Hi Gaby,
Both the 'Head-in-Pillow' and the 'Ball-in-Socket' effects have the same root 
cause—insufficient soldering temperature. In the 'Ball-in-Socket' cases, 
larger deposited solder paste volumes as well as BGA/PCB warpage 
exaggerate/heighten the effect.
More importantly, in both cases, increasing the soldering temperature solves 
the problem.

Werner

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