Beginners may easily neglect some steps before soldering which will cause trouble. Here we detail a necessary step, that is to remove the oxide layer before soldering.
Suppose you use a piece of wire which has been stripped and sitting out in the air for a long time, you can see it looks dark and dirty, that’s because the copper that has been exposed to air for a long times forms an oxide layer on its surface and that oxide layer tends to disrupt the alloying process of soldering. So it’s important to be able to get rid of that oxide layer before you begin soldering.
There are two ways to do that. The first is to mechanically remove it. You can do that by using sand paper to abrade away the oxide. And the other way is to chemically remove it and for that, you can use a rosin core.
If you tin solder to the clean copper wire, after adding some rosin and heating it with the iron, you can see the smoke forming as the flux burns off and finally get a nicely tinned piece of copper. But if you try to do the same process with the oxidized copper, you’re going to run into trouble. The copper that won’t be properly soldered and the reason for that is the oxides that have formed on its surface tend to disrupt that alloying process. So in order to make a better solder joint, what we need to do is we need to remove that oxide layer.