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In this paper, the effects of various conditions on underfill flow including the substrate surface, temperature-dependent underfill properties, nonuniform bump patterns (e.g., missing columns), irregular bump shapes (e.g., slanted bumps), and filler particle redistribution are investigated for flip-chip packages. Experimental studies are first presented for underfill flow in silicon dies with different types of substrate surfaces (ceramic versus organic). Results show that the underfill material cannot wet organic packages well compared to their ceramic counterparts. This is also observed in simulations employing a 2-D global underfill flow model for an isothermal underfilling process. The effects of missing bump columns, temperature-dependent underfill viscosity and surface tension, and irregular bump shapes on underfill flow-out time, flow front shape, and void formation are then investigated in detailed underfill flow models, with each factor resulting in ±4~7% variation in underfill filling time. Finally, the influence of filler particle inhomogeneity due to settling and shear migration is modeled using a full 3-D particle suspension model. The simulated filler particle distribution around solder bumps agrees well with the cross-sectional images of cured underfill samples.