Quantification of the natural convective heat transfer for the tilted and wire-bonded QFN32b-PCB electronic assembly

The version of the basic Quad flat non-lead QFN32 electronic device equipped with wire bondings, denoted as QFN32b, is more efficient thermally. This is due in part to the modification of the heat transfer phenomena occurring in the assembly. Although this interesting device is widely used in electronics, there are no specific correlations leading to determine accurately its associated natural convective heat transfer coefficient. This is the main objective of the present study, which considers a QFN32b generating a power ranging from 0.1 W to 1.0 W by steps of 0.05 W. It is welded in various positions of a printed circuit board (PCB), which could be inclined at different angles varying between 0° and 90° corresponding to the horizontal and vertical positions, respectively, by steps of 15°. These power and inclination angle ranges correspond to the normal operation of the device for the intended application. Calculations done by means of the finite volume method allow the determination of the free convective heat transfer coefficient on the different areas constituting the considered package. The results of the present study, compared with those of the basic QFN32 device quantified in a previous work, clearly show the influence of the wire-bonding technique on the QFN32b's thermal performance. The proposed correlations improve the design of this electronic device widely used in electronics for various applications covering many engineering fields.     

Simulation Studies on Multimode Heat Transfer from a Square-Shaped Electronic Device with Multiple Discrete Heat Sources

ABSTRACT

The results of a numerical study of the problem of multimode heat transfer from a square-shaped electronic device provided with three identical flush-mounted discrete heat sources are presented here. Air, a radiatively nonparticipating fluid, is taken to be the cooling medium. The heat generated in the discrete heat sources is first conducted through the device, before ultimately being dissipated by convection and surface radiation. The governing partial differential equations for temperature distribution are converted into algebraic form using a finite-volume based finite difference method, and the resulting algebraic equations are subsequently solved using Gauss-Seidel iterative procedure. A grid size of 151 × 91 is used for discretizing the computational domain. The effects of all relevant parameters, including volumetric heat generation, thermal conductivity, convection heat transfer coefficient, and surface emissivity, on various important results, such as the local temperature distribution, the peak temperature of the device, and the relative contributions of convection and surface radiation to heat dissipation from the device, are studied in sufficient detail. The exclusive effect of surface radiation on pertinent results of the present problem is also brought out.     

Reliability of thermal interface materials: A review

Thermal interface materials (TIMs) are used extensively to improve thermal conduction across two mating parts. They are particularly crucial in electronics thermal management since excessive junction-to-ambient thermal resistances can cause elevated temperatures which can negatively influence device performance and reliability. Of particular interest to electronic package designers is the thermal resistance of the TIM layer at the end of its design life. Estimations of this allow the package to be designed to perform adequately over its entire useful life. To this end, TIM reliability studies have been performed using accelerated stress tests. This paper reviews the body of work which has been performed on TIM reliability. It focuses on the various test methodologies with commentary on the results which have been obtained for the different TIM materials. Based on the information available in the open literature, a test procedure is proposed for TIM selection based on beginning and end of life performance.     

Electronic structure and assessment of thermoelectric performance of TiCoSb

Abstract

First principles band structure calculations coupled with the Boltzmann transport theory are used to study the thermoelectric properties in TiCoSb under pressure. The density of states and band structure were studied in detail. The thermoelectric power, electrical conductivity and electronic thermal conductivity were analysed using the Boltzmann transport equation with the assumption of the constant relaxation time approximation and the rigid band model. The enhancement of the thermoelectric properties of TiCoSb by adjusting pressure is predicted.     

Application of phase change materials in thermal management of electronics

Application of a novel PCM package for thermal management of portable electronic devices was investigated experimentally for effects of various parameters e.g. power input, orientation of package, and various melting/freezing times under cyclic steady conditions. Also, a two-dimensional numerical study was made and compared the experimental results. Results show that increased power inputs increase the melting rate, while orientation of the package to gravity has negligible effect on the thermal performance of the PCM package. The thermal resistance of the device and the power level applied to the PCM package are of critical importance for design of a passive thermal control system. Comparison with numerical results confirms that PCM-based design is an excellent candidate design for transient electronic cooling applications.     

