The importance of material selection for flip chip on board assemblies

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. However, the success of the encapsulation is dependent on many factors. Among these are the encapsulant´s material properties, and the compatibility with the chip and substrate surfaces. Thus, the selection of the basic materials to be used in the FCOB assembly are interrelated and critical to achieving the desired performance from the completed assembly. This paper highlights the key considerations involved in the identification of suitable materials for each of the basic constituent elements: printed circuit board (PCB), silicon chip and encapsulation material