Meaning
Interfacial fracturing that occurs in solder joints formed on electroless nickel immersion gold surface finishes represents a critical reliability hazard in microelectronics packaging. The occurrence of enig embrittlement is closely linked to the formation of a hyper phosphorus rich layer directly beneath the intermetallic compound. This defect, often referred to as black pad, develops during the immersion gold displacement reaction when the nickel plating undergoes excessive corrosion.
The fragile layer cannot withstand mechanical shock, leading to sudden adhesive failures at the pad interface during drop testing or handling.
Chemical Cause
Plating chemistry imbalances during the displacement deposition step drive the aggressive degradation of the underlying nickel deposit. When the gold bath is too acidic or runs at a high temperature, the gold ions displace nickel atoms in an uncontrolled manner, leaving behind a highly porous and unalloyed nickel structure. This porous region accumulates phosphorus because the chemical process selectively removes nickel while leaving phosphorus behind.
Structural Impact
Mechanical stress on the board assembly initiates rapid crack propagation along the weak boundary of the corrupted interface. The crack travels through the interface without significant plastic deformation in the solder bulk. This brittle separation happens instantly during transportation or product drop events.
Process Control
Regular analysis of the plating bath chemistry and the stabilization of the immersion gold deposition rate mitigate the risk of this joint failure. Modern assembly lines use alternative finishes like electroless nickel electroless palladium immersion gold to prevent the displacement reaction from attacking the nickel layer. These finishes provide a robust physical barrier that preserves the integrity of the metallic connection.
