Often, constraints and rules in PCB design tools are underutilized or not used at all. This often leads to errors in the design of the board, which can ultimately affect how the board is assembled. There is a reason for placing these PCB layout limitations, and that is to help you design better boards. Let’s take a look at what design rules and constraints can do for your design and how to best use them.

PCB layout limits requirements

PCB layout limitations Initially, the PCB designer is responsible for finding and correcting all design errors in the design. This works well when you design straps at 4x speed on a light table and can be corrected by Exacto cutting the mat off. However, in today’s multi-layer, high-density, high-speed PCB layout world, this is no longer possible. You may be able to remember all the different rules, but detecting each violation is beyond anyone’s ability. Too much searching.

Fortunately, every PCB design tool on the market today comes with a system of layout rules and constraints built in. With these systems, it is often easy to set global parameters, such as default line width and spacing, and depending on the tool, you can get even more advanced Settings. Most tools will allow you to set rules for different networks and network categories, or to set constraints to help you comply with design techniques such as network length and topology. More advanced PCB design tools will also have rules and constraints that you can set for specific manufacturing, testing, and simulation conditions.

Another benefit of these rules and constraints is that they can often be highly configurable for each design, giving you great flexibility. They can also often be reused from design to design. By saving or exporting rules and constraints outside of the PCB design CAD system, they can be arranged and saved in the same way as using library parts. It is important to use them, and to do so, you must know how to set them up.

How to set PCB design rules and constraints

Every PCB design CAD system is different, so it would be useless to give specific command examples on how to set design rules and constraints. However, we can provide you with some basic knowledge of how these constraint systems work and how to use them.

First, it’s always best to get as much design information as possible before you start. For example, you will need to understand board layer stacking. This is important for any controlled impedance routing constraints that must be set, as adding, removing, or reconfiguring layers after the design has begun is a heavy workload. You also need to look for default rule values for width and spacing, as well as any other values for a particular net, layer, or unique area of the board. Here are some key points for setting rules and constraints:

Schematic: Enter as much rule and constraint information as possible into the schematic capture system before entering the layout as possible. These rules are usually transferred when you synchronize the schematic with the layout. If schematics drive rules and constraints, as well as component and connectivity information, your design will be more organized.

Step by step: When entering rules into a CAD system, start at the bottom of the design and work your way up. In other words, start with the layer stack and build the rules from there. This is much easier if you have layer specific rules and constraints configured in your CAD system.

Part placement: Your CAD system will set different rules and constraints for you to place parts, such as height limits, part-to-part spacing, and part-to-class spacing. Set as many of these rules as you can, and don’t forget to change them to suit your manufacturing requirements. If the manufacturing requirement is 25 mils, then using your rules to maintain 20 mils of clearance between parts is a recipe for disaster.

Routing constraints: You can set multiple routing constraints, including default values, specific net values and net class values of width and spacing. You can also set net-to-NET and Net class-to-class values. These are just the rules. You can also configure design constraints for the type of technology you want to design. For example, controlled impedance cabling would require you to set up certain networks to be routed at a specific layer with a predetermined line width.

Other constraints: Use all available constraints in the PCB design CAD system whenever possible. If you have constraints you can check screen clearance, test point spacing or solder strip between pads, use them. These rules and constraints will help you avoid design errors on the board that must eventually be corrected for production.