All About Gold Fingers on PCBs

Posted October 31, 2022

Rush PCB Inc makes a large variety of printed circuit boards for the computerized and mobile-activated world. Most of these PCBs carry signals to and from other devices. For proper functioning of these boards, it is necessary for them to communicate without interruption. Gold fingers on PCBs, serving as connecting points between boards, make this possible.

Compared to earlier technologies, gold fingers are a huge leap forward for effective transfer of signals and commands. Gold fingers make it possible to communicate flawlessly between different boards and modules. This was a difficult earlier.

Gold Fingers

Printed circuit boards that must communicate with other boards, usually have gold fingers along their edges. These are the multiple gold-plated columns visible along the edge of the board where it will connect to a connector. For instance, a secondary PCB, such as a graphics card, has gold fingers that allow it to be plugged into a connector on the computer main board. Smartphones and smartwatches also have gold fingers on their PCBs, and so do many other devices that communicate via digital signals. To enable good connectivity along the PCB edge, the plated alloy offers high conductivity.

Types of Gold Fingers

PCB manufacturers offer two types of processes for creating gold fingers on board edges:

Electroplated Hard Gold — This is more suitable for boards that will be repeatedly plugged-in and removed. The gold offered is hard, and its thickness is typically of the order of 30µ inches. It can withstand the abrasive effects of repeated use.
ENIG or Electroless Nickel Immersion Gold — This gold is much softer, and its thickness is typically of the order of 2-5µ inches. Although this gold is more cost-effective and easier to solder, it cannot withstand the abrasive effects of repeated use.

As signals and power must travel to different circuit boards, they often move through various contacts. That means, each contact must present as low resistance as possible to prevent a voltage drop. The gold plating on the fingers ensures the contact resistance remains low.

Gold Finger Plating Process

The plating process for gold fingers involves many meticulous steps. This ensures that each circuit board in the production line presents very low resistance to conduct signals and power, free of error. Manufacturers typically follow international standards for the plating process, to ensure a perfect fit between each gold finger on the board and its corresponding slot in the connector.

To enable a smooth fit between the fingers and the slots in the connector, each board must pass a series of inspections and tests for defects. Lack of smoothness on the gold plating on a circuit board, or if the gold is not adhering adequately to the board surface, may result in rejection of the board.

In the manufacturing process, the fabricator can complete the plating process only after completing all other activities on the board. Before beginning the actual gold plating process, the fabricator plates the copper fingers with a layer of nickel. Then they apply the gold surface finish. The board then undergoes inspection under a magnifying glass and is subject to adhesion tests.

Using Gold Fingers

Gold fingers are the connecting contacts between two PCBs. Apart from providing high conductivity, the gold plating serves to protect the communicating pins from wear and tear over several operations. The strength of hard gold and its extra thickness on the gold fingers allow connecting, disconnecting, and reconnecting the board up to more than 1000 times in its slot, without any wear or damage on the fingers.

Gold fingers offer multiple functions. For instance, in a computer setup, it is possible to connect many peripherals to the computer through various types of gold fingers. Gold fingers are useful in many applications, such as:

Internal Connections — Secondary boards typically connect to the main motherboard through various types of female connectors such as AGP, ISA, or PCI. Gold fingers on the PCB interconnect signals between the secondary board and the motherboard or from peripheral devices and the computer.
Special Adapters — Using gold fingers, it is possible to add performance enhancements to personal computers. For instance, secondary PCBs like graphics and sound cards connect perpendicular to the mother board to deliver high-quality graphics or high-fidelity sound. As users seldom unplug and replug these cards, most use soft gold fingers, which present a long life.
External Connections — Many outer peripherals connect to a personal computer. These include speakers, scanners, monitors, printers, and USB devices. Most of these devices plug into various sockets in the computer. Gold fingers on these sockets and devices often face abrasive use due to frequent unplugging and replugging. Consequently, most such connectors typically use hard gold fingers.

Power is necessary for any device to function when connecting to a motherboard. The connector slot on the motherboard usually supplies the power. Here also, gold fingers carry power from the motherboard to the secondary board, allowing it to operate and function flawlessly.

Motherboard manufacturers typically offer flexibility through various connector systems, allowing connection of different types of cards. This allows the user to upgrade their computer systems periodically over a period of years. Updating a sound or graphics card involves unplugging the old card and replacing it with a new and improved model. In the upgrading process, the gold fingers on the PCBs remain the universal contact.

Gold fingers go beyond the realm of personal computers. For instance, industrial machinery also use gold fingers as connecting contacts. For instance, an automotive factory or a pressing plant will typically have a good deal of mechanized equipment with internal cards connecting to a main power source, and using gold fingers in the process. The industry uses robotic arms that receive power and control signals via gold fingers on cards.

Specifications for Gold Fingers

For the gold fingers to function properly, the fabricator must follow certain standards during the plating process. The design of the PCB must also be conducive to proper finger length and alignment. Some rules applicable to design of gold fingers, irrespective of the size or purpose of the PCB, are:

No PTH or plated through holes near gold fingers.
No solder mask or screen printing near gold fingers.
All gold fingers must end on the edge from the middle of the board.

Not complying with the above rules may make the PCB incapable of being inserted into the parent circuit board. For instance, the daughter board may not fit appropriately in the corresponding slot in the motherboard.

Superior strength and conductivity of gold is the main reason for using it on connecting fingers. Gold allows the insertion and ejection of the fingers many hundred times without causing wear and tear on the contacts. Without the gold protection, copper fingers on a board will easily wear out after a few uses.

Although gold is one of the most expensive and rare metals for use on PCB fingers, it provides the necessary functionality on a printed circuit board. This is due to several factors. The main benefit from the use of gold comes from its high electrical conductivity and its resistance to corrosion. The added strength comes from gold application in combination with nickel or cobalt, both providing it the necessary resistance to wear during repeated activity.

IPC has established the production standards for PCB gold fingers. For instance, the latest release of IPC A-600 and IPC-6010 define the chemical composition, thickness, and tape tests for any gold plating process on PCBs.

Gold Finger Plating Restrictions

Plating gold fingers must follow certain restrictions. These include:

Inner layer copper must not extend to the PCB edge. This is to prevent copper exposure during the beveling process.
No SMD parts, plated holes, or pads allowed within 1 mm of the gold fingers.
All plated gold fingers must be below 40 mm in length.
Any gold finger must be at least 0.5 mm away from the PCB outline.

Deviating from the above spacing requirements for gold fingers can result in the card becoming dysfunctional or physically weak.

Conclusion

With technology increasingly relying on gold fingers, Rush PCB Inc recommends plating and testing gold fingers to the highest standards. This is the way we ensure maximum performance from gold fingers in our high-quality circuit boards.