Some background information on the requirements of PCB routing.
Routing challenges that PCB designers handle daily, and what they need for success.
Methods of routing using advanced PCB design routing tools.
Routing is one of the core functional characteristics of a PCB
For years now my wife has described my PCB design work as “he just plays video games all day.” She is kidding, of course, but when you think about it, there is a game-like quality to PCB design, especially when it comes to routing. After all, we are working with shapes, colors, and lines on a screen where the goal is to get everything connected before you run out of space. I’ve never been given any bonus points or an extra life after routing my design to 100%, but maybe someday.
In all seriousness, completing the routing on a PCB design can be difficult. Although it may look like a game to some, it isn’t. Instead, it requires a lot of patience and skill on the part of the designer. So how can a PCB designer guarantee success when routing a board? Like anything else, the secret is in using the right tools for the job. We’re going to look at some of the routing challenges that designers have to deal with regularly and what kind of PCB routing tools can help.
Connecting the Nets Together with PCB Routing
Converting schematic nets into physical traces on a printed circuit board has been the responsibility of layout engineers for a long time. This used to be done manually and designers would draw and redraw their circuitry at an enlarged size on sheets of gridded mylar. When ready, those circuit drawings would then be covered with opaque tape and photo-reduced in order to create the required fabrication tooling. Over the years, the performance requirements of new electronics forced the use of smaller board elements that could not be accurately created using tape. This, along with many other reasons, forced circuit board design to evolve to the next level; enter the PCB design CAD system.
With the introduction of these systems, many new benefits were realized. For one, designers could automatically transition from schematic data to layout data without having to manually input connectivity from spreadsheets. Another advantage was that clearance tolerances between objects on the board, such as trace to trace or trace to pad, could be automatically checked for and maintained. On a tape-up board, these clearances were estimated, and if needed, an Exacto knife was used to trim a little tape to create more room.
With PCB design CAD tools, the amount of work being produced grew at an exponential rate. Everything changes, however, and soon the continually evolving electronics technology began to demand more and more of the PCB design tools, especially in the area of routing.
3D capability and advanced routing features can cut a lot of time off your PCB design schedule
The Challenges for Designers During PCB Routing
Connecting traces together during printed circuit board layout can be a lot of fun. Pulling the traces around and hooking them together to complete the nets can even be cathartic at times. But more and more, the simpler board designs that allowed for basic routing with minimal rules are disappearing. In their place are high-speed design rules, tight manufacturing requirements, and other constraints that require a lot more attention than just connecting the dots.
Here are some of the challenges of circuit board routing that designers are now faced with:
Shorter design times: To keep the competitive advantage, circuit board design times need to be trimmed back as much as possible. In addition, board spins must be held to a minimum to reduce prototyping expenses. This puts a lot more pressure on the designer to get the routing done correctly the first time.
Multiple design constraints: At one time, every net on the board had the same trace width and spacing rules, except for power and ground, which simply had to be wider. This very simplistic use of design rules has now been replaced with different width and spacing rules for multiple nets, and in some cases, in different areas of the board as well.
Analysis expectations: In order to get the trace routing done correctly the first time, layout designers need to know ahead of time how to configure their designs. This requires simulation and analysis tools in addition to the PCB routing tools.
Complex routing requirements: A lot of routing on today’s high-speed boards will require specific routing patterns or topologies. For instance, older DDR memory routing required the use of T-topology routing patterns, while DDR3 and DDR4 need fly-by-termination topologies. Beyond that, there are differential pairs that must be routed together, controlled impedance lines that have to be configured to match the board layer stackup, and many other requirements.
The only way to effectively manage all of these different routing challenges is to arm yourself with PCB design tools that are equipped for advanced circuit board design requirements. Now, let’s take a look at some of the features that are available in advanced tools like Cadence’s Allegro PCB Designer that can help you.
