Metal-halide is considered a promising material for electronic devices, particularly in the fields of solar cells and LEDs. In particular, organic-based metal-halide perovskites have shown a dramatic increase in device performance within a short period. Although formamidinium (FA) is known for its higher stability, it can also degrade easily during film formation and thermal treatment processes. As a result, it is crucial to demonstrate stable phase transition during these processes. To address this issue, we employed in-situ Grazing-Incidence X-ray Diffraction (GIXD), a powerful analysis technique for tracing phase transitions and degradation during film formation and thermal treatment. Through our analysis, we were able to identify specific conditions that facilitate phase transition and prevent thermal degradation. Additionally, we discovered the influence of intermediate phases and solution aging on the final film conditions and the performance of solar cells.