Effect of filler settling of underfill encapsulant on reliability performance

Flip chip technology has been evolving for some time, and underfill material has contributed a lot to its development. By filling the gap between silicon die and organic substrate, the polymeric underfill can dramatically enhance the reliability of the flip-chip device, as it couples the solder interconnection with die and substrate. Underfill materials are normally composed of a polymerizable/curable organic matrix, such as the epoxy/anhydride system, normally pre-filled with inorganic filler, such as silica. Filler settling is very likely to happen for silica of a much higher density than that of the organic matrix at typical processing temperatures. Pre-filling the underfill with silica filler would modify the overall properties of the underfill material, and filler settling within underfill would also result in completely different local material properties. This paper investigates the corresponding effect of silica-pre-filling as well as the potential consequence of filler settling on the reliability performance of the flip chip device and solder joint fatigue life under thermal cycle/shock conditions.