18.104.22.168. PCIe Gen5 Add-in Card Edge Finger Breakout Design Guidelines
Ensure inner layer ground under edge-fingers in the high-speed region comprising pins A12/B12 and beyond. The inner layer ground plane must extend to cover the full length of the edge finger region from the main routing area of the board. The inner layer ground plane must lie at least 0.52 mm (20.5 mil) below the edge finger copper. This requirement applies to both sides of the add-in card, so a symmetric pair of shielding planes is used.
Connect a row of plated vias to the inner layer ground plane along the bottom of the edge fingers in the high-speed region comprising pins A12/B12 and beyond. These vias are known as fingertip south vias. The vias must be plated thru holes (PTH). You can share them among ground pads on both surfaces of the add-in card. The upper boundary of the via pad must align with the 3.20 mm dimension. Join ground vias in the “I bar” with surface metal.
Align add-in card ground vias serving the north edge-finger ground conductors with the gap between adjacent edge-fingers, to reduce obstruction to signals routed from non-ground edge fingers. The axes of the north ground vias must be no more than 0.38 mm (15 mil) from the boundary of the edge finger pin field. Connect the edge fingers to the ground via with a length of trace whose width matches or exceeds the via pad diameter to minimize the inductance of the ground connection.
Implement a lateral ground bar to join all the fingertip south vias on the first inner layer (N-1) on each side of the board (Metal 2, for example). The ground bar must align with the north edge of the vias with a distance of 3.20 mm. The ground bar should be 0.71 mm wide, to ensure adequate clearance from the chamfer region.
Ensure the edge-fingers that are not assigned to ground in the region A12/B12 and beyond are 3.00 mm long and 0.60 mm wide (±0.038mm) (refer to Add-in-Card Edge Fingers Indicating Edge Finger Length). Ensure the upper of the edge-finger is 5.60 mm above the south edge of the add-in card (refer to Add-in-Card Edge Fingers Indicating Edge Finger Length). Small amounts of residual surface metal are permitted in the region extending 0.13 mm beyond the lower end of the edge finger.
Ensure the trace length from the top of an auxiliary signal, or unused edge-finger, to the DC blocking capacitor in the termination circuit is as short as practicable. PCI CEM 5.0 specifies no maximum trace length. Maintain a 42.5Ω trace impedance for traces between edge fingers and DC blocking capacitors. The ground via for the termination network must lie within 1.0 mm (39.4 mil) of the resistor component pad or through-hole.
Did you find the information on this page useful?