Most bits are made of copper, which is suitable because of its excellent thermal conductivity and high heat content per volume. Some bits are plain copper, while others incorporate various additives or have a protective plating applied.
One of the most common problems associated with plain copper bits, is that tin-lead alloys (more specifically the tin in the alloy) will attack the copper, dissolving it away. This makes it necessary to continually file the bits to maintain the required shape, giving these bits a shortened working life. Another concern is the amount of impurity that is imparted to the solder joint when using bare copper bits.
Adding tellurium to the copper improves both wear and oxidation resistance, but does not protect the tip from rapid deterioration. It has been determined that both iron and nickel, despite their low conductivity, are wettable, offer a high level of resistance to erosion and their heat per volume is close to that of copper.
Because of these facts it is possible to maintain good conductivity, while increasing the erosion resistance by plating copper bits with either nickel or iron. These plated bits are generally referred to as nickel-clad, or iron-clad and make up a large majority of the bits in use for modern soldering applications.
Courtesy of American Beauty Tools