Natural convection on inclined QFN32 electronic package generating constant volumetric heat flux

Qualification and quantification of the natural convective phenomena are examined in the case of a Quad Flat Non-lead (QFN32). This active electronic package is inclined with respect to the horizontal plane by an angle varying between 0° and 90° corresponding to the horizontal and vertical position respectively. It generates during its operation a constant volumetric heat flux leading to Rayleigh numbers varying in the range 1.31x10⁷ ‐ 1.01x10⁸. The walls of the large air-filled cubic cavity containing this device are maintained isothermal. The temperature and velocity fields are presented for different combinations of the Rayleigh number and inclination angle. The convective heat transfer concerning the whole component exchange surface is determined for all the treated configurations. Correlations of Nusselt–Rayleigh type are proposed. They allow optimizing the thermal design of electronic assemblies used in various engineering domains.     

Interface thermal characteristics of flip chip packages – A numerical study

Flip chip ball grid array (FC-BGA) packages are commonly used for high inputs/outputs (I/O) ICs; they have been proven to provide good solutions for a variety of applications to maximize thermal and electrical performance. A fundamental limitation to such devices is the thermal resistance at the top of the package, which is characterized θ_JC parameter. The die-to-lid interface thermal resistance is identified as a critical issue for the thermal management of electronic packages. This paper focuses on the effect of the interface material property changes on the interface thermal resistance. The effect of package’s junction to case (Theta-JC or θ_JC) thermal performance is investigated for bare die, flat lid and cup lid packages using a validated thermal model. Thermal performance of a cup or flat lid attached and bare die packages were investigated for different interface materials. Improved Theta-JC performance was observed for the large die as compared to the smaller die. Several parametric studies were carried out to understand the effects of interface bond line thickness (BLT), different die sizes, the average void size during assembly and thermal conductivity of interface materials on package thermal resistance.     

Mean Free Path–Thermal Conductivity Accumulation Calculations for Wurtzite Gallium Nitride: Two Approaches

ABSTRACT

Understanding the mean free path distribution of the dominant heat carriers is very important in determining the ballistic to diffusive heat transport transition in nanoscale devices. This is true for the high electron mobility transistors made from GaN where both the thickness of the buffer layer and localized heating causing ballistic-diffusive heat transfer may complicate the transport properties needed to describe the device thermal response. In this work, we obtain the mean free path–thermal conductivity relation of phonons in bulk wurtzite GaN crystals using two different, ab-initio-based calculations. While the Vienna Ab-initio Simulation Package (VASP) is used in both approaches at the initial stage, the first method does not calculate the third-order force constants (FCs) and approximates the anharmonicity with a single fitting parameter in determination of discrete phonon properties thermal conductivity and relaxation time, while the second method uses third-order force constants directly. Results show that the third-order FCs are important in modeling the contribution of high-frequency optical phonons with relatively short MFPs, to the thermal conductivity of the material. Yet these effects are more significant at high temperatures and at samples without crystallographic disorders, and they can be omitted while modeling the real samples at low temperatures.     

Bottom up Approach Toward Prediction of Effective Thermophysical Properties of Carbon-Based Nanofluids

ABSTRACT

Carbon-based nanofluids, mainly suspensions of carbon nanotubes or graphene sheets in water, are typically characterized by superior thermal and optical properties. However, their multiscale nature is slowing down the investigation of optimal geometrical, chemical, and physical nanoscale parameters for enhancing the thermal conductivity while limiting the viscosity increase at the same time. In this work, a bottom up approach is developed to systematically explore the thermophysical properties of carbon-based nanofluids with different characteristics. Prandtl number is suggested as the most adequate parameter for evaluating the best compromise between thermal conductivity and viscosity increases. By comparing the Prandtl number of nanofluids with different characteristics, promising overall performances (that is, nanofluid/base fluid Prandtl number ratios equal to 0.7) are observed for semidilute (volume fraction ? 0.004) aqueous suspensions of carbon nanoparticles with extreme aspect ratios (larger than 100 for nanotubes, smaller than 0.01 for nanoplatelets) and limited defects concentrations (<5%). The bottom up approach discussed in this work may ease a more systematic exploration of carbon-based nanofluids for thermal applications, especially solar ones.     

