Oxidation is a big problem with the harsh no-clean fluxes in many cheap lead-free solder wires. Switching to a rosin flux (RMA or RA) name brand solder should f

Oxidation is a big problem with the harsh no-clean fluxes in many cheap lead-free solder wires. Switching to a rosin flux (RMA or RA) name brand solder should fix that (did for me.) Metcal have a great big doc on tip care that covers this.