Why Use Gold Rather Than Silver and Copper in PCB Manufacture?

Printed circuit boards (PCBs) form the backbone of electronics, providing mechanical support and electrical connections between components. The pads and traces on PCBs are typically made from copper, which readily oxidizes when exposed to air. This oxidation can impair solderability and electrical performance. Therefore, surface finishes like gold, silver or organic coatings are applied to protect the copper from corrosion.

This article explores the rationale behind using expensive gold plating versus cheaper silver or copper in PCB fabrication.

Overview of Common PCB Surface Finishes

The exposed copper traces on PCBs are plated with metal coatings or organic films to prevent oxidation. The popular metallic surface finishes include:

• Immersion silver – Chemical silver plating process
• Organic solderability preservatives (OSP) – Organic protective films
• Electrolytic nickel/gold – Electroplated nickel underlying gold
• Electroless nickel/immersion gold (ENIG) – Electroless nickel then immersion gold

Of these, ENIG and electrolytic nickel/gold utilize gold to maximize corrosion resistance and conductivity. But gold is also the most expensive finish.

The Need for Surface Finishes on PCBs

PCBs act as the platform for mounting and interconnecting electronic components. The copper traces etched on the PCBs provide the conductive paths for signals and power.

However, copper readily forms non-conductive copper oxide when exposed to oxygen and moisture. This oxidation can already begin during PCB fabrication itself when copper is exposed.

The oxidation process continues as bare copper is left exposed to ambient environments. This is especially true for the soldering pads which need to be soldered later to attach components. Oxide formation on pads deteriorates solderability.

Furthermore, the oxidation increases resistivity, which impairs electrical connectivity. Poor solder joints as a result can cause device failures or reduced reliability.

Therefore, PCB surface finishes serve multiple functions:

• Prevent copper oxidation prior to soldering
• Enable reliable solder joint formation
• Maintain low contact resistance at joints
• Protect copper from corrosion over product lifetime

Why Gold is Used in PCB Fabrication

Gold provides unique advantages that make it the metal of choice for PCB surface finishes in many applications:

Excellent Corrosion Resistance

Gold is highly inert and does not oxidize or corrode under typical use conditions or ambient environments. This preserves the purity and conductivity of the gold-plated traces over decades of product lifetime.

High Electrical Conductivity

Gold has the highest electrical conductivity rating after silver and copper. But both silver and copper are prone to surface corrosion. Gold’s excellent conductivity combines with its corrosion resistance.

Flexible Deposition Options

Gold coatings can be easily applied on PCBs through processes like electroless or electrolytic plating. Thickness can be controlled as needed.

Outstanding Solderability

Gold allows formation of highly reliable solder joints without any wetting issues. The solderability remains intact over storage periods.

Long Shelf Life

Gold maintains its purity and properties indefinitely. This enables exceptionally long shelf lives for gold-plated PCBs.

Ductile Deposits

Gold provides ductile coatings that are less prone to cracking or fracturing under thermal cycling stresses.

Wire Bonding Ability

The gold finish permits high yield wire bonding, important for microelectronics.

Reworkability

Gold coatings allow multiple rework and repair cycles without losing surface integrity or properties.

Lead-Free Compatibility

Gold is fully compatible with lead-free solders and soldering processes.

Drawbacks of Silver and Copper Finishes

Silver and copper provide advantages like higher conductivity and lower cost versus gold platings. However, both silver and copper suffer from limitations:

Oxidation and Corrosion：Silver and copper readily oxidize and corrode in ambient environments. This leads to increased contact resistance and poor solderability over time.

Poor Soldering Reliability：Oxide formation interferes with proper wetting when soldering. Dissimilar metal interfaces with solders also increases failure risks.

Reduced Shelf Life：Oxidation and corrosion impacts on silver and copper surfaces start immediately after plating. This necessitates soldering soon after plating.

Inferior Wire Bonding：The oxidation risks pose wire bonding challenges with silver/copper surfaces compared to gold.

Thermal Cycling Reliability：Silver and copper oxides are more prone to damage under temperature fluctuations versus ductile gold.

Environmental Constraints：Tarnishing of silver coatings can release toxic silver compounds into the environment during usage and disposal.

Higher Lifecycle Costs：Frequent corrosion and soldering issues with silver/copper surfaces translate into higher repair, rework and warranty costs.

