Why does solder smell of incense?
"Resin core" is incorrect, it is "Rosin core", and rosin is different, albeit from the same source. Pine tree resin is heated to remove the volatile elements in it such as turpentine, leaving behind a solid brittle substance called rosin. That rosin will melt again when heated but is difficult to burn and it contains a mild acid. When it melts, the acid etches the metal surfaces so that the solder sticks. If solder had resin inside, it would result in flames when you used it and leave behind carbon deposits.