The effect of flow properties on filler settling of underfill in the flip chip package

As the bump diameter and bump pitch of flip chip packages get smaller, the underfill becomes more resistant to flow. It is also necessary for the underfill to have low viscosity. High throughput also requires the underfill to have low viscosity. Problems associated with low viscosity underfills include filler settling and flow induced voids due to fast edge flow. In this paper, we discuss how the rheological properties can affect underfill filler settling and flow voids. The viscosities of several underfill materials were measured at both room and dispensing temperatures. The effects of yield stress of underfill on filler settling, and shear thickening of underfill at large shear rate on underfill flow voids, were investigated. SEM and CSAM were used to study the filler settling and flow voids. It was shown that the underfills with small fillers have shear-thickening viscosity and yield stress. The filler settling of underfill with a yield stress was greatly reduced. The flow voids were also reduced for underfill with shear-thickening viscosity.