Connectivity is critical for computers, mobile phones, and other devices in the modern technological world. Each connected component or circuit board usually has specific functionality, so bringing them together is essential to enhancing the overall performance of any device and increasing the available features. The feature that enables this kind of connectivity is the gold finger. Found at the edge of PCBs, this section provides a durable and reliable connection point to allow data and power signal transfer between the peripheral device and the motherboard. We will take a closer look at gold finger PCBs, their types, and how to design them.
What are Gold Finger PCB Boards?
PCB gold fingers are narrow, gold-plated connectors found at the edge of PCBs, and their purpose is to enable connectivity and communication between the boards. They contain flesh gold, the most solid and durable gold form with superior conductivity.
But why gold? The material has the highest electrical conductivity and corrosion resistance levels after copper and silver. Usually, the gold gets combined with nickel and cobalt to make the finger surfaces more resistant to wear and tear.
The gold finger PCB edge of a graphics card
This feature is crucial because the constant insertion, removal, and reinsertion can peel the gold over time to expose the contacts underneath.
How Are Gold Fingers Used?
- Interconnection Points: Gold fingers are usually the contact points in several slots, such as AGP, PCI, or ISA, to conduct signals from secondary PCBs to the main motherboard. Thus, they come in handy when linking an internal card or peripheral device to a computer.
PCI slots for attaching RAM to a motherboard
Source: Wikimedia Commons
- Special Adapters: Gold fingers make it possible to connect numerous performance enhancement features to your computer. For instance, your computer can deliver high-fidelity sound and better graphics via the secondary PCBs found in sound cards and graphics cards. The cards slot perpendicularly to the motherboard, and their fingers usually outlast them because there's not much wear since the cards seldom get unattached and reattached.
Sound card gold fingers
- External Connections: All other outer peripherals (monitors, printers, scanners, etc.) link to the motherboard through gold fingers. They connect to the computer through ports or slots. These slots then direct the signals to the motherboard via circuit boards.
What is PCB Gold Finger Beveling?
When making a PCB, the gold finger plating process comes right after depositing the solder mask, and it consists of the following three steps.
- Nickel Plating: Initially, two to six microns of nickel get plated to the finger connector edges.
- Gold Plating: Next, hard gold plating between one and two microns thick gets applied over the nickel. In most cases, manufacturers add cobalt to the gold to enhance the surface resistance.
A gold-plated PCB. Note the fingers
Source: Wikimedia Commons
- Beveling: Beveling is the last step, and it involves angling the edge connector fingers for easy insertion into the respective slot. The tapering is usually 30° - 45° depending on the port, size of the circuit board, or the client's requirements.
After beveling, the next step is applying the surface finish.
Design Specifications for Gold Finger PCBs
The gold plating process has certain restrictions and design specifications to ensure the fingers function as expected. They include the following:
- Pads, plated holes, and SMD should never be close to gold fingers (within 1 mm).
- The fingers should not contact the screen printing or solder mask.
- Plated pads should not exceed 40 mm lengthwise.
- The inner layers along the PCB edges must be free of copper material to prevent exposure during the beveling stage.
- Maintain a distance of at least 0.5 mm between the PCB outline and the fingers.
- The fingers on PCBs should always face the opposite direction or outward from the center of the board.
Compromising these standards can result in weak or malfunctioning circuit boards.
Types of Gold Finger PCBs
There are three types of gold fingers based on their appearance or the plated gold.
Non-Uniform PCB Gold Fingers
As the name suggests, these fingers are not uniform. They have unequal lengths, with some being longer than others. The connection is desirable in PCBs that need to link with the power connection earlier than the rest to get the energy flowing in. A good example is the fingers found in memory card readers.
The non-uniform gold fingers in a memory card and a memory card reader
Uniform PCB Gold Fingers
These fingers are the opposite of the non-uniform type described above. All the connection points are equal in length and width because there's no priority link. Thus, the finger connections power up at the same time.
Segmented PCB Gold Fingers
Apart from varying in length, some segmented gold fingers appear disjointed among other fingers in the same PCB. The design is usually present in rugged and water-resistant electronics. They are also the primary connection in modern hybrid technological devices.
Quality Measures for PCB Gold Fingers
In addition to the specifications explained earlier, there are PCB gold finger production standards. First established in 2002 and amended several times over the years, these standards include the following.
- Chemical Composition: Use a gold and cobalt alloy with 5-10% cobalt in the plating to maximize rigidity along the edges of PCB contacts.
- Thickness: Ensure the plating thickness is within 2-50 microinches. Most standards are 0.031, 0.062, 0.093, or 0.125 inches, with the thinner ones being ideal for board prototypes. The thicker ones are ideal for "busy" contact edges (regularly inserted, removed, and reinserted).
- Visual Test: Visual inspection is vital for defect tests. Use a magnifying lens to check whether the edge contacts are smooth, clean, and free of excess plating or any nickel appearance.
- Tape Test: To check the adhesiveness of the gold plating, place a tape strip along the contact edges, then remove it and do a visual inspection of the tape. If there are traces of the gold plating, the layer lacks enough adhesiveness with the contacts underneath.
Use transparent tape to test the adhesiveness.
It is worth noting that these are IPC standards, and they are the most widely used when manufacturing PCBs. As the technology improves, always check for the latest IPC standard amendments. Also, read and understand other standards because several exist.
How are Gold Fingers Changing The World?
Gold fingers are all about providing reliable connectivity between peripheral devices and motherboards. This link is crucial in modern-day devices because the rapid development of computers, smartphones, and other technological devices demand high peripheral connectivity to enhance performance.
Improvements in gold plating technology allow faster insertion and ejection while also enabling more connectivity, leading to more productivity in industries. However, as these technological advances continue to emerge, it is critical to have the circuit board connection points plated and tested according to the highest standards.
At OurPCB, we adhere to these standards and have the experts plus equipment to ensure you get only the best quality.
The gold fingers on two RAM modules
As you can see, gold fingers are critical in connecting peripherals or cards to the motherboard, and this could be RAM, graphics cards, memory cards, etc. But you must follow the latest design specifications and standards when plating them to your circuit board to ensure perfect functioning.
We recommend getting experts to do the plating and beveling for you, and our team is best suited for this task. Reach out for any queries or send your PCB designs, and we will give you a free quote.