A soldering iron is a simple device for applying heat to a specific point. The intention is to melt solder at this point, joining two materials. This joint is then allowed to cool, thus soldering the two materials together. Generally, soldering is used to create an electrically conductive joint between electrical components and a circuit board. It should be noted that although the term soldering also refers to joining pipes, a plumber would use a blowtorch rather than a soldering iron.
Solder is generally a mixture of tin and lead with a melting point just below 190 degrees Celsius. The tip of the soldering iron, therefore, must reach this temperature, and the most common method of achieving this is with electrical power. Other options include butane powered soldering irons, which have the advantage of being cordless, and there are other devices available which are designed to be heated in a furnace.
Depending on the type of work at hand, differently shaped tips can be fitted to the soldering iron. For working with fine electronics components, a slender and rounded tip is useful, whereas a triangular, flat face is useful for work with sheet metal. The tip itself should be coated with a thin layer of solder, a process known as tinning. This ensures a good transfer of heat from the tip of the iron to the surface being soldered, and keeps the tip in good condition.
The high temperatures at which soldering takes place means that certain safety considerations must be taken into account. The majority of soldering irons feature a hook by which the device can be hung whilst hot, or come with a stand into which the iron should be placed when not in use. Additionally, there is always risk of the solder splashing, and so protective goggles are essential.
Although soldering irons come in a range of sizes, the majority of these are visible with the naked eye. New Scientist, however, recently reported the smallest device in the world. Formed from carbon nanotubes with a diameter of 20 millionths of a millimetre. Gaseous iridium is condensed into solid droplets between one and ten nanometres wide, then electrically forced along the nanotube's surface where they collect as a bubbling liquid. The inventor, Alex Zettl, plans to use them to solder tiny parts together, enhancing current work on a 'nano-structure production line'. | 
