A three-dimensional mathematical model simulating the self-alignment mechanism in the flip-chip soldering of a single joint has been developed. Based on the principle of energy minimization, it calculates the quasi-static surface profiles defined by the size of the solder pads, the solder volume, the surface tension coefficient, the vertical loading and the misalignment. The model is capable of calculating initial, intermediate and final profiles and their associated restoring forces during the solder reflow. It can provide the guidelines to improve the assembly yield and the stress-related reliability of the flip-chip soldering technology.

Flip-Chip connections using gold-to-gold, gold-to-aluminum, or gold-to-solder bondings or contacts enhanced by epoxy are low-cost alternatives to soldering. To assist their technology advancements, we have developed yield models for a representative assembly process with flip-chip, thermocompression bondings. Based on bonding mechanics, a physical yield model has been developed to characterize the process. Then, a fuzzy logic model has been established to improve the modeling’s accuracy by including experimental data. The physical yield model can predict the assembly yield as a function of forces and planarities of the end effector, bump height variations, bump geometries, mechanical properties corresponding to different materials and temperatures, and distribution patterns of bumps. Consistent with our experimental experience, the calculated force level for a high-yield process was around 3000 gmf for a 30-gold-bump chip with a bump diameter of 60 μm and a height of 50 μm. The fuzzy logic model can be trained and adjusted by the results of physical models and experiments. It correlates very well to the nonlinear relationships between the yield and the assembly parameters, and has a self-learning capability to update itself with new data. Such capabilities have been demonstrated by studying the bonding on a substrate with or without a compliant layer.

Based on an energy minimization principle, a mathematical/numerical model has been developed to study the impact of design and process variations associated with flip-chip solder joint on its ability to align in lateral and axial direction. The minimum-energy shape needed for joint evaluation is computed by a novel numerical method based on motion by mean curvature. The analysis shows that (1) the magnitude of the reaction force in lateral and axial direction reduces with increase in solder volume, (2) the normal reaction is an order of magnitude higher compared to the lateral reaction (restoring force) thus making the joint more susceptible to lateral misalignment compared to the axial misalignment, and (3) the axial misalignment is primarily dictated by the accuracy of the solder deposition height.

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Thermal management of high power electronics is becoming a critical issue as the power density of semiconductors increasing. The flat heat pipe (FHP) is widely used in the electronic cooling because it is possible to interface with flat electronics packages without additional conductive and interface resistances. The heat flux of the next generation electronics may exceed 100 W/cm2, which is significantly beyond the cooling capabilities of commercially available FHP today. A novel micro scale hybrid wick was developed in this study to improve the effective thermal conductivity and working heat flux of FHP. The hybrid wick consists of multilayer of sintered copper woven meshes to promote the capillary pressure and microchannels underneath to reduce the flow resistance. The analysis indicates that the effective thermal conductivity and the capillary limit of flat heat pipe (FHPs) with this novel micro scale hybrid wicking structure can be significantly enhanced as compared to the reported FHPs. In this paper, the design of this innovative micro scale hybrid wick is illustrated. The fabrication and charging processes are also outlined. The preliminary experimental results show that the effective thermal conductivity can approach 12,270 W/(m·K), which is more than 30 times better than pure copper at approximate 91.3 W input heat.