C_3F_8 plasma fluorination of lead free solders for fluxless soldering

摘要

Fluorination of Sn-3.6 wt.% Ag lead free solder was achieved using a C_3F_8 and O plasma treatment. X-ray photoelectron spectroscopy (XPS) was used to analysis fluorinated surface before and after oxidation in air. Plasma treatment formed SnF_2, Sn oxide, CO and CF_X groups such as CF, CF_2, CF_3 and C-CF_X on the surface. The CO and CF_X groups only existed on the outermost surface. The native oxide layer was 56 nm thick. Plasma treatment reduced this oxide layer to the value between 16 nm and 31 m. Fluorine diffused to a depth of 427 nm. Increasing exposure time to air reduced the SnF_2 peak in the top surface layer, in contrast, Sn oxide peak was increased. These results suggest that oxidation of the fluorinated surface took place by replacement of F with O atoms. Fluorine atoms diffused through the solder and reached a depth of 733 nm after 4 days. Wettability tests by reflow experiments in a nitrogen atmosphere showed that the affect of plasma treatment on fluxless solder bonding lasted at least 4 days in air.