Soldering - Processes

Processes

There are three forms of soldering, each requiring progressively higher temperatures and producing an increasingly stronger joint strength:

1. Soft soldering, which originally used a tin-lead alloy as the filler metal,
2. silver soldering, which uses an alloy containing silver,
3. brazing which uses a brass alloy for the filler.

The alloy of the filler metal for each type of soldering can be adjusted to modify the melting temperature of the filler. Soldering differs from gluing significantly in that the filler metals alloy with the workpiece at the junction to form a gas- and liquid-tight bond.

Soft soldering is characterized by having a melting point of the filler metal below approximately 400 °C (752 °F), whereas silver soldering and brazing use higher temperatures, typically requiring a flame or carbon arc torch to achieve the melting of the filler. Soft solder filler metals are typically alloys (often containing lead) that have liquidus temperatures below 350°C.

In this soldering process, heat is applied to the parts to be joined, causing the solder to melt and to bond to the workpieces in an alloying process called wetting. In stranded wire, the solder is drawn up into the wire by capillary action in a process called 'wicking'. Capillary action also takes place when the workpieces are very close together or touching. The joint's tensile strength is dependent on the filler metal used. Soldering produces electrically-conductive, water- and gas-tight joints.

Each type of solder offers advantages and disadvantages. Soft solder is so called because of the soft lead that is its primary ingredient. Soft soldering uses the lowest temperatures but does not make a strong joint and is unsuitable for mechanical load-bearing applications. It is also unsuitable for high-temperature applications as it softens and melts. Silver soldering, as used by jewelers, machinists and in some plumbing applications, requires the use of a torch or other high-temperature source, and is much stronger than soft soldering. Brazing provides the strongest joint but also requires the hottest temperatures to melt the filler metal, requiring a torch or other high temperature source and darkened goggles to protect the eyes from the bright light produced by the white-hot work. It is often used to repair cast-iron objects, wrought-iron furniture, etc.

Soldering operations can be performed with hand tools, one joint at a time, or en masse on a production line. Hand soldering is typically performed with a soldering iron, soldering gun, or a torch, or occasionally a hot-air pencil. Sheetmetal work was traditionally done with "soldering coppers" directly heated by a flame, with sufficient stored heat in the mass of the soldering copper to complete a joint; torches or electrically-heated soldering irons are more convenient. All soldered joints require the same elements of cleaning of the metal parts to be joined, fitting up the joint, heating the parts, applying flux, applying the filler, removing heat and holding the assembly still until the filler metal has completely solidified. Depending on the nature of flux material used, cleaning of the joints may be required after they have cooled.

Each alloy has characteristics that work best for certain applications, notably strength and conductivity, and each type of solder and alloy has different melting temperatures. The term silver solder likewise denotes the type of solder that is used. Some soft solders are "silver bearing" alloys used to solder silver-plated items. Lead based solders should not be used on precious metals because the lead dissolves the metal and disfigures it.