Thermoelectric coolers (TECs) are solid-state cooling devices that operate on the Seebeck effect. They can be used in electronic cooling applications as well as other refrigeration systems. Among the various factors that affect TEC performance within a system, it has been shown that the thermal conductance is an important parameter, which can also be easily altered during the design of a TEC to deliver optimal TEC performance for a given application. However, these studies have considered only a fixed heat load and heat sink temperature, whereas in many realistic applications these quantities can vary. A procedure has been developed for optimizing the thermal conductance of a TEC based on a typical operating cycle of time-varying heat load and sink temperature, while permitting constraints that ensure that one or more worst-case operating conditions can also be met. This procedure is valid for any arbitrary heat load and sink temperature functions; however, for illustrative purposes, a simple heat load function at fixed sink temperature (and a sink temperature function at fixed heat load) is used. The results show that the optimal conductance can strongly depend on the operating cycle, and the corresponding reduction in electrical input work (and corresponding increase in net coefficient of performance (COP)) can be significant.

This short communication addresses a numerical investigation of the thermal behavior of an electronic unit. The unit consists of several parallel planes and on the top and bottom planes heat is generated by a number of electronic chips. The heat is transported by conduction through plastic and copper-invar layers. Finally, the heat is rejected by a forced air stream in the center of the unit. The channel system for the cooling air is designed as an offset strip fin surface. A three-dimensional numerical method based on a thermal resistance or conductance network has been developed. The grid points on the cooling air side are staggered compared to the grid points in the solid materials. Details of the numerical method as well as some temperature distributions on the chip planes are provided.

Thermal interface materials (TIMs) are particulate composite materials widely used in the microelectronics industry to reduce the thermal resistance between the device and heat sink. Predictive modeling using fundamental physical principles is critical to developing new TIMs since it can be used to quantify the effect of particle volume fraction and arrangements on the effective thermal conductivity. The existing analytical descriptions of thermal transport in particulate systems do not accurately account for the effect of interparticle interactions, especially in the intermediate volume fractions of 30–80%. An efficient random network model (RNM) that captures the near-percolation transport in these particle-filled systems, taking into account the interparticle interactions and random size distributions, was previously developed by Kanuparthi et al. The RNM approach uses a cylindrical region to approximate the thermal transport within the filler particles and to capture the interparticle interactions. However, this approximation is less accurate when the polydispersivity of the particulate system increases. In addition, the accuracy of the RNM is dependent on the parameters inherent in an analytical description of thermal transport between two spherical particles and their numerical approximation into the network model. In the current paper, a novel semispherical approximation to the conductance of the fillers is presented as an alternative to the cylindrical region approximation used earlier. Compared with the cylindrical model, the thermal conductivities of the semispherical model are more closely to the finite element (FE) results. Based on the FE analysis, the network model is improved by developing an approximation of the critical cylindrical region between two spherical particles over which energy is transported. Comparing the RNM results with FE results and experimental data, a linear relationship of the critical parameter with the thermal conductivity ratio and the volume fraction was found that provides a more accurate prediction of the effective thermal conductivity of the particulate TIMs.

Generally, microelectro mechanical systems (MOEMS) devices require encapsulation for protecting their fragile and tiny inner components in a hermetically sealed cavity. Cavity hermeticity can be critical to the device performance and plays a vital role with respect to reliability and long-term drift characteristics of the MOEMS products. The paper presents a theoretical approach for estimation of lifetime of MOEMS devices in terms of cavity’s hermeticity to gases and water. The results are summarized as working maps for MOEMS packaging engineers, in terms of device cavity (internal package volume), equivalent leak rates, and equivalent size of interconnected defects in the bonding zone.

Wax (predominantly tricosane paraffin wax, with a melting temperature of 48 ° C ) filled with hexagonal boron nitride (BN) particles ( 5 - 11 μ m ) was found to be an effective phase-change thermal interface material. The thermal contact conductance, as measured with the interface material between copper surfaces, decreased with increasing temperature from 22 to 48 ° C , but increased with increasing temperature from 48 to 55 ° C . The melting of the wax enhanced the conductance, due to increased conformability to the mating surfaces. For a given BN volume fraction and a given temperature, the thermal contact conductance increased with increasing contact pressure. However, a pressure above 0.30 MPa resulted in no significant increase in the conductance. The conductance increased with BN content up to 6.2 vol. % , but decreased upon further increase to 8.6 vol. % . The highest conductance above the melting temperature was 18 × 10 4 W ∕ m 2 . ° C , as attained for a BN content of 4.0 vol. % at 55 ° C and 0.30 MPa . Below the melting temperature, the highest conductance was 19 × 10 4 W ∕ m 2 . ° C , as attained for a BN content of 6.2 vol. % at 22 ° C and 0.30 MPa .

