In this article, flip chip assembly using nonconductive films (NCFs) was evaluated for chip-on-board (COB) packages using Cu/Sn-Ag hybrid bumps. In general, significant thermomechanical stress can be induced at the Cu-pillar/Sn-Ag solder joint because of a large coefficient of thermal expansion (CTE) mismatch between the Si chip and printed circuit board (PCB). If a concave-shaped solder joint morphology is generated at the solder joint, thermomechanical stress could be concentrated at the solder joint, resulting in an early failure of T/C reliability. To obtain the solder joint morphology without a concave-shaped solder joint, NCFs' curing properties and viscosity should be controlled. In particular, the NCFs' viscosity at the solder melting temperature, at which metallurgical bonding between molten solder and Cu pads occurs, will significantly affect the solder joint morphology. For this reason, solder joint formation was investigated at each viscosity of NCFs and bonding temperature during the thermo compression (TC) flip chip bonding process. In addition, specific viscosities at certain bonding temperatures during the flip chip bonding process were obtained from measured degrees of cure and viscosity approximation. Finally, the material properties of NCFs were optimized without NCF traps at the solder joint and concave-shaped solder joint, and T/C reliability was evaluated for each silica powder content. As a result, it was confirmed that the COB packages using Cu/Sn-Ag hybrid bumps and optimized NCFs show stable resistance changes within 10% of the initial resistance after the T/C reliability test.