Experimental investigations on phase change material based finned heat sinks for electronic equipment cooling

This paper reports the results of an experimental investigation of the performance of finned heat sinks filled with phase change materials for thermal management of portable electronic devices. The phase change material (PCM) used in this study is n-eicosane and is placed inside a heat sink made of aluminium. Aluminium acts as thermal conductivity enhancer (TCE), as the thermal conductivity of the PCM is very low. The heat sink acts as an energy storage and a heat-spreading module. Studies are conducted for heat sinks on which a uniform heat load is applied for the unfinned and finned cases. The test section considered in all cases in the present work is a 80 × 62 mm² base with TCE height of 25 mm. A 60 × 42 mm² plate heater with 2 mm thickness is used to mimic the heat generation in electronic chips. Heat sinks with pin fin and plate fin geometries having the same volume fraction of the TCE are used. The effect of different types of fins for different power level (ranging from 2 to 7 W) in enhancing the operating time for different set point temperatures and on the duration of latent heating phase were explored in this study. The results indicate that the operational performance of portable electronic device can be significantly improved by the use of fins in heat sinks filled with PCM.     

High Density Polyethylene/Paraffin Composites as Form-stable Phase Change Material for Thermal Energy Storage

Abstract

This article focuses on the preparation and thermo-physical properties of paraffin/high density polyethylene (HDPE) composites as form-stable solid-liquid phase change material (PCM) for thermal energy storage. In the paraffin/HDPE blend, the paraffin (P) dispersed into the HDPE serves as a latent heat storage material when the HDPE, as a supporting material, prevents the melted paraffin leakage thanks to its structural strength. Therefore, this type composite is form-stable and can be used as a PCM without encapsulation for thermal energy storage. In this study, two paraffins with melting temperatures of 48°C–50°C and 63°C–65°C were used. The mass percentages of paraffins in the composites could go high as 76% without any seepage of the paraffin in melted state. The dispersion of the paraffin into the network of the solid HDPE was investigated using scanning electronic microscope (SEM). The melting temperatures and latent heats of the form-stable P1/HDPE and P2/HDPE composite PCMs were determined as 44.32°C and 61.66°C, and 179.63 and 198.14 Jg^?1, by the technique of differential scanning calorimetry (DSC), respectively. Furthermore, the thermal conductivity of the composite PCMs were improved as about 33.3% for the P1/HDPE and 52.3% for the P2/HDPE by introducing the expanded and exfoliated graphite to the samples in the ratio of 3 wt%. The results reveal that the prepared form-stable composite PCMs have great potential for thermal energy storage applications in terms of their satisfactory thermal properties, improved thermal conductivity and cost-efficiency because of no encapsulation for enhancing heat transfer in paraffin.     

NUMERICAL ALGORITHM FOR ESTIMATING TEMPERATURE-DEPENDENT THERMAL CONDUCTIVITY

Abstract

The hybrid scheme of the Laplace transform technique and the central difference approximation is applied to estimate the temperature-dependent thermal conductivity by utilizing temperature measurements inside the material at an arbitrary specified time. In the present study the functional form of the thermal conductivity is not known a priori. Thus, this problem can be regarded as the functional estimation in inverse calculation. The accuracy of the predicted results is examined from various illustrated cases using simulated exact and inexact temperature measurements obtained within the medium. Results show that a good estimation on the thermal conductivity can be obtained with any arbitrary initial guesses of the thermal conductivity. The advantage of the present method in the inverse analysis is that, for most types of boundary conditions, the relation between the thermal conductivity and temperature at any specified time can be determined without measuring the early temperature data.     

