PCB Routing Essentials for the Modern Designer

Key Takeaways

• Considerations for routing complex ICs and differential pairs
• Tips and tricks for using your PCB editor’s autorouter
• Transmission line and high-speed signal routing techniques

There’s a lot that goes into routing within a modern PCB. Read on for tips and methodologies to help your PCB routing process.

Early in the development of printed circuit boards, PCB routing was a fairly straightforward process. All the traces had a fixed width and spacing, save for a couple power and ground traces. At worst, on occasion, a couple traces had to be made wider or narrower manually.

Now, however, PCB and miniaturization technology has grown by leaps and bounds and designers face a multitude of new challenges in PCB routing. These challenges require them to utilize multiple new tools. Simple routing rules remain in the past, and in place are tight manufacturing requirements, high-speed design rules, and various constraints that require designers to utilize advanced PCB layout and routing software.

We’ll be examining some of the tools, regulations, and skills that PCB designers must utilize to route the next generations of PCBs, including differential pair routing, complex ICs, autorouting, transmission lines, design rules, and other tips that may benefit designers doing routing.

Differential Pair Routing

Differential pairs kept symmetrical as per good practice

Generally, when routing a variety of traces to their components, the ground plane often provides a return path for the signal. This is known as single-ended routing, and is how many PCB nets are routed. The issue with single-ended routing is that it experiences a variety of integrity issues including electromagnetic interference (EMI), low signal-to-noise-ratio (SNR), and crosstalk. This is where differential pair routing comes in.

Differential routing utilizes two complementary signals, with one inverted from the other to transfer a single data signal. The receiver utilizes the difference between the two signals to extract the data. Any interference on the lines will exist equally in both signals, such that when the receiver subtracts the two signals, the noise is essentially canceled out. Sounds great, doesn’t it?

Routing differential pairs, however, is more challenging than simply adding another signal. It requires specific rules such as trace width, spacings, and more to ensure performance and retain these previously discussed benefits. Here are key rules to keep in mind:

1. The first essential requirement of differential pair PCB routing is ensuring that the lengths of the two traces are equal. If the line lengths are not kept equal, then the pair might experience interference and integrity issues. One line might be noisier than the other, resulting in common-mode noise and other EMI problems. The greater the rise and fall times of the signal are, the more this is exacerbated. In order to ensure equal lengths, utilize trace tuning to lengthen or shorten one of the pairs until they are both equally long.
2. Similarly, the spacing and widths of the differential pair traces must be consistent throughout their lengths. If the spacing or widths of traces change, this affects the differential impedance between the two traces, creating a mismatch that can result in EMI, crosstalk, and noise. To retain integrity, traces must be routed together and retain the same widths. This can become a problem when needing to route around various other components and vias, so make sure to utilize your PCB software’s trace tuning tool.
3. Route both parts of the differential pairs together and avoid using vias, as it creates easy opportunities for length and impedance mismatches. In case vias are necessary, keep them close together and place them equally far from pads. Furthermore, isolate vias from other traces by specifying a clearance, oftentimes around three times that of the normal trace width. 
4. As a whole, differential pairs should be as symmetrical as possible; plan pad entry and exit routing so that they continue to mirror each other. Avoid obstacles to maintain symmetry, maintaining the same trace width throughout.

Routing Complex ICs

Complex ICs such as BGAs may require smaller trace widths

When routing complex ICs with high-speed signals, start by planning the board layers and configuration. Squeezing compact routing into a couple layers may be great for miniaturization, but may not be ideal for signal integrity.

Set up the rules and high-speed constraints for your design into your PCB software. Many of the rules should already be specified in the schematic depending on the specific ICs you may use.

When planning out your signal paths, consider the whole path including the return, rather than simply point-to-point connections. Consult your schematic often to place parts sequentially, keeping the entire path from the driver to the receiver as close as possible. This is ultimately to help keep your PCB routing short and maintain signal integrity.

There will be significantly more nets to route, especially with complex ICs, so in layout, plan to have room for via escape patterns. Memory devices and ICs should be kept close together and placed in order, starting with the lowest data bit and ending with the highest.

