In the flip chip on board assembly method, an underfill encapsulant material is applied in the gap between the chip and substrate to lower the coefficient of thermal expansion (CTE) mismatch between the IC chip and substrate. Differenttechnologies such as fast-flow, no-flow, and reworkable underfills are currently being studied for flip-chip underfill encapsulant materials. This paper looks at the fast-flow method of underfilling the chip/substrate gap. The effect of filler loading,particle size, and particle size distribution on the flow rate and CTE of the fast-flow underfill material are discussed in this work. The material used for the experiments is an epoxy resin with added filler to decrease the CTE. This study focuses onwhat effect different characteristics of the filler have on the underfill encapsulant. Also, an underfill encapsulant that flows faster than one of industry's fastest fast-flow underfills was developed as a result of this work.
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