Avram Bar-Cohen

Advances in Thermal Modeling of Electronic Components and Systems, Vol. 1

This volume opens with a sweeping overview of the physical design of electronic systems-methodology, technology, and future challenges-thermally induced failures in electronic systems. Subsequent chapters examine the causes for thermally induced failures of electronic components and the techniques used to analyze and prevent such failures. It gives a comprehensive bibliograhy of project managers and lead engineers, packaging engineers and mechanical analysts, consultants, and academic, industrial, and government laboratory researchers.

Direct Liquid Cooling of High Flux Micro and Nano Electronic Components

The inexorable rise in chip power dissipation and emergence of on-chip hot spots with heat fluxes approaching 1 =kW/cm2 has turned renewed attention to direct cooling with dielectric liquids. Use of dielectric liquids in intimate contact with the heat dissipating surfaces eliminates the deleterious effects of solid-solid interface resistances and harnesses the highly efficient phase-change processes to the critical thermal management of advanced IC chips. In the interest of defining the state-of-the-art in direct liquid cooling, this paper begins with a discussion of the thermophysics of phase-change processes and a description of the available dielectric liquid cooling techniques and their history. It then describes the phenomenology of pool boiling, spray/jet impingement, gas-assisted evaporation, and synthetic jet impingement with dielectric liquids. Available correlations for predicting the heat transfer coefficients and limiting heat transfer rates, as well as documented empirical results for these promising techniques for on-chip hot spot cooling, are also provided and compared

theta /sub jc/ characterization of chip packages-justification, limitations, and future

A discussion is presented of thermal figures-of-merit (FOMs) for chip package thermal resistance, for use in evaluating competing thermal designs, and analysis techniques, for use in determining operating temperature profiles. The junction-to-case thermal resistance, Theta /sub JC/, as well as the junction-to-fluid thermal resistance, have been used in both of these roles for first-level packaging. The use of a modified Theta /sub JC/ is proposed. Experimental data indicate that the relations developed are capable not only of accurately describing the chip temperature for a variety of thermal management strategies, but also of highlighting the impact of specific thermal design features.<<ETX>>

Effusivity-based correlation of surface property effects in pool boiling CHF of dielectric liquids

Abstract Although the effects of fluid properties, pressure, and subcooling, as well as heater geometry, on the pool boiling critical heat flux, or CHF, are relatively well established, explanations for the surface property effects remain controversial. Proposed formulations, embodying the dependence of CHF on the product of the heater thermal effusivity and thickness are described and compared with available data. A composite correlation for pool boiling CHF, accounting for the conduction and hydrodynamic limits, as well as the effects of pressure, subcooling, and length, is proposed. This effusivity-based correlation is found to predict a broad range of pool boiling CHF data for dielectric liquids, for thermal effusivity values between 0.2 and 120, pressure from 100 to 450 kPa, and subcoolings from 0 to 75 K, with a standard deviation of 12.5%.

Coffin-Manson fatigue model of underfilled flip-chips

The fatigue life of an underfilled flip-chip package has been evaluated using the Coffin-Manson relation and finite element modeling (FEM)-computed solder shear strain for typical flip-chip structures. In the course of this effort, numerical simulations were performed for underfill materials of varying thermo-structural properties, two chip sizes, and two solder bump heights. The results were used to examine the parametric sensitivity of the thermal strain in the solder joints and the axial, as well as shear stress in the underfill material. The predicted improvement in the number of cycles-to-failure of the underfilled flip-chip was found to agree with empirical observation. However, the maximum improvement achievable by underfilling was found to be limited by the adhesion strength of the underfill material.

Thermal challenges in next generation electronic systems - summary of panel presentations and discussions

The presentations made, as well as the discussions, in the panels at the workshop, Thermal Challenges in Next Generation Electronic Systems (THERMES), are summarized in this paper. The panels dealt with diverse topics including thermal management roadmaps, microscale cooling systems, numerical modeling from the component to system levels, hardware for future high performance and Internet computing architectures, and transport issues in the manufacturing of electronic packages. The focus of the panels was to identify barriers to further progress in each area that require the attention of the research community.

