Brian R. Burg

Steady-state low thermal resistance characterization apparatus: The bulk thermal tester

The reliability of microelectronic devices is largely dependent on electronic packaging, which includes heat removal. The appropriate packaging design therefore necessitates precise knowledge of the relevant material properties, including thermal resistance and thermal conductivity. Thin materials and high conductivity layers make their thermal characterization challenging. A steady state measurement technique is presented and evaluated with the purpose to characterize samples with a thermal resistance below 100 mm(2) K/W. It is based on the heat flow meter bar approach made up by two copper blocks and relies exclusively on temperature measurements from thermocouples. The importance of thermocouple calibration is emphasized in order to obtain accurate temperature readings. An in depth error analysis, based on Gaussian error propagation, is carried out. An error sensitivity analysis highlights the importance of the precise knowledge of the thermal interface materials required for the measurements. Reference measurements on Mo samples reveal a measurement uncertainty in the range of 5% and most accurate measurements are obtained at high heat fluxes. Measurement techniques for homogeneous bulk samples, layered materials, and protruding cavity samples are discussed. Ultimately, a comprehensive overview of a steady state thermal characterization technique is provided, evaluating the accuracy of sample measurements with thermal resistances well below state of the art setups. Accurate characterization of materials used in heat removal applications, such as electronic packaging, will enable more efficient designs and ultimately contribute to energy savings.

Ecological and economical advantages of efficient solar systems

A strategy to optimize the positive climatic effect of solar energy requires a minimization of embodied energy and a maximization of efficiency to compensate for the upward temperature forcing effect of low albedo solar collector surfaces. Climatic effects of low albedo surfaces are related to those of carbon dioxide emissions. The resulting positive climate forcing effect (cooling) then depends directly on the embodied energy, the absorption, and the efficiency of the system. This analysis favors high efficiency CPV, and even more so dual-use HCPVT systems, over low-efficiency flat panel and thin film technologies. The effect is most prominent in large low latitude cities, where urban heat islands and the subsequent demand for cooling negate positive effects of low-efficiency solar installations.

Enhanced Percolating Thermal Underfills Achieved by Means of Nanoparticle Bridging Necks

Efficient heat removal from integrated circuits arranged vertically in 3-D chip stacks requires thermally conductive underfill materials. The low-heat-transport performance of traditional capillary underfills can be improved by percolating the thermal conductive filler particles. We increased the thermal path by adding quasi-areal contacts using nanoparticle assemblies directed to the contact points of the percolating filler particles. We studied the formation and thermal effect of such nanoparticle neck assemblies in the filler-particle contact points using aqueous suspensions containing nanoparticles of different sizes, size distributions, and concentrations. An optimized binary mixture of small (28-43 nm) and large (200-300 nm) nanoparticles results in dense and defect-free neck assemblies. A neck-enhanced percolating thermal underfill (PTU) with a thermal conductivity as high as 2.4 W/mK was achieved using alumina filler and nanoparticles. Compared to a PTU, the addition of nanoparticle necks resulted in a more than twofold improvement in thermal conductivity.

Characterization of particle beds in percolating thermal underfills based on centrifugation

Heat dissipation in 3D chip stacks suffers from multiple thermal interfaces. The effective thermal resistance of the bond-line between individual dies, with the electrical interconnects can be minimized by the introduction of thermal conductive underfills. Up to now, only sequentially formed underfills result in true percolation and hence, thermal conductivities of more than 1 W/m-K. In this study, we report on various aspects to consider during the formation of percolating thermal underfills, by centrifugal filling of micron-sized particles and the subsequent backfilling of an epoxy by capillary action. Particle assemblies within silicon-glass cavities were investigated for mono and poly-dispersed spherical and facetted particles with characteristic dimension in the range of 15 μm to 50 μm. Clogging of particles between silicon pillars could be mitigated at low particle fluxes dispensed by the hour glass principle. Particle shadowing behind the silicon pillars could be eliminated by ultrasonic agitation. Finally, close to crystalline phases could be identified for the mono-dispersed particles, compared to a random packing for the poly-dispersed particles. The effective pore diameter of the particle beds was experimentally derived from a backfilling experiment with viscosity standards. A normalized pore diameter of 0.15, 0.17 to 0.20 and 0.11 was observed for mono and poly-dispersed spherical and facetted particles, respectively. The backfill dynamics can be predicted with those values and the Washburn equation. Cavities filled with particles down to 30 μm diameter could be filled completely with the available low viscosity epoxy system. Finally, we report on the re-arrangement of filler particles due to capillary action and viscous drag, during the backfilling process. Defects are minimal for fluids of low surface tension and high viscosity. Hence, only 1 area-% of defects were observed from the infiltration of epoxies.

Receiver-module-integrated thermal management of high-concentration photovoltaic thermal systems

Solar energy is typically converted into electrical energy or collected as thermal energy. Co-generation of electricity and low-grade heat allows a more efficient use of the solar spectrum. To this end, a prototype of a high-concentration photovoltaic thermal (HCPVT) system is demonstrated. It is based on low-cost optical concentrator materials, a high-efficient, densely-packed multi-cell receiver array and a hierarchically stacked hot-water-cooling structure embedded in the receiver body. As the distance and number of interfaces between the photovoltaic cells and the cooling channels was minimized, an efficient PV-cell-cooling was achieved. Besides stabilizing PV-cell operation at elevated temperatures, the low-grade heat generated by the hot-water cooling approach can be used further in applications such as, e.g. thermal energy storage, space heating, heat-driven cooling, desalination for clean-water generation. Here, a prototype system is characterized under outdoor real-illumination conditions regarding its optical concentration and photovoltaic performance by recording illumination patterns and current-voltage traces. Furthermore, thermal characterization is carried out indoor by using a heater structure to simulate systematically the expected thermal load on the receiver module. Using 4.3 m2 collector area, average concentrations of almost 1000 suns with receiver-localized illumination hot-spots exceeding 3000 suns were achieved on the receiver plane of 53 × 57 mm2 size in a 5 × 5 array of triple-junction cells. The overall electrical efficiency reaches 32.3 % at 955 suns and a water inlet temperature of 25° C. The HCPVT concept enables large concentration × area products to maximize the energy-generation density while simultaneously providing a high thermodynamic efficiency for heat recovery.

