Gold Finger

On computer memory sticks and graphics cards, we can see a row of golden conductive contacts, which are called “golden fingers”. The Gold Finger (or Edge Connector) in the PCB design and production industry uses the connector of the connector as the outlet for the board to connect to the network. Next, let’s understand how to deal with gold fingers in PCB and some details.

Surface treatment method of gold finger PCB
1. Electroplating nickel gold: thickness up to 3-50u”, because of its superior conductivity, oxidation resistance and wear resistance, it is widely used in gold finger PCBs that require frequent insertion and removal or PCB boards that require frequent mechanical friction Above, but because of the high cost of gold plating, it is only used for partial gold plating such as gold fingers.

2. Immersion gold: The thickness is conventional 1u”, up to 3u” because of its superior conductivity, flatness and solderability, it is widely used in high-precision PCB boards with button positions, bonded IC, BGA, etc. Gold finger PCBs with low wear resistance requirements 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 Immersion Gold is golden yellow.

Gold finger detail processing in PCB
1) In order to increase the wear resistance of gold fingers, gold fingers usually need to be plated with hard gold.
2) Golden 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; the 45° chamfer in the PCB is shown in the figure below:

3) The gold finger needs to be treated as a whole piece of solder mask to open the window, and the PIN does not need to open the steel mesh;
4) Immersion tin and silver immersion pads need to be at a minimum distance of 14mil from the top of the finger; it is recommended that the pad is more than 1mm away from the finger during design, including via pads;
5) Do not spread copper on the surface of the gold finger;
6) 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; it can be used for half-finger cut copper and whole finger cut copper.

Is the “gold” of gold fingers gold?

First, let’s understand two concepts: soft gold and hard gold. Soft gold, generally softer gold. Hard gold is generally a compound of harder gold.

The main function of the golden finger is to connect, so it must have good electrical conductivity, wear resistance, oxidation resistance and corrosion resistance.

Because the texture of pure gold (gold) is relatively soft, gold fingers generally do not use gold, but only a layer of “hard gold (gold compound)” is electroplated on it, which can not only obtain good conductivity of gold, but also make it resistant Abrasion performance and oxidation resistance.

So has the PCB used “soft gold”? The answer is of course there is use, such as the contact surface of some mobile phone buttons, COB (Chip On Board) with aluminum wire and so on. The use of soft gold is generally to deposit nickel gold on the circuit board by electroplating, and its thickness control is more flexible.