“It is important to note that assemblers may run multiple flux chemistries through the same cleaning equipment and cleaning media (i.e. heated DI water). Also, it makes environmental sense to reuse the cleaning media until it is exhausted or filtered, to avoid creating a significant waste stream. In such scenarios, the flux residue may come in contact with other chemistries during the cleaning process. These other chemistries can negatively affect the cleanability of a residue.
All flux residues “cure” with time, making them more difficult to remove. It is beneficial to clean the assembly as soon as possible, generally, within the same shift is suggested. Leaving the flux residue on the assembly for an extended period of time, without cleaning, also allows the possibility of flux induced corrosion to occur.” –Eric Bastow