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