Abstract:

The susceptibility to corrosion of electroless nickel / electroless palladium / immersion gold (ENEPIG) and immersion silver (ImAg) surface-finished copper on printed wiring boards was examined using mixed flowing gas (MFG) testing. For the ENEPIG finish, two palladium thicknesses were examined: 0.05 um and 0.15 um. Optical images and high resistance measurements revealed that the tested ImAg-finished boards provided better corrosion protection than the ENEPIG-finished boards. SAC 305 soldered ENEPIG-finished boards with a loss of surface insulation resistance for 4 and 6 mil spaced pads protected with 0.15 um palladium were found to have a 50% lower failure rate than boards with 0.05 um palladium. However, no failures were observed in the 0.05 um palladium thickness ENEPIG boards that were reflowed with SnPb. Corrosion, which causes bridging between oppositely biased fingers, was observed on some of the ENEPIG-finished boards and was dependent on the exposure period inside the MFG chamber. Interdigitated comb fingers under plastic ball grid array (PBGA) packages were less susceptible to MFG-induced failures compared to similar structures without coverage. No creep corrosion was observed on the ImAg-finished boards. This work concludes that the ENEPIG finishes, especially those with 0.05 um palladium layers, are not suitable for corrosive environment applications as ENEPIG does not protect the underlying copper in a corrosive environment.