Surface treatment methods and processing considerations of PCB gold finger

Definition of Gold Finger

(Gold Finger or Edge Connector) Insert one end of the PCB into the connector slot, use the connector pin as the outlet of the external connection of the pcb board, make the pad or copper sheet contact the pin at the corresponding position to achieve the purpose of continuity, and deposit gold or nickel gold on the pad or copper sheet of the pcb board, which is called gold finger because of its finger shape.

Surface treatment method of PCB gold finger

1. Nickel gold plating: the thickness can reach 3-50u ". Because of its superior conductivity, oxidation resistance and wear resistance, it is widely used on gold finger PCBs that need to be plugged or mechanically rubbed frequently. However, because of the high cost of gold plating, it is only used for local gold plating such as gold finger, also known as: selective gold plating process. The color of the gold plating process is silver white, not as yellow as the gold plating process, The disadvantage is that the weldability is slightly poor.

2. Immersion gold: the thickness is 1 u "in general, up to 3 u" in maximum. Because of its superior conductivity, flatness and solderability, it is widely used in high-precision PCB boards designed by key position, binding IC, BGA, etc. For gold finger PCB with low wear resistance requirements, you can also choose the whole board immersion gold process. The cost of the immersion gold process is much lower than that of the electrometal process. The color of the gilding process is golden yellow.

Precautions for Gold Finger Design

When seeing the shape and package similar to the figure below during PCB design, the first reaction is that there are gold fingers on the board. Simple judgment methods for golden finger: devices with PIN on TOP and BOTTOM surfaces; There will be a fool proof U-shaped slot as shown in the following figure.

When there are gold fingers on the board, the details of the gold fingers need to be handled properly.

Gold finger detail processing in PCB:

1) For PCB boards that often need to be plugged in and out, gold fingers usually need to be plated with hard gold in order to increase their wear resistance.
2) The gold finger needs to be chamfered, usually 45 °, and 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 make all solder mask as whole openings, and the PIN does not need to make the stencil.
4) The minimum distance between tin and silver pads and the tip of the finger is 14mil; It is recommended that the pad should be more than 1mm from the finger position, including via pad.
5) The surface of gold finger shall not be covered with copper.
6) All layers of the inner layer of the gold finger need to be copper chipped, usually 3mm wide; Can do half finger copper cutting and whole finger copper cutting. In the PCIE design, it is also indicated that the copper in the gold finger area should be cut off completely.
The impedance of the gold finger is relatively low. Cutting copper (digging holes under the finger) can reduce the impedance difference between the gold finger and the impedance line, and it is also good for ESD.

Suggestion: Fully cut copper under the gold finger pad.