Flux residues on soldered boards

Introduction
During soldering some overspread may occur to the component side. This cannot always be avoided, however this can be minimized when the process settings are optimized.
Besides this effect also some flux or reaction residues on the topside solder fillets after soldering are sometimes present. This is due to the fact that the solder that fills the joints will displace the flux from these joint gaps during soldering.
Reasons for overspread
When the start or stop position of the sprayfluxer is not exactly tuned with the board position in the conveyor, it may cause flux overspread to parts of the board. But also when a board has 'slots' or open holes, some flux may cover the component side. This is unavoidable unless these gaps are (temporary) closed.
Some flux overspread is not always avoidable. It is therefore important that these remaining 'unused' flux residues are harmless, just like the common flux residues from the flux that was part of the soldering operation.
Residues on solderjoints
A 'coloured ring' of residues can sometimes be found on the topside fillets. On the solder side most of these residues are washed away by the friction of the wave. On the topside however some residues might be left as a result of the chemical reaction between the flux activator and the metal (copper) oxides.
Behaviour of flux residues
Flux residues on the board after soldering can have a different appearance. This is depending on how and to what degree the flux was fully heated and in contact with the solder or not.
In any case the flux remainings of all different kinds should not affect the sound operation of the board neither should these residues give any harm.
All electronic fluxes should pass an insulation resistance test and a corrosion test. Proving that remaining residues after the soldering process are harmless for the equipment.
In case of doubt if these specifications meet the necessary product specification for a particular board, one should contact the flux supplier.