Experimental analysis of internal leakage current using a 100 cm2 class planar solid oxide fuel cell

This work studies the effect of the electronic conductivity of a commercially used yttria-stabilised zirconia (YSZ) electrolyte on solid oxide fuel cell performance. For the first time, an experimental method is used to measure the electronic conductivity of YSZ in commercially manufactured and operated single cells. An inert-gas step addition (ISA) method is employed to measure the electronic conductivity in the form of voltage changes at the open-circuit state. Since the electronic conductivity allows oxide ion transfer and water generation at the anode, a reduction in open-circuit voltage (E_OCV) occurs due to the increased water partial pressure. The step increase in the anode flow rate in the ISA method reduces water partial pressure and raises E_OCV. The E_OCV change is then converted into internal leakage currents and electronic resistivities. The YSZ's electronic resistivity obtained in this work is comparable to the results obtained using the Hebb-Wagner polarisation technique and the oxygen permeation method.     

Modeling Microstructure Effect on Thermal Conductivity of Aerogel-Based Vacuum Insulation Panels

Abstract

Understanding the effects of microstructural parameters on the heat transfer through an aerogel-based vacuum insulation panel (VIP) is important for the design and development of thermal insulation materials. The present work first prepared the aerogel-based VIP and characterize its microstructure through scanning electron microscopy and nitrogen gas adsorption analysis. A theoretical model for the thermal conductivity of the aerogel-based VIP was then presented and validated with experimental results. Based on the model, the effects of microstructural parameters, i.e. particle diameter and pore diameter, on the thermal conductivity of the aerogel-based VIP were explored. The results indicated an extremely low thermal conductivity with approximately 1.7?×?10^?3 W·m^?1·K^?1 can be achieved as the particle diameter and pore diameter are 1 and 10–15?nm, respectively. Furthermore, the microstructure effect under various service time of the aerogel-based VIP was considered for practical heat transfer engineering. It was found that the increase rate of the thermal conductivity decreases with a decreased particle diameter or an increased pore diameter. The microstructure effect modeling of the aerogel-based VIP could be of great advantage to heat transfer engineering applications aiming to reducing heat loss and saving energy.     

An investigation on the flow and heat transfer characteristics of nanofluids by nonequilibrium molecular dynamics simulations

ABSTRACT

The flow and heat transfer characteristics of nanofluids are investigated by nonequilibrium molecular dynamics simulations. Both the effect of chaotic movements of nanoparticle (CMN) on flow properties and its resulting heat transfer enhancement are analyzed. Results show that compared with the base fluid, the effective thermal conductivity of nanofluids is increased, and the increase ratio in the shear flow field is much higher than that in the zero-shear flow field. Based on the models built in this paper, the contributions of increased thermal conductivity and CMN to the heat transfer enhancement of nanofluids are 49.8–68.6% and 31.4–50.2%, respectively.     

Effect of Thermal Conductivity on Cooling of Square Heat Source Array under Natural Convection in a Vertical Channel

Abstract

This article presents experimental and numerical investigation on natural convection air-cooling of discrete square heat source array in a vertical channel. Conjugate heat transfer for three-dimensional laminar developing flows over an array of square heat sources representing integrated circuit components for electronic cooling has been studied. Experiments are conducted using three-substrate board materials viz. FR4, Bakelite, and copper clad board having thermal conductivities of 0.3, 1.4, and 8.8?W/m K to study the effects of substrate thermal conductivity on fluid flow and heat transfer. A finite element-based software is used to solve the coupling between heat transfer in solids and fluid region. Incompressible flow over discrete square heat sources is modeled using Navier–Stokes equations under Boussinesq approximation. Air-cooling of circuit boards populated with heat sources is modeled and simulated to present heat transport in combination with the fluid flow resulting from the natural air circulation at constant heat fluxes of 1,000, 2,000, and 3,000?W/m². Multilayer copper clad board of thermal conductivity of 40.5?W/m K have been studied numerically. The results show that single sided copper clad board is the preferred candidate. Experiments indicate a deviation of under 5% with simulations.     

