I bring to your attention already 83 questions for the heading "question-answer". This question came from the reader Alexei with just such a title (I did not change it). If you have your own answer to the question, then write it in the comments below. I and other readers of my channel will be happy to read it.
The text of the question itself (literally) is as follows:
I made a soldering iron from a ceramic resistor. Only I didn't have a copper bar, but an aluminum one. The solder melts perfectly, but does not get tinned (does not stick to the sting). Why?
After considering the issue, I consulted the reader within the framework of my knowledge and qualifications as follows:
Everything is correct. Due to the oxide film formed on the surface of aluminum, tin does not adhere to it. The situation can be corrected very simply and without any special tools. I am soldering aluminum in the following way. First, I clean the metal surface to a shine and then, with a well-heated soldering iron, with a lot of rosin, I rub the aluminum in the right place. The task of rosin is to prevent contact of aluminum with oxygen. After a while, tin adheres to aluminum no worse than to copper.
But in general, tin-lead solder does not wet aluminum. This is the property of aluminum - the solder does not wet it. The fact is that, with the exception of a deep vacuum, an oxide layer is always formed on the surface of aluminum - very thin - a few nanometers - but strong and stable. It is not "taken" by any fluxes, except for those containing hydrofluoric acid, it is very difficult to destroy it mechanically.
And it separates the molten solder from the metallic aluminum. Therefore, aluminum is not soldered or tinned. Using special fluxes with fluorides and an abrasive component, it is possible to irradiate aluminum, but even under the solder layer, the oxide layer is formed again due to the oxygen dissolving in it.
To understand why aluminum is badly tinned, you need to study physics a little deeper. The distances between the atoms of the crystal lattice of unmelted aluminum are too small for the "large" atoms of tin and lead. For example, the most common tin-lead solders have been created for copper tinning.
The molten solder atoms easily diffuse into the copper. If you make a "cut" of tinned copper, you can distinguish three layers: copper - copper alloy with solder - solder. To reliably solder aluminum, you need to use special solders containing atoms comparable to the size of the crystal lattice, for example, containing zinc. And to solder aluminum with POS is akin to nailing something to a concrete wall, it seems to hold, but it can fall off.