Part 4 of the 10 fundamental rules of high-speed PCB design deals with the routing of critical signals and return path discontinuities. Needless to say, matched delay and length, differential pairs, and other critical signals should be routed first with the precision they require before less important low-speed and static signals are completed. Maintaining this priority is imperative.
Note that the logic schematic diagram masks details crucial to the operation of unintentional signal pathways vital to the understanding of signal performance, crosstalk, and electromagnetic radiation emanating from the board. Therefore, it is just as important to understand the flow of the return current path of critical signals because these can influence the signal integrity and electromagnetic compliancy (EMC).
VI. Route the Board Based on Critical Signals: Adhere to the defined routing strategy. Clock signals should always have the longest delay of the group. Differential pairs should maintain constant impedance along the entire length.
As mentioned in Part 1 of this series, before starting placement and routing, detailed interconnect routing constraints should be established. These constraints—based on pre-layout simulation, manufacturing restrictions, and the IC manufacturer’s recommendations and guidelines—will control the placement and routing processes. Online design rule checks (DRCs) will warn the designer when a constraint is violated.
When we draw a schematic by functionality, I think that we should also place and route by functionality. By doing this, you can add your own creativity and make decisions on the fly while still taking advantage of the automation. The most efficient approach is to cross-probe between the schematic and the PCB/routing editor. Also, having two extended monitors or a new 34-inch curved screen monitor—as I just purchased—makes this very easy to implement. I read that curved monitors make a straight line look bent and are not suitable for CAD applications. However, that is definitely not the case as the screen curvature follows the natural profile of the eye and the graphics appear crystal clear and undistorted over the entire screen.
On a multilayer PCB, critical signals should be routed on a stripline (inner layer) adjacent to a solid reference plane to reduce radiation. The spacing between the signal trace and return plane should be as small as possible to increase coupling and reduce loop area.
To read this entire column, which appeared in the December 2018 issue of Design007 Magazine, click here.