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