PCB Surface finish Processes for Gold

Electroless Nickel Immersion Gold (ENIG)

Electroless Nickel Immersion Gold (ENIG) is a widely used surface finish for printed circuit boards (PCBs). It consists of a two-step process involving the deposition of nickel and gold layers onto the PCB’s copper traces. Here’s a breakdown of the ENIG process and its key characteristics:

Electroless Nickel Plating

• In the first step, the PCB’s copper traces are immersed in a chemical bath containing a solution of nickel salts, reducing agents, and stabilizers.
• The nickel ions in the solution are catalytically reduced onto the copper surface, forming a uniform and continuous layer of electroless nickel.
• The thickness of the nickel layer typically ranges from 3-6 μm (micrometers). This thickness provides good coverage and serves as a diffusion barrier between the copper and gold layers.

Immersion Gold Deposition

• After the nickel plating, the PCB is then immersed in a solution containing gold salts.
• The gold ions in the solution are chemically reduced, depositing a thin layer of gold onto the nickel surface through an immersion process.
• The gold layer typically has a thickness ranging from 0.03-0.15 μm, providing a thin but effective protective coating.

Key Characteristics and Benefits of ENIG

1. Diffusion Barrier: The nickel layer acts as a diffusion barrier, preventing the migration of copper into the gold layer and vice versa. This barrier helps to maintain the integrity of the gold layer and prevents the formation of intermetallic compounds that could impact solder joint reliability.
2. Solderability: ENIG provides excellent solderability due to the flat and smooth gold surface. The gold layer facilitates the wetting of solder during the assembly process, ensuring reliable solder joints.
3. Corrosion Protection: Gold is highly resistant to oxidation and corrosion, providing a reliable protective layer for the underlying nickel and copper. This corrosion resistance is particularly important for PCBs used in harsh environments or exposed to moisture.
4. Surface Flatness: The ENIG process results in a flat and planar surface, which is important for fine-pitch surface mount technology (SMT) components. The smooth gold surface helps ensure accurate component placement and reliable solder connections.
5. Good Electrical Performance: ENIG has good electrical conductivity and low contact resistance, making it suitable for high-frequency and high-speed digital applications.
6. Wire Bonding Compatibility: The gold layer in ENIG is compatible with wire bonding techniques used for connecting semiconductor chips to the PCB. The smooth gold surface facilitates reliable wire bonding connections.

Overall, Electroless Nickel Immersion Gold (ENIG) offers a reliable and versatile surface finish for PCBs, providing excellent solderability, corrosion resistance, and electrical performance. Its use is prevalent in various industries, including consumer electronics, telecommunications, aerospace, and medical devices.

Features of Gold-Plated PCBs process

Solderability Issues

While the gold plating layer exhibits good chemical stability, this stability also means that gold does not readily form a good alloy with solder, thereby affecting the quality of solder joints.
During the soldering process, if the gold layer is too thick, gold may dissolve into the solder, resulting in brittle solder joints. This phenomenon is known as “gold embrittlement.”

Usage Limitations

Due to solderability issues, gold plating is typically not used in areas that require extensive soldering. Instead, it is more commonly employed for connector contact points or areas intended for aluminum wire bonding.
In areas where soldering is necessary, alternative surface treatment methods such as Organic Solderability Preservatives (OSP), lead-free solder, or tin-lead alloys are often utilized.

Design Considerations

When designing a PCB with gold plating, careful consideration needs to be given to the thickness of the gold layer and its application location to ensure that the benefits of gold can be utilized while avoiding its drawbacks in soldering.
Therefore, despite the superior performance of gold-plated PCBs in certain aspects, there are indeed limitations in terms of soldering performance. These limitations require special attention during the design and manufacturing process.

Key Applications of Gold-Plated PCBs

Some examples of critical applications relying on gold-plated PCBs include:

• Aerospace and Avionics: Gold’s reliability ensures lifelong performance in space missions.
• Medical Devices: Gold coatings prevent corrosion in implanted devices and diagnostics.
• Undersea Electronics: Gold withstands high-pressure and saline conditions experienced in oceanographic equipment.
• High-Speed Digital Circuits: Gold’s conductivity enables error-free data transmission up to multi-GHz frequencies.
• Microwave and Radar Modules: Gold plating provides lowest loss at microwave frequencies.
• Connectors and Electromechanical Relays: Hard gold on contact points handle millions of switching cycles.
• Photonic/Optoelectronic Systems: Gold coatings provide high reflectivity needed in precision optical instruments.

Conclusion

In summary, gold is the ideal PCB surface finish for applications demanding ultimate reliability, conductivity, corrosion resistance, solderability and wire bonding performance. The higher initial material cost of gold is justified by enhanced product quality, lifespan, and reduced failure risks in critical systems. This makes gold worth its weight in enabling next-generation electronics to keep pushing the limits of performance and durability.

Related Posts

1.What is The PCB Manufacturing Process?
2.The advantages and disadvantages of PCB surface finish
3.A Comprehensive Guide to PCB Surface Finishes: Selection, Properties, and Reliability Considerations