This post will serve to address various questions I have seen floating around the internet regarding how solder adheres to various surfaces. Will Solder stick to Aluminum? Copper? Steel? and if so, how can I make it "stick".
What makes solder stick?
It may be helpful to begin by discussing what is really happening that makes you solder "stick" to the metals you are soldering together. A process called "wetting" occurs when we heat our PCB component up to a temperature hot enough to melt our solder alloy and then apply said solder to the components we are looking to join. By creating a molten solder and a hot metal surface, the atoms in the metals will begin to bond together. When done correctly, an intermetallic bond is created between the surface of the component and the solder alloy. This intermetallic bond is what creates our solder joint, and without it we would simply be able to pluck our bead of solder right off the surface of the component.
The key to creating a intermetallic bond is making sure that there is no interference between the atoms in our alloy and the atoms in our components. The thing about metals is that, generally speaking, when placed in contact with each other, the atoms will do their best to connect and create new metal compounds. This is why it is so essential to make sure that our metals are able to make direct contact with one another in order to create that bonded layer between them. This is also why some metals are easier to solder and some prove to be more difficult.
So, which metals will my solder stick to?
One of the things that makes a metal more or less solderable is it's tendency to oxidize. What this means is that when exposed to oxygen, a layer of oxygen molecules form over the surface of the metal. This layer sits on top of the atoms and will prevent proper wetting. We can easily picture what an oxidized metal might look like if we imagine the rust on an old car or the green layer that has formed over the copper statue of liberty. However, it is not always so clear, and even a small layer of oxidization can prevent our solder from "sticking".
Let's take a look at a few metals and see how effective they are when used in soldering projects:
Copper
If you are planning to solder electrical components, it is likely you will be creating joints on copper or copper compounds. Copper is often used in electronics because of its electrical conductivity and it can withstand heat well without breaking down. As mentioned above, a key aspect of this process will be making sure those pesky metal oxides don't get in the way of our intermetallic bond. The most common and popular way to do this is by using a flux. Rosin Flux is actually derived from pine trees and has a naturally acidic property when heated. It has a lower melting point than the solder alloy itself, and so it is activated before the solder melts. When activated, the flux will react with the metal oxides on the surface of the copper, removing them, and allowing the intermetallic bond to be created. Because copper oxides are soluble in the rosin flux, it is an effective solution to clearing them out to make way for our wetting process. Many solders are Flux cored, which means the center of the solder wire is filled with a flux that activates as you melt the wire.
Aluminum
Unlike copper, aluminum is not frequently used in electronics, if at all. Like all metals, when exposed to the air a protective layer of oxides is formed on its surface. However, this oxide layer is more tricky to deal with than the one found on copper for a variety of reasons. Aluminum oxides are less soluble in acids than copper alloys are and the oxide layer is formed more quickly on aluminum than it is on copper. So, not only is it more difficult to break down, it reforms almost instantaneously. While it isn't impossible to solder to aluminum, the process requires a solder and flux specifically designed for work with aluminum and will require cleaning the surface you are soldering rigorously with a steel brush before you begin.
Stainless steel
Similarly to our issues with Aluminum, stainless steel has a thick oxide layer to compete with. So our method would be much the same as that for Aluminum, requiring specialty products and a deliberate cleaning method to remove any grease or oxides present before applying our flux and beginning the soldering process. Because you need the strong acidic fluxes to work with steel, it is generally not recommended for electronics as the flux can breakdown the components.
Ultimately, from a chemistry perspective, each of these metals are solderable. However, the difficulty you might have in doing so and their use cases will vary, so it's important to keep in mind what your objectives are for the project you're working on, and select resources that best suit your project.