Enhanced thermal underfills by bridging nanoparticle assemblies in percolating microparticle beds

A high thermally conductive underfill material is key for the efficient removal of heat generated by a 3-dimensional chip stack. Improved thermal properties are achieved by creating a percolating microparticle network within the composite underfill material. In this work, the directed assembly of nanoparticle necks formed by capillary bridging is investigated in order to improve the thermal transport in microparticle to microparticle contacts. The morphology of the formed necks using different alumina nanoparticle sizes and distributions, as well as a sol-gel binding system are characterized. High density and defect free nanoparticle necks were formed by using a mixture of small (28 - 43 nm) and large (200 - 300 nm) nanoparticles. The formation of such necks in the percolating alumina microparticle network increased the thermal conductivity of the underfill material from 1 W/mK without necks to 2.4 W/mK, a 2.4 × improvement in thermal conductivity.

Placement and efficiency effects on radiative forcing of solar installations

The promise for harnessing solar energy being hampered by cost, triggered efforts to reduce them. As a consequence low-efficiency, low-cost photovoltaics (PV) panels prevail. Conversely, in the traditional energy sector efficiency is extremely important due to the direct costs associated to fuels. This also affects solar energy due to the radiative forcing caused by the dark solar panels. In this paper we extend the concept of energy payback time by including the effect of albedo change, which gives a better assessment of the system sustainability. We present an analysis on the short and medium term climate forcing effects of different solar collectors in Riyadh, Saudi Arabia and demonstrate that efficiency is important to reduce the collector area and cost. This also influences the embodied energy and the global warming potential. We show that a placement of a high concentration photovoltaic thermal solar power station outside of the city using a district cooling system has a double beneficial effect since it improves the solar conversion efficiency and reduces the energy demand for cooling in the city. We also explain the mechanisms of the current economic development of solar technologies and anticipate changes.

Thermal characterization of percolating thermal underfills: Bulk and cavity

Three dimensional chip stacking benefits from thermally conductive intra-stack bondlines. Accurately determining the thermal conductivity of percolating thermal underfill (PTU) layers, made up of dielectric filler particles and epoxy, is challenging due to their reduced dimensions. The constrained dimensions may also influence the effective composite material properties. This study investigates how bulk samples compare to constrained cavity samples of identical material combinations, characterized by a steady state measurement technique. It is shown that the bulk thermal conductivity is reduced by over 35% in cavity layers. Reduced particle fill fractions at the chip interfaces due to cavity boundary effects which limit possible particle settling locations are the primary reason for this observation. Reduced dimensions in the size range of the filler particles consequently play a very significant role in effective medium properties of composite materials and need to be taken into account during material development.

Review of percolating and neck-based underfills with thermal conductivities up to 3 W/m-K

Heat dissipation from 3D chip stacks can cause large thermal gradients due to the accumulation of dissipated heat and thermal interfaces from each integrated die. To reduce the overall thermal resistance and thereby the thermal gradients, this publication will provide an overview of several studies on the formation of sequential thermal underfills that result in percolation and quasi-areal thermal contacts between the filler particles in the composite material. The quasi-areal contacts are formed from nanoparticles self-assembled by capillary bridging, so-called necks. Thermal conductivities of up to 2.5 W/m-K and 2.8 W/m-K were demonstrated experimentally for the percolating and the neck-based underfills, respectively. This is a substantial improvement with respect to a state-ofthe-art capillary thermal underfill (0.7 W/m-K). Critical parameters in the formation of sequential thermal underfills will be discussed, such as the material choice and refinement, as well as the characteristics and limitations of the individual process steps. Guidelines are provided on dry vs. wet filling of filler particles, the optimal bi-modal nanosuspension formulation and matrix material feed, and the overpressure cure to mitigate voids in the underfill during backfilling. Finally, the sequential filling process is successfully applied on microprocessor demonstrator modules, without any detectable sign of degradation after 500 thermal cycles. The morphology and performance of the novel underfills are further discussed, ranging from particle arrangements in the filler particle bed, to cracks formed in the necks. The thermal and mechanical performance is benchmarked with respect to the capillary thermal and mechanical underfills. Finally, the thermal improvements within a chip stack are discussed. An 8or 16-die chip stack can dissipate 46% and 65% more power with the optimized neck-based thermal underfill than with a state-of-the-art capillary thermal underfill.

On the Evaporation of Colloidal Suspensions in Confined Pillar Arrays

The thermal and electrical transport capabilities of materials in electronic packaging are key to supporting high-performance microelectronic systems. In composite and hybrid materials, both of these transport capabilities are limited by contact resistances. We propose a directed nanoparticle assembly method to reduce contact resistances by transforming point contacts between micrometer-sized objects into quasi-areal contacts. The nanoparticle assembly is directed by the formation of liquid bridges in contact points during the evaporation of a colloidal suspension. In this work, we experimentally study the evaporation of colloidal suspensions in confined porous media to yield uniform nanoparticle assembly, as required for electronic packaging. The evaporation pattern of liquids in confined pillar arrays is either branched or straight, depending on the surface tension of the liquid and on the pore size defined by the pillar size and spacing. Stable evaporation fronts result in uniform nanoparticle deposition above a bond number threshold of 10