Meaning
Surface treatment processes for printed circuit boards involve the deposition of a nickel barrier and a protective gold layer to ensure long-term solderability. Printed circuit boards often utilize enig finish to provide a flat, coplanar surface that is ideal for mounting small components and fine-pitch packages. This coating protects the underlying copper from oxidation during storage and prevents the formation of thick intermetallic layers before assembly.
It is widely used in high-density applications where pad uniformity is a requirement.
Oxidation Barrier
The immersion gold layer serves as the primary shield against atmospheric moisture and chemical contaminants. Because the gold is inert, the enig finish remains stable for over twelve months in controlled warehouse conditions. This thin layer dissolves into the molten solder during the reflow process to expose the fresh nickel underneath.
Solderability Retention
The nickel layer provides the actual surface where the metallurgical bond is formed with the solder. It acts as a barrier that prevents the copper from migrating into the solder and forming weak chemical compounds.
Defect Susceptibility
Improper control of the plating chemistry can lead to a condition known as black pad where the nickel is corroded during the gold immersion step. This flaw weakens the mechanical strength of the solder joint and causes intermittent electrical failures. Quality departments monitor the phosphorus content of the nickel bath to prevent such issues.
Consistent monitoring of the bath concentration and the immersion time is the only way to ensure the integrity of the coating.
