Soldering and welding are often used interchangeably in metal fabrication, but each technique is different and has a distinct application. While both methods require the use of heat, soldering and welding are two very different procedures for securing two or more pieces of metal together.
There are several basic methods used to join two or more pieces of metal, or sometimes other materials, together. Without applying heat, the options include: gluing, via epoxies, polyurethane, or silicon-based glues, which tend to work well for joining lightweight metal parts; and the mechanical method of fastening using nuts, bolts, and washers. Riveting is a method of joining metal together that can be used with or without heat, depending on the project. Overlapped sheet metals are joined by a straight piece of metal (the rivet) inserted through both sheets, which is then formed over the connection, joining the sheets securely together. Smaller rivets can be secured solely by using force, like with the use of a rivet gun, whereas larger and thicker rivets may require heat to be applied first before it is pliable enough for forging. The remaining methods of joining metal together are soldering and welding, which both require heat. Soldering and welding are often used interchangeably in metal fabrication, but each technique is different and has a distinct application, and it is crucial to use the appropriate procedure.
While there are different techniques for welding, they all involve the same basic principle: the application of extremely high heat (we’re talking upwards of 3000°C!) to metal components causes the metal to melt and permanently fuse together. Welding creates the strongest bond possible between two pieces of metal, allowing the joint to withstand all types of stresses and strains, which makes the process ideal for car bodies, industrial pipes, and shipbuilding. However, there are some limitations, one of the biggest barriers to the welding process is that both metals must be similar. Differences in the melting points or electrochemistry of two metals can make welding difficult or nearly impossible, for instance, titanium and steel cannot be joined by fusion welding.
Soldering, in contrast, is a method of using a filler metal to join two others. Unlike in welding, the base metals remain solid throughout the process, and rather it is the filler material (the solder) that melts and fills the area between the base metals, joining them together. Soldered bonds are not as strong as welded ones, simply because soldering does not form a very strong mechanical connection, but an electrical one instead. However, soldering allows for any two metals to be connected together, as long as the solder material is below the melting point of both, and at relatively ‘low’ temperatures (generally below 450°C). This can be advantageous as the joint can be easily melted, separating the connection, and allowing for rework.
In terms of strength and reliability, welding stands in contrast to soldering, but is entirely improper for delicate electronics where excess heat can damage the components. Using a solder material is perfectly suitable to join the base metals mechanically, securing the connection so it does not break loose due to vibrations or other mechanical forces, and electrically, so the electronic signal can travel through the connection without interruption. No matter the specific function, a stable mechanical and electrical connection between the component and substrate is vital to the overall performance of the device. A poor solder connection can lead to an unreliable solder joint, causing issues such as delamination, bridging, insufficient heat transfer, and many more.