What Are PCB Gold Fingers?
Have you ever looked at a printed circuit board and noticed shiny, gold-colored traces on the edges? These traces are known as gold fingers, but what exactly are they and why are they important in PCB manufacturing?
Printed circuit boards are essential components in modern electronics. They provide a platform for connecting and controlling electronic components. PCB come in all shapes and sizes, and different designs require different features to function optimally. One such feature is the gold finger.
In this article, we will take a closer look at PCB gold fingers and their role in electronics manufacturing. We will explore what gold fingers are, why they are crucial for the proper functioning of electronic devices. Whether you’re an electronics hobbyist or a manufacturer, understanding the importance of PCB gold fingers is essential in creating reliable and efficient electronic devices.
Table of Contents
What Are Gold Fingers or Edge Fingers on a PCB?
Gold fingers are gold-plated contact surfaces found at the edge of a printed circuited board (PCB). The contacts are rectangular pads arranged in a column, resembling small gold fingers.
The gold fingers often sit on an extended portion of the PCB, allowing the connectors to fit into a female slot on a separate PCB. They are also sometimes called “edge fingers.
Gold fingers typically act as connectors for connecting the PCB to another board. Many desktop computer components connect to the PC motherboard using these narrow connectors. Video game cartridges also include gold fingers. The game cartridge fits into a slot with a female connector.
Gold fingers allow one PCB to communicate or read data from another PCB. The gold fingers act as a bridge between circuits on two separate boards.
Benefits of Using Gold Fingers PCB
PCB gold fingers offer many advantages for a wide range of industrial, commercial, and consumer electronics. Some of the reasons to use PCB gold plating for edge connectors include:
- Electrical conductivity
- Superior connectivity
- Durability and reliability
- Cost-effectiveness
Here is a closer look at each benefit of using gold fingers on PCBs.
Electrical Conductivity
Gold-plated pads offer greater electrical conductivity compared to using other materials for connectors on the edge of a PCB.
The high conductivity of gold helps it provide a reliable connection between the two PCBs. Increased electrical conductivity is essential for high-speed applications. The increased conductivity of gold helps reduce the risk of interruptions in the signal.
Superior Connectivity
Gold fingers provide a solid connection when properly applied to the board. It has low contact resistance. Gold may even improve the signal quality.
The combination of increased electrical conductivity, low impedance, and superior connectivity reduces signal loss. You get the best possible signal from a temporary connection, which could be useful in extreme environments or situations where the board may face stress.
Durability and Reliability
Gold is a durable metal and helps increase the reliability of the contacts on the edge of a PCB. It protects the connecting edge of the PCB, allowing it to be used many times.
Gold offers greater resistance to corrosion and general wear and tear compared to other metals, such as nickel and copper. It holds up better.
Durability is important for applications where the contacts are likely to experience a lot of mechanical stress, including the frequent removal of the PCB.
Removable components, such as RAM sticks and video game cartridges, are often inserted and removed multiple times. Gold contacts are less likely to wear quickly in these situations.
Cost-Effectiveness
Applying gold fingers to a PCB is often a cost-effective design solution. It can save money on the production of large quantities of circuit boards.
The male connectors on the edge of the board are created using gold-plated pads. Incorporating gold fingers into a PCB design may be more cost-effective compared to adding separate components.
Design Specifications for Gold/Edge Fingers
Manufacturers typically follow a few specific design specifications when designing gold fingers for use on a PCB.
For example, manufacturers often maintain a distance of at least 1 mm between plated through holes (PTHs) and the gold fingers. A distance of at least 0.5 mm is kept between the outer edges of the board and the gold fingers.
Maintaining spacing around the gold fingers helps protect the gold-plated pads and allows for a more secure connection.
The gold fingers typically face outwards from the center of the PCB. The inner layers of the board also need to be copper-free to help prevent exposure during the beveling process.
The arrangement of the PCB gold fingers depends on the needs of the project. With normal gold fingers, the fingers appear in an even array with all the pads featuring the same length, width, and spacing.
With uneven PCB gold fingers, the pads are different lengths or spaced differently, which allows some pads to come into connection with the pads on the female socket early. This may allow for easier insertion of the PCB.
Production Standards for Gold/Edge Fingers
The Institute for Printed Circuits (IPC) publishes standards for the electronics manufacturing industry. The IPC also has standards when it comes to the use of PCB gold fingers.
When including gold fingers on a PCB, the gold-plated contacts should meet the following requirements:
- Chemical composition of 5% to 10% cobalt
- A thickness of 2 to 50 microinches
- A clean, smooth surface along the edges
- Adhesiveness testing
Ensuring that the gold contains cobalt improves the strength of the edges of the gold fingers. The edges are less likely to crack, peel, or lose their adhesiveness.
The thickness of the gold fingers also impacts their ability to withstand damage when inserted into a female slot. This is especially important if the PCB is regularly removed and reinserted into another board.
The typical thickness of gold fingers is between 2 and 50 microinches. Standard sizes include 0.031 inches and 0.125 inches. Thinner pads may be used during prototyping, while thicker pads are often used for the final product.
A clean, smooth surface is also required, especially around the edges. A smooth surface without excess plating helps ensure a stable connection and creates less friction on the pads.
PCB Manufacturer may perform a tape test to analyze the adhesiveness of the PCB gold plating. A strip of tape is applied to the edges of the contacts. The tape should peel from the contacts without removing any traces of the plating.
Following these standards helps improve the reliability of products. It also leads to greater efficiency, as manufacturers have a set of guidelines to follow to ensure quality results.
Application of PCB Gold Finger
PCB gold plating is applied after adding the solder mask but before adding the surface finish. The solder mask and screen printing mustn’t have any contact with the PCB gold fingers.
There are two main types of gold used for PCB gold fingers—electroless nickel immersion gold (ENIG) and electroplated hard gold. ENIG is more affordable but offers less wear resistance compared to hard gold plating. Applications that require frequent removal of the PCB may work better with hard gold instead of immersion gold (ENIG).
Also See: HASL vs ENIG: What is the Difference?
There are typically three main steps involved in the PCB gold plating process when using electroplated hard gold:
- Applying nickel plating
- Applying gold plating
- Beveling the edges
Three to six microns of nickel plating is applied to the edges of the connectors. The nickel plating has good oxidation resistance. It is followed by one to two microns of hard gold or gold with cobalt for superior strength and wear resistance.
The edges are beveled for easier insertion without causing wear on the pads. The bevels may include an angle of 30 to 45 degrees.
Summary
PCB gold fingers are often used as interconnection points for connecting a secondary PCB to a motherboard. The gold fingers are gold-plated pads on an extended portion of the PCB.
The gold pads often connect to a female socket on a separate board, which may be necessary for connecting peripheral components or upgradable parts.
If you need to include a second PCB in your design, consider using gold plating for increased strength, reliability, and performance.
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