All you need to know about PCB Gold Fingers

In today's highly connected, tech-driven world, seamless communication between devices is essential, and it all starts at the circuit board level. A critical element in enabling this communication is the use of gold fingers, the gold-plated connectors that link circuit boards to motherboards, making the flow of signals possible. Although it looks nice, gold plating is not merely for aesthetic purposes, it serves a practical function critical to the connector performance. Without gold fingers, critical components like graphics or sound cards wouldn’t be able to interact with the main processing units in computers and other electronic devices.

Gold fingers allow instant communication between circuit boards, enabling automation in industries such as manufacturing, automotive, and even consumer electronics like smartphones. Gold, prized for its superior conductivity and resistance to oxidation, ensures reliable performance and longevity in these critical connectors. In this blog, we will explore the role of gold fingers in PCB design, why they are essential to modern technology, and the careful selection of materials that make them so effective.

Types of Gold Plating on a PCB:

The standards involved in the plating process also helps to ensure a perfect fit between the gold fingers on each circuit board with the corresponding slots on a given motherboard. Here are two main types of PCB over which gold plating process can be carried out:

1. Electroless Nickel Immersion Gold (ENIG):

Its the most popular PCB surface finish used by Electronic Engineers, as it is economical and relatively feasible to solder than electroplate gold as shown below. EING finish provides reliable electrical connection and better resistance from corrosion and oxidation. But due to its soft and thin (typically 2-5u” composition) nature, ENIG became unsuitable for the abrasive effects of circuit board insertion and removal.

2. Electroplated Hard gold:

The gold offered is hard, and the thickness can reach 3-50u. This is more suitable for gold finger PCBs that will be repeatedly plugged-in and removed.  It’s also suitable for PCB boards that require frequent mechanical friction. The cost of gold plating is extremely high, and it is only suitable for partial gold plating such as gold fingers.

How Are Gold Fingers Used?

Gold fingers serve as the connecting contacts between two PCBs, ensuring conductivity while protecting the edges from wear during multiple uses. Due to the durability of hard gold at its specified thickness, gold fingers enable a PCB to be connected, disconnected, and reconnected up to 1,000 times. Some common applications include:

Interconnection points: Gold fingers connect secondary PCBs to the motherboard through female slots like PCI, ISA, or AGP, allowing signal transmission between peripheral devices or internal cards and the computer.

Special adapters: Gold fingers allow secondary PCBs to enhance computer performance, such as improved graphics and sound. These connectors typically outlast the cards themselves.

External connections: Peripherals like speakers, printers, and monitors connect to the motherboard via slots with PCB gold fingers, ensuring smooth communication with the computer.

Beyond personal computers, gold fingers are also used in industrial machinery for connecting contacts in automated systems.

Classification of Goldfingers:

1. Conventional gold fingers (flush fingers): Rectangular pads of the same length and width are neatly arranged at the edge of the board. Commonly used in physical objects such as network cards and graphics cards.

2. Long and short gold fingers (i.e. uneven gold fingers): Rectangular solder pads of varying lengths located on the edge of the board, commonly used in physical objects such as memories, USB flash drives, and card readers. ​

3. Segmented gold fingers (interrupted gold fingers): There are rectangular pads of different lengths located at the edge of the board, and the front section is disconnected.

The IPC Standards and Rules for Gold Fingers:

The production standards for PCB gold fingers were established in 2002 by the Association Connecting Electronics Industries (IPC). The standards were amended with the release of IPC A-600 and IPC-6010, which are currently the most widely employed standards in PCB production.

Chemical composition: For maximum rigidity along the edges of PCB contacts, the gold plating should consist of between 5 and 10% cobalt.

Thickness: The plating thickness of gold fingers should always fall within the range of 2 to 50 microinches. The standard thicknesses by size are 0.031 inches, 0.062 inches, 0.093 inches and 0.125 inches. The lower thicknesses are generally used for prototypes while the higher thicknesses are used along connecting edges that are regularly inserted, unplugged and reinserted.

Visual test: The visual test is conducted through a magnifying glass. The edges should have a smooth, clean surface with no unwanted plating or the appearance of nickel.

Tape test: To test the adhesiveness of the gold plating along with the contacts, the ICP recommends a test whereby a strip of tape is placed along the contact edges. After removing the tape, inspect the strip for traces of plating. If any gold plating is evident on the tape, the plating lacks sufficient adhesiveness along with the contacts.

As technology improves, further standards of testing are commonly introduced to the process, so it is best to check back with the IPC for updates periodically.

Processing Steps of a PCB Gold Finger:

On a circuit board, the PCB gold finger plating process is used after the solder mask and before the surface finish. The plating process generally consists of the following steps:

Nickel plating: 3 to 6 microns of nickel are plated first to the connector edges of the fingers.

Gold plating: 1-2 microns of hard gold are plated over the nickel. The gold is usually enhanced with cobalt for boosted surface resistance.

Beveling: The connector edges are bevelled at specified angles for easier insertion on corresponding slots. Beveling is typically done at angles of 30 to 45 degrees. More information about beveling the edges of a PCB can be seen here.

There are certain restrictions to the plating process. For example, certain distances are needed between the gold fingers and other parts of the circuit board.

Process restrictions and  include the following:

• Plated holes, SMD and pads should not be placed within 1.0mm of the gold fingers.
• Plated pads cannot exceed 40mm in length.
• A distance of 0.5mm should exist between the gold fingers and the outline.

Deviating from these standard spacing requirements can compromise the physical strength and functionality of the PCB.

Manufactures Design Specifications for Gold Fingers:

In order to ensure the reliability of the gold finger , PCB Design engineers must follow some design rules when designing. Regardless of the purpose or size of the PCB itself, the following rules are always applicable in the design of gold fingers:

• Gold-plated connectors should not be placed near PTH (plated-through holes).
• Gold fingers need to be chamfered, generally 30°, 45°, 20°, etc.
• The gold finger is at least 1.0mm away from the PCB outline.
• Gold fingers should be placed outward from the centre of the PCB.
• In order to bevel the PCB edge, the gold fingers should never face in the direction of the middle of the PCBs.
• There is no solder mask or screen printing near the gold fingers, they should be kept at a distance from the gold fingers.

Here are the commonly used gold finger chamfering parameters in JLCPCB:

Application of PCB Gold Fingers:

It is not possible to conceive today’s world without PCB Gold Fingers. While scanning a photo from a handheld device and sending it to a printer, many gold fingers have already been utilized for communicating between devices. They have not only made communication faster and cheaper but also reliable. Today, we see PCBs in all sectors of life including:

• Consumer Electronics
• Industrial Equipment
• Medical Equipment
• Automotive Components
• Aerospace Components
• Safety and Security Equipment
• Telecommunications Equipment
• Personal Computers and more

Conclusion:

Each time you print out an article or scan a photo to upload to a social media account, signals are sent from peripheral devices to motherboard cards, which receive these signals via PCBs. Thanks in large part to gold fingers, technology has been able to advance to the modern array of mobile devices that millions now use on a daily basis.

In conclusion, PCB gold fingers are essential components that enable reliable connections and seamless communication across a wide range of electronic devices. They play a crucial role in enhancing the performance and convenience of modern technology, from everyday mobile devices to industrial machinery. As technology evolves, the significance of gold fingers will only grow, ensuring that devices remain efficient and interconnected. To maintain this level of performance, it is important to ensure that gold fingers are plated and tested to the highest standards, supporting the continued advancement of technology in our daily lives. Turn your PCB designs into real products with JLCPCB amazing PCB services.