Soldering is the act of joining two pieces together with a melted component. Solder can seal the links of a tiny necklace chain or join giant pipes on a ship. Soldering is different from welding. In welding, the pieces to be joined are actually melted together. Solder, though, is the material that actually melts between the two solid pieces, forming a joint, of which there are several different kinds.
In a butt joint, two pieces of metal are joined with their ends abutting each other. The ends must be clean and flat in order for the solder to adhere properly to both edges. This joint doesn't have a lot of strength, and shouldn't be used for any type of load-bearing project. However, it's useful for smaller projects. A butt joint is often used to close jump rings in jewelry.
Strapped Butt Joint
When a supportive piece of metal is soldered over or under the butt joint, it becomes a strapped but joint. The added support makes the butt joint stronger. Pieces soldered over both sides of the butt joint seam are said to be joined in a double-strapped butt joint.
A scarf joint is similar to a butt joint, but instead of the metal joined edge to edge, both pieces are cut on a diagonal. This gives more surface area for the solder to join. Ultimately, this makes a stronger joint. When both pieces are cut in a stair-step pattern, the joint is called a step joint.
A lap joint is formed when two pieces overlap each other and solder is applied between the two pieces. The strength of this joint depends on how much of an overlap is created. The larger the overlapped area, the stronger the soldered joint.
Side Seam Joint
Similar to the lap joint, the side seam joint is stronger than a simple lap joint. The extra strength is due to the fact that one piece is folded over, giving the joint three layers rather than two. You might recognize this joint from the side seam of tin cans.
Pipes need to be soldered together to provide a water-tight seal. The pipe joint fulfills this need. The pipes fit as one is telescoped into the other and the joint is filled with solder. In this case, a heat gun is used to melt the solder and seal the joint. Heat guns have a small flame that can be concentrated and controlled, in contrast to the heated wand of a soldering iron.