The residues after soldering depends on the flux system used and the soldering temperature profile used. The images of acids used in solder flux show how heating will deactivate the acid parts. The acid is placed in petri-dishes and the heated at different temperatures and then Residues RAT is added.
Effect of temperature on decomposition of WOAs (activators in solder flux systems). Red coloration indicates amount of acid
The images show clear red coloration for all the residues evaporated at room temperature. At 170 °C, all of them give more or less same residue levels as at room temperature, showing very little degradation of acid. However, after heating at 250 °C, the amount of residues varied depending on the type of acid or flux system. These observations have been confirmed by Ion-Chromatography analysis. In general, DL-malic acid showed a more intense colour, being the acid type having a more aggressive nature.
Source: [Decomposition studies of no-clean solder flux systems in connection with corrosion reliability of electronics, H. Conseil, V. Verdingovas, M. S. Jellesen, R Ambat, Proceedings of EuroCorr 2013]