I was looking into soldering wires. I found out that even when soldering wires are, for example 60% Tin and 40% lead, the wires contain 2-5% flux. Even "pure tin" soldering wire contains 2-5% "flux in core"! This is not the flux that you add later on, but flux apparently integrated into the "core" of the soldering wire itself. But what does that mean exactly?

How does this:

  1. Affect the soldering wire’s ability to solder
  2. The physical properties of the soldering wire (e.g. more brittle)
  3. Where is the flux located inside the soldering wire?
  4. Is it possible to solder with just the pure element (e.g. Tin) and no flux?

2 Answers 2

  1. You need flux, solder won't wet most things well without it. Solder paste is tiny balls of solder in a matrix of flux. Solder is sold by mass and flux is relatively low density so the flux represents a bigger part of the volume than the small percentage quoted.

  2. The properties of the alloy wire are not significantly affected- namely it won't fall apart on the way to the tip of your iron. That's all we really care about.

  3. One or more hollow 'tubes' filled with liquid flux. One manufacturer made a feature out of having several and calls themselves "Multicore" (even though they sell other things such as solder paste). You can see the arrangement right in their logo.

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  1. Perhaps you could under some kind of ideal conditions (perhaps in a vacuum with perfectly clean non-oxidized surfaces- or soldering noble metals together). But it's not practical in most situations. So we use flux and deal with cleaning or residue in virtually all soldering situations.

In wave soldering the liquid flux is added first and then the board is exposed to the solder which does not have any flux in it (just a layer of oil on the top to keep it from oxidizing).

In repair situations we often add additional copious quantities of liquid flux, more is sometimes better and very fine solder wire may not have the best distribution of flux.

Plumbing soldering generally used solid solder wire (and a cheap non-eutectic alloy) and nasty strongly acidic paste flux. Activated rosin fluxes used in electronics are much more benign at room temperature.

  • \$\begingroup\$ I was going to suggest that one can see the cores if the solder is cut with something very sharp so that the metal isn't smeared over, but I think the "classic" Multicore logo does a better job. I'd add that Multicore started off /only/ making cored solder ("soldering wire" in USA English I believe) with paste etc. being a recent addition. Before cored solder, i.e. in the 1950s, separate solder and flux were used as they still are by many metalworkers, see nrhf.no/Hallo-Hallo/HH%201986-5.pdf page 4. \$\endgroup\$ Jul 8, 2023 at 7:04
  • \$\begingroup\$ @MarkMorganLloyd My father was an electrical engineer from the 1940s to 1980s and I remember him talking about 'killed spirits' being used as a flux when jointing large cables, made by dissolving zinc in dilute hydrochloric acid ('spirits of salt') until it ceased to evolve hydrogen. An acid flux, not suitable for electronics work! \$\endgroup\$ Jul 8, 2023 at 9:19
  • \$\begingroup\$ My father was- among other things- a jeweller, and I remember him soldering gold in the 1980s with a spirit lamp and blowpipe roughly as shown at mindat.org/imagecache/f9/20/01382480016330461893070.jpg . Younger members of the community might not appreciate that in the other link I gave the implement being carried is a soldering iron (not a poker) with copper bit scarfed into an iron handle, I remember somebody's home-made iron in the '50s which wasn't much smaller. \$\endgroup\$ Jul 8, 2023 at 9:53
  • \$\begingroup\$ @MarkMorganLloyd: ""soldering wire" in USA English I believe". No--I've never heard anybody refer to it as "soldering wire" before. In the US (just as in GB, I believe) it's normally just referred to as "solder". \$\endgroup\$ Jul 8, 2023 at 16:26
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    \$\begingroup\$ @MichaelHarvey Ah, it's the good old Severn-Trent regional demarcation :-) \$\endgroup\$ Jul 16, 2023 at 15:07
  1. It improves solderability by removing oxides as they form, allowing the solder to thoroughly wet the work.

  2. Insignificant.

  3. Usually it's right in the middle. You might never see it unless you cut the solder wire with snips, though.

  4. You can use solid-core solder (your question makes it sound like all solder wire is flux-core, but solder without flux isn't that hard to find). But you still need the workpiece to be thoroughly free of oxides or you will get a bad solder joint — or the solder will simply refuse to flow into the joint at all. Although there are some oddball alternatives (like mechanical scrubbing right before soldering), a bit of flux added at the beginning of soldering is the easiest and most common way to remove oxides.

  • \$\begingroup\$ Hi... how do you find solder without flux? The linked product from the question (sra-shops.com/…) is 2-5% polyethylene glycol, but is described as "solid core". Is it an exception? Another product on Amazon is the same way \$\endgroup\$ Jul 7, 2023 at 22:27
  • \$\begingroup\$ @nickcarraway Metalworking supplies, cored solder was originally an electronics thing and was based on rosin etc. rather than various acid mixes which don't play nicely with fine work. Even today there are many solder alloys- and even more fluxes which depend very much on what metals are to be "wetted" during the joining process. \$\endgroup\$ Jul 8, 2023 at 7:07
  • \$\begingroup\$ @nickcarraway see for instance Kester 14-6337-0031. 63/37 tin/lead, nothing but. Or check anything else in their "solid wire" product series for other alloys. \$\endgroup\$
    – hobbs
    Jul 8, 2023 at 7:16

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