Hi Ingemar,
Looking at pic1, I would not call what I am seeing a "good joint"—it is an 
interfacial separation, and thus the interface [whatever it is] is the weak. and 
unpredictable, link.
Pic3 looks more like a ductile separation—see the directionality of the 
cup-like 'voids', in the solder [easy to confirm if the composition is the same on 
both fracture surfaces.] Pic4 looks like the Ni layer is showing through—>the 
solder properly wetted in some areas, but not all.
Sn spheres—beats me.



Werner



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