Stress Concentrations in Electronic Packaging
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RAVI MAHAJAN,* ERDOGAN MADENCI,*" LEVENT ILERI,'* AND MARK THURSTON* *INTEL Corporation, Assembly Technology Development, Chandler, AZ 85226 "The University of Arizona, Center for Electronics Packaging Research and Department of Aerospace and Mechanical Engineering, Tucson, AZ 85721
ABSTRACT The reliability of ceramic and plastic packages used in microelectronics is compromised by interface (delamination) and homogeneous (bulk) cracking that is initiated during processing and stress testing. These cracking failures have certain characteristics in that they begin at locations of geometric stress concentrations and propagate along typical failure paths. Characterization of these stress concentrations is therefore essential to a good design and for defining material response parameters such as bulk and interfacial fracture toughness. In this paper, applications of a general-purpose finite element technique for the characterization of stress concentrations are presented. The technique uses the full twodimensional elasticity solution for different materials that are bonded together. It is possible to use this technique to extract relevant material parameters, such as stress intensity factors, the J-integral, or energy release rate, and hence to eventually define the conditions necessary for crack initiation and propagation.
INTRODUCTION Microelectronics packaging is used to provide an electrical connection path from the silicon device (die) to the external circuit board, as well as to provide structural integrity and protection for the die The packages are composite structures, which are subjected to from external contamination. temperature excursions during manufacturing and testing.' These excursions, combined with the different thermal expansion coefficients of the package components, result in residual thermal stresses. Reliability of the packages is significantly compromised by delamination and bulk cracking failures due to thermal stresses. Hence, it is essential to understand the effect of geometry and material parameters on the structural integrity of the package. A linear elastic stress analysis of the package structure indicates that stress concentrations exist at junctions where different materials are bonded together (Fig. 1). It has been observed that delamination and cracking failures usually initiate at such junctions that 2 occur at the edges and comers of the package structure. In this paper, attention is confined to a twodimensional, linear, elastic stress analysis of the stress concentrations. Locally, the junctions can be considered as wedges of different materials bonded together (Fig. 2). High stress gradients are known to exist in the vicinity of the wedge apex due to the mismatch in thermal expansion coefficients and in elastic stiffness. It is further known that the stress state at the apex of the wedge often has a singular behavior and that the nature of the singularity is a function of 34 Traditional finite element analyses of the the wedge angles and the elastic constants of the materials.
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