>>941580376
make the solder stick to that shit first without joining it its called tinning, if it wont stick add flux and sit with the heat on it while it smokes and add solder. maybe turn the iron down if you have it on full.
it might be that the flux isnt suitable for the temps you are using anyway it might just be getting in the way idk
you can see if the solder has flux in in its cross section if you cut some and look, it'll have a core