In metallurgy, a flux (derived from Latin fluxus meaning “flow”), is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining. -- Courtesy of Wikipedia.
This is the stuff which no reprap(ed)household/solderer should be without. Even your solder contains it.
- It's cheap.
- Readily available.
- It's there for you, your reprap and your electronics in dire need.
- It's toxic.
Get two packages when you think of getting one.
Did I say its essential?