What the Flux: How Does Solder Work Anyway?

I’ve been soldering for a long time, and I take pride in my abilities. I won’t say that I’m the best solder-slinger around, but I’m pretty good at this essential shop skill — at least for through-hole and “traditional” soldering; I haven’t had much practice at SMD stuff yet. I’m confident that I could make a good, strong, stable joint that’s both electrically and mechanically sound in just about any kind of wire or conductor.

But like some many of us, I learned soldering as a practical skill; put solder and iron together, observe results, repeat the stuff that works and avoid the stuff that doesn’t. Seems like adding a little inside information might help me improve my skills, so I set about learning what’s going on mechanically and chemically inside a solder joint.

Solder != Metal Glue

It should come as little surprise that like other metal working methods, soldering has a strict definition. Soldering is the joining of metals by melting a filler metal into the joint. Unlike in welding, only the filler metal — the solder — melts. The metals being joined usually have a much higher melting point than the solder. Brazing is similar to soldering in this regard; even though the filler metal in brazing melts at a much higher temperature than solder, the joined metals still don’t melt.

The metallurgical details of solder itself could take volumes to discuss completely, but for our purposes it’s pretty simple stuff. Solder is just an alloy that has been engineered to melt at a specific temperature. For electronics uses, the king of solders for years was an alloy of 60% tin and 40% lead. New regulations in response to environmental concerns have led to the development of different lead-free alloys, but whatever the composition, solder’s job is pretty simple. Solder needs to melt at a predictable temperature and maintain its mechanical and electrical properties when it solidifies. In other words, solder needs to be strong enough to physically hold a joint together without introducing any undesirable electrical properties to the joint.

Intermetallic bonding. Source: Indium Corp
Intermetallic bonding. Source: Indium Corp

Solder needs to do more than just melt and solidify, though. People seem to think of solder as some kind of “metal glue” — apply it as a liquid and let it become solid to lock a joint together. That’s only part of the picture, though. For a soldered joint to be electrically and mechanically sound, the solder needs to wet the metals to be joined. In the context of soldering, wetting is the process whereby the molten solder partially dissolves into the copper base metal, forming a region that’s part solder and part copper. This creates intermetallic bonding and it’s the key to soldering. In most solders, the molten tin is the primary solvent that dissolves into the copper substrate and forms the intermetallic bond that electrically and mechanically stabilizes the joint.

Intermetallics are necessary to a good solder joint, but like so many things, too much of a good thing can be a bad thing. Intermetallics tend to be brittle, so if the intermetallic layer is too thick, the joint can be mechanically weak. There can also be voids within the intermetallic layer that add to mechanical instability.

Keeping It Clean

We all know that flux is critical to quality solder joints. But what exactly is flux and why do solder manufacturers go to the trouble of stuffing it into the core of solder wire?

The importance of flux is due to its ability to fight the mortal enemy of solder: metal oxides. Metal oxides are no good for solder joints — solder will not adequately wet a joint when there’s a metal oxide coating. Fluxes are designed to remove metal oxides, and to do so while the joint is being soldered. Pre-cleaning the metals doesn’t cut it, by the time the solder flows atmospheric oxygen has rebuilt the metal oxide layer enough to spoil solder wetting.

Electronic solder usually has a flux made of rosin. Rosin is a natural product derived from pine trees, notably the loblolly and longleaf pines for US-made rosin. It has the advantage of being more or less inert at room temperature but highly acidic when liquified, and has a melting point slightly lower than solder. The rosin core of electronic solder will therefore melt before the solder, flowing into and around the joint. The acidic liquid reacts with metal oxides, exposing clean metal for the solder to wet into. The acidic liquid flux converts the metal oxides to metal salts and water, which are typically locked up in the flux when it solidifies. The reaction products are generally harmless at that point, but some processes still require the used-up flux to be removed.

Of course there’s a lot more to soldering than this, but these are the basics about what’s going on inside that blob of solder at the end of your iron.

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