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This paper summarizes a study on several factors that affect capillary underfill flow and cause underfill voids in a large die (19.7 times 19.7 mm) flip chip ball grid array (FCBGA) package, including substrate solder mask roughness, substrate plasma clean, flux residue amount & its compatibility with underfill and the filler size of underfill material. The study showed that rougher substrate surface would generate more obvious underfill flow striation, a telltale sign of uneven underfill flow front that might increase the chance of underfill void formation. Various underfill dispense patterns and process related parameters (plasma and dispense temperature) were studied with respect to the occurrence of underfill voids due to uneven flow front. It was found that by increasing the plasma time and underfill dispense temperature, the occurrence of underfill voids could be reduced. The compatibility between underfill material and solder flux used in flip chip attach was another focus of the study. It was found that the higher the flux residue amount, the more obvious the flow striation of the underfill material. The time taken for the underfill material to fill the die/substrate gap also became longer when more flux residue was present. Two underfills of the same chemistry but different filler sizes (max lOum vs 3um) were assessed and the latter gave a more even flow front and a more homogeneous underfill layer. The use of the finer filler underfill material overcame the underfill void/flow striation issue induced by rougher solder mask surface and higher flux residue content and shortened underfill flow time by 29-35%. Reliability test showed moisture preconditioning level 3 @ 260degC reflow and temperature-cycling 1000x (condition B) passed without any underfill delamination.