Gold plating PCB boards has the metal covering designated areas, but why gold? Also, how do manufacturers apply the coating? In addition to answering these critical questions, we will explore the different types of gold plating and when to use them.
What is a Gold Plating PCB?
A gold-plated PCB board uses solid gold as a surface finish. The finish is a thin layer deposited chemically or through an electrolytic process. It protects the copper-coated areas (pads, traces, etc.) against corrosion.
Surface finishes also make assembly pads solderable. They can be of various materials, such as tin, silver, OSP, and HASL. So, why gold, you may ask. Let’s find out below as we explore its benefits.
Why Use Gold on Printed Circuit Boards?
We all know gold (Au) as a valuable material that jewelers use to make luxury products. But while that is true, it also makes a good PCB surface finish. The following reasons explain why it outperforms other choices.
Excellent Oxidation Resistance
Au is a chemically inert material. This property makes it resistant to oxidation, protecting the copper layer from corrosion. The plated surface maintains its quality in unstable conditions, including moist environments.
Good Electrical Conductivity
Copper is only slightly more electrically conductive than Au. It has a conductivity of 5.9×107 S/m compared to copper at 4.5×107 S/m. Therefore, using it to coat copper layers doesn’t affect signal transmission and quality.
Enhanced Solderability
Because it prevents oxidation, Au plating improves solderability, creating sturdy joints. Oxidation affects wetting, reducing bonding, and causing component assembly problems.
Extended Shelf Life
Au coatings last longer, given the material’s exceptional chemical stability. This benefit is more apparent if the manufacturer intends to store or ship bare boards for extended periods: the coated surface remains in good condition for a long time.
Resource: https://www.mdpi.com
Types of PCB Gold Plating
PCB makers use two procedures to coat circuit boards: chemical (electrolless immersion) and electrolytic (hard plating). Each application method has advantages and disadvantages.
Electroless Immersion
Electroless immersion, or soft gold plating, requires dipping the PCB in a gold salt bath. The solution contains Au ions. It also has a reducing agent to supply electrons. The electrons reduce the Au ions, converting them into atoms.
Electroless finish is typically ENIG or ENEPIG and sometimes EPIG. ENIG stands for electroless nickel immersion gold, while ENEPIG means electroless nickel electroless palladium immersion gold.
EPIG is electroless palladium immersion gold. ENIG only uses nickel as the barrier layer. ENEPIG adds an extra Palladium layer, achieving the desired qualities. EPIG omits nickel, using palladium to produce thinner layer profiles.
Hard Plating
Hard plating is an electrolytic process. It utilizes electricity and chemical reactions to deposit nickel and Au atoms from an aqueous solution. The plating process begins with placing the board in the solution.
Applying an electrical current to the solution deposits a thin layer of gold on the desired surfaces. The process continues until Au covers the exposed area.
Hard-plating a PCB produces thick and hard-wearing coats, suiting regular use applications. A good example is edge connectors, such as gold fingers. However, it costs more to produce than immersion.
Comparison Feature | Immersion Gold | Electroplated Gold |
Corrosion Resistance | Good | Better |
Hardness | Lower | Higher |
Wear Resistance | Lower | Higher |
Solder-ability | Good | Better |
Adhesion | Good | Better |
Lifespan | Long | Longer |
Overall Costs | Moderate | Higher |
Requires power source | No | Yes |
Best Applications | HDI PCBs, low-cost electronics | High-reliability and durable PCBs |
Immersion Gold vs. Electroplating
The two PCB plating methods offer varying benefits. Understanding their differences is crucial when choosing a surface finish material. Here’s a brief examination of their pros and cons.
Immersion Advantages and Disadvantages
Advantages:
- Thinner layers
- Uniform deposition
- Faster process cycles
- More economical than electroplating
Disadvantages:
- Reduced lifespan compared to electroplating
- Works with simpler geometries
Electroplating Advantages and Disadvantages
Advantages:
- Harder coating that resists wear
- Excellent corrosion resistance
- Better soldering performance
- Suitable for complex board geometries
Disadvantages:
- Requires a power source
- Costs more than immersion
- Slower process sessions
Resource: https://forum.kicad.info
PCB Gold Plating Process
PCB manufacturers use a four-stage process to apply the Au coating on copper surfaces. Let’s briefly examine each coating step, starting with surface preparation.
Surface Preparation
- The PCB undergoes cleaning to remove contaminants.
- A clean surface enhances adhesion during coating.
- Preparation steps include surface treatment to activate the copper surface.
- Coating the surface with solder mask before immersion ensures only exposed sections come into contact with the coating solution.
Nickel Barrier Deposition
- The board goes through an electrolytic or chemical process for a nickel coat.
- Cu resists many materials, including Au. The nickel layer makes it easier to plate.
- After nickel, the surface is now ready for Au coating.
Gold Deposition
- The board gets coated with a solid Au layer.
- The process is either chemical (ENIG) or based on electrolysis.
- Various measures control the plated layer thickness.
- The measures include regulating ionic concentration, limiting the coating period, etc.
Inspection and Testing
- Coated surfaces pass through quality control checks.
- The checks include manual and automated inspections.
- Various tests determine if the coating meets the required reliability standards.
- Examples include tape testing to determine the adhesion strength.
Resource: https://www.mdpi.com
PCB Gold Plating Applications
Gold plating PCB boards has varying applications in modern electronics where performance and reliability are key features. The following are examples of key uses.
Contact Fingers
These are finger-like contacts on the edges of high-performance board connections. Au makes them resist corrosion. They also withstand regular insertions or similar uses. Adding cobalt increases the coating’s hardness.
Edge Connectors
Edge connectors are contacts that designers place on PCBs for electrical connections. Unlike Au fingers, you can find them anywhere other than the edges. The material excellently resists corrosion and wear.
Wire Bonding Pads
The metal provides a good surface for bonding chip carrier wires to circuit board substrates. It typically involves coating copper pads with a nickel barrier and an upper Au layer. The result is a conductive and highly effective soldering surface.
High-Wear Interfaces
The interfaces include regular-use connections, such as the memory device contacts. The Au’s hardness suits these coatings, ensuring longer device life and reliability.
PCB Gold Plating Thickness and Specification Standards
A PCB’s immersion and electroplated gold must not be too thin or excessively thick. The coating must comply with specific electronic and material standards. In that regard, the following are typical requirements for each case.
Plating Thickness
Thickness influences cost, tolerance levels, and durability. Designers must, therefore, specify the limits based on the application’s needs. It ensures the plating matches end-product demands.
For instance, gold plating PCB fingers produces a 0.005 to 5 microns layer thickness. Thinner layers suit consumer electronics while thicker types are best for high-reliability needs.
Plating Standards
The IPC and MIL standards specify various coating requirements by PCB type, class, and usage. On the other hand, RoHs guidelines prohibit harmful substances like lead. Hard and soft Au plating satisfy this requirement, making the material and process compliant.
Conclusion
Gold-plated PCBs provide various advantages, such as strong conductivity, good oxidation resistance, and long shelf life. It also resists wearing, making the coated surfaces more durable. With all these benefits, we can see why PCB fabricators and assemblers prefer the material for the surface finish.
