PCB Gold Finger
What is PCB Gold Finger?
On computer memory sticks and graphics cards, we can see a row of golden conductive contacts, which are called “gold fingers”. The Gold Finger (or Edge Connector) in the PCB design and production industry uses the connector as the outlet for the board to connect to the network. Make the pad or copper contact the pin at the corresponding position to achieve the purpose of conduction, and plate the pad or copper on the PCB board with nickel and gold, which is called a gold finger because it is shaped like a finger. Gold was chosen because of its superior conductivity, oxidation resistance and abrasion resistance.
PCB Gold Finger Plating Thickness
- Electroplating Nickel Gold: The thicknessof goldup 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.
- Immersion Gold(ENIG): the thicknessof gold is conventional 1u”- 3u”, because of its superior conductivity, flatness and solderability, it is widely used in high-precision PCB boards with button positions, binding IC, BGA, etc. Gold finger PCBs with low wear resistance requirements can also choose the whole board immersion gold process, which is much lower than the electro-gold process cost. The color of Immersion Gold is golden yellow.
Gold Finger Details Processing in PCB
- In order to increase the wear resistance of gold fingers, gold fingers usually need to be plated with hard gold.
- 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 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 stencil;
- Immersion tin and immersion silver pads need to be at a minimum distance of 14mil from the top of the finger; it is recommended that the pad be designed to be at least 1mm away from the finger, including via pads;
- Do not cover the surface of the gold finger with copper;
- All layers of the inner layer of the gold finger need to be cuttedcopper, usually the width of the cut copper is 3mm larger; it can be done half-finger to cut copper and whole finger to cut copper.
PCB Gold Finger Repair
There are several instances where the gold contacts on PCBs need to be repaired. IPC A-610 discusses several of these cases. One of the more common defects is when solder “splashes” onto the contact during the wave or selective soldering process, thereby contaminating it. This requires that the operator strip away the solder, first by wicking off excess solder and then to a greater degree via mechanical and chemical stripping means. This is then followed by a repairing of the contacts. Another defect is when “pitting” occurs in the contact area. This pitting is a result of a defective plating operation. Many times, this defect can be buffed out and then it can be replated. Other times, the contact area is scratched, which similarly requires the buffing out of the scratch followed by replating.
Gold repairing of these contacts is a process that requires setup, special chemistry that requires advanced PCB repair tech skill, and the correct materials and training.
The operator skills required for gold contact replairing are similar to those required for micro device rework and PCB repair. Patience, dexterity and the ability to improvise correctly if the standard process followed does not go as planned are all attributes of a PCB rework and repair technician. The most experienced and skilled techs are the right ones for being trained to perform PCB repair techniques such as gold finger replating.
PCB Gold Finger Beveling
On a circuit board, the PCB gold finger plating process is used after the solder mask and before the surface finish. The plating process generally consists of the following steps:
- Nickel plating: Between three and six microns of nickel are plated first to the connector edges of the fingers.
- Gold plating: Between one and two microns of hard gold are plated over the nickel. The gold is usually enhanced with cobalt for boosted surface resistance.
- Beveling: The connector edges are beveled at specified angles for easier insertion on corresponding slots. Beveling is typically done at angles of 30 to 45 degrees.
On some circuit boards, certain gold fingers will be longer or shorter than others. For example, a circuit board may have longer fingers at one end. This way, the PCB is easier to insert into a slot, as the end with the longer fingers will snap into place more readily. From there, you simply push the other end of the PCB into place.
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