Effects of PCB Substrate Surface Finish, Flux, and Phosphorus Content on Ionic Contamination

摘要

The ionic contamination on printed circuit boards (PCB) having different surface finishes was examined using ionograph. The study was performed at the RT on three types of PCBs covered with: (i) hot air solder leveling (HASL LF), (ii) electroless nickel immersion gold (ENIG), and (iii) organic surface protectant (OSP), all on Cu substrates, as well as two types of fluxes, namely EF2202 and RF800. In the group of boards without soldered components, the lowest average value of contamination was for the ENIG 18 A mu m surface (0.01 mu g NaCl/cm(2)). Boards with soldered components were more contaminated (from 0.29 mu g NaCl/cm(2) for the HASL LF 18 A mu m surface). After spraying boards with fluxing agents, the values of contaminants were the highest. The influence of phosphorus content in Ni-P layer of ENIG finish on ionic contamination was examined. In the group of PCBs with Au coating, the smallest amount of surface contaminants (0.32 mu g NaCl/cm(2)) was for Ni-2-5%P layer. PCBs with Ni-11%P layer were higher contaminated (0.47 mu g NaCl/cm(2)), and another with Ni-8%P layer had 0.81 mu g NaCl/cm(2). PCBs without Au coating, had the lowest contamination (0.48 mu g NaCl/cm(2)) at phosphorous content equal 11%P. Higher contamination (0.67 mu g NaCl/cm(2)) was at 2-5%P, up to 1.98 mu g NaCl/cm(2) for 8% of P. Boards with Au finish have lower value of contamination than identical boards without Au layer thus contributing to better reliability of electronic assemblies, since its failures due to current leakage and corrosion can be caused by contaminants.