Transformation of reflow solder flux residue under humid conditions

The use of reflow flux system for soldering process of SMT electronics results in the concealed residues located underneath of the components, which changes its characteristics upon exposure to humidity. In this paper, the transformation of reflow process-related solder flux residues was investigated under varied humidity and temperature conditions. Evolution of potentially corrosive active components from the residue under high humidity and temperature conditions was investigated for different solder pastes using the acidity indication gel test. Changes in the concealed morphology of the residue were visualized using light optical and scanning electron microscopes, and the observations were correlated to the results from the gel indication test. Quantification of the release of active components from the residue was performed using the extract conductivity measurements. The results show that the evolution of acidic residues from the reflow-soldered pads occurs upon exposure to humidity and temperature due to opening up of the concealed residue, however, temperature alone was found to have no effect on this phenomenon. The hygroscopic nature of the evolved solder flux activators cause the absorption of moisture under humid conditions, accelerating the formation of a conductive electrolyte that provides a potential chance for an undesired corrosion process.