PCB Tech - Best practice techniques for high-speed routing of PCB boards

The circuit boards design also require extra effort when laying out high-speed circuits. Sensitive networks must be routed according to specific high-speed rules and many other high-speed design requirements that must be followed. These include everything from the organization of the schematic to the placement of components. We will study all of these when discussing high-speed routing technology to help you complete the finish line of your next PCB design.

High-speed design considerations before PCB layout

Compared to those used on standard circuit boards, successfully wiring high-speed circuits requires more preparation. While balancing the usual manufacturing and assembly requirements of circuit boards, signal paths, controlled impedance routing, and EMI must be considered in high-speed designs. It is essential to start preparing before the layout even starts to keep all these needs in order:

Schematic: To help your high-speed wiring, the first thing you can do is to start with a clean schematic. In the PCB layout process, there should be an easy-to-follow high-speed circuit logic flow. Any instructions should also be communicated to the layout staff to avoid any confusion in the future.

Layer stacking: High-speed wiring usually requires stripline or microstrip layer configuration. This provides a shielding layer for sensitive trace routing, which helps prevent EMI problems and maintain the signal integrity of the circuit. Before starting the layout, you should reach an agreement with your PCB contract manufacturer on stacking to provide you with a working foundation and ensure the manufacturability of the circuit board.

Design rules: In addition to the standard trace width and spacing rules, there will also be a new set of high-speed design rules and constraints. These will include specific network types, differential pairs, trace length and topology, and impedance-controlled routing rules. There may also be specific requirements for blind and buried vias, micro vias and other high-speed restrictions.

Once these items are selected from your to-do list, you can start the PCB layout.

Layout and routing techniques for high-speed PCB design

Although there are many high-speed wiring technologies to discuss, the first topic to be discussed is component placement. Good wiring starts with good component layout, regardless of whether the circuit board is designed for high speed or not.

Component placement

Use standard component placement methods, starting with connectors and large CPU and memory devices. To obtain the best signal path while continuing to place parts, follow the logic flow of the schematic. Many of the more important CPU and memory devices will require a large number of bypass capacitors, so be sure to place them immediately, otherwise there may not be enough space to place them in the future. When placing, remember to reserve space for routing channels and vias in the entire board stack. In addition to high-speed requirements, remember that your placement still needs to comply with the design for manufacturability (DFM) rules, and consider the heat dissipation requirements of heat-generating components.

Escape route

Now you are ready to route, but you need to create escape routes for all fine-pitch devices before you start laying the traces. If you are dealing with a large part, such as a BGA package with hundreds or thousands of pins in it, you can access each pin for routing. This accessibility is usually achieved by routing diagonally from the external pin row to the through hole.

For the next row of pins, very short traces are usually used to connect to the through holes between the BGA pads, which are called dog-bone patterns. However, if the BGA pin pitch is too fine, you may need to use vias, micro vias, or both in the pad technology, but you must first obtain the approval of these PCB technologies from the manufacturer. A useful tip here is that component manufacturers usually provide recommended wiring patterns for their parts, so be sure to check there to save some time.

Trace route

After completing the escape routing, it is time to route the remaining boards. If you have fully set up the design rules, you can use automatic interactive routing tools or batch routing tools to manually complete this routing. No matter which method you use, the following points should be kept in mind to ensure successful routing:

High-speed signal paths must be kept short and routed from point to point.

Sensitive traces should be routed on the inner layer sandwiched between the reference planes in the stripline configuration.

Differential pairs must be wired in pairs. Use the automation capabilities of your design system to route these traces and ensure that the pairs are not interrupted by vias or other obstacles.

For network groups whose lengths must all match, start with the longest connection. For the remaining networks in the group, an adjustment function is added to each trace to match the first network routed to the same length. Tuning is usually done by adding a waveform or trombone topology to the trace to extend the trace, and it is usually done automatically by a CAD tool.

Do not route sensitive digital lines through noisy power supplies or analog areas of circuits.

Power and ground plane

Designing a clean power distribution network (PDN) for high-speed PCBs is critical to the overall success of the design. High-speed components generate more noise on the circuit board due to their switching rate, which is controlled by bypass capacitors. It is also important to remember that the ground plane will be used as the reference plane for signal return. Be careful not to route sensitive traces where these signal return paths are blocked by dense via placement, circuit board cuts or split planes, as this will reduce the signal integrity of these traces.

As you can see, high-speed wiring is more than just laying some unique traces on the board. Many aspects of PCB layout must be completed by routing to complete high-speed design. As we said at the beginning, it all starts with setting up the board correctly with your PCB contract manufacturer before the layout begins.

Work with your PCB CM to get the best high-speed routing technology

Although it is usually possible to make old PCB designs work by transforming the layer stacks into layouts, high-speed designs should start with clearly configured stacks. Although PCB designers are usually familiar with different board layer configurations, there are many other variables that need to be considered for high-speed design. These include circuit board materials, controlled impedance wiring, layer pairs, and assembly processes. The best thing you can do is to consult your PCB contract manufacturer first to ensure that you are using the most optimized layer configuration for your design.