Michael,

Is it possible that you are using a standard tin-lead reflow process at a
peak temperature below 220C.  Pure tin requires 232C to melt.  Since you
are really trying to solder to the nickle, if you don't thoroughly melt the
tin, you will not get a good joint.  In fact, using silver instead of tin,
I have seen the "buttery" appearance that you noted.  I increased the
temperature to melt the silver and achieved a good joint.

Glen

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