Capillary-driven micro flow allows liquid transport by interfacial force without external pressure or momentum. Theoretical and experimental studies have been conducted to predict the movement of the flow meniscus in the application of capillary underfill flows. In a flip chip package, two-dimensional motions of flow front through solder bumps can result in unwanted air void formation because the meniscus and the arrangement of the solid surface affect the interface dynamics. This study introduces analytical models of filling time and discusses their verification and limitations. Recent developments in underfill flow visualization are also presented to analyze flow phenomena, including the racing effect and void formation.