This paper reviews the existing knowledge base about thermal contact resistance in cooling electronic equipment, and also highlights some novel issues that are emerging with the advent of compact electronic equipment. Where a high contact pressure is tolerable, such as in cooling power electronic devices, the experimental data and the theoretical models that have been developed to this day provide useful guides for the management of contact resistance. In such applications the compression load and a technique to enhance interface heat transfer need be examined, weighing their relative importance in the entire heat transfer system. Using the Yovanovich correlation for contact resistance and assuming water-cooled or air-cooled heat sinks, the contact pressure ranges of practical importance are identified. The case studies revealed that contact pressures around and less than 1–4 MPa are often sufficient to make the contact conductance comparable to the convective conductance in the water-cooled channel. The threshold pressure is much lower for the air-cooled case, around 0.2–0.6 MPa. However, heat transfer data in such intermediate pressure ranges are relatively few. For compact electronic equipment, such as laptop computers, the contact conductance to a thin heat spreader plate is becoming an issue of prime importance. In a constrained space the heat flow across the interface is affected by the heat conduction paths beyond the interface. This is illustrated using an example where warped heat sources are in contact with a heat spreader. It is shown that, with decreasing heat spreader thickness, the warping of the heat source has an increasing influence on the contact resistance.

Thermal interface pastes based on silicone, lithium doped polyethylene glycol (PEG), and sodium silicate were evaluated in their performance before and after heating up to 120°C. The thermal contact conductance of any of the pastes between copper disks decreased after heating, such that the fractional decrease was less for the silicone-based paste than the PEG-based and sodium-silicate-based pastes. Nevertheless, the conductance was lower for the silicone-based paste than the other pastes both before and after heating up to 100 cycles.

This study presents a numerical investigation of the effects of wall conduction on laminar natural convection heat transfer in a two-dimensional rectangular enclosure. The heat transfer is driven by a constant-temperature heat source in the center of the enclosure. The time dependent governing equations in the primitive form are solved numerically by the use of a finite-volume method. The numerical algorithm is first validated by comparing our predictions with those of Kim and Viskanta for a square cavity surrounded by four conducting walls. A parametric study is then conducted to examine the effects of wall conduction on the natural convection heat transfer. The parameters include the Rayleigh number, wall thickness, wall thermal conductivity ratio and diffusivity ratio. In addition, the effects of varying thermal boundary conditions on the outside walls are reported. Results indicate that the qualitative features of natural convection heat transfer in the laminar range are not significantly altered by the inclusion of wall conduction. However, the quantitative results may be significantly modified by the wall conductance. In general, the wall conduction reduces the rate of heat dissipation from the enclosure. The average Nusselt number decreases as the wall thickness ratio is increased and/or the wall thermal conductivity is reduced. Results also indicate that it may be possible to define an effective Rayleigh number that includes the effects of wall thickness and conductivity.

This study examines experimentally the quality of contact of mechanical joints in a diode heat sink assembly. Steady-state contact conductance, h, is used as a quantitative measure of the quality of contact of a joint. The emphasis of the work is on determining the contact conductance, h, for a nonideal joint using a noncontact method of recording temperature distribution in a complex geometry. Thermal contact conductance for an interface is known to depend on parameters such as contact pressure, mean interface temperature and the surface roughness characteristics of the mating surfaces. The results are presented for three different conditions of the interface namely (i) the mating surfaces are bare (ii) aluminum foil is inserted between the mating surfaces, and (iii) a high thermal conductivity grease is applied to the mating surfaces. Two levels of contact pressure are used for the interface with aluminum foil. The results indicate that the contact conductance increases with the mean temperature of the interface in all the cases. At low interface temperature, the contact conductance was greatest for the bare interface conditions. At high interface temperature, the contact conductance was greatest for the aluminum foil interface condition.

The effect of variations in stream-wise spacing and component length on convection from rectangular, surface mounted components in a channel flow are reported. Component dimensions are the same order of magnitude as the channel wall-to-wall spacing. The channel Reynolds number, with air as the coolant, ranged from 670 to 3000. Flow visualization showed that under the above conditions the channel flow is transitional. The effect of variations in component stream-wise spacing on the level of turbulence in the channel and on the interaction between the core of the channel flow and the recirculating flow in cavities between components is discussed. Pressure drop measurements show that the dominant loss mechanism is due to form drag caused by the components. Local heat transfer measurements are made using an interferometer. Analysis of the results shows that the overall heat transfer is properly correlated in terms of a flow Reynolds number based on the component length. At small component Reynolds number, the overall conductance tends towards the laminar smooth wall value. An overall correlation is proposed which includes the effect of component Reynolds number, channel wall-to-wall spacing, and component stream-wise spacing.