The importance of material selection for flip chip on board assemblies : Advancing MCM technologies

Flip Chip On Board (FCOB) technology, where the unpackaged silicon chip is assembled directly to an organic substrate, provides an unique interconnect structure that significantly reduces the substrate area required by traditional through hole or surface mount integrated circuit packages. Without the protection for the chip that a package body affords however, reduced mechanical robustness and a decrease in reliability performance are potential concerns. In addition, the inherent thermal mismatch between the chip and substrate, particularly with an organic printed circuit board, will produce an accumulation of stress in the solder joints under normal operating conditions that can result in relatively premature failures due to solder fatigue. Coupling the chip and substrate together with an encapsulation material under the chip face has successfully overcome these mechanical and reliability issues