390C is far more than hot enough, even for lead free solder.
Your tip is probably too small from the photos. You need a large tip with large thermal mass to store more thermal energy since it gets sucked into the workpiece and drops the temperature of the tip because the heating element can't pump heat in fast enough. A really hot, small tip with a lot of wattage behind it either won't heat up a large workpiece, or will damage things before the entire piece comes to temperature.
Also, use rosin flux.