Sorry for the general nature of this question. But I have always asked myself what makes solder fumes smell of incense.
closed as primarily opinion-based by Leon Heller, PeterJ, Enric Blanco, Dmitry Grigoryev, Voltage Spike Jul 5 '17 at 15:31
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"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.
Assuming that you use the standard spools of solder, look closely at the label. It will say "resin-core solder", or something like it. The solder actually contains a core of resin which acts as flux to make a good solder joint possible. It is the hot resin which you smell.