Extracting gold from discarded electronic waste may sound somewhat bizarre, but the fact is that this is a real and viable process. According to statistics, approximately 200 grams of gold can be found in every ton of electronic waste. These precious metals exist in various forms within electronic waste, including gold wires used for COB packaging and PCB gold fingers for slot connections. In this article, TechSparks will introduce you to the concept, electroplating process, and specifications of gold fingers, aiming to help readers better understand how to efficiently utilize these metal resources and prevent them from being wasted.
What Are Gold Fingers in PCB
PCB gold fingers, also known as edge fingers, are gold-plated columns located at the edge of a circuit board. Their primary function is to facilitate communication between different circuit boards. When the power is on, current flows along these metal contacts from one board to another, enabling the transmission of digital signals.
One of the most typical applications is in computer memory modules. The memory module is essentially a PCB, and during use, its bottom gold fingers are inserted into the motherboard‘s card slot. When a computer experiences slowdowns, a common practice is to vigorously clean the gold fingers’ surface with an eraser or tissue to restore their luster.
From the appearance and application examples, several key features of PCB gold fingers are evident:
Multi-contact Design: To meet the complex requirements of connection and communication, each contact represents an independent channel or signal line. This design enhances reliability, ensuring that even if one contact encounters an issue, the others can still function normally. Additionally, for modular designs, each contact can be utilized for different types of data transmission.
High Reliability: Gold on PCB is divided into soft gold and hard gold. Soft gold, representing pure gold with lower hardness, is not suitable for frequent plugging and unplugging or friction. Therefore, hard gold, an alloy composed of metals such as gold and nickel in certain proportions, is used for PCB gold fingers.
Superior Conductivity: Since the purpose of gold fingers is to facilitate signal communication, conductivity is crucial. This is why the expensive metal gold is chosen as the raw material. The conductivity of gold fingers is closely related to the cleanliness and neatness of their surface.
Gold Finger Design and Specifications
Chamfer Design
When designing gold fingers, always maintain a safe distance from the board edge, with the specific distance depending on the finished board thickness and chamfer angle. Chamfer angles commonly used are 20⁰, 30⁰, 45⁰, and 60⁰, with 45⁰ being the most common.
If the gold fingers are too close to the board edge, copper shaving may be required, but caution is needed not to shorten the gold fingers. It is recommended to perform copper shaving with the following parameters to ensure that the chamfer design does not affect the performance of the gold fingers.
| Chamfer Angle | Board Thickness (mm) | Remaining Thickness (mm) | Chamfer Depth (mm) |
| 20° | 1.6 | 0.5 | 1.51 |
| 2.0 | 0.65 | 1.85 | |
| 2.5 | 0.7 | 2.47 | |
| 30° | 1.0 | 0.5 | 0.44 |
| 1.2 | 0.5 | 0.60 | |
| 1.6 | 0.5 | 0.95 | |
| 2.0 | 0.65 | 0.16 | |
| 2.5 | 0.7 | 1.56 | |
| 45° | 1.0 | 0.5 | 0.25 |
| 1.2 | 0.5 | 0.35 | |
| 1.6 | 0.5 | 0.55 | |
| 2.0 | 0.65 | 0.68 | |
| 2.5 | 0.7 | 0.9 |
Solder Mask
Due to the repeated plugging and unplugging of gold fingers during use, the presence of a solder mask in the gold finger area may create resistance and pose a risk of solder mask ink peeling off. Therefore, an opening design is typically used in the PCB’s gold finger area.
- The margin for the opening should be approximately 10 mil from the board edge.
- The opening size should be about 4mil larger than the trace on one side, ensuring that no copper is exposed. Copper removal processing can be performed.
- It is not recommended to opening within 2mm of the gold fingers.
Bevel Design
Using gold fingers can result in PCB having more edge angles. These angles are more fragile and have sharp edges, posing a potential risk of injury. Therefore, it is common to choose beveled or rounded corners to address these edge angles. The benefits include:
- Avoiding the generation of greater force during insertion and removal, making the process smoother.
- Mitigating impacts on slots and gold fingers to extend product life.
- Rounded bevels look more refined, aligning with aesthetic standards in some product designs.
- Preventing sharp edges from causing harm to users or maintenance personnel.
Panelization
In the process of single-board production, it is common to consider orienting the gold finger end of the PCB outward for convenience in subsequent processing or testing. For bulk projects, an inverted panelization method is adopted to have the gold finger of the PCB facing inward for the addition of gold wires.
When the single-board size of the gold finger PCB is less than 40*40mm, it is recommended to prioritize chamfering by milling the chamfer into a strip shape before performing the outline milling. During this process, designing locating holes is necessary to ensure accurate positioning during the second outline milling. Before chamfering, through a reasonable CAM process arrangement, ensure that the width of the gold fingers can be maintained above 40mm during automatic chamfering.
PCB Gold Finger Plating Process
- Blue Masking
Before the gold finger plating process, a layer of blue masking is applied to cover the non-plating areas. This gel-like substance effectively blocks chemical agents and plating solutions, protecting the surface properties of these areas.
- Acid Etching
This step involves rinsing the copper surface of the plating area with sulfuric acid. This not only removes any potential oxide layers but also activates the copper surface, improving the plating quality and ensuring a more robust metal adhesion.
- Nickel and Gold Plating
During the plating process, nickel plating is performed first, followed by gold plating. Since nickel has good affinity with copper, it is more suitable for direct contact with the copper surface. In contrast, gold has stronger anti-oxidation properties, making it suitable for the outermost layer. This step involves placing the PCB into an electroplating bath containing the corresponding metal ions. Under the influence of electric current, metal ions are reduced to a solid metal state and deposited on the PCB surface.
- Blue Mask Removal
After gold finger plating is completed, the blue masking needs to be removed. This can be achieved by softening the blue mask with a solvent and then mechanically removing it. Finally, surface cleaning is performed to ensure cleanliness.
This process shares similarities with the electroless gold plating technique, but they are, in fact, two distinct technologies. Electroless gold plating does not involve the use of external electric current, whereas gold finger plating requires the application of electric current.”
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