Having a fully configurable constraint management system available will prove invaluable for routing
The PCB Routing Tools That Will Guarantee Success
To successfully route a printed circuit board, you need a lot more than just advanced routing features. Routing starts long before you lay down a single trace, so we’ll start this list with some of the other important functions that your PCB design tools should also have:
Circuit simulation: Not only will SPICE tools enable you to get your design to market sooner by revealing design problems before you build a prototype, but they can also help with your routing as well. Most high-speed, high-density designs have their trace routing tightly packed. This is essential for signal timing purposes, impedance control, and a host of other reasons. When you have to rip up and reroute traces that were captured incorrectly, however, it can throw the entire design off-balance. With circuit simulation tools you can work with correct schematic data from the start.
Component placement aids and checks: Routing a trace correctly starts with having the components placed correctly. This means the proper spacing between parts needs to be observed, the board has to pass manufacturability checks, and signal paths have to be optimized. Just as with circuit simulators, having to rip up your routing to correct component placement problems can have a ripple effect through your entire design that you can’t afford.
Design rules and constraints: Not only do you need to control trace widths and spacings for multiple nets and areas, but there are many other design parameters that you need to manage as well. Constraint managers, like what is shown above from the Cadence line of PCB design tools, will give you control over all of the physical attributes of the design as well as the electrical. You can set up trace lengths, length matching, differential pairs, and many other rules and constraints.
Analysis tools: Waiting for the prototype to be built before you analyze the effectiveness of your routing is kind of like closing the barn door after the horse has escaped. The best design process is to work with embedded analysis tools in your PCB design system to check your signal and power integrity while you are routing. This will allow you to make real-time corrections as you work instead of re-designing the board later.
With those features in place, you will be well set up for routing your board. Here are some of the routing tools that you will find most useful:
Manual interactive routing: No matter what, you will always need to hook up some traces manually. Your design tools should allow for this to be done easily with a variety of editing options to compliment your routing.
Slide routing: To clean up traces quickly, the ability to grab a trace segment and pull it to the desired location is invaluable. Even better is when it will move other design objects like vias and traces out of the way while you are sliding.
Fanout routing: Also known as escape routing, this automatic feature can help a lot by quickly pulling traces out of high pin-count parts and connecting them to vias.
Differential pair routing: Diff pairs must be routed together with consistent spacing between the traces of the pair. These routers work in conjunction with the diff pair rules set up in the design constraints for the trace width and spacing values.
Bus routing: Here is another useful feature where you can grab a group of traces and route them together.
Auto-routing: There are many forms that auto-routing can take, from single trace routing to full batch auto-routing, which can route the entire board if the user desires.
Trace tuning: These routers also work in conjunction with the design constraints and add small serpentine segments to a trace in order to increase its overall length.
Cleanup routing: There are a whole host of tools that will clean up the routing on your board. Some will miter the corners of your traces, while others will remove unnecessary jogs and segments. You can also add teardrop features to traces that connect to vias or gloss the routing to make it more presentable.
There are many other routing features available, and they may be called by different names depending on what PCB design tools you use. The key, of course, is to use an advanced CAD system that provides the routing power and flexibility that you need.
CAD tools like Allegro PCB Designer offer many different routing tools and features
How You Can Put These Tools to Work
The more routing tools and features that your PCB design system offers you, the more capabilities you will have to get the job done. As you can see in the picture above, Allegro PCB Designer offers many different routing features and functions in their standard PCB editor. Usually, designers will use combinations of these tools to get the results that they are looking for in their design.
For instance, you may want to start off using the fanout editor to get your escape routing done. Next, you may want to use the manual interactive router together with the constraints that you set up to route the diff pairs and controlled impedance lines in the proper locations and at the correct width and spacing. Later on, you may use various auto-routing features to put your main routing in and then fine-tune your high-speed transmission lines to the correct lengths using the tuning features. Lastly, you would optimize the routing using different combinations of route cleanup tools.
Whichever approach you take will of course depend on the needs of the PCB design. For more information on how to set up and execute routing on your next PCB design, take a look at the information in this E-book on routing.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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