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Nonconductive films (NCFs) are one of the polymer interconnect materials for flip-chip interconnection. NCFs containing no conductive particles play several roles such as adhesion, insulation, and underfilling at the same time. The most important issue of NCF flip-chip-on-board (FCOB) assemblies is thermal cycling reliability. Thermomechanical properties of cured NCFs, such as glass transition temperature (Tg), storage modulus (E'), and coefficient of thermal expansion (CTE), significantly affect the thermal cycling reliability of NCF FCOB assemblies. In this paper, the improvement of thermomechanical properties of NCFs was investigated by controlling the number of functional groups of NCF resins. To compare the reliability of conventional and modified NCF FCOB assemblies after thermal cycling test, electrical analysis and scanning acoustic microscopy investigation were performed. Thermal deformations of NCF FCOB assemblies under thermal cycling environment were also investigated, and quantitatively compared using high sensitivity Twyman-Green interferometry and portable engineering Moire interferometry. According to these results, the functional groups of NCFs have significant effects on thermomechanical properties of cured NCFs, thermal deformation, and thermal cycling reliability of NCF-bonded FCOB assemblies. As a result, the functionality modified NCF FCOB assemblies showed significantly enhanced reliability compared conventional NCFs in thermal cycling test.