What are PCB Gold Fingers?

PCB Basics 
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Editorial Team - PCB Directory

Jan 15, 2020

Gold Fingers are the gold plated narrow connectors found on the edge of printed circuit boards to enable connections between multiple boards. They are made from flesh gold, the hardest form of gold available and work for a long time with superior conductivity. The thickness of gold fingers usually ranges from 3 to 50 microns.

Gold is chosen for these fingers as it has the highest corrosion resistance and electrical conductivity after Copper and Silver. Sometimes, Gold is combined with Cobalt and Nickel to increase the resistance of the fingers for wear and tear. PCBs are connected/disconnected from each other multiple times. So these connection points (fingers) need to be able to handle some wear and tear.

What is PCB Gold Finger Beveling? 

The PCB Gold Fingers plating process starts after the solder mask deposition and before the surface finish. It includes the following steps:

  1. Nickel Plating: Initially, between 2 to 6 microns of Nickel is plated to the connector edges of the fingers.
  2. Gold Plating: In this step, between 1 to 2 microns of hard Gold is plated over the Nickel layer. In general practice, Cobalt is also added to Gold to boost the surface resistance.
  3. Beveling: The edges are then beveled/ tapered at a particular angle (30 to 45 degree) to make easier insertion to the corresponding slot

Design Specifications for PCB Gold Fingers:

  • The inner PCB layers towards the PCB edges must be Copper-free, to prevent exposure at the time of Beveling.
  • It is not advisable to include the plated through holes (PTH) within 1 mm of Gold Fingers.
  • Maintain at least 0.5 mm of distance between the gold fingers and the board outline.
  • Any compromise with the standard spacing values can lead to weak and malfunctioned PCB board.
  • No soldermask or screen printing should be performed near the Gold Fingers.
  • The Gold Fingers should be placed facing outwards from the PCB’s center.  

PCBs with Non-uniform Gold Fingers:

For some PCBs, the gold fingers are intended to be shorter than others. The most relevant example of such PCB is the one used for memory card readers, where the device linked with long fingers must be powered first to those connected with the shorter fingers. 

PCBs with Segmented Gold Fingers

Segmented gold fingers vary in length and some of them are also disjoined within the same fingers of the same PCB. Such PCBs are suitable for water-resistant and rugged electronics.

pcb gold finger.jpg

Quality Measures for PCB Gold Fingers:

The Association Connecting Electronics Industries (IPC) has prescribed some standards for the production of PCB Gold Fingers. The IPC standards are summarized as follows:

  • Chemical composition: To achieve maximum rigidity along the edges of PCB Gold Fingers, the gold plating should consist of between 5 to 10% Cobalt.
  • Thickness: The plating thickness should be in the range of 2 to 50 microinches. The standard thicknesses by size are 0.031 inches, 0.062 inches, 0.093 inches, and 0.125 inches.
  • Visual test: The visual inspection is performed through a magnifying lens. The contact edges should be smooth, have a clean surface and also be free from excess plating like Nickel.
  • Tape test: It is conducted to check the adhesiveness of the gold plating along the contacts. In this test, a strip of tape is fastened over the contact edges followed by removing the same. In the next step, the tape is inspected for traces of plating. If any gold is visible on the tape, then the plating is considered as insufficient for continuous injection and ejection.
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