1. What is Soldering?

This is the first post in a series on soldering for the neophyte and the perpetual learner. I would be very grateful for feedback on this series of any kind that you find relevant and particularly on factual errors, omissions, and unclear explanations. Bear in mind that many very relevant things will be progressively covered in the series, so it might be what you want included is planned—nonetheless, please mention it so it won't be overlooked. Thank you for reading and participating in creating what I hope will be a resource for the members of AAC—new and old.

It is obvious to even the most uninformed eye that soldering makes a mechanical connection between to metallic items. It doesn't take much more experience to know that this connection is also a good conductor of electricity. This is all true, but what exactly is happening when we solder?

As a general case, there are four ways we join metallic items when making something:

  • Gluing
  • Soldering
  • Brazing
  • Welding

While there is overlap among the categories, there is enough distinction to make each more suited to particular applications than the others. For our purposes, they can be characterized by a couple of attributes to make that clear.

First is the application of heat. In this list, the heat requirements increase from first to last. Gluing requires little or no applied heat, soldering more, brazing even more, and welding the most. The amount of heat is baed on the reason for the heating.

In gluing, a third substance is applied to two pieces of metal (or any other material—unlike soldering which is exclusive to metals). This substance makes a mechanical bond between the two pieces which can be very strong if the glue and target are compatible and properly prepared.

Glues can even be made conductive, and this is often used to advantage in electronics applications where the very low heat needed for gluing is desirable. The only heat needed comes from limits on ambient temperature (not too cold), need for curing (heating above ambient but generally not very much) or self-heating due to an exothermic reaction that is part of the curing process.

As a rule, we can consider gluing to be a very low heat method, that can be very strong, and can be electrically conductive. Gluing, though, generally requires a long time to become mechanically sound, is not specific to metals (and so can be messy affecting plastics and other undesirable targets), and will be more expensive than soldering.

In soldering, the heat must be sufficient to melt the solder. This is, by definition, far less than the melting point of the joined materials but very high compared to ambient temperature. The purpose of the heat in soldering is to melt the solder and to cause wetting. Wetting is the process of the solder forming an alloy layer with the material being soldered.

Unlike gluing, soldering actually creates a new metal which joins the solder filler material and the soldered material together. This makes solder joints very strong. Generally, about as strong as the solder filler, which will be some alloy of tin and lead (or other, less desirable materials in the case of "lead free" solder, more of this in the post on materials). This will usually be a lot stronger than a glue joint, though it is possible that some advanced glues are competitive in strength.

Managing heat while soldering is a critical aspect of hand soldering art and craft and you'll be hearing more about that. However, in the evolution of electronics materials and components, practical high quality soldering was one of the goals and so it is quite possible to solder things well without burning them up. In fact, it is surprising how much heat those tiny components can tolerate—until you burn something up and then it seems they are incredibly delicate. Believe me, it can be done.

Progressively, brazing and welding require even more heat but brazing is more like soldering while welding is something different.

Brazing is a sort of high temperature soldering using a filler with a much higher melting point than solder wire (around 450℃, vs, the ~300℃ of solder, based on various authorities). Traditionally, this is brass, hence the name. For brazing to work, the pieces to be joined must have a higher melting point than the brass. The heat is applied, and the brass wets the work joining it. The joint is much stronger than soldering because the filler is brass not tin/lead.

An intermediate step between soldering and brazing is silver soldering used in jewelry and other things that required a great deal of mechanical strength but can't tolerate the heat of brazing. Some tin (Sn)/lead (Pb) solder alloys also include silver (Ag) for special applications we will discuss.

Welding is something different. In welding the work pieces are intentionally heated to their own melting point so they can fuse together. Some welding uses a filler of an alloy very similar to what is being welded, though methods like spot welding don't.

Welding is particularly unsuited to the applications that call for hand soldering but it is not absent from electronics. For example, it is very common today because spot welding is the method of choice to create Li-Ion battery packs from cells.

The second distinction is the metals to be joined. Gluing of varying materials is common, even metallic and non-metallic work can be joined. But in the metal specific methods, this is one of the critical areas that might make one better than another.

In soldering and brazing, it is possible to join dissimilar metals. This is because the formation of the alloy layer is the key to the connection so as long as the filler material can alloy with each, it will form the mechanical (and electrical) connection. Different metals have different solderability which is a measure of how hard it is to wet that material with the solder.

Copper, brass, gold, and silver, for example, are "easy" to solder while iron, some steels, and nickel are more difficult. Decreasing in solderability there are stainless steels and aluminum. You may have guessed this aligns with the thickness and hardness of the oxide layer that forms on them. Sometimes a workpiece can be plated with a more solderable metal to facilitate soldering. Plating is an entirely different process to which these materials are far more susceptible. Managing oxide layers is a critical activity in soldering which will be discussed later.

In welding, the materials must be very similar, as must the filler. That's because a weld is formed by the fusing of the materials involved, not by wetting and the formation of an alloy. Interestingly, this means you can weld thermoplastics and in fact this is a very useful and common technique for both fabrication and repair.

We will discuss the physical process of soldering, in particular, in more detail going forward to help understand why soldering works—or more importantly doesn't—and how to make it work best. The choice of solder alloys, flux, and surface preparation are all critical. When you understand what each does you will be more capable of deciding the best route when things don't seem to be going well.

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