Nickel palladium gold (ENEPIG) surface finish is being referred to as the “universal finish.” The finish is an excellent soldering surface, forming a Ni/Sn intermetallic (IMC). It is a wire-bondable surface for both gold and aluminum wire. It also serves as a good contacting surface. ENEPIG was also the answer to the nickel corrosion “black pad” occasionally encountered with electroless nickel/immersion gold (ENIG) deposits.
As the finish gained more market share due to its flexibility—particularly as a gold wire bonding surface—occasional bond failure was observed in cases where the dwell time in the immersion gold was extended in an attempt to meet design requirements of > 3.0 µins (0.075 µm) of gold. The failure was manifested as wire bond lifts. Failure analysis of the failed bonds showed a separation at the Ni/Pd interface. The nickel surface was black and was clearly corroded “black pad.” Nickel corrosion in ENIG occurs in the immersion gold deposition step and is usually the result of a compromised nickel surface (uneven), in combination with an aggressive immersion gold bath (low gold concentration, low pH) and an extended dwell time in the gold bath. The extended dwell time is used in instances where the design requirements demand a thicker immersion gold. How can the nickel capped with an electroless palladium layer corrode when it is theoretically not available to the immersion gold step?
Immersion gold deposition is a displacement reaction where one atom of nickel metal is oxidized to the nickel ion giving up two electrons. The two electrons are picked up by two positively charged gold ions in solution and, in turn, are reduced to the metal and deposited on the surface of the nickel substrate.
The driving force of the oxidation-reduction displacement reactions can be derived from the electromotive series. The electromotive series is a listing of chemical species (atoms, molecules, and ions) in the order of their tendency to gain or lose electrons (be reduced or oxidized, respectively), expressed in volts and measured with reference to the hydrogen electrode—which is taken as a standard and arbitrarily assigned the voltage of 1.0 v.
To read this entire column, which appeared in the May 2020 issue of PCB007 Magazine, click here.