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.
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.
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.
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 ( firstname.lastname@example.org)
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: -
1.. Remove oxide as it is formed by the use of a suitable brazing flux.
2.. Prevent oxidation during brazing by heating in a protective atmosphere
3.. Use a self-fluxing brazing alloy - only applicable when brazing copper
to copper. Brazing fluxes are only designed to remove oxide films. Where
other contaminants such as oil, paint, lacquer etc... are present these
should be removed before brazing, using either mechanical or chemical
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