The product information

PCI Express board PCB design

2021-12-16

PCI Express board PCB design

The routing length of the card from the edge of the golden finger to the PCIe chip pin shall be limited to 4 inches (about 100mm). After exceeding this length, the high-frequency differential transmission line needs to be used, and we can provide a technical scheme to extend it by more than 300mm.
● the perpn, petpn and peckpn of PCIe are three differential pairs. The peckpn is a differential signal line with a frequency of 100MHz, which needs to be protected. The first two pairs are differential signal lines with a frequency of 2.5GHz, which needs to be protected.
● the two routing lines in the differential pair shall be wired synchronously. If the route needs to turn, the two routes should turn synchronously, and the sharp angle and right angle should be avoided. Instead, arc or obtuse angle should be used.

● via vias shall be avoided as far as possible in the process of differential alignment routing. If it is necessary to change layers through vias, the two routing lines shall make vias simultaneously, and GND signal vias shall be placed near the signal alignment vias. If conditions permit, the number of peripheral GND signal vias shall be appropriately increased.

● the length difference between the two routing lines in the differential pair should be controlled within 5MIL, with a maximum of 10mil (about 0.25mm). The recommended line width of PCB routing is 7mil (about 0.18mm), and the recommended net spacing between the two routing lines is 7mil. For detailed analysis of line width and line spacing, please refer to PCIe specification.
● the distance between two pairs of differential pair lines (such as per pair and peck pair), or the distance between differential pair lines and other non PCIe signals, is recommended to be no less than 20MIL (about 0.5mm) to reduce mutual crosstalk and electromagnetic interference (EMI). It is recommended to isolate two pairs of differential pairs with GND ground wire, for example, GND, peck pair, GND, per pair, GND, pet pair and GND from left to right.
● PCIe chip, especially the PCB reverse side of PCIe signal line, should avoid high-frequency signal line as far as possible, and it is best to lay copper on the whole GND floor. For example, the SCL signal line and iord signal line of ch367 chip are relatively high-frequency signal lines. It is recommended not to run through PCIe chip.
● PCIe needs to be at the sending end (petpn) and the receiving end of the other party shall be AC coupled. The two AC coupling capacitors of the differential pair must have the same package size, be symmetrical, and be placed near the golden finger. It is recommended to select a high-frequency capacitor with a capacity of 0.1uF. The package size is recommended to use 0402. In addition, 0603 is also acceptable, but the capacitor of in-line package is not allowed.
● when designing PCB, a high-frequency decoupling capacitor with a capacity of about 0.1uF shall be placed near each pair of power pins (vcc18gnd and vcc33gnd) of PCIe chip, and the distance from the chip shall not be too far. In addition, vcc18 and vcc33 of the whole chip need a tantalum capacitor with a capacity of not less than 10uF for medium and low frequency decoupling.
● due to the high resistance of PCB via (via) and its instability due to long-term high-temperature aging, in order to reduce the voltage drop consumed, considering the hundreds of ma current between vcc18vcc33 and GND, it is recommended to use large via, double via or double loop power supply (two paths) for the power supply connecting PCIe chip or the via on GND wiring.

● some chipsets and mainboards support PCIe cards with hot plug. In order to support hot plug, two additional processing needs to be done during card design: first, when designing the circuit schematic diagram, please jumper a 1000pf chip capacitor between the perst# pin signal line of PCIe chip and GND; Second, when designing PCB, please design the gold finger of perst# pin signal of PCIe as a short gold finger with the same length as prsnt1# length.