Hi Neil,
Indeed, Au over Ni will result in diffusion. However, while AuSn IMC is 
indeed brittle and weak and weakens the solder structure, I do not know whether 
Cu-Au is particularly brittle. it would also be very thin. Ni barrier would stop 
the Au diffusion, but brings with it a whole new set of problems, among which 
are a thicker possibly equally brittle Ni layer, Black Pad with too muc P, Ni 
'hyper-galvanic' corrosion with too little P, etc.
My question about your problem is—have you determined whether the Cu is not 
brittle itself. If you use un-annealed rolled Cu, it will be brittle in the 
cross-rolling direction.



Werner



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