Not just the wire...the entire joint needs to be heated equally (ideally) and then the solder is touched to the heated joint. Any time you touch the solder to the iron it will burn off the flux and cause cold joints.
Never put solder on the iron tip...although...I do it sometimes to increase heat transfer to larger joints (emulates a larger tip) but you can expect that the flux has burned off and may require liquid flux and reflow.
Sidenote: if you coat your tips with solder before shutting off your iron it will prevent oxidation and increase tip life.
Never put solder on the iron tip...although...I do it sometimes to increase heat transfer to larger joints (emulates a larger tip) but you can expect that the flux has burned off and may require liquid flux and reflow.
Sidenote: if you coat your tips with solder before shutting off your iron it will prevent oxidation and increase tip life.