Modeling and Prediction of Two-Phase Microgap Channel Heat Transfer Characteristics

A detailed analysis of microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids, gathered from the open literature and sorted by the Taitel and Dukler flow regime mapping methodology, is performed. Annular flow is found to be the dominant regime for this thermal transport configuration and to grow in importance with decreasing channel diameter. A characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity) for the flow of refrigerants and dielectric liquids in miniature channels is identified. The inflection points in this M-shaped curve are seen to equate approximately with flow regime transitions, including a first maximum at the transition from Bubble to Intermittent flow and a second maximum at moderate qualities in Annular flow, just before local dryout begins. The predictive accuracy of five classical two-phase heat transfer correlations for miniature channel flow is examined. Selecting the best fitting of the classical correlations for each of the flow regime categories is seen to yield predictive agreement with regime-sorted heat transfer coefficients that does not depart significantly from the agreement found in large pipes and channels.

Gas-assisted evaporative cooling of high density electronic modules

Reliable operation of advanced microelectronic components in three-dimensional packaging configurations necessitates the development of cooling systems capable of removing high heat fluxes and very high heat densities. A recently patented thermal management technique, using high velocity flow of a liquid-gas mixture in the narrow channels between populated substrates, appears to provide such a thermal transport capability. A prototype, high packaging density module, relying on this approach, has been successfully operated and a research study, focusing on the heat transfer rates attainable with this technique in a single, asymmetrically-heated channel has been completed. This paper begins with a description of this gas-assisted evaporative cooling approach, its advantages in thermal packaging of microelectronics, and its implementation in a prototype high-performance computer module. Attention is then paid to theoretical considerations in the flow of gas-liquid-vapor mixtures in narrow parallel plate channels and to the design and operation of an appropriate experimental apparatus. Next, experimental results for the wall temperature, heat transfer coefficients, and pressure drops are presented and compared to theoretical predictions. The paper concludes with a discussion of the thermal packaging potential of this novel thermal management technique. >

Coffin-Manson based fatigue analysis of underfilled DCAs

The continuing drive toward high-density, low-profile Integrated Circuit packaging has accelerated the spread of flip-chip technology to laminated substrates, creating direct chip attach (DCA) configurations. However, the substantial difference in the coefficients of thermal expansion (CTE) between the chip and the laminated substrates makes DCA configurations vulnerable to thermally-induced strains and the resulting solder joint fatigue. The reliability of flip-chip technology is dramatically improved by “underfilling” the gap between the chip and substrate with epoxy. The present effort is aimed at exploring the benefits of underfilling in DCA configurations. The thermo-structural behavior of an underfilled DCA is evaluated using FEM and employing an axisymmetric model of a typical DCA structure. Numerical simulations are performed for different sets of underfill material properties. The results are used to determine the parametric sensitivity of the thermal strain in the solder joints and the axial and shear stresses in the underfill material and to define the desirable range of underfill material properties. These results together with the Coffin-Manson relation are used to predict the theoretical improvement in cycles to failure. The results suggest that to minimize fatigue failure, the CTE of an underfill material should match that of solder material and its Youngs Modulus should be as high as the adhesion strength of the underfill allows.

Contact angle effects on boiling incipience of highly-wetting liquids

Abstract The difficulty in predicting the boiling incipience of highly-wetting liquids has slowed the application of immersion cooling technology in the electronics industry. The present effort sheds new light on this phenomenon by examining the influence of the dynamic solid/liquid contact angle and contact angle hysteresis on the incipience superheat. The results suggest that variations in contact angle, induced by changes in the direction and magnitude of the liquid/vapor interface velocity, can substantially affect the formation of bubble embryos and may well explain the wide experimental scatter in incipience superheat values reported for highly-wetting liquids.