To assess the contribution of the thermomechanical stress on the long-term reliability of photovoltaic (PV) modules, we applied extended thermal cycling tests up to 600 cycles to 13 models of commercially available PV modules. The extensive testing, using 5-10 PV modules for each model, revealed that the levels of power loss, induced by thermal fatigue during this extended testing, differed in each model of the PV module. Degradations by solder bond failure and bypass diode failure, which were observed in a few models of PV modules, were most likely to result from incorrect soldering that may be completely avoidable through appropriate implementation of a quality management system. Therefore, we suggest that, in most of the latest PV modules, their durability to thermal fatigue was established, as evidenced by the extended thermal cycling test with 600 cycles. However, solder bond failure due to inappropriate manufacturing process quality, which is not identifiable by the conventional thermal cycling test (200 cycles), would be detected by this extended test in a few models. Based on these suggestions, we discuss the evaluation procedures of these failures caused by the quality-control and design issues of PV modules.