What does flux do?

If I understand it correctly, the flux chemically performs a final cleaning of the surfaces, prevents copper oxides from forming, and also does help draw the solder into the joint. You can even buy what's called a "tinning flux" which contains a small amount of solder and helps make a more reliable joint.

nate

As I understand it, nate is correct. The solder flows into the joint because the joint is hot and it melts the solder. The heat along with the close surfaces (capillary action I assume) causes it to draw into the joint. You can solder without flux, but it is not recommended for the exact reasons that nate mentions.

It also serves as a wetting agent which is what helps the capillary action and to ensure full joint coverage.

What he said. It keeps the copper from oxidizing, or 'rust'ing. And wen you add the heat it would oxidize faster. Solder does not take to copper oxide as I understand it. so flux helps to keep the weld area clean.

Stick the hot tip of your soldering gun/iron in a tin of flux and see how clean & shiney it comes out. That's what it does for the joint.

Red

Keeps oxygen from getting on the copper

The main thing I see is preventing oxidation of the metal, even reacting with oxygen in the oxide to remove some of any existing oxide for some metals, including copper and lead/tin, whichever of those in "ordinary" solder tends to oxidize when such solder is molten. Some metals are reactive enough for their oxides to be unable to be reduced by flux, with aluminum being a prime example.

- Don Klipstein ( snipped-for-privacy@misty.com)

True, but the wetting action is also important in helping w/ solder flow...similar to detergent w/ water on dry glass--it tends to "clump" rather than flow alone, but a drop of detergent "breaks" the surface tension. That same action is what helps to ensure full coverage and the takeup in the joint.

1. Why Use a Flux? A molten brazing alloy will only wet and flow over the parent metal's if both are substantially free of surface oxide. Simply removing surface oxide before brazing is not effective, since a new oxide layer is rapidly formed on heating. Thus, to achieve an oxide free surface, it is necessary either to: -