Gold finger design and processing of PCB

                                  Gold finger design and processing of PCB

This article comes from Jeddah

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1. The definition of gold finger

   (Gold Finger or Edge Connector) Insert one end of the PCB board into the connector card slot, use the pins of the connector as the outlet for the external connection of the pcb board, and make the pad or copper contact with the pins at the corresponding position to achieve conduction. For the purpose of communication, and immerse gold or plate nickel gold on this pad or copper skin of the pcb board, because it is in the shape of a finger, it is called a gold finger.

2. The surface treatment method of gold finger PCB

Electroplated nickel gold: the thickness can reach 3-50u", because of its superior electrical conductivity, oxidation resistance and wear resistance, it is widely used on gold finger PCBs that need to be plugged in and out frequently or PCB boards that need to be mechanically rubbed frequently. However, because the cost of gold plating is extremely high, it is only used in local gold plating treatment such as gold fingers. Jeddah PCB is called: selective electro-gold process, the color of electro-gold process is silvery white, not as yellow as immersion gold, the disadvantage is Slightly less weldable.

Figure: Selective electric gold finger PCB (halving tin + gold finger area electric gold)

2. Immersion gold: The thickness is generally 1u", up to 3u", because of its superior conductivity, flatness and solderability, it is widely used in high-precisi high- on PCB boards with button positions, bound ICs, BGAs, etc. Gold finger PCBs that do not require high wear resistance can also choose the whole board immersion gold process. The cost of the immersion gold process is much lower than that of the electro-gold process. The color of the immersion gold process is golden yellow.

Figure: full board immersion gold PCB

3. Matters needing attention when designing gold fingers

When seeing a shape and package similar to the figure below during PCB design, the first reaction is that there are golden fingers on the board. A relatively simple method for judging cheats: devices with PINs on the TOP and BOTTOM sides of the device; there will be a fool-proof U-shaped groove as shown in the figure below.

When there are gold fingers on the board, it is necessary to do a good job in the details of the gold fingers.

PCB gold finger detail processing:

1) For PCB boards that often need to be plugged and unplugged, in order to increase the wear resistance of the gold fingers, the gold fingers usually need to be electroplated with hard gold.

2) Gold fingers need to be chamfered, usually 45°, other angles such as 20°, 30°, etc. If there is no chamfer in the design, there is a problem; as shown in the figure below, the arrow shows a 45° chamfer:

3) The gold finger needs to be treated with a whole piece of solder mask and window opening, and the PIN does not need to be stenciled;

4) The immersion tin and immersion silver pads need a minimum distance of 14mil from the top of the finger; it is recommended to design

When the pad is more than 1mm away from the finger position, including the via pad;

5) Do not spread copper on the surface of the gold finger;

The following picture shows the design of a golden finger for reference:

6) All layers of the inner layer of the gold finger need to be copper-cut, and the copper-cut width is usually 3mm larger; half-finger copper-cut and whole-finger copper-cut can be done. In the PCIE design, it is also specified that the copper in the gold finger area should be completely cut off;

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

Suggestion: remove all copper under the gold finger pad.

4. Matters needing attention in Jeddobang PCB golden finger processing

1. Plates with a thickness of 1.2-2.4m can be bevelled. Beveled edges cannot be processed outside this plate thickness range.

2. The depth and angle of the hypotenuse are generally between 20-45 degrees by default. There should be sufficient distance between the gold finger and the edge of the board. According to the board thickness or design requirements of different gold finger PCBs, we recommend that the distance between the gold finger and the edge of the board is 0.6-1.5mm during design, so as to avoid damage to the gold when the bevel is used. Fingers, if the distance between the gold finger and the edge of the board is less than 0.6mm, the beveled edge treatment is required, and there is a risk of injury to the gold finger. The following are several common bevel angles and depth diagrams for your reference.

Plate thickness 1.6mm, bevel depth 1.4mm, bevel angle 25 degrees

Plate thickness 1.6mm, bevel depth 1.3mm, bevel angle 25 degrees

Plate thickness 1.8mm, bevel depth 1.8mm bevel angle 20-25 degrees

Plate thickness 1.2mm, bevel depth 0.4mm, bevel angle 45 degrees

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Professional PCB manufacturer--Shenzhen Jedobang Technology Co., Ltd. research and arrangement