As the high density package is moving towards miniaturization, high I/O density, better thermal and high reliable system, the conventional wire bonding technology can not satisfy the product need already. The advanced flip chip technology is highly expected due to its high area array I/O interconnection, short signal path, high thermal dissipation, high electrical and thermal performance. In order to enhance the reliability of a flip-chip on organic board package, underfill is used between the chip and the substrate to redistribute the thermo-mechanical stress created by the coefficient of thermal expansion (CTE) mismatch between the silicon chip and organic substrate. However, the conventional underfill relies on the capillary flow of the underfill material and has many disadvantages. In order to overcome these disadvantages, no-flow underfill has been invented to improve the flip-chip underfill process. This paper reviews the development of flipchip technology and expounds the behavior of flow and no-flow underfill.