Che cos'è un PCB Gold Finger?

Pubblicato il 14/07/2020

Conoscenza di base di Gold Finger PCB

Sulla barra di memoria del computer e sulla scheda grafica, possiamo vedere una fila di contatti conduttivi dorati, chiamati "dita d'oro". Il Gold Finger (o Edge Connector) nel settore della progettazione e produzione di PCB utilizza il connettore del connettore come presa per il collegamento della scheda alla rete esterna.

Gold Fingers PCB is the gold-plated terminal of the card edge connector. Usually, the fingers are made of flash gold (hard gold), and the thickness of gold is required to be 3u to 50u, because the fingers are mainly used for multiple insertions and removals.

When the PCB is repeatedly installed and removed, gold plating is used for the edge connector contacts. As we all know: gold fingers are the edge connector contacts used to plate the pads onto the circuit board. The idea behind the Golden Finger is to help protect the printed circuit board prototype from wear and tear. If the coating is correct and the thickness is appropriate, it is expected to last more than 1,000 times before maintenance is required.

Gold fingers classification and recognition

Gold fingers classification: regular gold finger (flush finger), long and short gold finger (uneven gold finger), segmented gold finger (intermittent gold finger).

Conventional gold finger (flush finger): The rectangular pads of the same length and width are arranged neatly on the edge of the board. There are many types of physical objects such as network cards and video cards. Some small plates have fewer gold fingers.

Long and short gold fingers (uneven gold fingers): rectangular pads with different lengths located on the edge of the board.

Segmented gold fingers (intermittent gold fingers): rectangular pads with different lengths located on the edge of the board, and disconnected in the previous stage.

There is no character frame and label, and the window is usually opened by solder mask. Most shapes have grooves. The golden fingers part protrudes from the edge of the board, or close to the edge of the board. Some boards have gold fingers at both ends. Normal gold fingers are on both sides, and some PCB boards only have single-sided gold fingers. Some gold fingers are wider than others.

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At present, the commonly used gold finger gold plating process mainly has the following two types:

  • Lead the wire from the gold finger as a gold-plated wire. After the gold plating is completed, the leads are removed by milling the shape or etching. However, for products made by this process, there will be lead residues around the gold fingers, resulting in exposed copper, which cannot meet the requirement of not allowing exposed copper.
  • Do not lead from the gold finger, but from the inner or outer circuit of the circuit board connected to the gold finger to realize gold plating of the gold finger. So as to avoid exposing copper around the gold finger. However, when the circuit density is high in the circuit board and the circuit is very dense, it may not be possible to make leads in the circuit layer using this process. Moreover, this kind of process is powerless to isolate the gold finger (that is, the gold finger is not connected to the line).

Gold finger PCB surface treatment

  1. Nickel plating: thickness up to 3-50u”. Because of its excellent conductivity, oxidation resistance and wear resistance, it is widely used in gold finger PCBs that require frequent insertion and removal or PCBs that require frequent mechanical friction. But because the cost of gold plating is extremely high, it is only used for local gold plating such as gold fingers.
  2. Immersion gold: conventional thickness 1u”, up to 3u”. Because of its superior conductivity, flatness and solderability, it is widely used in high-precision PCB boards with buttons, IC, BGA and other designs. For gold finger PCBs that do not require high wear resistance, you can also choose the whole board immersion gold process. The immersion gold process cost is much lower than the electrical gold process cost. The color of the immersion gold process is golden yellow.

Gold finger PCB detail processing

  • In order to increase the wear resistance of gold fingers, gold fingers usually require hard gold plating.
  • Gold fingers need to be chamfered, usually 45°, other angles such as 20°, 30°, etc. If there are no chamfers in the design, there is a problem.
  • The gold finger needs to be processed with a whole piece of solder mask to open the window, and the PIN does not need to open the steel mesh.
  • Immersion tin and immersion silver pads need to be at least 14 mils from the top of the finger. It is recommended that the pad be more than 1 mm away from the finger during the design, including via pads.
  • All layers of the inner layer of the gold finger need to be cut copper, usually the width of the cut copper is 3mm larger.
  • You can do half finger copper cutting and whole finger copper cutting. In the PCIE design, it is also indicated that all copper in the gold finger area should be cut off.

The impedance of the gold finger will be relatively low. Cutting copper (hollowing under the finger) can reduce the impedance difference between the gold finger and the impedance line, and it is also good for ESD. Recommendation: Fully cut copper under the gold finger pad.

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How does gold finger prevent oxidation?

The golden finger is the connecting part between the graphics card (memory module) and the slot. Memory is an important part of the computer. If something goes wrong, it will cause the computer to malfunction, or even not be used normally. The most common cause of memory failure is the oxidation of memory fingers.

Generally seen in machines that are used for half a year or more than one year. Such a fault occurs when the weather is wet or the weather temperature changes. The failure phenomenon is mainly manifested in: unable to boot normally, the display is blank, and the machine can hear the beeping “memory alarm” sound.

Gold fingers are oxidized. Sharp objects cannot be used, but they must have a certain hardness. Therefore, the experience that people find in use is that the effect of using rubber is the best. It can be erased without hurting golden fingers.

To prevent gold fingers from being oxidized, you can start from the following three aspects:

  •  Solve by tightening method.
  •  When we install and overhaul the memory, we must pay attention not to directly touch the golden fingers of the memory slot with our hands. Because the sweat on our hands will stick to the gold fingers of the memory stick, if the gold fingers of the memory are not well-made or not gold-plated at all. Then the memory stick is prone to oxidation of the gold finger during use. Long time will lead to poor contact between the memory module and the memory slot, and finally the memory will be alarmed after booting.
  • If the computer is not used frequently, it is recommended to turn on the machine at least once in a certain period of time, let the machine run for an hour or two, use the heat generated by the machine to drive off the moisture inside the machine, and keep the machine in good operating condition.

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