Hi Günter,
There are a number of possible reasons for what you see:
1. the Ni layer is not solderable, because
  a. the Au layer is too thin to protect the Ni,
  b. the Ni layer was contaminated/oxidized prior to the Au plating,
2. the maximum reflow temperature reached is too low/not sustained long enough
to affect solder wetting the Ni layer (remember that Au only promotes the
spreading of the solder on the Ni surface, not the wetting.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask]

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