BSE SEM image of BGA microsection.
The entire interface fractured between the Ni-Sn intermetallic layer and the Ni-P of the ENIG board pad. This is the corner ball where warpage strains are at maximum typically.
Evidence of “hyper-etching” of the Ni-P grain boundaries, which is a symptom of black-pad-syndrome.
Check out SEM Lab, Inc. to learn more.