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Flip chip packages are becoming more popular due to many factors such as electrical performance, functionality and high I/O interconnections. To fulfill such needs in different applications, chip sizes are gradually becoming larger. Due to the large die sizes with high pin count, small bump pitch and low-K inter-metal-dielectric material, reliability concerns are arising at the interfaces of die, solder bumps and substrate. Of concern are package warpage issues, bump cracks, underfill void/delamination/cracks and die cracks, etc. The reliability issues can be solved by selecting more appropriate underfill materials to relief mechanical stress from CTE mismatch. Many commercial brands of underfill materials are available in the market and the underfill properties such as Tg, modulus, CTE, viscosity, flow characteristics, and adhesion need to be characterized before implementation. In this project, the underfill properties are studied and discussed and stress modeling for large dice in large packages is performed. Package data such as warpage, bump crack and delamination are measured for verification. The optimum underfill material for large die flip chip packages has been implemented in mass production.