Adhesion issues in flip-chip on organic modules

Flip chip attach on organic carriers is a novel electronic packaging assembly method which provides advantages of high I/O counts, electrical performance and thermal dissipation. In this structure, the flip chip device is attached to the organic laminate with predeposited eutectic solder. Mechanical coupling of chip and laminate is done via underfill encapsulant materials. As the chip size increases, the thermal mismatch between silicon and its organic carrier become greater. Adhesion becomes an important factor, since the C4 joints fail quickly if delamination of the underfill from either the chip or the solder mask interface occurs. Newly developed underfills have been studied to examine their properties, including interfacial adhesion strength, flow characteristics, void formation and cure kinetics. This paper describes basic investigations into the properties of these underfills and also how these properties related to the overall development process. In addition, experiments were performed to determine the effects on adhesion degradation of flip chip assembly processes and materials such as IR reflow profile, flux quantity and residues. Surface treatment of both the chip and the laminate prior to encapsulation were studied to enhance underfill adhesion. Accelerated thermal cycling and HAST (highly accelerated stress testing) were conducted to compare various underfill properties and reliability responses.