Daisy-chain routing can be used for address and control routing, with point-to-point for other sensitive circuits such as differential pairs. Depending on the IC you use, it will likely be important to keep your trace lengths a precise distance, especially with clock-based ICs.

Utilizing Your Autorouter

Utilizing an autorouter can save you lots of time and energy if done correctly.

Routing traces by hand can be quite challenging when designing a larger, more complex board. This is where automated PCB routing comes in, freeing up employee resources and allowing for more time on other parts of the design. Many autorouters are now built into CAD systems and contain many advantages such as ease-of-use and superior results.

Before starting the autorouter, it is important as a designer to go through the rules and constraints to set up net classes with specific trace widths and spacing for each class. The autorouter will pull its trace widths and spacings from the existing rules and constraints, so ensure it reflects how you want your design to behave.

Make sure to set up specific keep-out zones and areas, as you’d do with regular component placement and routing. When operating, the autorouter will avoid running traces through these areas. Take some time and familiarize yourself with your autorouter’s routing options. Some popular options include escape routing, trace routing, and routing cleanup, further discussed in this article.

Transmission Lines and High-Speed Signals 

Long traces at high-speeds can act as transmission lines

As electronics reach higher and higher operating speeds, transmission lines come more into the forefront. A high-speed trace that is relatively longer can easily become a transmission line. When high-speed signals traveling through the line and back take more time than the waveform's rise and fall time, this can create signal reflections.

A good go-to rule is if you are working with high-speed signals with a defined impedance, design the routing signal to meet the impedance requirement without worrying about the length of the trace. After all, the impedance is a function of the trace length – if your impedance value is okay, then your length is likely appropriate.

The width of the trace is determined using the copper weight, depth between the two conductors, and the dielectric material properties. Ensure that the line has the same impedance throughout using controlled impedance routing. PCB routing for transmission lines can either be accomplished through a microstrip configuration or a stripline configuration.

After you’ve configured your design rules, keep the following rules in mind for PCB routing for transmission lines:

• Avoid routing across breaks, splits, or crowded zones in the reference planes, as this will ruin the return paths.
• Keep your transmission lines on a single layer if possible.
• In case of a mandatory layer transition, use a ground transfer via next to the trace to continue the uninterrupted return path.
• Don’t break up differential signals (that can also be transmission lines) around vias and other obstacles.

Additional PCB Routing Techniques and Tips

Routing can seem like a daunting task before it’s taken on, but with these tips you’ll be able to achieve any routing challenge.

There are a variety of additional PCB routing tips that can be applicable to your designs, depending on your use of analog parts, connectors, or other multi-terminal components.

• Other analog ICs may require their own specific width and spacing requirements (consult your schematic).
• In general, power and ground connections will need wider traces, as more current will be flowing through them.
• Power and ground will also need to be kept as short as possible to reduce inductance and noise in the circuit.
• Depending on the circuit, power supplies might even have multiple trace widths.
• Isolate analog and digital routing from each other by giving them extra space.
• Connectors may require small trace widths to reach specific pins.
• Fine-pitch parts such as quad-flat packages (QFP) or small-outline packages (SOP) may similarly require smaller trace widths for escape routing.
• Ball grid arrays (BGA) will likely also require shrunken trace widths around the packaging.
• High-speed traces should have larger spacing to prevent crosstalk.
• Look into fanout routing for high-pin count components, cleanup routing to remove unnecessary parts, and bus routing when you have a group of traces.

Design Rules and Constraints

Constraint managers will give you control over all the physical values required by the design. Set up trace lengths, length matching, differential pairs, and other rules and constraints beforehand to ensure that everything will meet the requirements while you route.

The Benefits of an Advanced PCB Software for Routing

Having an advanced PCB software can significantly ease your routing experience

There’s a lot to plan when preparing to design a circuit board with differential pairs, complex ICs, and transmission lines. Another method that can help you prepare for such design complexity is using a PCB design tool with advanced built-in routing features with automated differential-pair routing tools.

You can easily set up your design rules and constraints with many features that can help you route both manually and automatically. Cadence software has all the autorouting capabilities you need for PCB routing. With advanced routing tools and features, you’ll be equipped to get even your most difficult designs accomplished.