FAST-FLOW UNDERFILL ENCAPSULANT - FLOW-RATE AND COEFFICIENT OF THERMAL-EXPANSION

In the flip-chip on board assembly method, an underfill encapsulant ma terial is applied in the gap between the integrated circuit (IC) chip and substrate to distribute the shear stresses at the solder interconn ects. These shear stresses are imposed on the solder interconnects due to a coefficient of thermal expansion (CTE) mismatch between the IC c hip and substrate. Different technologies such as fast-flow, no-flow, and reworkable underfills are currently being studied for flip-chip un derfill encapsulant materials, This paper looks at the underfill encap sulant used in the fast-flow method of underfilling the IC chip/substr ate gap. The effect of filler loading, particle size, and particle siz e distribution on the flow rate and CTE of the fast-flow underfill mat erial are discussed in this work. The material used for the experiment s is an epoxy resin with added silica filler to decrease the CTE. This study focuses on what effect different filler characteristics have on the underfill encapsulant. Also, an underfill encapsulant that can co mpete with one of industry's faster fast-how underfills was developed as a result of this work.