How to Design PCB Layout Rules That Work

Key Takeaways

• Your design rules will only be as good as the information that you have to work with.

• Some practical tips on using design constraints to help you with your work.

• How the rules and constraint features of PCB design tools can help.

Designing the right PCB layout rules are crucial to laying out a dense circuit board like this.

Watching educators teach children to read is an amazing experience. I’m used to a set routine in life where step A leads to step B, which is then followed by step C. But that approach doesn’t always work when teaching. If a child isn’t responding to one instructional method, then the teacher will try another and adjust the rules of learning as they go. The important thing is to help the child to read, and if that means re-working the rules to best fit the child’s needs, then that is what they will do.

PCB design is dependent on a set of rules and constraints that govern how the board will be laid out. These rules cover everything from how close components will be to each other to how thick or thin a specific net will be routed. The only way this will succeed, however, is if the rules are designed specifically for each job. What worked previously may not be a good fit for the current board design, and the PCB designer needs to know how best to design PCB layout rules for the board they are working on.

The Best Way to Design PCB Layout Rules Is to Start With Good Information

The design rules and constraints in a PCB design system will do a great job of helping you to stay on track to create a design that is without errors, as long as you start with good information. Too often, design rules are either copied from existing designs without any modifications, or the default rules are used without any changes and the board ends up with easily avoidable errors. This is where having a reliable and documented workflow can help, as it is not unusual for people to cut corners when there isn’t a posted workflow for them to follow.

The next question is, where can this information be found? Here are some ideas to consider:

• Work with all of the design notes and input that you can find, including datasheets, and other documentation.

• Consult with your entire design team. It is not unusual, especially in bigger companies, to have cross-departmental communication that isn’t at its best. In situations like these, you need to work at bringing everyone together.

• Bring in your manufacturer early on during the design phase. These people understand what it takes to make a workable and manufacturable printed circuit board. They will be able to advise you on trace widths and spacings, via sizes, and board layer stackups in order to design an error-free board.

The more information you can gather, the better off you will be. Not only do you need to know what rules and constraints your design should be set up for, but also what rules can be bent, should the need arise. A case in point is setting up a rule for narrowing traces to route escape patterns out of a BGA. Once all of your design information is gathered, the next step is to load it into the CAD tools.

Having a constraint management system in your design tools will help immensely with PCB routing.

Manage the Design Rules, Don’t Let Them Rule You

Traditionally, PCB design rules have been in the domain of the board layout engineers and they would enter all of the constraint data before placing and routing began. Today’s schematic capture applications also have a lot of these rules and constraint capabilities built into them, but this task is often still pushed off to layout. Entering the rules and constraints in the schematic first, however, can be a real help to the overall design process. The same engineers that are capturing and configuring the connectivity of the design can also create the net classes that the rules will attach to, instead of letting the layout team figure it out later. And although you should customize your rules for each design, don’t be afraid to save those rules for later use. It’s generally much better to work from a template than to start from scratch.

Another helpful tip in designing PCB layout rules for your design is to fully understand the capabilities of your rules and constraints system. Some rules are entered into a constraint manager, while others, such as board layer stackups, will be generated when the board outline is created. To enter the rules that you need, you have to first understand where they go. You should know that competing design systems will name their rules and constraints differently than others. What may be a net type in one system could be described as a net class in another. Therefore, you can increase your efficiency by fully understanding the rules and constraints system in your tools before you start working with them.

Generally speaking, the more rules that you can set up, the more you will help yourself during layout. It is true that you can go too far with setting up your rules, like having multiple instances of power and ground net classes when in fact your design needs only one. But usually, the problem with constraints is that the designer doesn’t leverage the full power of the tools by setting up a complete set of rules to work with. You can set up rules to govern component placement, escape routing, trace lengths, differential pairs, high-speed design topologies, and much more. The key, of course, is using PCB design software that gives you a wide range of these rules and constraints to work with.

With the right design tools, DRC reports and status are a click away.

PCB Software Tools That Are Designed for Rules

One of the best tools to work with in PCB design CAD software is a constraint management system that gives you full control over your design rules. Imagine having to set impedance constraints on a board with over a thousand nets on it. Doing this one by one would take a lot of time, but a constraint manager gives you the ability to easily target groups of nets that have been set up by classes in order to add the constraint once. In addition, you need the ability to quickly report on your rules to verify the status of your design, as shown above.

You will be happy to know that a PCB design system that has these advanced rules and constraint features is already available for you from Cadence. OrCAD PCB Designer has the Constraint Manager and Design Rule Checking (DRC) capabilities that we have been talking about. These features will ensure that your design will be completed without error and ready for manufacturing.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.