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This chapter focuses on the understanding of the key parameters of the underfill material selection to minimize moisture-induced failures in flip chip ball grid array (FCBGA) packages during reflow soldering. An integrated study has been carried out, including interface adhesion measurements (pull/shear), moisture properties characterization, moisture/reflow sensitivity test of FCBGA, failure analysis, and finite element modeling. Moisture vaporization during reflow soldering process is the key element in understanding the failure mechanisms. It has been found that the vapor pressure in the plastic package saturates much faster than the moisture diffusion during the reflow soldering process. This implies that the vapor pressure reaches the saturated pressure level at an early stage of moisture absorption, when the package is far from being moisture saturated. However, the interfacial adhesion degrades continuously with moisture absorption. The interface adhesion characte-rization, and, in particular, the measurement at high temperature with moisture, is a challenge. Our analysis is aimed at the development of a methodology for the adhesion measurements in pull/shear under high temperature and high humidity conditions. A detailed matrix study for the adhesion between the chip (polyimide passivation) and the underfill (PI/UF), as well as between the solder mask at the board side and the underfill (SM/UF), has been conducted under different moisture and different temperature conditions. Flip chip daisy-chain test samples were built and tested for moisture sensitivity performance to investigate the impact of the underfill material selection, plasma grafting surface treatment, the flux residue effect, the overmolding effect, and the voids effect on the underfill. An integrated study shows that the interfacial adhesion strength at high temperature conditions with elevated moisture content is a comprehensive factor. This factor includes the effects of adhesion degradation by moisture, vapor pressure, thermo-mechanical stress, hygroscopic stress, and underfill material properties. Such a factor represents therefore the overall resistance of the plastic package to the moisture. A failure criterion was proposed and it has been shown that there exists a critical adhesion level, which may be independent of the particular molding material and moisture conditions for an FCBGA package. [ABSTRACT FROM AUTHOR]