Board Plating - Tin-Nickel

Has anyone had any experience with 60/40 Sn/Ni(300 u in.) over Ni(300 u in.)? If so, what kind of solder issues might exist with 63/37 Sn/Pb reflow? Is a different paste needed? Also, what is the benefit of using the same platings with a top plating of gold(100 u in.) on fingers (compared with the more normal Au over Ni)?

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