If you are looking to build a circuit using copper pipe, then you may be unsure which kind of solder is the best. Most solders are available in two different types, the acid core and the rosin core. There is also a lead solder which differs in its make up from the other two, but most people prefer not to use a dangerous compound like lead in their pipework. If you want a secure solder, then you will be considering either acid core or rosin core for your copper pipe.
Acid core is a very aggressive solder, and may be too forceful for circuits and for older pipes. While it may be suitable for mending holes in aged pipes, it should be cleaned off totally once the seal is made. Acid core should never be used if you are sweating your pipes, rather than making a simple repair. It is best suited to use on plumbing work.
Rosin is considered to be more gentle on the pipe work which it is soldering, and is sometimes known as 'no-clean'. This kind of flux is dissolved in a combination of alcohol and water that make it more suitable for long-term use on a copper pipe. Anything conducting electricity should be soldered using rosin core.