This paper presents recent results on underfill flow characterization. The flow properties of a number of commercial and experimental underfills were recorded and analyzed using quartz test chips with specially designed bump patterns (e.g., peripheral, full array, and mixed designs). Each was bonded onto an organic laminate substrate to form a flip chip package. Underfill was then applied to the packages and flow time, filler settling, and air entrapment were evaluated. Good flow can be described in terms of three measurable parameters, namely, viscosity, contact angle, and more importantly, filler size and distribution. Viscosity and contact angle are commonly used in Hele Shaw and Washburn models. However, these models do not take filler properties into consideration. In general, underfills with particles less than 5 /spl mu/m exhibited faster and more uniform flow fronts than materials with larger particles. The best flowing materials worked well with standoff heights between 50 and 75 /spl mu/m, while the poorer flowing materials showed streaking, voiding, and fingering at these heights. At gaps of 25 /spl mu/m, however, nearly all the materials exhibited pronounced and reproducible streaking.