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In this paper, effects of internal stress on material properties such as CTE, modulus, and glass transition temperature (Tg) of various NCPs and underfill materials for flip chip applications are discussed. And a simple method for the internal stress estimation using material properties is introduced. Usually, internal stresses are generated inside NCPs and underfill materials during a curing process. And these internal stresses affect thermomechanical properties of cured NCPs and underfill materials. Using thermo-mechanical analyzer (TMA), dimensional changes of all materials were measured. Dimensional changes after 1st heating cycle rapidly increased near Tg, however, it was not observed after the 2nd and 3rd heating cycles. This difference between the 1st and 2nd cycles is due to excess free volume built during curing processes. Also, using dynamic mechanical analyzer (DMA), modulus and Tg were measured. The modulus of the 1st cycle was smaller than that of the 2nd cycle for all materials. Based on these differences in the 1st and 2nd cycles, internal stresses of various materials were theoretically estimated. The estimated stresses build-up of the 1st cycle was different from those of the 2nd cycle for all materials. It is considered that internal stresses generated during a curing process alter the stress state inside materials.