Morphology and Electrochemistry Drive Corrosion of Electroless Nickel Immersion Gold Films: A Multi-Technique Analysis
Authors: Fowler James Elliott , Rosenberg Samantha Gayle , Desai Choundraj Jahnavi , Rajendran Rupesh , Singh Preet M , Kacher Josh 
Published: November 2022

Electroless nickel immersion gold (ENIG) is a common surface finish for high-reliability microelectronics circuitry due to its good solderability, corrosion resistance, layer thickness uniformity and applicability for fine pitch applications1. ENIG deposition is a multistep process in which a 3 to 6 micron (118-236 μ-in) layer of electroless nickel (i.e. Ni-P) is deposited onto the substrate material (typically copper), followed by a galvanic displacement reaction replacing the surface layer of Ni with 0.04 to 0.1 microns (1.6 to 4 μ-in) of gold (Au). Despite ENIG’s widespread use, it continues to suffer from a failure mode known as “black pad” or Ni hypercorrosion during immersion gold deposition2. A cohesive mechanistic explanation for the occurrence of Ni hypercorrosion is not yet established despite extensive study. Recent approaches have attempted to characterize in-situ the influence of bath chemistry; however, the competing influence of all variables of the ENIG process (e.g. temperature, pH, solution chemistry, volume mixing, surface morphology, etc.) make global mechanistic conclusions challenging if not impossible experimentally3. Instead, the work here aims to develop tools to assess an ENIG film of unknown origin or deposition conditions for the risk of corrosion. Therefore, this study utilizes a combination of surface characterization via x-ray photoelectron spectroscopy (XPS) depth profiling and transmission electron microscopy (TEM) imaging and electrochemical characterization via cyclic polarization (CP) and scanning vibrating electrode technique (SVET) to build a multi-technique analysis of ENIG failures.

ENIG samples with identical geometries and plating requirements from two domestic vendors (A & B) were obtained. XPS with argon ion sputtering allows for comparison of atomic concentration of Ni, Au and P in ENIG films through their depth as a function of the number of sputter cycles. XPS results show that Vendor A films have a high concentration of Ni in the Au layer and a lower concentration of P near the Au-Ni interface, whereas Vendor B films have a low concentration of Ni in the Au layer and a higher concentration of P near the Au-Ni interface. This indicates a more porous morphology in Vendor A versus B films. SVET is an electrochemical scanning probe tool which allows for real-time observation of cathodic and anodic currents (in a 0.35% NaCl solution) representing existing and developing defect sites on ENIG surfaces. SVET experiments show that Vendor A and B samples exhibit preferential anodic currents on ENIG surfaces corresponding to Ni hypercorrosion defects. Focused ion beam (FIB) lift-outs from SVET samples viewed in TEM give ex-situ confirmation that SVET observations align with the real metal layer structure and chemistry as well as confirming the presence and location of corrosion products. CP curves for samples of each film in a 0.35% NaCl solution demonstrate that both Vendor A and Vendor B films exhibit effective corrosion rates above 1.0 mil per year which is on the order of bare Cu wire in the same electrolyte. Overall, a distinct difference in film morphology between Vendor A and B samples exists; however, both films exhibit substantial electrochemical activity in solution indicating corrosion susceptibility. Taken together, these results show that Ni hypercorrosion can be driven by at least two distinct mechanisms: high porosity in the Au film leading to enhanced Ni migration and reaction at the Au surface; or, P enrichment at the Au-Ni interface leading to enhanced P migration and a more electronegative surface composition. Future work will build out characterization of additional ENIG samples to establish a failure matrix for ENIG films, encompassing a wider variety of unique mechanisms.

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