28/04/2026
Why Must Nickel Be Plated Before Gold on PCB Boards?
In PCB manufacturing, plating a layer of nickel before gold plating (typically ENIG or electroplated gold) is a common and critical process step. This nickel layer plays several key roles. So why is nickel necessary?
一、Preventing "Gold Embrittlement"
1.Diffusion Issue: Copper (Cu) and gold (Au) are metals that readily diffuse into each other. If gold is plated directly onto copper, during subsequent storage or high-temperature assembly (e.g., reflow soldering), copper atoms will rapidly diffuse into the gold layer, and gold atoms will diffuse into the copper layer.
2.Formation of Brittle Alloys: They form a series of brittle intermetallic compounds. These compounds are very brittle and severely weaken the mechanical strength of solder joints or contact points.
3.Oxidation Risk: Once copper diffuses to the gold surface, it oxidizes to form copper oxide, leading to a sharp decline in solderability and an increase in contact resistance.
4.Role of Nickel: Nickel (Ni) forms a dense diffusion barrier layer between copper and gold. The diffusion rate between nickel and gold is extremely slow, effectively preventing the migration of copper and gold, thus ensuring the purity and long-term stability of the gold layer.
二、Providing a Smooth and Hard Substrate for Gold Plating
1.Planarization: After pattern transfer and etching, the copper surface is microscopically rough. Plating gold directly onto it would cause the gold layer to "copy" this roughness, affecting the mounting of precision components or electrical contact performance. Plating a layer of nickel first fills in tiny cavities, providing a smoother, more uniform surface.
2.Mechanical Support: Gold itself is a very soft metal. The hard nickel layer provides a rigid "foundation" for the soft gold layer, improving the wear resistance, scratch resistance, and fretting resistance of the gold-plated surface. This is critical for connectors that require frequent insertion and removal (e.g., "gold fingers").
三、Enhancing Corrosion Resistance
1.Copper is Prone to Oxidation: Copper readily oxidizes, sulfidizes, or halogenates in air, forming non-conductive or poorly conductive compounds.
2.Double Protection: Nickel itself has good corrosion resistance. The nickel layer first isolates the copper from air and moisture. Even if the external gold layer has tiny pinholes, the underlying nickel provides protection, preventing the internal copper from corroding, thus forming a double layer of protection.
四、Enhancing Soldering and Bonding Reliability
1.Solderability: For areas requiring soldering (e.g., ENIG surfaces), the nickel layer provides a stable, solderable surface. Gold dissolves rapidly in solder; the actual soldering occurs on the nickel layer. A clean, unoxidized nickel surface forms a strong nickel-tin alloy solder joint, which is much stronger than soldering directly onto copper.
2.Gold Wire Bonding: For gold wire bonding in chip packaging, a hard substrate (nickel) is also required to support the soft gold, achieving better bonding strength and consistency.
五、Reducing Gold Consumption to Lower Cost
Gold is a precious metal and very expensive. With nickel as the primary layer, only a very thin gold layer (typically 0.05-0.2 µm) is needed on the surface to achieve good conductivity, solderability, or contact performance. This significantly reduces production costs without compromising performance.
Plating nickel before gold on PCBs is by no means an unnecessary step; it is an essential process in modern high-reliability electronics manufacturing. By acting as a diffusion barrier, a mechanical reinforcement layer, and a corrosion protection layer, the nickel layer fundamentally solves the many reliability problems caused by direct copper-gold contact, ensuring the electrical performance, solderability, and mechanical stability of PCBs under long-term use and harsh environments.
Manufacturing Plating Plating Barrier Embrittlement # Solderability Resistance (Electroless Nickel Immersion Gold) Gold
# Reliability
