Meaning
Substrate feature utilizing electroless nickel immersion gold to provide a solderable surface on an integrated circuit carrier. The enig package pad consists of a nickel layer that provides a diffusion barrier and a thin gold layer that prevents oxidation. This finish supports both wire bonding and high-density flip-chip assembly.
Plating Chemistry
Deposition occurs through a displacement reaction where gold ions replace surface nickel atoms. An enig package pad must maintain a gold thickness between 0.05 and 0.15 microns to ensure proper wetting. If the gold layer is too thin, the nickel underneath will oxidize before soldering begins.
If the gold is too thick, it can lead to brittle solder joints.
Joint Integrity
Nickel acts as the primary structure for the intermetallic compound formed during reflow. The enig package pad prevents copper from migrating into the solder, which preserves the mechanical strength of the connection. Solder attaches directly to the nickel after the gold dissolves into the melt.
Solderability Failure
Hyper-etching of the nickel layer leads to a condition known as black pad. An enig package pad affected by this defect will separate under minimal stress. Monitoring the phosphorus content in the nickel bath is the standard method for preventing this failure mode.
