This chart shows some common metals along with their varying degrees of soldering difficulty. The higher the difficulty, the tougher the metal oxides are to remove. Kester flux-cored wire formula numbers are shown on the right of the slide and are highlighted with the color corresponding to their ability to solder to the various metals. Most electronic applications will be within the “easy to solder” category; however, nickel is commonly found as a finish on components and connectors.