Flux serves two purposes, which ultimately, make things easier.
First, the flux combines with oxygen (burns) near the metal. Since the flux is more attactive/reactive than the metal it prevents the metal being worked from oxidizing. All the oxygen involved ends up burning off with the flux. If you run out of flux, or try to resolder an existing joint without supplying new flux, you will usually end up with a dull looking poor joint, which is oxidized.
Second, flux helps the heat conduction in the same way that heat sink grease does. The contact between the soldering tip and the metal is usually a tiny surface area. Maybe one or two little points that happen to intersect. This can make it very difficult to heat the work. Tinning your soldering tip helps with this, but the tinned tip can oxidize after a while. Using plenty of flux in your work helps even more.
Ultimately, you're much better off using lots of flux and making a mess than using too little and getting a poor solder joint. Of course, just the right amount is even better, but erring on the side of excess is better than erring on the side of insufficient.
Flux is also extremely useful when desoldering. Existing solder joints don't contain flux (if they were done right) and you aren't supplying any with new solder. So adding flux is almost a necessity to get the solder to melt and run properly when desoldering.
No matter how much flux you use, you should clean your work thoroughly when you're done soldering. Left over flux can be corrosive.
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http://amzn.com/B004RIF3BM
First, the flux combines with oxygen (burns) near the metal. Since the flux is more attactive/reactive than the metal it prevents the metal being worked from oxidizing. All the oxygen involved ends up burning off with the flux. If you run out of flux, or try to resolder an existing joint without supplying new flux, you will usually end up with a dull looking poor joint, which is oxidized.
Second, flux helps the heat conduction in the same way that heat sink grease does. The contact between the soldering tip and the metal is usually a tiny surface area. Maybe one or two little points that happen to intersect. This can make it very difficult to heat the work. Tinning your soldering tip helps with this, but the tinned tip can oxidize after a while. Using plenty of flux in your work helps even more.
Ultimately, you're much better off using lots of flux and making a mess than using too little and getting a poor solder joint. Of course, just the right amount is even better, but erring on the side of excess is better than erring on the side of insufficient.
Flux is also extremely useful when desoldering. Existing solder joints don't contain flux (if they were done right) and you aren't supplying any with new solder. So adding flux is almost a necessity to get the solder to melt and run properly when desoldering.
No matter how much flux you use, you should clean your work thoroughly when you're done soldering. Left over flux can be corrosive.
I prefer this:
http://amzn.com/B004RIF3BM