ENIG Anomalies and Excessive Intermetallic Growth Can Weaken Corner BGA Joints

What the evidence preview establishes

Corner joints fractured between nickel-tin intermetallic and electroless nickel. Irregular nickel plating, low bulk phosphorus in one assembly, excessive intermetallic thickness, mechanical or thermal-mechanical corner stress, and under-BGA residue formed a multifactor failure condition.

Why this case matters

BGA interfacial fracture should not automatically be labeled black pad. Bulk and fracture-surface chemistry, nickel morphology, intermetallic thickness, joint location, and assembly stress must be integrated.