Thermal conductivity and dynamic viscosity modeling of Fe2O3/water nanofluid by applying various connectionist approaches

Abstract

Thermal conductivity and dynamic viscosity play key role in heat transfer capacity of nanofluids. In the present study, thermal conductivity and dynamic viscosity of Fe₂O₃/water are modeled by applying various artificial neural network algorithms. The applied algorithms are MLP, GA-RBF, LSSVM, and CHPSO ANFIS algorithms. The data for modeling procedure are extracted from several experimental studies. Obtained results by the different algorithms are compared and it was concluded that the highest R-squared values belonged to GA-RBF algorithm which were equal to 0.9962 and 0.9982 for thermal conductivity ratio and dynamic viscosity, respectively.     

Thermoelectric performance of polyparaphenylene/Li0·5Ni0·5Fe2O4 nanocomposites

Abstract

Polyparaphenylene/Li_0·5Ni_0·5Fe₂O₄ (PPP/LiNi-ferrite, Li_0·5Ni_0·5Fe₂O₄ and LiNi-ferrite were used interchangeably) nanocomposites were synthesised using an in situ compounding method, and their thermoelectric (TE) properties were measured. Compared with single phase LiNi-ferrite, nanocomposites have significantly enhanced electronic transfer capability and substantially lowered thermal conductivity. The low thermal conductivity of the added PPP and the very large boundary between the conductive polymer and the oxide nanoparticles play a dominant role on the thermal conductivity reduction. The figure of merit ZT of the PPP/LiNi-ferrite nanocomposite has been improved several orders in a wide temperature range from 300 to 850 K. Fabrication of nanocomposites consisting of electrically conductive polymer and oxide nanoparticles may provide a promising way for realising high ZT TE performance.     

An experimental study on the thermal conductivity and dynamic viscosity of TiO2-SiO2 nanofluids in water: Ethylene glycol mixture

The hybrid nanofluid has been thriving among researchers due to its potential to improve heat transfer performance. Therefore, various studies on heat transfer properties need to be carried out to provide a better understanding on hybrid nanofluid performance. In this paper, the experimental work is focused on the thermal conductivity and dynamic viscosity of TiO₂-SiO₂ nanofluids in a mixture of water and ethylene glycol (EG) with volume ratio of 60:40. The stable suspension of TiO₂-SiO₂ prepared at volume concentrations of 0.5 to 3.0%. The measurements of thermal conductivity and dynamic viscosity were performed at a temperature range of 30 to 80 °C by using KD2 Pro Thermal Properties Analyser and Brookfield LVDV III Ultra Rheometer, respectively. The thermal conductivity of TiO₂-SiO₂ nanofluids was improved by increasing the volume concentration and temperature with 22.8% maximum enhancement. Besides, the viscosity of TiO₂-SiO₂ nanofluids showed evidence of being influenced by nanofluid concentration and temperature. Additionally, the TiO₂-SiO₂ nanofluids behaved as a Newtonian fluid for volume concentration up to 3.0%. The properties enhancement ratio suggested that TiO₂-SiO₂ nanofluids will aid in heat transfer for concentrations of more than 1.5% and within the range of the temperature studied. A new correlation for thermal conductivity and dynamic viscosity of TiO₂-SiO₂ nanofluids were developed and found to be precise.     

Effects of the particle size and temperature on the efficiency of nanofluids using molecular dynamic simulation

ABSTRACT

Nanofluids are conventional heat transfer fluids with suspended nanoparticles to enhance their thermal conductivity. However, enhancement of thermal conductivity is coupled with increased viscosity. This study investigates the efficiency of nanofluids (ratio of thermal conductivity and viscosity enhancement) with the effects of particle size and temperature using molecular dynamic (MD) simulation. The efficiency of nanofluids is improved by increasing particle size and temperature. The thermal conductivity enhancement increases with increasing particle size, but is independent of temperature; the viscosity enhancement decreases with increasing particle size and temperature. Particle size variation is therefore shown to be more effective than temperature control.