<h2>Summary</h2> Heterojunction (HJT) solar cells still encounter challenges related to long-term operational stability despite their remarkable power-conversion efficiency. Here we report the corrosion of indium tin oxide (ITO) electrode in HJT solar cells caused by commercially available solder fluxes. A significant increase in sheet resistance is observed in the rear ITO layer but not in the front ITO layer. The variation in corrosion resistance is explained by differences in the ITO phase. The corrosion process primarily occurs through the dissolution of the indium content. Furthermore, even after encapsulating HJT solar cells through lamination, degradation spots appear in electroluminescence images after just 45 h of damp heat testing, confirming that the lamination process does not effectively remove solder-flux residues. Additionally, it is found that a proper drying process for solder flux can mitigate the degradation of HJT solar cells.