Guidelines for routing PCB traces are necessary when trying to maintain signal integrity and lower EMI. Although there seems to be an endless number of precautions, this section provides only a few main recommendations as layout guidance.
- Reduce intra-pair skew in a differential trace by introducing small meandering corrections at the point of mismatch.
- Reduce inter-pair skew, caused by component placement and IC pinouts, by making larger meandering correction along the signal path. Use chamfered corners with a length-to-trace width ratio of between 3 and 5. The distance between bends must be 8 to 10 times the trace width
- Use 45° bends instead of right-angle (90°) bends. Right-angle bends increase the effective trace width, which changes the differential trace impedance creating large discontinuities. A 45° bends is seen as a smaller discontinuity.
- When routing around an object, route both trace of a pair in parallel. Splitting the traces changes the line-to-line spacing, thus causing the differential impedance to change and discontinuities to occur
- Place passive components within the signal path, such as source-matching resistors or AC coupling capacitors, next to each other. Routing as in case a) creates wider trace spacing than in b). However, the resulting discontinuity is limited to a far narrower area.
- When routing traces next to a via or between an array of vias, make sure that the via clearance section does not interrupt the path of the return current on the ground plane below
- Avoid metal layers and traces underneath or between the pads off the DisplayPort connectors for better impedance matching. Otherwise, they cause the differential impedance to drop below 75 Ω and fail the board during TDR testing.
- Use the smallest size possible for signal trace vias and DisplayPort connector pads as they have less impact on the 100 Ω differential impedance. Large vias and pads can cause the impedance to drop below 85 Ω.
- Use solid power and ground planes for 100 Ω impedance control and minimum power noise.
- For 100 Ω differential impedance use the smallest trace spacing possible, which is usually specified by the PCB vendor.
- Keep the trace length between the DisplayPort connector and the DisplayPort device as short as possible to minimize attenuation.
- Use good DisplayPort connectors whose impedances meet the specifications.
- Place bulk capacitors (for example, 10 µF) close to power sources, such as voltage regulators or where the power is supplied to the PCB.
- Place smaller 0.1-µF or 0.01-µF capacitors at the device.