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Plating Nickel over silver

I use a nickel sulfamate bath to plate approximately 250 µ" of nickel onto a silver epoxy termination of an electronic component. The terminations are then dipped into a tin/lead solder pot (Temp=230° C). 95% of the time, the nickel covers the silver terminations completely , however, approximately 5% of the time I will have "spots" on the terminations that do not nickel plate. I believe the "spots" are due to the resin in the silver epoxy coming to the surface of the silver and not allowing this "spot" to plate. I surmise this because if I abrade the spot it will plate. If this is true, is there a way to treat the surface of the silver (without abrading) before nickel plating to solve this issue? Or, have I missed the issue altogether and there is something I am not seeing/doing during this plating process?

The component is next to a scale with 1/64 graduations to give you an idea of the size of the part and of the area that did not nickel plate and subsequently did not wet with solder (The picture is of a termination that has been dipped in hot tin/lead solder). The "circle" (approx: 5/64") is the base layer of silver. A much smaller non-plated area can also be seen on the termination.

Dan Johnson
Electronic component Manufacturer - Sanford, Maine, USA
February 14, 2011