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DECEMBER 2019 I PCB007 MAGAZINE 49 The Electroless Nickel Bath Electroless nickel (EN) baths are formulated to give a specific range of incorporated phos- phorus, expressed as a percentage range, in the deposit. A low-phos deposit would range from 4–6%, a mid-phos deposit would range from 6–8%, and a high-phos deposit would range from 8–10%. Mid-phos nickel is widely used for ENIG application, where soldering and cor- rosion control are required. With the emphasis on eliminating nickel corrosion, newer formula- tions now favor a higher phos content (>8%) to further expand the corrosion control window. EN is a well understood multi-component bath. The primary constituents are nickel sul- fate as the source of nickel and sodium hypo- phosphite (hypo) as the reducing agent. The latter supplies the electrons needed to reduce the nickel ion to the nickel metal. The hypo reaction also produces phosphorus and the by- product orthophosphate. The phosphorus is incorporated in the electroless nickel deposit. The buildup of the orthophosphate byproduct in the bath determines the life of the bath. The nickel deposition reaction requires a specified temperature (ranging from 175– 185°F) and a weak acidic pH medium. In addi- tion, there are a series of other important pro- prietary constituents that have a direct impact on the quality of the nickel deposit, including stabilizers, surfactants, complexing agents, accelerators, and buffers. Different vendors or suppliers may use various ingredients to achieve the goal of a uniform deposit with an optimized deposition rate and achievable/re- producible operating conditions. The nickel bath requires good management as it is a dynamic, ever-changing bath. The nickel and the hypo are consumed during the deposition; in addition, there is a buildup of the byproduct orthophosphate. The pH of the bath must be controlled within a narrow range. It is necessary to replenish those ingredients that are being used up. This is best achieved by a controller that will automatically replen- ish the desired components as well as maintain the pH. To compensate for byproduct buildup, newer, more sophisticated controllers increase the nickel concentration and the pH operating ranges, as the bath ages to maintain a consis- tent rate of deposition. The Immersion Gold Bath To minimize corrosion, new developments include: • Neutral pH formulations for reduced corrosion • Reduced gold concentration for cost control • RAI gold for corrosion elimination RAI gold is a mixed reaction bath that initi- ates as an immersion bath and continues as an electroless bath. It is capable of depositing 4–6 µins of gold with no corrosion. RAI gold is ideally suited to deposit thicker gold if it is a design requirement. Conclusion The ENIG line is one of the most complex chemical lines in a board shop. It requires a good understanding of how the process works and the critical parameters that must be main- tained. The ENIG line has little tolerance to de- viations, particularly to extending the bath life of any of the process steps. Shops that have good engineering and documentation of the manufacturing process, coupled with a dedi- cated, experienced ENIG operator and backed by a capable analytical laboratory, run defect- free ENIG day in and day out, producing a con- sistent product that meets customer require- ments. PCB007 George Milad is the national accounts manager for technology at Uyemura. To read past columns or contact Milad, click here. The nickel bath requires good management as it is a dynamic, ever-changing bath.