Immersion gold (ENIG) and electroplated gold (hard gold) are two common surface finishes in PCB manufacturing. As IC integration increases and component pins become denser, traditional HASL (Hot Air Solder Leveling) struggles to achieve flat pads for ultra-fine SMT assembly (e.g., 0603/0402 components). Additionally, HASL has a short shelf life. Gold finishes address these challenges, offering superior flatness and extended shelf life (critical for prototypes or delayed assembly). While initial costs for gold finishes are comparable to HASL, their long-term benefits justify widespread adoption.
Definitions
Electroplated Gold (Hard Gold):
Process: Electroplating nickel and gold (via electrolysis) onto copper pads.
Types: Soft gold (general use) and hard gold (wear-resistant, e.g., gold fingers).
Features: High hardness, oxidation resistance, and durability.
Immersion Gold (ENIG):
Process: A chemical redox reaction deposits a thicker gold layer over nickel (no electrical current).
Features: Uniform thickness, excellent flatness, and superior solderability.
Differences Between ENIG and Gold Plating
The crystal structures formed by ENIG (Electroless Nickel Immersion Gold) and gold plating are different. ENIG results in a much thicker gold layer than gold plating and appears more yellowish, which is generally preferred by customers.
Due to their different crystal structures, ENIG is easier to solder than gold plating, reducing the risk of soldering defects and customer complaints. Additionally, ENIG allows for better stress control, making it more suitable for bonding applications. However, because ENIG is softer than gold plating, it is less wear-resistant when used for gold fingers.
In ENIG, nickel-gold is only present on the pads, and signal transmission primarily occurs in the copper layer due to the skin effect, ensuring no signal interference.
ENIG has a denser crystal structure than gold plating, making it more resistant to oxidation.
As PCB trace widths and spacing become narrower—now reaching 3-4 mil—gold plating is more prone to gold wire short circuits. Since ENIG deposits nickel-gold only on the pads, it eliminates the risk of gold wire short circuits.
Because ENIG applies nickel-gold only on the pads, the solder mask adheres more firmly to the copper traces. This ensures that engineering compensation adjustments do not affect spacing.
ENIG is generally used for high-requirement PCBs that demand excellent surface flatness. It also prevents black pad defects after assembly. The flatness and shelf life of ENIG boards are comparable to those of gold-plated boards.
