We have newly developed a plating-free bumping process by Cu nanopaste with injection molded solder (IMS) technology. In the new process, Cu pillars are fabricated by filling photoresist openings with Cu nanopaste followed by pressure-free Cu sintering. The Cu nanopaste in the photoresist opening forms a unique cup-shaped Cu pillar after the sintering process due to the evaporation of solution in the paste and to the aggregation of Cu nanoparticles. Molten solder is injected, by the IMS technology, into the space created by the cup-shaped Cu pillar in the photoresist opening. The photoresist layer is then stripped for solder bump formation. In this paper, we demonstrated the fabrication of 60 μm pitch and 33 μm diameter bumps, and of 80 μm pitch and 46 μm diameter bumps by new bumping process. Mechanical integrity of the bumps was confirmed by bump shear testing. Reliability of the joints was also confirmed through preconditioning test, temperature cycling test, high temperature storage test, or highly accelerated stress test. Thermomechanical analysis of 2D/2.5D packages using the joints with sintered Cu nanoparticles was conducted.