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In this paper, flip-chip interconnections were made on very flexible polyethylene naphthalate substrates using nonconductive films (NCFs). In general, the NCF/underfill materials are both epoxy thermosetting substances, and it was found that the curing process would increase the internal stress, shrinkage, and warpage. Therefore, the use of thermalplastic materials might solve these problems. Such materials provide lower water absorption at 0.56 ± 0.05% and curing for 4 h at 150 ± 10°C for the elimination of voids. The chip-on-flex (COF) parameters are a bonding temperature of 400 ± 10°C, a bonding force of 95 ± 5 N, and a bonding time of 0.4 ± 0.1 s. The workability test shows that the Au/Sn eutectic condition has fine adhesion for leads and bumps, and the bond accuracy is controlled in less than one-third bump space. The COF products were able to pass all the reliability tests, including the highly accelerated stress, high-temperature storage life, low-temperature storage life, temperature cycling, and pressure cooker tests. The results indicate that the COF assemblies exhibit fine performance in the reliability, workability, and simulation tests. Thus, the NCF thermoplastic materials are suitable for replacing thermosetting materials, and can be applied in fine pitch to improve the reliability issues associated with COF products.