Michael Osterman and Robert Utter
CALCE, Center for Advanced Life Cycle Engineering, Department of Mechanical Engineering, University of Maryland, College Park, Maryland 20740, USA
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