Polyimide (PI) is a curable polymeric dielectric, which is a popular material for wafer level packages (WLP). Low-curable-temperature polyimide (LTP) attracts increasing interest in assembly applications and is one of the critical materials for 2.5D IC and fan-out (FO) applications because of its curing temperature constraint during the packaging process. As the possible curing temperature is approached, the curing level percentage is an important index of the polymer film. Herein, we aimed to study the thermal properties of PI under various curing conditions (curing level 30–100%) and choose the curing conditions (curing level 80–100%) to apply on WLP for component-level reliability (CLR), chemical resistance and board-level reliability (BLR) tests. According to the CLR results, the lower curing level (about 80%) passes scanning acoustic tomography (SAT) inspection for the 1000-h temperature humidity test (THT), the 96-h u-bias highly accelerated stress test (uHAST), and the 1000-h high-temperature storage test (HTST) despite its lesser thermal stability and chemical resistance than the fully cured condition. In the BLR test, ball drop test results show no gap between the 80% and 100% curing level conditions. In the thermal cycle test (TCT) the lower curing level gives a better TCT result. Finally, since reliability issues minimize chemical resistance capability and thermal process reduction, curing level criteria might be redefined in the polyimide curing process.