The purpose of flux used in soldering is to remove oxidation from the copper and the devices being soldered. The ability of flux to react with and remove oxidation is called its “activity”. Highly active flux does a good job removing oxidation, but it can also corrode metal, such as the iron coating on soldering iron tips.
Water soluble fluxes have a high activity. They are even more corrosive when in contact with the high temperatures of a soldering tip, where it can damage the tip’s iron coating by corrosion. Water soluble flux is not recommended for hand soldering as it can significantly shorten the life of your soldering iron tip.
Water soluble flux is often used in wave soldering systems that are followed by an automated cleaning step.
No-clean fluxes have low activity. They are designed to boil away during soldering, leaving very little residue. Because it rapidly boils off of a hot soldering iron tip, no-clean flux may not have time to clean off a buildup of oxidation. Using no-clean solder requires using good soldering and cleaning techniques to prevent oxidation build up on the tip. If the tip becomes oxidized and won’t wet with no-clean solder, rosin flux can be used to clean the tip, then you can tin the tip with no-clean solder.
You can read more about it on his blog. Great tips, Greg!