Chemical Deposition
Electroless nickel immersion gold functions as a plated metallic surface finish applied to copper traces during printed circuit board fabrication. Immersion plating replaces surface atoms of nickel with gold through a displacement reaction, leaving a thin protective barrier that protects underlying metal from oxidation during storage. Fabrication requires careful control of bath chemistry because hypophosphite reduction rates dictate the phosphorus content within the deposited nickel layer.
That phosphorus concentration typically ranges between seven and twelve percent by weight to secure optimal resistance against corrosion. Immersion gold thickness must remain within specific operational limits to provide a flat coplanar pad surface for subsequent component attachment without generating excessive brittleness.
Intermetallic Formation
Soldering operations rely on the rapid dissolution of the thin gold layer into molten solder during reflow. Molten solder dissolves the gold completely within seconds, allowing the underlying nickel to react directly with tin to form a stable nickel tin intermetallic compound. Excessive gold thickness prevents complete dissolution and leaves brittle intermetallic phases inside the solder joint, which degrades mechanical strength and causes premature structural failure under thermal shock.
Nickel acts as a diffusion barrier that stops copper from migrating upward into the joint while protecting the trace from excessive consumption. Porosity within the nickel deposit accelerates corrosion when moisture penetrates micro cracks during field exposure.
Surface Acceptance
Visual inspection procedures evaluate finished circuit boards for defects such as black pad, corrosion spots, and incomplete gold coverage across component pads. Black pad occurs when excessive bath activity attacks the underlying nickel surface during immersion plating, creating a hypercorroded phosphorus rich layer that destroys solder joint integrity. Optical systems combined with X ray fluorescence spectroscopy measure deposit thickness and composition to verify that production lots conform to established acceptance criteria.
Component placement machines depend on the resulting flat pad topography for accurate optical recognition and stable lead seating during surface mount assembly.
