Common Soldering Mistakes
One common soldering job might be to put a connector on something like this. The copper that has been exposed to air for a long time 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. Begin by adding some rosin, placing it in your vice, and heating it with your iron. You will have a nicely tinned piece of copper when a smoke forms as the flux burns off.
Now, if you try to do the same process with the oxidized copper, you’re going to run into trouble. As you heat the joint after adding the flux, your solder is still tries to find its way. That copper hasn’t been 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 you need to do is to remove an oxide layer.