I'm glad to see some people know to use flux. For doing electrical work, a rosin flux cored solder is great but, when dealing with heavier stuff, a true flux can't be beat in getting solder to stick. I have used silver solders that claim flux isn't needed, only to have the joint fail. Something you can also do, in a pinch, is to use brake cleaner to clean the parts being soldered together. It does work but not as well as true flux