Oxidation behavior of ENIG and ENEPIG surface finish

The worldwide transition to lead-free electronics has increased the usage of several lead-free pad finishes for electronic packaging industry, including ENIG (Electroless nickel immersion gold), ENEPIG (electroless nickel electroless palladium immersion gold), immersion Sn, immersion Ag, and OSP (Organic solderability preservatives). The ENIG process generally involves cleaning the bare Cu, etching, applying a catalyst, acid dipping, and electroless Ni and immersion Au deposition. If an electroless plating of Pd is applied between the electroless Ni plating and the immersion Au plating, then the finish will become Ni/Pd/Au (ENEPIG). Copper is the commonly used metallization material for PCBs (printed circuit boards). Copper can have good solderability; however, it oxidizes rapidly when exposed to the air. This oxidation quickly reduces the wettability and solderability, and raises contact resistance which leads to electrical failure. ENIG and ENEPIG surface finish are then used as a final surface finish of PCBs or UBMs for solder bumps/balls because electroless Ni serves as a diffusion barrier for Cu metallization, and Au provides good solderability and oxidation resistance. However, after annealing we could find that there existed Ni oxides on the ENIG surface. The existence of Ni oxides could lead to non-wetting of the solder even with strong flux. To solve this problem, reducing the Ni oxides, we replace ENIG by ENEPIG surface finish. In this paper, we put the ENIG and ENEPIG surface finish samples to study their oxidation behavior and annealing effect. After cutting and cleaning, we put ENIG and ENEPIG surface finish samples into the oven for Oh and 200h at 150°C to study the influence of annealing. By using AES (Auger electron spectroscopy) and SIMS (Secondary ion mass spectrometry), we investigate element species and element distribution, and determine the composition of oxides on the surface. The AES mapping and depth profile results indicated that EN- - IG sample were covered with a continuous layer of nickel oxides, and grew thicker after annealing; for ENEPIG, there existed Ni and Pd oxides and grew thicker after annealing. However, total nickel oxides thickness of ENEPIG is thinner than that of ENIG under the same annealing condition. This result indicates that Pd suppresses the growth of Ni oxides on surface. SIMS results showed the compositions of oxides on ENIG surface were NiO and NiO₂, and those of ENEPIG were NiO PdO and PdO₂.