Oh K,

I have never soldered to a Nickel finish.  Gold over Nickel, but not just
Nickel.  Well, actually not intentionally.  In the way back machine, (AKA
through -thru- hole parts) we were so smart we had our external layer a
Nickel plane.  We learned Nickel is lousy to solder to and also what sticks
to Nickel... nuttin.

Good luck,

Boston Brad