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February 2015 • The PCB Magazine 15 OSP AND SELECTIVE ELECTROLESS NICKEL continues OSP. The second consideration is the ability of the OSP to properly coat the bare copper while leaving the gold deposit virtually untouched. Finally, the photoresist used in the secondary imaging step must be capable of withstanding the aggressive nature of the ENIG chemistry and temperatures. With respect to the photo- resist used in the mixed-metal finish applica- tion, engineers should not assume that the pri- mary resist used in the outerlayer processing sequence will be effective in such a processing environment. Consult the resist supplier for recommendations related to secondary image transfer (SIT). This type of resist must possess several qualities including: • Good resist adhesion to multiple surfac- es of different compositions such as laminate, copper, soldermask. Thus, many adhesion-en- hancing process adjustments may not be practi- cal because of stripping difficulties or resist em- brittlement • The resist has to survive rather harsh plat- ing conditions, e.g., 20–30 minutes at 85°C, pH 4.5–4.8 in the nickel bath and equally high temperature in the gold bath, which can cause loss of adhesion • The resist should be compatible with the various plating processes that are used in the selective plating process. Note that there is a possibility of resist leach products that may interfere with the plating process. These leach products can lead to skip plating, peeling and poor solderability Corrosion Resistance of the Selective EN It is necessary that the nickel process em- ployed in the SENIG/OSP process be highly cor- rosion resistant. Keeping in mind the various process steps outlined in Figure 1, the engineer should be wary of the potential for corrosion of the nickel (even with a thin deposit of im- mersion gold over the nickel). Immersion gold deposits are somewhat porous and it is possible for the pre-cleaning and micro-etching steps of the OSP process to attack and corrode the un- derlying nickel. (Note: At this point where the OSP is applied, the nickel-gold is exposed.) In addition, dry film leach by-products can cause corrosion of the nickel (Figure 2). The SEM in Figure 2 is that of a mid-phos EN process that is used for a selective application (6–9% by weight phosphorous). One way to prevent such a condition is to employ a high phosphorous content electroless nickel process (generally phos content in the range of 10.2–11.5%). Figure 3 shows the pre- ferred condition. Exposed nickel was subjected to dry-film leaching and OSP acid cleaning and micro-etching. feature Figure 2: Mid-phos En deposit that is susceptible to corrosion by resist leach products and acids and micro-etchants. Pitting in the deposit is due to corrosion of the nickel. Figure 3: high-phos En formulation designed for SEnIG and mixed-metal finish applications.