How brazing and Soldering Work
Brazing and soldering join materials, usually metals, together by flowing a filler metal into the joint. In both cases the filler metal has a lower melting point than the base metals being joined. And for both metal joining processes, capillary, or the ability of a liquid to be drawn through a narrow gap, is the driving force. Gaps can vary, but in general they tend to be in the range of .002-.005" for best results.
So what is the difference between brazing and soldering? The American Welding Society (AWS) defines brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C) and below the solidus of the base metals.
Soldering has the same AWS definition as brazing, except that the filler metal used has a liquidus below 840°F (450°C) and below the solidus of the base metals. Soldering can be considered the low-temperature cousin to brazing.
Comparative Advantages. Although there are similarities between brazing and soldering, the temperature difference between the processes yields different behavior. Base metals involved in soldering are typically stronger than the solder itself; under the stress and fatigue of service, failure may occur through the solder joint. This means a that a soldered assembly may exhibit less joint strength and lower fatigue resistance than a brazed assembly
Should you braze or solder?
There are many factors that impact this decision including the service loading and temperature to name two. Many delicate substrates are damaged by the high temperatures required by brazing and fare better when combined with the lower temperatures of soldering materials. Wettability of the substrate by either the solder or brazing filler metal is another key consideration in selecting the appropriate process. The ability to remove flux residue can be an important factor such as in certain HVAC and other fluid transport systems; closed loop systems which cannot be readily cleaned after joining must often be brazed or soldered in vacuum or under a protective atmosphere, or a self-fluxing filler metal such as the Lucas Milhaupt Sil-Fos alloys (BCuP-5) in copper-based assemblies must be used.
And what about other options? Mechanically fastened joints (threaded, staked or riveted) generally don’t compare to brazed joints in strength, resistance to shock and vibration, or leak-tightness. Adhesive bonding and soldering will provide permanent bonds, but generally, neither can offer the strength of a brazed joint –equal to or greater than that of the base metals themselves. Nor can they, as a rule, produce joints that offer resistance to temperatures above 200°F (93°C). When you need permanent, robust metal-to-metal joints, brazing is a strong contender.