Solution based printing methods using metal ink provide advantages of applicability to curved surfaces, a low manufacturing cost and large-area patternability as well as simple manufacturing processes. Despite of these advantages of metal ink printing, mechanical reliability issues have always been a concern due to its limited mechanical characteristics for flexible electronics. In this research, mechanical reliability of Ag ion ink films on flexible substrates was evaluated. To suggest a proper thermal sintering method for mechanically stable metal ink films, mechanical behavior of Ag ion ink films with different thermal sintering methods was studied. Besides, to achieve mechanically robust flexible electronics using metal ion ink, mechanical reliability of Ag ion ink films on flexible polyimide (PI) substrates was evaluated. Adhesion reliability of Ag ion ink films on PI substrates was analyzed by measuring adhesion energy as a function of sintering temperature by a double cantilever beams (DCB) fracture mechanics test and understanding fracture mechanism. Humidity reliability of the films was evaluated by a moisture assisted subcritical debonding test. Bending endurance of the films was analyzed by a bending test and a bending fatigue test. This research could provide guidelines to design mechanically robust flexible electronics using metal ion ink.