Future generations of electronics are expected to include flexible, bendable, and wearable devices. There have been very few studies, however, on the bonding technology and reliability of bonding joints between a chip and a flexible substrate. In this study, we investigated the properties and reliability of joints formed with Sn-58Bi solder microbumps between a Si chip and a flexible substrate. For fine-pitch bonding, we formed Cu pillar bumps and Sn-58Bi solder microbumps with diameter of 25 mu m by electroplating. The Si chips were then bonded on a flexible substrate finished with electroless nickel immersion gold (ENIG) using flip-chip technology, processing at 170A degrees C under a force of 1 N, 2 N, or 3 N for 15 s. Cross-sectional images of bump joints were analyzed using field-emission scanning electron microscopy. While the Cu6Sn5 intermetallic compound (IMC) was formed on the Cu-pillar-bump side, another IMC, Ni3Sn4, was formed on the ENIG-substrate side. In addition, we performed a shear test, thermal shock test, and bending test to evaluate the joints' mechanical properties and reliability. The bending test was performed by a machine designed for joints on flexible substrates. Unit shear force of 2 N was the nominally highest value obtained from joints prepared under the three bonding conditions. After the thermal shock test, we observed cracks initiated at the Cu6Sn5/Sn-58Bi interface, which then propagated within the solder bumps or at the interface. In the case of the bending test, failure occurred at the Ni3Sn4/Sn-58Bi interface or within the solder bumps.