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46 The PCB Magazine • May 2015 releasing the electrons that reduce the positively charged ions of the second metal present in so- lution. Driven by the electrochemical potential difference, the metal ions in solution (e.g., gold ions in ENIG or ENEPIG process) are deposited onto the surface of the board, simultaneously displacing ions of the surface metal into solu- tion. This type of reaction will continue as long as the base metal is available to supply electrons to the displacement reaction. ENEPIG has the advantage over ENIG of be- ing a gold wire bondable surface. ENIG is not an ideal gold wire bonding surface because over time the electroless nickel will diffuse through to the surface along the crystal boundaries of the immersion gold and the wire would not stick. ENEPIG's palladium layer is a diffusion barrier to the nickel, the gold remains unadul- terated and is bondable with gold thickness as low as 1.2 μin or 0.03 microns. A gold coating as thick as 0.1–0.2 microns or 4–8 μin would open the wire bonding operating window. Thicker gold is beyond the capability of standard im- mersion gold. If a thicker gold is specified, then modi- fied immersion gold, such as reduction-assisted immer- sion gold, would be the pre- ferred choice. The electroless palladium requires a reducing agent. The most commonly used reduc- ing agent is sodium hypo- phosphite, which produces a phos-palladium deposit with 4–5% phosphorous in the de- posit. The presence of phospho- rous renders the palladium deposit amorphous (non-crystalline), making it the ideal diffusion barrier. Other reducing agents may produce a non-phos palladium, which tends to be crystal- line in structure. The ENEPIG IPC-4556 Specification 2013 For ENEPIG thickness, IPC-4556 states: Nickel: 3–6 µm [118.1–236.2 µin] at ± 4 sig- ma (standard deviations) from the mean. Palladium: 0.05–0.15 µm [2–12 µin] at ± 4 sigma (standard deviations) from the mean. Gold: Minimum thickness of 0.025 µm [1.2 µin] at - 4 sigma (standard deviations) below the mean. All measurements to be taken on a nominal pad size of 1.5 mm x 1.5 mm [0.060 in x 0.060 in] or equivalent area. An amendment that sets an upper limit for gold thickness for ENEPIG at 2.8 μin (0.7 μm) is presently in final draft for peer review. The amendment was necessary because too many designers were specifying a much higher immer- sion gold value. The higher thickness involved increased dwell time in the gold bath, resulting in nickel corrosion under the palladium layer. ENEPIG Fabrication The copper surface coming to the ENEPIG line in most cases follows tin stripping and the application of soldermask. Residual tin left on the surface will interfere with catalyza- tion of the copper surface. Soldermask application in- volves cleaning and roughen- ing the copper surface, the ap- plication of a photoimageable mask, tack drying, imaging, developing and curing. Atten- tion to the details of process- ing soldermask is paramount to achieving the desired EN- EPIG deposit. The proper ad- hesion between the mask and the copper surface has to be achieved. After development there should be no organic residues left on the pad surfaces, and the side wall should be straight with no signs of negative or positive foot. This is particularly important if the design includes soldermask defined pads. Process Sequence 1. Cleaner: The purpose of this step is to clean the copper surface in preparation for pro- soldermask application involves cleaning and roughening the copper surface, the application of a photoimageable mask, tack drying, imaging, developing and curing. attention to the details of processing soldermask is paramount to achieving the desired EnEPig deposit. " " FEaturE ENEPIG: THE PLATING PrOCESS continues