Showing papers by "Avram Bar-Cohen published in 1998"

Coffin-Manson based fatigue analysis of underfilled DCAs

Abstract: 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 Young's Modulus should be as high as the adhesion strength of the underfill allows.

Least-material optimization of vertical pin-fin, plate-fin, and triangular-fin heat sinks in natural convective heat transfer

Abstract: A least-material optimization of pin-fin, plate-fin, and triangular-fin array geometries is performed, by extending the use of least-material single fin analysis to multiple fin arrays. The heat dissipation from vertical fin arrays in natural convection is calculated using the Nusselt number correlation by Aihara et al. (Int. J. Heat and Mass Transfer vol. 33, no. 6, pp. 1223-32, 1990) for pin-fins, by Bar-Cohen and Rohsenow (Trans IEEE CHMT vol. 6, pp. 154-8, 1983) for rectangular plate fins, and by Karagiozis et al (Air, vol. 116, pp. 105-11, 1994) for triangular plate fins. Comparisons of the thermal capability of the three different array geometries are carried out on the basis of total heat dissipation and material-specific volumetric heat dissipation. Manufacturability constraints of these heat sinks, and their effects on the final design, are briefly discussed.

Immersion cooling of high heat flux microelectronics with dielectric liquids

Abstract: This paper explores the use of direct liquid cooling, by immersion of the components in inert, non-toxic, high dielectric strength perfluorocarbon liquids. Boiling heat transfer with the candidate liquids provides heat transfer coefficients that are as much as two orders of magnitude higher than achievable with forced convection of air. Unfortunately, the highly effective nucleate boiling domain terminates at the so-called Critical Heat Flux, approximately in the range of 20 W/cm/sup 2/ at atmospheric conditions and saturation temperature. Consequently, if immersion cooling is to be used for next generation chips, ways must be found to increase the pool boiling CHF of these dielectric liquids. Use of a pool boiling CHF correlation, developed in this laboratory, points to the possibility of reaching a CHF of nearly 60 W/cm/sup 2/, using elevated pressure and subcooling, along with a dilute mixture of a high boiling point fluorocarbon, and applying a microporous coating to the surface of the chip.

Parametric dependence of fatigue of electronic adhesives

Abstract: Adhesives used in electronic applications are subject to high cyclic shear stresses resulting from differential thermal expansion of the bonded materials. In this study, an attempt has been made to experimentally determine the dependence of the fatigue life of two packaging epoxy adhesives on: temperature, peak cycling stress as a fraction of adhesion strength, and cyclic frequency. Both epoxies were used to bond surfaces of aluminum and silicon. Tensile lap-shear specimens were used.

A national course on thermal design of electronic products: recent experiences and a proposal

Abstract: The concept of a national course on thermal design of electronic products was presented by the authors at the last ECTC. Since that time, additional experience has been gained on this topic through student feedback and continuing advancements in instructional technologies. The focus of the present paper is on an Internet based approach, supplemented with live interactions for the development of a series of learning modules. Advances in Internet based instructional technology and satellite tele-conferencing offer unprecedented opportunities to provide high quality learning where and when it is needed. A high degree of reconfigurability is possible, which makes lifelong learning a real possibility. The concept of a national course and its basic elements are first summarized. Examples of modules and suggested implementation methodologies are then presented. Also discussed are methods for the assessment of the effectiveness of these modules.