Packaging and 3D Assembly

Packaging and 3D Assembly

PA-1 Optimum Filler Geometry for Suppression of Moisture Diffusion in Molding Compounds

W. Ahn, S, Shin, R. Asadpour, L. Nguyen*, D. Varghese*, S. Krishnan*, M. Alam, Purdue University, *Texas Instruments

Inorganic fillers, such as fused silica or organic clay, help tailor/co-optimize the mechanical toughness, thermal conductivity, and moisture diffusivity of polymer mold compounds used to package microelectronic integrated circuits. Despite long history and wide-spread current use, the optimization of filler-infused composites is generally empirical and therefore time-consuming. A physics-based predictive modeling will improve application-specific design of composites that would offer optimum performance and reliability. As an illustrative example, in this paper, we develop a general theory of polymer composites that anticipates the suppression of moisture diffusion as a function of fill-fraction, size-dispersion, shape, and topology of filler nanoparticles. Our results show that the best performance is obtained by incorporation rod-shaped fillers, randomly closed packed at maximum density (~60%). Our numerical results are succinctly captured by the analytical model based on generalized Maxwell Garnett effective medium theory. The analytical model can be used for initial optimization of mold compounds before large-scale numerical modeling is invoked and characterization experiments are designed.

PA-2 A Finite Element Method Study of Delamination at the Interface of the TSV Interconnects

S. Papaleo, H. Ceric, TU Wien

Through Silicon Vias (TSVs) are the interconnections in three dimensional integrated circuits responsible for the vertical lines inside the dies. In particular, the open TSV has been developed in order to reduce thermo-mechanical issues. This interconnect structure has interfaces where the possibility of a device failure due to delamination needs to be considered. The Critical Energy Release Rate Gc determines the condition for a fracture to propagate. When the Energy Release Rate G exceeds Gc, a fracture will propagate. Experimental measurements were used to calculate Gc. The experimental Gc, was calculated at the interface between silicon dioxide and tungsten; materials used for Open TSVs. We have developed a model to calculate the G and compared the experimental data with our results. The results obtained are in good agreement with experimental measurements. Therefore, the model developed provides a convenient tool for the study of delamination issues in TSVs.

PA-3 Electromigration Induced Thermomigration in Microbumps by Thermal Cross-talk Across Neighboring Chip in 2.5D IC

M. Li, D.W. Kim*, S. Gu*, K.-N. Tu, University of California, Los Angeles, *Qualcomm

This paper investigates the thermal cross-talk between the powered microbumps under one chip and the unpowered microbumps under the neighboring chip. Both chips were on a Si interposer for 2.5D IC. The Joule heating from the powered chip was found to be transferred laterally along the interposer to the unpowered chip and produced a temperature gradient in the microbumps in the unpowered chip. Void formation is observed in both the powered and the unpowered microbumps. The latter is due to thermomigration (TM), and the former is due to electromigration (EM). The amount of voids is bigger by TM than by EM. The void nucleation and growing is studied by examining the un-powered microbumps at different stages during electromigration tests. The nucleation of voids at the cold end in TM is observed, which indicates that Sn atoms diffuse from cold end to hot end. The current-enhanced surface electromigration of Sn along the side walls of Cu pillars to form intermetallic compound is observed in the powered microbumps that were subjected to a 5.3×104 A/cm2 current density at 150 ℃ for a period of time. The depletion of Sn will cause serious void formation in these powered microbumps.