Electronic – What are the advantages of this gold finger shape

connectorpcb-designpci

Some PCBs, like the PCI card specification have gold fingers which start very narrow near the bottom edge, and gain their usual width much higher, where the actual contact is expected to be made.

What is the advantage of having the narrow part?

PCB with gold fingers narrow at bottom, widening progressively.

Why not make the pad fully wide all the way to bottom, like ISA cards, DDR, etc.? Or simply make the finger shorter, only in the area of contact? What is better in gradually increasing the width?

My speculation:

  • To connect ground pins first – All the pins have this shape.
  • Resistance against peeling the pad off – The smaller trace seems much more susceptible to damage
  • Insertion force – I expect the narrow part to be made of equally thick gold, which would require the same amount of force.
  • Insertion force – Can it be that some number of the connector contacts (in motherboard) get pushed sideways in each stage as the card goes in, lessening the amount of force needed to insert the board?

Can't seem to find any evidence or description why this is designed this way. Some high frequency high pin count stuff (DDR modules) use rectangular pads.

Note: See page 196 of the linked PCI card specification document.

Best Answer

To electroplate the fingers with gold they must all be joined together electrically. This is done with a "plating bar" trace outside the final board area, which is cut off afterwards.

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Usually the board edge will be chamfered for easier insertion in the socket. Since chamfering removes the lower part of the fingers they only have to be wide enough to carry the electroplating current. Making them narrower saves gold, which makes the board cheaper. If the board is not intended to be plugged in often then chamfering may not be applied, and then the narrow parts remain.

Gold plating for edge connectors