Nickel corrosion in ENEPIG (electroless nickel/electroless palladium/immersion gold) deposits has been reported on multiple occasions. This was originally observed in cases where the desired gold thickness was in excess of 2.0 µin (0.05 µm). The boards were left in the gold bath for an extended dwell time until the desired gold thickness was achieved. In those cases, the gold ion reaches through micro-pores, that are common in thinner palladium deposits, below the palladium substrate to the underlying nickel and continues to deposit, corroding the nickel.
This article is an attempt to reproduce the defect, and to determine the necessary mitigation action to avoid nickel corrosion in the ENEPIG finish.
Nickel corrosion, also referred to as black pad, has been associated with the electroless nickel/immersion gold (ENIG) surface finish. Nickel corrosion occurs during the immersion gold deposition step. It occurs when the nickel deposit is compromised (uneven with deep crevices) in combination with an extended dwell time in an immersion gold bath, particularly if the bath is run beyond its intended life and outside the specified operating parameters such as temperature, pH and chemical concentrations. The expectation with ENEPIG is that these conditions would not be available due to the presence of the electroless palladium layer. However, nickel corrosion was observed in some instances with ENEPIG. Excessive nickel corrosion in an ENEPIG compromises the solderability of the surface finish and can give rise to wire bond lifts at the nickel interface.
The immersion gold reaction is an exchange reaction between the gold ions in solution and the substrate basis metal. The substrate metal (Ni or Pd) is oxidized to the respective metal ion, giving up electrons. The gold ion picks up electrons and is reduced to the gold. The driving force for these reactions follows the electromotive series.
To read the full version of this article which originally appeared in the August 2018 issue of PCB007 Magazine, click here.