Meaning
Surface finish chemistries deposit a thin, protective layer of precious metal over an underlying nickel barrier to preserve the solderability of printed circuit boards. An immersion gold finish utilizes a displacement reaction where gold atoms replace nickel atoms on the copper pad surface. This process creates a flat, highly solderable layer that is ideal for mounting fine-pitch ball grid arrays and surface-mount components.
Solderability Protection
Oxidation of the nickel layer is prevented by the dense gold topcoat, which remains stable during long-term storage before assembly. If the immersion gold finish is too thin or porous, the nickel beneath it oxidizes, which prevents proper solder wetting and leads to weak mechanical joints. Ensuring a continuous gold layer preserves the solder pads during multiple reflow cycles, making it a standard choice for multi-layered assemblies that undergo complex manufacturing sequences.
Intermetallic Formation
During the reflow process, the gold layer dissolves completely into the liquid solder, allowing the solder to bond with the nickel layer. This reaction produces a reliable intermetallic compound that provides the necessary mechanical strength for the joint.
Thickness Control
Accurate deposition rates are maintained by monitoring the chemistry of the plating bath. Excessive gold thickness is avoided to prevent solder joint embrittlement.
