A few years ago I remodeled my bathroom and learned how to sweat copper pipes. I've always been able to do it successfully since then, but I've always wondered exactly how and why it works. I've done some searching on the web but most of what I find are "how to" articles and videos, but nothing that explains the specifics of how and why it works. Can anyone give me some answers to these questions that have bugged me?

  1. What exactly does the flux do?

    • What happens if all the flux burns away before the solder melts?
    • What if you didn't use flux at all?
    • What is flux in the first place?
  2. Where exactly should you aim the flame? (I've seen differing reports on this one)

    • Right at the joint (where the flux is)? (in my limited experience, this seems to burn away the flux immediately and sometimes causes problems)
    • At the copper pipe, an inch or so away from the joint?
    • At the fitting that the pipe is being soldered into?
  3. What exactly is the capillary action that draws the melted solder into the joint?

    • What causes it and how does it work?
    • Sometimes I find myself in a situation where the solder melts but just drips right off the pipe instead of getting sucked into the joint. What have I done wrong? I usually have to clean everything off and start over when this happens
  • 2
    the copper pipe, not the flame, has to melt the solder .... google "capillary action"
    – jsotola
    Jan 20, 2018 at 5:25
  • 2
    And it's not the copper, nor the solder, needing flux to be soldered. Anything that is not copper or solder needs it :) Jan 20, 2018 at 10:04

2 Answers 2


I think @ArchonOSX has a good answer, but I'd like to expand a bit more on it (having a background in welding, brazing, and soldering).

Going point by point:

  1. Flux typically promotes bonding between the base metals and the solder. It does this by cleaning the metal (removing surface contaminants), deoxidizing the metal, and preventing further oxidation. Hot metal tends to oxidize more quickly than cold metal, but the flux will remove oxygen (either by absorption, adsorption, or oxidation of the flux itself) where it is applied.

    • If all your flux burns (or gets oxidized) before you've finished brazing the joint, then the solder will be less likely to flow where you need it because the base metals will tend to oxidize. Solder doesn't bond as well with oxidized surfaces, so instead, it balls-up and drips off.
    • This is possible, but difficult. Unless you were in an anaerobic environment (and even then it might not work) you'd probably not get a good bond.
    • Wikipedia is pretty good on this one.
  2. You want the metal to be uniformly heated so thermal expansion is uniform, so wicking is uniform, so everything is uniform. Copper, being a good electrical conductor, is also a very good heat conductor. If you only heat the joint, you might find it cools relatively quickly due to surrounding copper cooling the joint.

    • Not right at the joint (right away). When you are promoting solder flow, then you do want to focus the flame very tightly.
    • Yes, to heat the surrounding pipe initially.
    • Yes, because the metal here is also often a bit thicker than anywhere else, so it takes the most energy to heat up to brazing temperatures.
  3. Capillary action is an interesting phenomenon that occurs when one material can "wet" another. Wetting is a process where the surface energy is lowered when the liquid-phase material is in contact with the solid-phase material. Some liquids, like mercury, don't wet others, like glass (forming an upward meniscus). Water does the opposite, wetting glass. The surface energy must be lower than the surface tension of the liquid to prevent beading (balling-up, which you see when the flux burns off). In small spaces, this wetting capability can cause a column of liquid to rise against gravity. For non-wetting materials, the opposite tends to happen.

    • I think I answered this part above.
    • When this happens, it's because the molten solder cannot wet the copper. Flux lowers the surface energy of the bare copper (another of its functions), which is how it promotes wicking, but if the copper isn't hot enough, it won't wet either.

HOWEVER, everything I just described indicates that the solder is on the surface of the copper (adsorptive or adhesive). In fact, the solder is able to form a localized alloy with the base metal that penetrates some distance into the base metal. This is why brazing (and soldering) can be quite strong, even compared to welding. This is an absorptive process and occurs because the higher temperatures allow copper atoms to move enough within their lattices to allow for an alloy to form. This process adds to the effects described before.

  • 2
    Today I learned...
    – Gabe Evans
    Jan 20, 2018 at 8:19
  • I know we're supposed to avoid comments just to say thanks, but damn, that was such a great answer. Thanks!! I feel much more confident in sweating pipes now that I actually understand what's happening Jan 24, 2018 at 21:36

Flux helps keep the oxygen from contaminating the joint. Any time you heat things, oxygen wants to help them burn. Welding and soldering requires some method to prevent the oxygen from contaminating the weld.

The flame should be aimed into the fitting ~1/2" with the tip of the light blue part that is the hottest.

The solder is drawn into the heat if you do it right and it will wick right into the joint. The exact scientific process is beyond me.

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