Solder, usually an alloy of lead and tin, has a low melting temperature. Placed between two metallic objects and heated, solder melts and wets the two metallic surfaces. On cooling, solder forms a bond between the two objects. Originally, a heated iron piece brought the solder to its melting point, hence the name soldering iron. Later, people found copper to be a better replacement for iron piece.
People working with electronic components are the biggest users of the soldering iron today. To suit their needs, the soldering iron has had to undergo several improvements. The latest models can be those of the uncontrolled type or ones where the user can set the temperature of the tip.
The simplest form of uncontrolled soldering iron has an insulated hollow handle that has an electric cable passing through. One end of the cable terminates in a resistive heating coil wrapped around an iron rod, but insulated from it by a layer of mica. A metal tube attached to the handle and insulated from the heater protects the user from the heating coil. At the front of the iron rod, there is a special copper tip that heats up when electricity is allowed to flow through the heating coil. When the tip is sufficiently hot, it is able to melt a solder.
There are several disadvantages of the uncontrolled soldering iron. The continuous heating of the tip causes a layer of oxides to form on it, reducing its ability to melt solder, unless the oxide layer is frequently scraped off. Solder reacts with the impurities in the copper tip and causes pits to appear on its surface. This requires occasional filing to keep the tip free of pits. Some components are temperature sensitive and can be damaged because the soldering iron tip touching them while soldering is much hotter than they can withstand.
The above disadvantages led to the development of temperature-controlled soldering irons, where a temperature feedback from the tip controlled the power fed into the heater, enabling the tip to remain at a certain temperature. By controlling the amount of feedback, the user now had the ability to allow the tip to become cooler or heat up further.
The tip too underwent a lot of change. Rather than use a copper piece throughout, the tip was made with multiple layers of different metals, such as iron, aluminum, and hardened copper. A soldering station helped to house the control electronics to set and adjust the tip temperature, as well as to detect when the iron was actually resting between two bouts of active soldering so the control electronics could reduce the tip temperature at times of rest. As soon as the user picked up the soldering iron, the control electronics pumped in more power input to bring the tip temperature quickly up to the set point.
Earlier, the control electronics for the soldering iron was predominantly analog. The latest models feature a digital control. Analog control was simple where the user could turn a knob to set the temperature. However, the digital ones have more features. Apart from a digital display showing the tip temperature in either Centigrade or Fahrenheit that the user can select, the soldering stations usually have a few preset selections.