Soldering is a process for joining metal items together by melting a flowing a filler into the joint between work pieces.
For basic soldering tips, watch this demonstration presented by GalcoTV:
For electronic purposes, soldering is the most common method for connecting components to circuits and other precision metalwork. Printed circuit boards in particular rely heavily on soldering.
In the soldering process, the filler metal is called solder. Specifically, solder is a metal alloy with a lower melting point than that of the work pieces it is meant to join together. Thus, heating the work pieces won’t damage them, while still allowing the solder to melt between them and fill the joint. When the solder cools and solidifies, the result is an electrically-conductive, water- and gas-tight joint.
The most common solder used for electronics is a tin-lead alloy. A 63/37 mix of tin and lead, respectively, is ideal because of its eutectic properties, meaning its melting point exists at a single temperature rather than over a range. The soldering process is expedited because the alloy has no plastic phase – there is quicker wetting of the solder as it heats up, and quicker setup as it cools. This reduces error and simplifies creating reliable joints. A tin-silver-copper solder has also become increasingly common as regulators push for lead-free solutions in electronics and manufacturing.
Accompanying the solder during most applications is a reducing agent called flux. Flux “cleans” the metal surfaces being joined by reducing metal oxides at the point of contact. This improves both the electrical connection and the mechanical strength of the joint. Flux may need to be cleaned from the surface after soldering, depending on the application. Some flux is corrosive, conductive, or reacts differently to high temperatures. In electronics, a rosin flux is most common because it is non-corrosive and non-conductive, but water-soluble flux is being used increasingly often to ease the cleaning process. Traditionally, flux is supplied within the solder wire, known as a flux-core solder. As the solder melts, the flux is allowed to flow with it onto the joint.
In the electronics industry, where many components are mass-produced, wave soldering or reflow soldering are preferred. This is especially the case with printed circuit boards. With wave soldering, parts are held into place with an adhesive and the whole assembly is over flowing solder. The solder is shaken into waves so the alloy is applied to various pins and pads but not the circuit board itself. For reflow soldering, a solder paste is applied to components that adheres them to attachment pads. The assembly is then heated by infrared lamp or passing it through an oven.
For some precise applications, or spot repairs, hand soldering is still routine. A soldering iron is used to heat the metals to be joined by pressing the iron’s heated tip to the joint. Heating is done electrically, by passing a current through a resistive heating element. Soldering irons come in several variations, from simple irons to cordless configurations. Soldering stations are common, providing temperature control and means to clean the iron tip.