Children usually don’t appreciate the constraints their parents put on them. Often, they can’t see any purpose in these annoying rules. They don’t understand, for instance, why their parents won’t allow them to play in the street when it looks like it would be so much fun. As adults, we understand the value of these constraints. Yet, when it comes to PCB design, we often fail to apply them to ourselves.
All too often, the constraints and rules in PCB design tools are underutilized or not used at all. This usually results in errors in the board design that will ultimately affect how the board is assembled. Those PCB layout constraints have been put there for a reason, and that is to help you to design a better board. Let’s take a look at what the design rules and constraints can do for your design, and how best to use them.
The Need for PCB Layout Constraints
Originally, PCB designers were responsible for finding and correcting all design errors in their design. That method worked fine when you were doing tape-up designs at 4X on a light table, and corrections could be made by shaving down a pad with an Exacto knife. In today’s world of multilayer, high-density, high-speed PCB layouts, however, it just isn’t possible anymore. You might be able to remember all of the different rules, but detecting each and every rule violation is beyond anybody’s mental capabilities. There’s just too much to look for.
Fortunately, every PCB design tool on the market today has some layout rules and constraints system built into it. With these systems, you can usually easily set the global parameters, such as default trace widths and spacings, and depending on the tools, you can get even more advanced than that. Most tools will let you set up rules for different nets and net classes, or set up constraints that will help you to adhere to design technologies like net lengths and topologies. The more advanced PCB design tools will also have rules and constraints that you can set up for specific manufacturing, test, and simulation conditions.
Another benefit to these rules and constraints is that they are usually highly configurable for each design, giving you an enormous amount of flexibility. They also are usually reusable from design to design. By saving or exporting the rules and constraints out of the PCB design CAD system, they can be arranged and saved in the same way that you would do with a library part. The important thing is to use them and to do that you have to know how to set them up.
How to Set Up PCB Design Rules and Constraints
Each PCB design CAD system is different, so it would be useless to give specific command examples on how to set up the design rules and constraints. But we can give you some of the basics of how these constraint systems work, and what you can do with them.
First of all, it is always best to get as much pertinent information on your design as possible before you start. For instance, you will want to know the board layer stackup. This is important for any controlled impedance routing constraints that have to set up because adding, deleting, or reconfiguring layers after you’ve started the design is a lot of work. You will also want to find out your default rule values for width and spacings, as well as any different values for specific nets, layers, or unique areas of the board. Here are some of the finer points of rules and constraints setups:
- Schematic: When possible, enter as much of the rule and constraint information into the schematic capture system before you get into the layout. These rules usually transfer over when you synchronize the schematic with the layout. Your design will be more organized if the schematic is driving the rules and constraints along with the component and connectivity information.
- Work your way up: When entering the rules into your CAD system, start from the bottom of the design and work your way up. In other words, start with your layer stackup and build your rules up from there. It’s a lot easier to configure specific rules and constraints for layers if those layers are already configured in the CAD system.
- Component placement: Your CAD system will give you different rules and constraints to set up for placing your components, such as height restrictions, component-to-component spacing, and component class-to-class spacings. Set up as many of these as possible and don’t forget to change these rules to match your manufacturing requirements. Using your rules to maintain a 20-mil clearance between parts will result in disaster if the manufacturing requirement is 25 mils instead.
- Routing constraints: There will be several routing constraints you can set up, including default values, specific net values, and net class values for width and spacing. There will also be net-to-net and net class-to-class values you can set up. And those are just the rules. Design constraints for the type of technology that you are designing can be configured, too. Controlled impedance routing, for instance, will require you set up certain nets to route on specific layers at predetermined trace widths.
- Additional constraints: Use as many of the constraints available to you in your PCB design CAD system as possible. If you have constraints that will allow you to check for silkscreen clearances, testpoint spacing, or solder mask webbing between pads, use them. These rules and constraints will help you avoid design errors in your board that will eventually have to be corrected for the board to be manufactured.
Working Together with Your Contract Manufacturer
The next question that usually comes up is where can you get the information you need for setting up these design rules and constraints? A lot of the electrical engineering rules will be driven by the performance requirements of the board. Your engineering team will most likely be the ones to specify controlled impedance routing, differential pairs, and other design performance constraints.
The rest of the rules and constraints, however, must satisfy the manufacturing requirements of the board. To get that information, you should work ahead with your PCB contract manufacturer to get the data you need very early on in the design of your board.
Here at VSE, we have a long history of working together with our customers to not only make the board as manufacturable as possible, but we can help with the electrical performance of your design, as well. Our engineers are available to work with you on rules and constraint recommendations to help you get started, and they will continue that help through the design. By using a carefully constructed set of rules and PCB layout constraints to align your board with our design for manufacturing (DFM) requirements, your board will be easier to assemble and you will end up with a more robust design.
