PCB gold fingers are a common PCB manufacturing requirement of memory sticks, graphics cards, and other product designs requiring edge connections or contact points.
“Fingers” are neat, narrow, usually, long row of PCB pads along the edge of a PCB card (either on one or both, sides of the PCB).The rectangular row of conductive PCB pads are commonly referred to as fingers.
PCB gold fingers have a higher gold thickness than other common PCB pads. According to IPC-4556, the normal PCB gold finger thickness is 32u" (microinch), and the gold thickness range of PCB gold fingers is 3u" to 50u". This gold plated deposit is an alloy, which has a higher hardness than typical immersion gold plating. There are 3 common types or classes of PCB gold fingers.
- Normal - most common PCB gold fingers in a level, even arrangement with the same length, width, and spacing.
- Uneven ones - the pads have the same width but different lengths, and sometimes, different spacing.
- Segmented PCB gold fingers - the PCB pads have different lengths or positions (such as shown below)
PCB edge contacts or fingers provide an electrical connection between the PCB board and system or device. Generally, they have better conductivity and mechanical strength than connectors or wires. They are used primarily because they provide:
- Excellent conductivity
- Strong wear resistance
- Good oxidation resistance due to the hard-gold plating process
Hard gold plating (electroplating) is a surface finish to electroplate gold alloy deposited on the PCB pads.
Hard gold plating is electroplated gold over nickel, which has a higher hardness than immersion gold plating which is gold over copper.
- For an equal thickness of gold plating, immersion gold looks more yellow than hard gold.
- Hard gold plating has better wear resistance and is harder than immersion gold.
- Hard gold plating is applied prior to the solder mask. Immersion gold applied after solder mask is applied.
Processing:
- All but the PCB gold fingers are covered with an adhesive.
- All oxidation is removed from the PCB pad copper surfaces to receive electroplating using a strong acid and thorough wash with water. Following which mechanical abrasion is used to further clean the PCB pads with water wash and a final deionized water rinsh.
- Nickel is electroplated over the copper surface first. Water wash, and a final deionized rinse cleans the nickel plating
- Then gold is electroplated on the already nickel plated, copper pads.
- Finally the adhesive is removed and solder mask printing begins.
Some PCB manufacturers do not process PCB gold fingers because
- The gold salt potassium cyanide (Au(CN)2) used in the hard gold plating is toxic.
- The hard gold plating process discharges chemical wastewater, which requires proper treatment
Some limitations associated with Gold Fingers are:
- Plated pads need located on the edge of PCBs for electroplating (i.e. linked to the panel frame).
- Generally, the maximum pad length is limited to about 40 nm.
- The internal edge connector layers should not contain copper to avoid exposure during edge beveling.
- Apart from flex circuit boards, when plating the top and bottom PCB sides, the minimum allowable space between pads is generally about 150nm.
General Design Guidelines:
- Ensure the internal PCB layers do not contain copper. Copper creates too much contact during beveling when it’s found in the inner layers.
- Ensure a 1mm distance between gold fingers and any Plated Holes.
- Maintain clearance of solder mask and silk screen printing from the gold fingers.
- Keep the fingers oriented on the outside edge of your board.
Applications:
Technical Applications
- A primary application of edge contacts is to link a module circuit board to a power source from another PCB assembly.
- To facilitate the transmission of signals generally between a peripheral and main PCB.
- To interconnect modules together.
- To provide the capability for future features or enhancements by adding new cards or modules for expansion.
Commercial Applications
- To provide connection for data transmission on network-enabled gadgets such as network sim cards and other networking circuit board modules.
- To link domain adapters. Edge contacts offer an attachment place for special adapters such as flash disks.
- To link other modules to the primary processing PCB.
For all your sensitive electronic assembly needs, including specialized processing such as low temperature applications, please send us your project requirements now, either through our online quote tool, or to orders@optimatech.net. We strive to produce the highest quality PCBs and assemblies to meet all our customer needs.
