Novel surface coatings to enable fluxless soldering of copper surfaces [PCBs]

In this paper, novel organic coatings have been produced for the preservation of active copper surfaces to enable soldering without the need to use flux. These coatings are effective in protecting the metal surface from oxidation during exposure in air for up to several weeks, however, they are displaced by the soldering process, allowing the oxide free copper to be wetted. An intensive study has been carried out to investigate the properties of the coatings, in particular, the time dependence of the solderability of such coated copper surfaces. Wetting balance testing under a nitrogen atmosphere has been used to measure the wetting force of preserved copper coupons stored in air. Both eutectic Sn-Pb and Sn-Ag-Cu lead free solder has been used. Copper coupons stored in air at different temperatures for up to three months have been tested to investigate the mechanism of the preservation gained from the coatings. XPS has also been employed to monitor the amount of oxide formed on the preserved copper surface due to prolonged exposure in the air. The storage environment can significantly affect the degradation of the coated surface, and its solderability.