Seeking Perfection
What is flux and why do I need it?
Flux is a gel-liked substance used to make solder flow on the work surface. It cleans the metal so molten solder sticks to it and creates a strong bond.
Most "high-tech" solder is self-fluxed meaning they contain rosin inside the solder, the user can work with parts without the need to mess with sticky flux gel, but this rosin core is good only for the initial use, recycled solder needs to be used with flux.
If you just bought some solder from the dollar store, or have a braid of unknown type of solder, here's how you can test it for self-contained-rosin-flux.
Make a blob of solder ball on your soldering iron, then use the iron to wipe this molten blob onto a piece of paper. If you notice some amber or brown residues, that is flux, and you're ready to go.
For those who has to work with flux, sorry I do not have a picture of it, but here's how it works: simply smear the work surface with a little flux, and then solder. Read the lable of your flux carefully, some flux needs to be cleaned off, some can stay on your work surface and won't do any damage.