The Hakko FS100-01 Tip Cleaning Paste re-tins oxidized soldering iron tips and is completely lead-free. The flux does not contain diammonium phosphate and does not leave a corrosive residue. (Conforms to JIS-Z-3197 6.6.1 test method.)
For details on how to use this product, see the user instructions (2MB pdf).
Note: This product is Hakko item number FS100-01.