PCB Routing Techniques: Best Practices

Using GPS when planning a trip is utterly convenient: just plug in a destination, and the gentle automated navigator will offer turn-by-turn guidance. It’s hard to believe that only a generation ago, people relied on physical maps where the information was static, and a budding cartographer had to invest time and chart the route with a pen. Routing signals on a PCB is like the lost art of manual navigation mixed with connect-the-dots (and possibly the 3-houses, 3-utilities puzzle for the mathematically inclined). Determining the best solution through a multicolored maze of traces can be challenging; thankfully, some of the same tricks from analog route mapping (like highlighting the route against the background/surrounding features) are built-in features on modern ECAD tools. PCB routing techniques must leverage these tools and general best practices to handle the demands of modern HDI boards effectively.

Pre-Routing Checklist for DFM Design

Schematic	A thorough schematic review will help reduce in-design revisions for component footprint updates/corrections.
Stackup	Determining the impedance structure of the various transmission lines by layer will give designers their first insight into constraint-led layout. Primarily, this will be the trace width according to impedance.
Design rules	Designers can determine the ruleset to maximize production without impacting performance from the schematic, stackup, and communication with the manufacturer.
Placement	Keeping traces short and direct will improve signal performance; careful arrangement will minimize via transitions for components with large fanouts.

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Start with a Solid Foundation in the Schematic

In the same way that a road trip needs the right map to follow, your PCB design needs an accurate and complete schematic for the layout to follow. Without it you won’t just be lost in your layout, you won’t even be able to get started. In addition to the circuitry, here are some of the things that your schematic should have to successfully drive the layout:

✓    Logical circuitry flow: The schematic should follow a logical flow of its circuitry to layout the components of a PCB design and route their nets correctly.

✓    High-speed signal paths: These critical nets must be routed precisely in the layout for the best performance of the circuit board. The schematic must identify these nets (clocks, data, memory, etc.) so that the layout team can prioritize their routing.

✓    Power delivery network: Another critical aspect of PCB routing is to create a clean power delivery network (PDN) and adequate ground planes for signal return paths and noise control. The layout team must identify potential areas of concern for power, ground, and isolation between digital and analog PDNs.

✓    Design rules: Some manufacturing constraints established in the schematic pass into the PCB layout through the netlist. These can include high-speed constraints, groups or classes of nets, and specifying trace widths for controlled impedance lines or power and ground routing.

✓    Cross-probing: A helpful tool for the layout designer is the ability to cross-probe with the schematic. Any setup required on the schematic side should be ready for the layout team to use this feature.

With a good schematic, the PCB layout designer is now ready to begin the first step of successful routing by carefully placing the components.

A Good Component Placement Will Lead to Successful Trace Routing

Tracing routing can complicate PCB design when component placement lacks optimization for the most direct routing paths, adding unnecessary impedance and potentially endangering signal integrity. To do this, the PCB layout designer needs to place components as represented by the logic flow in the schematic. Not only will this optimize the trace routing, but it will promote better signal integrity performance.

The first step towards good component placement is to work backward from connectors and other fixed components and place the processors and memory chips connected to them. Next, place the parts directly associated with those already on the board, followed by the remaining non-critical components. Remember to reserve enough space between components for routing, especially for dense parts like high pin-count BGA devices. Don’t forget that in addition to the room necessary for trace and PDN routing, placement must optimize passing design for manufacturability (DFM) rules.

PCB Routing Techniques That Can Help

While a successful PCB trace routing strategy relies on a solid schematic and good component placement, it also takes experience, like most things. Simply put, the more boards designers encounter, the more techniques they’ll have for optimizing the layout. There are, however, some features in PCB design software that can provide a lot of help with trace routing, including:

• Automatic routing features such as batch auto-routing and auto-interactive routers can provide a springboard for a general solution that designers can then fine-tune. Careful use of this tool is key: designers should not be confident in the results of an automated layout without review and routing clean-up.
• Editing features that control options (like how much of a trace to rip up when deleting them). These are often user-configurable and can be set to delete the entire line or just to a particular segment.
• Push and shove features that will move existing traces and vias out of the way when routing in new traces. This tool can help align routes and squeeze the maximum routable area from the board without constantly placing new tracks.
• Display features that control the color of different layers or highlight specific nets.

In addition to the functionality of the CAD tools that can help, remember that trace routing is also an artistic task. Embrace the freedom to be creative in routing, as the design is rarely simple enough for a clear-cut best solution. Don’t be afraid to rip up and re-route or move large portions of circuitry to create a better routing pattern. While at it, please use those design rules and constraints included in the system’s online error checking (ideally set up before the layout begins).

Many other PCB routing techniques can help increase design productivity; your local PCB contract manufacturer can also assist with the trickier aspects of DFM layout. At VSE, we have made a career of working with PCB design engineers who want to create the best possible layouts for their designs. Our engineering team is ready to work with you to optimize your board’s layout for its best performance and error-free manufacturing.