How Solder Works in Electronics
Soldering is especially useful for electronics because not only does it create a strong physical connection between metals, but it also creates an excellent conductive path for electric current to flow from one conductor to another. This is because the solder itself is an excellent conductor.
Soldering refers to the process of joining two or more metal objects by heating them, and then applying solder to the joint. Solder is a soft metal made from a combination of tin and lead. When the solder melts, which happens at about 700° Fahrenheit, it flows over the metals to be joined. When the solder cools, it locks the metals together in a connection.
700° Fahrenheit is pretty hot — certainly hot enough to burn your skin instantly on contact — so soldering is an inherently dangerous task. It isn't extremely dangerous, as the amount of solder you actually use and the tools you use to solder are fairly small. So any burns you do receive are likely to be small. But they can be painful, so you should take great care whenever you're soldering.
You can create a reasonably good connection between two wires simply by stripping the insulation off the ends of each wire and twisting them together. However, current can flow through only the areas that are actually physically touching. Even when twisted tightly together, most of the surface area of the two wires won't actually be touching.
But when you solder them, the solder flows through and around the twists, filling any gaps while connecting the entire surface area of both wires.
Soldering isn't the same as braising or welding. Braising is similar to soldering, but instead of solder, metals with a higher melting point (usually 850° Fahrenheit or more) are used. Braising forms a stronger bond than soldering. Welding is an entirely different process. In welding, the metals to be joined are actually melted. In liquid form, the metals intermix and they cool to form an extremely strong bond.