PCB Layout Considerations: Differential Pair Length Matching Tolerance and Avoiding Violations

Created: June 29, 2018
Updated: December 9, 2020

 High density trace routing on green PCB

Detangling my daughter’s hair reminds me of detangling PCB traces. Everything needs to be set into layers, and crossover should be avoided. Detangling the hair of a 9-year old doesn’t take as long as routing PCB traces, but the results are just as painful if not done correctly. Ideally, though, your daughter’s hair isn’t causing short-circuiting of electronics or small fires to spark up.

Routing traces in a layout is arguably the most important and time-consuming design activity. In high speed design, trace lengths net need to be extremely accurate and fall within particular tolerances. Keeping track of characteristic impedance, signal integrity, layer stackups and trace width can become tedious, time-consuming, and extremely difficult for any tool to accomplish automatically.

You can overcome the toughest routing bottlenecks and maximize productivity when you work with the best PCB design software. In addition to using the best automating routing and length tolerance tools, you need access to simulation tools the industry demands. This way, you can trust your designs to get past whatever impedance is obstructing your device’s path and provide the best circuit board designs possible.

High Speed Routing and Length Tolerance Matching

High speed signals are most likely to interact with other signals, and vice versa. Most engineers layout these signals first to ensure that prescribed routing rules can be followed. Among the most critical high speed routing rules is length tolerance matching, also known as length tuning. Single-ended and differential pair routing on your PCB is that critical task that connects disparate components together into a working system.

Routing multiple traces must be extremely precise in high speed digital systems. When digital data is sent through components with multiple inputs, like a logic gate IC, the signals need to arrive at all the inputs at the same time. As data transfer speeds increase in electronic devices, the acceptable amount of mismatch between multiple traces gets successively smaller. Trace lengths need to be precisely matched to avoid creating data errors in your device.

Length tolerance matching in differential pairs is also a must in order to ensure signals are synchronized. When component interconnections in a net are not matched, your software should notify you with an indicator directly on the layout. Not all software makes correcting a length mismatch easy. Different programs have different procedures and tools for tuning trace lengths and adjusting trace lengths should be as easy as any drag-and-drop command.

Not all nets are created equal. Your PCB design software should balance automating routing and rules checking with providing freedom to customize your design as your device application requires. When you group multiple differential pairs or single-ended traces into nets, you need a PCB rules and constraint editor that makes it easy to define length matching constraints on each net.

Screenshot of differential pair routing in Altium

Differential pair routing in Altium 

Realizing Where the Pain Comes From

If you start routing directly in the layout editor, traces are placed using predefined design rules. Some problems with routing arise due to conflicting rules settings, especially in shoddy design packages. Poor design software can actually automatically route your traces incorrectly throughout the entire board, and you would have no idea this happened except in the most extreme cases. This is especially true for differential pairs.

Not only does poor design software route traces and differential pairs incorrectly, you won’t even be provided with the routing features unless you purchase them as add-ons. This leaves you stuck routing by hand, checking length tolerances by eye, and inserting meanders manually. You’ll need to buy these essential features if you want to route even a small number of traces in a reasonable amount of time, not to mention a wrist brace for that new case of carpal tunnel.

Once you are actually in the process of fixing any routing mismatches in your nets, you’ll need a clear indicator that shows the mismatch between neighboring single-ended traces and between each trace in a differential pair. This should be as simple as placing a visual indicator in the layout view that adjusts as you drag meanders into your traces. There is no reason for your tolerance indicator to be buried three dialogs deep.

Routing Tolerances in a Unified Design Environment

Since routing differential pairs, length tracing, defining nets, and interlayer routing are typically the most time-consuming portions of PCB design, your design software should include interactive features that speed up the entire process while still ensuring that your length tolerances on your signal nets will be met. These tools should be built into your software package; there is no reason that this critical feature should only be available as an add-on.

Having your software notify you of a length mismatch in your net is one thing, correcting this mismatch is another. Correcting a trace length mismatch requires placing meanders in the shorter traces in the net so that they match the length of the longest trace. Your length matching settings and meander geometry should be easily accessed directly from the layout. Adding a miter for length tuning should be as easy as dragging the mouse across the mismatched trace.

Once you have routed all your connections and cleaned up your mismatched traces, a unified design environment integrates your layout into industry-standard simulation and analysis packages. No one should have to export from their design package into their simulation program. Working in a unified environment gives you access to these tools and lets you generate all your manufacturer deliverables within a single program.

Screenshot of auto-interactive routing in Altium

Auto-interactive routing in Altium

Routing in Altium’s Unified Design Environment

Altium uses a unified interface that makes routing between components intuitive, while still integrating your design with advanced simulation and rules checking features. Even though length tolerance is extremely sensitive in high speed devices, the routing tools, CAD tools, and rules checking features are to make sure that your device will operate correctly. All commands are intuitive and are easy to locate within the program.

You can define your differential pairs directly in your schematic and easily capture these specifications in your layout. Defining design rules is simplified by specifying directives directly in the schematic. This provides you with a visual cue to indicate your differential pairs directly in the schematic and layout. Rules violations can be checked individually or in batches, and violations have an intuitive display that makes it easy to locate and fix any design errors.

If you’re looking for an advanced routing solution that gives you the power to define tolerance rules, you need a PCB design software package like Altium. The routing features and the
® tool in Altium allows you to define your signal nets, automatically lay traces, and route high pin density components like FPGAs and memory modules. Critical routing constraints are maintained using automatic design rules checking features.

So what is the difference between Altium and other companies? Altium makes sure that you have the resources you need for success. Altium is easy to read, easy to access, and includes examples and use cases for advanced design features. You’ll have access to the AltiumLive forum, user groups, video, and webinars provided by industry experts. A strong support network is just a few clicks away.

Don’t let your boards wither away from the stress and demands of signal integrity. Use PCB design software which cares about you, the PCB, as its user and manages tools designed to make every aspect of your design process to feel interconnected and unified.

Now you can download a free trial of the PCB layout software that you want to use for your designs. To find out more about Altium and how its routing features can help you design your next PCB talk to an expert at Altium.

Start the journey to switch over to Altium Designer today.

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