don't want to "contaminate" the lead-free iron)
Why not you are not a factory having to conform to stupid regulations. Are you victim of the "lead solder is dangerous" delusion.
Lead solder is perfectly safe and produces more reliable joints, why do you think there is an exception for military and medical use?
The only potential issue with lead solder in mass consumer devices centers on the disposal and rotting in land fill of large amounts of solder and even their the legislators have a lot of muddled thinking. Lead in petrol was several million times more dangerous.