Solder is a fairly low-heat, convenient way to join metal bits together. It requires less equipment than welding, and it can be done safely without much training. Along with rosin core solder, acid core solder is one of the two most important types.
Acid core solder is used to attach pieces of sheet metal or plumbing together. The acid core is a type of flux—a chemical agent that helps create a stronger bond between the pieces being joined. A thin section of flux is surrounded by a combination of tin and lead, silver or other metals depending on what the solder is used for.
Solder melts at a fairly low temperature and bonds well to metals, which makes it useful for joining different pieces of metal together. Unfortunately, it does not bond well to rust and other metal oxides. Metals left sitting can build up a layer of oxidization. Heating the metal can also cause oxidization to build up. The flux dissolves the oxide layer, then burns off. This allows the solder layer to form a strong, metal-to-metal bond.
The technician does not heat the solder directly. Instead, he uses a soldering iron, torch or other implement to heat the metals where he wants to join them. He then touches the solder to the metals while continuing to heat the joint. The solder will melt along with the flux core and flow onto the connection. It will start to smoke as the flux burns off, taking the oxidization with it. Once it stops smoking, the heat is removed and the solder cools and joins the metal pieces.
Any dirt or contamination can stop the solder from forming a good connection. If the metal is very dirty or rusty, the technician may clean it, sand it, or apply a separate layer of flux. The soldering iron will also build up a layer of oxidization. It will need to be cleaned frequently with steel wool and/or cleansers to make sure it doesn't contaminate the soldering joint.
The main benefit of acid core solder is the convenience. Because the flux comes wrapped up in the solder, it can all be applied together. Without a rosin core, the flux has to be applied separately, which is more time-consuming. A soldering job can involve a lot of individual connections, so saving a little time on each can really add up.