Effect of Nano-Particles on Heterogeneous Void Nucleation in No-Flow Underfill Materials

The large number of voids induced by the thermal effect from solder reflow process is one of the technical challenges for flip chip package techniques typically with no-flow underfills. The underfill voids could extensively reduce the life cycle in thermal cyclic test. This paper reviewed classical heterogeneous bubble nucleation theories to model voids nucleation occurring on the phase boundary of solid solder and liquidus underfill to control the nucleation during flip chip assembly process using no-flow underfill materials. On the heterogeneous voids nucleation, the properties of nucleation sites (such as density and surface tension) that determine void formation were explored using nano-sized fillers. Thus, this experiment investigated the effect of nano-sized fillers, which are potential nucleation sites in the no-flow underfill material, on the void formation by changing the nano-sized filler type and concentration. Eventually, we proposed a formulation guideline of no-flow underfill materials using nano-sized fillers to effectively prevent the underfill voiding during flip chip assembly process.