Soldering is the process of joining metal parts by means of a molten filler metal (solder) whose melting point is lower than that of the metals to be joined. The latter are wetted by the molten filler without themselves being melted (as in welding). The solder is employed in the form of rods, wires, strips, sheets, granules, powder or paste.

In contrast with welding, different metals can be joined by soldering. A distinction is made according to the type of solder employed :
1. Soft solders usually a mixture of lead and tin
2. Hard solders, which comprise brass solders copper-zinc alloys, silver solders, copper solders, nickel silver solders, solders for light alloys, etc.

The solder must be suitably chosen in relation to the metals to be joined. In particular, the melting point of the solder should be well below that of the metals. In soft soldering, the heat may be supplied by a soldering iron (Fig.1) or a blowpipe. Another method consists in placing the assembled work on a plate along with a piece of solder and flux. The work is then heated in a furnace, so that the solder melts, and is then allowed to cool. The last-mentioned technique may also be used in hard soldering. More usually the heat to melt a hard solder is supplied by a blowpipe (Fig.2) or torch.

In certain cases heat generated by electrical resistance is used. The term brazing is applied more specifically to a form of hard soldering using brass i.e., a copper-zinc alloy as the jointing medium. A flux is generally employed in conjunction with the latter. In brazing, borax is used as a flux; it serves as a means of preventing the formation of an oxide coating on the joint faces, as a cleaning agent, and as a wetting agent to aid the flow of the molten metal. In some cases a shielding gas may be used to prevent oxidation of the faces of the joint. Dip brazing is a technique in which the assembled parts to be joined are immersed in the molten jointing medium (Fig.3). It is widely used in industrial mass-production processes. In other industrial techniques the workpiece, provided with solder at the joint, is heated to the appropriate soldering temperature by immersion in a salt bath (Fig.5) or an oil bath.

In another method, the molten solder is poured through the highly heated joint until the metal cools and unites the two parts (Fig.4). In electrical-resistance soldering (Fig.6), the solder, flux and workpiece are heated between tungsten or copper electrodes. Induction soldering (Fig.7) utilizes a high-frequency alternating current to induce a heating current in the workpiece. A more recent method is ultrasonic soldering, which is used, for example, for the soldering of aluminum. The ultrasonic vibrations are transmitted by a nickel rod through the solder on to the surface of the workpiece, destroying the oxide film on the aluminum.

Soldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a relatively low melting point. Soft soldering is characterized by the melting point of the filler metal, which is below 400 °C (800 °F). The filler metal used in the process is called solder.

Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is distinguished from welding by the base metals not being melted during the joining process. In a soldering process, heat is applied to the parts to be joined, causing the solder to melt and be drawn into the joint by capillary action and to bond to the materials to be joined by wetting action. After the metal cools, the resulting joints are not as strong as the base metal, but have adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an ancient technique mentioned in the Bible and there is evidence that it was employed up to 5000 years ago in Mesopotamia.