Soldering is a well known and widely used process where two or more metal items are joined together using a fusible alloy with a melting temperature that is lower than their own. The most commonly used solder is a fusible alloy consisting essentially of a tin and lead mixture. It is the solvent action (the solder actually dissolves a small amount of the metals surface, at a temperature that is well below its melting point and joins with it) of the solder alloy that causes it to fuse with and attach to the surface of the metal items being joined. The solvent action that takes place, between the solder and the metal, makes the joint chemical (not just physical) in nature and causes the properties of the joint to differ from the original solders properties and from those of the surface of the metal items being joined. When metal parts are joined by solder, a metallic continuity is established as a result of the interfaces where the solder is bonded to the metallic surfaces.
The metal joining process that is generally referred to as soldering (or soft soldering) requires temperatures between 361°F to 842°F. The joining of metals at temperatures above 842°F (and below the melting point of the metals being joined) is more commonly referred to as brazing (or hard soldering). The actual melting and fusing of the metal items that are being joined together is considered welding. There are, of coarse overlapping situations that may occur when classifying a process. The actual joining characteristics that take place are physically different in each of these processes. Soft solders attach to metals by what is referred to as a solvent action that takes place at relatively low temperatures. Hard solders, or brazing alloys contain metals that require higher temperatures to cause the solvent action to take place and fuse the alloy with the metal being joined. Because welding involves actually melting and fusing the surface of the metals that are being joined together, a filler, or fusible material is not always used.
Soldering is used primarily when the expected operating temperature of a joint will not exceed around 300oF and thermal or electrical continuity cannot be adequately achieved, or maintained, by the use of a mechanical joint. It is one of the most ideal methods available for the creation of a physical, electrical, or hermetically sealed bond between various metal items that are being joined together. Soldering is quite often used, in addition to other mechanical methods (twisting, crimping, etc.) to improve electrical continuity, to help protect the joint from the effects of vibration, or to encapsulate the joined metals preventing oxidation. Although soldering may be used to provide some minor support to an assembly, the solder should not (excluding sheet metal applications) be used as the primary mechanical support of a finished joint.
The soldering process may be accomplished in a wide variety of ways, but the four primary ingredients required will remain the same:
The base metal is the metal that is in contact with the solder and forms an intermediate alloy. There are many metals that will react willingly with solders to form a strong chemical and physical bond, while others can be very difficult, or even impossible to solder.
Flux is used to eliminate minor surface oxidation and to prevent further oxidation of the base metals surface during the heating process. Although there are many types of flux, each will include two basic parts, chemicals and solvents. The chemical includes the active portion, while the solvent is actually the carrying agent. It is the solvent that determines the cleaning method required to remove the remaining residue after soldering.
Solder is the alloy used to create the solvent action, which generates the bond between the base metals. The type and form of the solder is very important and must be determined by the individual application being performed, as well as the base metals and soldering method being employed.
It is important to match the soldering method and the equipment that will be used, to the soldering application that is being considered. There are several methods, as well as a wide variety of tools available to perform the task of soldering. Some of the current methods that are available include induction, conduction, ultrasonic, flame, dipping, resistance, oven and wave soldering. Some of these methods involve the use of small inexpensive hand tools, while others may require large and expensive machinery, equipment and tools. It is a good idea to become educated on the various methods and tools that are available, in order to insure that you are utilizing the best, safest, most efficient and economical means available for your specific soldering application.