fracture between (Pd,Ni)Sn₄ and (Cu,Ni)₆Sn₅ interfaces when the Pd thickness was greater than 0.3 μm. In the samples reflowed at 260°C for 20 s, only (Cu,Ni)₆Sn₅ formed at the solder interfaces and the (Pd,Ni)Sn₄ phase was not observed in the solder interfaces, regardless of Pd thickness. The shear strength of the SAC/ENIG solder joints was the lowest of the joints, and the mechanical strength of the SAC/ENEPIG solder joints was enhanced as the Pd thickness increased to 0.1 μm and maintained a nearly constant value when the Pd thickness was greater than 0.1 μm. No adverse effect on the shear strength values was observed due to the interfacial fracture between (Pd,Ni)Sn₄ and (Cu,Ni)₆Sn₅ since the (Pd,Ni)Sn₄ phase was already separated from the (Cu,Ni)₆Sn₅ interface. These results indicate that the interfacial microstructures and mechanical strength of solder joints strongly depend on the Pd thickness and reflow conditions.     

Thermal Conductivity of Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Affected by Grain Size and Pores

A fine measurement system for measuring thermal conductivity was constructed. An accuracy of 1% was determined for the reference quartz with a value of 1.411 W/m K. Bi_0.5Sb_1.5Te₃ samples were prepared by mechanical alloying followed by hot-pressing. Grain sizes were varied in the range from 1 μm to 10 μm by controlling the sintering temperature in the temperature range from 623 K to 773 K. The thermal conductivity was 0.89 W/m K for the sample sintered at 623 K, while a grain size of 1.75 μm was measured by optical microscopy and scanning electron microscopy. The thermal conductivity increased on the sample sintered at 673 K because of grain growth and decreased on those sintered at the temperatures from 673 K to 773 K because the increase of pore size caused to decrease thermal conductivity. The increase of thermal conductivity for the samples sintered at temperatures above 773 K was affected by the increase of carrier concentration.     

Fabrication of Nickel/Gold Multilayered Shells on Polystyrene Bead Cores by Sequential Electroless Deposition Processes

Core-shell conductive beads composed of a polystyrene (PS) core and a metallic shell of nickel-gold were investigated to identify the metal-polymer interface and the electromechanical response under a large deformation. Using dispersion polymerization, the average PS bead diameter was controlled to between 1.5 μm and 4.1 μm by adjusting the water and initiator concentrations, to give a monodispersed particle size distribution. In the electroless deposition processes of Ni followed by gold, the Ni-P shell was initially formed with a thickness of 43 nm, part of which was replaced by gold to give Ni-P and Au shell layers with thicknesses of 39 nm and 43 nm, respectively, as confirmed by transmission electron microscopy (TEM) observation. The electromechanical indentation test of a single core-shell bead showed that its resistance was 9.8 Ω/bead at a compressive deformation of 35%.