Flip chip underfill flow characteristics and prediction

This paper will present recent results on underfill flow characterization. With quartz test chips of different bump patterns (peripheral, full array, and mixed designs) flip chip bonded on organic laminates, the flow properties of a number of commercial and experimental underfills were recorded and analyzed. Flow time, filler settling, and air entrapment were three major features evaluated. Good flow can be described by 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. These models, however, do not take into consideration the features of the fillers. In general, underfills with particles less than 5 μm exhibited faster and more uniform flow fronts than materials with larger particles. The best materials worked well with standoff heights between 50 and 75 μm, while the poorer flowing materials showed streaking, voiding and fingering. However, at gaps of 25 μm, nearly all the materials tested incurred pronounced and reproducible streaking