Showing papers by "Avram Bar-Cohen published in 1994"

Gas-assisted evaporative cooling of high density electronic modules

Abstract: 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. >

A shear based optimization of adhesive thickness for die bonding

Abstract: In low-cost integrated circuit (IC) packaging, a thin layer of adhesive is often used to bond the die to a leadframe or substrate. Because of the poor thermal conductivity of the adhesive, a thick layer will result in elevated chip temperatures. However, due to the differential expansion between the silicon and metal, a thin layer will result in high shear stress, in the adhesive, and along the bonded surfaces. In this study, first order thermal and thermostructural analytical relations for shear stress are used to determine the appropriate thickness of the adhesive layer between a die and a leadframe paddle, as encountered in PDIP packages. Numerical simulation, with finite element (FE) models, is used to examine the assumptions underpinning the analytical relations and to verify the analytical results. Using a filled-epoxy with an adhesion strength of 25 MPa, adhesive layer thicknesses between 0.027 and 0.2 mm are found to be suitable for this application. Assuming equal loss of reliability, due to chip temperature increases and shear stress increases, an optimum die-bond thickness of 0.16 mm is found. The FE computations appear to support the assumption that elevated shear stress is the most likely cause of die-bond structural failure for this subject package. >

Analysis and Prevention of Thermally Induced Failures in Electronic Equipment

Abstract: This paper begins with a discussion of the physical phenomena associated with thermally-induced/accelerated failures, including diffusion-driven and thermo-mechanical failure mechanisms. Attention is then turned to the results of ongoing Northern Telecom “field-return” surveys and to the identification of the factors controlling device field reliability for newly designed equipment, mature technologies in mild environments, and equipment operated under harsh environmental conditions. It is concluded that new reliability models and figures-of-merit, based on the phenomenological understanding of the failure mechanisms, are needed in order to predict the thermal dependence of the probability of failure.

Thermal Stress In Convectively-Cooled Plastic-Encapsulated Chip Packages

Abstract: The present study focuses on the analytical determination of the thermally-induced stresses in a convectively-cooled PDIP package. A model of thermal conduction inside the plastic package is combined with a methodology for the computation of the thermal resistance in the convective boundary layer to yield the internal temperature field. The component temperature distribution, obtained in this manner, is then used, via first-order elastic thermal stress relations, to determine the stresses both within the constituent materials and along the interfaces among them.