PCB Gold Finger Design: A Complete Specifications Guide

How well do you understand PCB gold finger design? This article explores its key requirements, such as material choice, finger size, and thickness. Additionally, it examines their uses in electronic devices or systems.

What is a Gold Finger in PCB?

Gold fingers on PCB boards are finger-like metal contacts on the board’s edge. They usually come arranged in rows and plated with gold, which explains the name “gold fingers.”

The fingers connect the PCB to various devices, including motherboards, allowing the efficient transfer of digital signals between circuits. We also call them edge fingers since they sit on the board ends, ready to create external connections.

They differ from edge connectors in location and construction. You can place an edge connector anywhere on the board, including the middle. On the other hand, edge fingers only sit on the edges, away from the center.

Why Use Gold in PCB?

Gold is a precious metal we usually associate with jewelry. However, it’s also conductive and has various other unique characteristics, making it useful in electronics. The reasons below explain why we use it for PCB fingers.

• High electrical conductivity for improved signal quality.
  
• Excellent hardness that allows it to resist damage from frequent use.
  
• Corrosion resistance, making the fingers usable in different conditions.
  
• Soft and malleable for improved wear resistance.
  

The gold also contains about 5-10% cobalt to improve its rigidity. This chemical composition makes it more durable. Other material options include silver, palladium, and tin.

Sometimes, the material is an alloy of two metals, such as tin and lead or nickel and silver. The choice of material depends on the required performance and other factors like cost.

PCB Gold Finger Design and Specifications

The quality of any PCB finger electrical connection depends on its design. The following are key specifications to know about, including their correct choices.

1. Finger Type

We group the fingers by their design and plating method. In line with that, we have rectangular, uneven, and segmented fingers. We also have electroless nickel immersion and electroplated hard gold types based on the plating technology.

• Rectangular types have identical lengths and widths. They are popular in computer graphic cards, network cards, etc.
  
• Uneven fingers combine long and short contacts. You’ll usually find them in USB flash drives, card readers, and similar devices.
  
• Segmented variants have a disconnected front section where they connect with an external device.
  
• Electroless nickel immersion gold, ENIG, fingers are thin and soft. These features make it prone to abrasion damage but a more cost-effective option.
  
• Electroplated hard-gold types are solid gold and, therefore, more expensive. They are also thick and wear or abrasion-resistant.
  

2. Finger Location and Direction

The PCB board designer must correctly position the fingers to ensure proper function and circuit reliability. These rules apply when designing a printed circuit board with these contacts.

• Place them 1 inch from the edge.
  
• Ensure beveled types face away from the board’s center.
  
• Site them away from plated through holes, solder mask, and silkscreen or surface finish.
  
• Connect them to an 8-mil trace for easier placement.
  

3. Finger Bevel Angle

Beveling the outer edges improves their functionality by making insertion easier. Designers must follow these recommendations to ensure correct finger design.

• Bevel angles vary, ranging from 20-60 degrees.
  
• It’s usually advisable to avoid acute or sharp angles.
  
• Rounded corners improve functionality by allowing easier insertion.
  

4. Finger Size and Thickness

We measure circuit board fingers by their size or length and width. Depth also impacts their performance and durability, making it an essential consideration.

• Sizes vary depending on the end-user device.
  
• On the other hand, standard thickness ranges from 3-125 mils.
  
• Thickness is low in consumer electronics and high in heavy-duty applications.
  
• The thickness level depends on the usage cycles, contact density and resistance, cost, etc.
  
• Increasing thickness lowers electrical resistance and improves lubricity.
  

PCB Gold Finger Plating Process

Making circuit board contact fingers involves a series of distinct steps: plating, beveling, coating, and testing. Here’s more about them, including their best practices to achieve better results.

Plating with Nickel

• The process begins with nickel-plating bare copper strips.
  
• The plating process is either electrolytic or electroless.
  

Plating with Gold

• In this step, the strips receive a coating of hard gold.
  
• Like nickel, you can electroplate gold using an electrolytic or electroless process.
  

Finger Beveling

• This step bevels the ends to a desired angle.
  
• This design improves insertion.
  
• The bevel angle depends on specific application needs.
  
• The areas between the fingers also receive a solder mask coating to protect them.
  
• A lubricant applied to the surface helps reduce friction.
  

Testing and Inspection

• After plating and beveling, it’s time to inspect and test the projections.
  
• The visual test uses a magnifying lens to examine the surfaces and edges.
  
• The tape test checks the plated surface for adhesion strength.
  
• The test involves sticking an adhesive film on the plated surface.
  
• Upon removal, the film should not contain plating material.
  

Gold Finger Applications in PCB

PCB finger contacts have widespread applications in electronics. They are common in computers, telecommunication equipment, and industrial settings.

Computer Systems

They make secondary board-to-board connections possible, allowing users to increase mainboard capabilities. They move data between motherboards and memory cards, peripheral devices, and specialized adapters.

Industrial Applications

Many industrial circuits utilize these contacts to connect modules, allowing signal exchange. This communication is the backbone of many automation control systems. PCBs with these contacts create robust, wear-resistant connections.

Telecommunication Equipment

They serve as adapters in telecommunication systems, transferring data between devices or circuits. An example is an audio or display adapter. These move signals from the primary device to the reproducing gadget.

Conclusion

Gold finger implementation on PCBs demands meticulous design and fabrication processes. For instance, the fingers must be correctly sized, located, and angled. Depending on the application, the material should match insertion frequency requirements, resisting wear or other damage.