Whoever said “rules are mostly made to be broken” obviously wasn’t a PCB designer. General Douglas MacArthur said it, which proves the point about not being a designer. Circuit board manufacturing relies on holding minimum object clearances, but the board’s electrical performance requires specific trace widths and object clearances. And although there may be exceptions to the rules from time to time, for the most part, you must follow PCB design rules and constraints.
To help designers create functional and manufacturable circuit boards, the PCB design software creators will include a variety of design rules and constraints in their tools. The appearance and use of these constraints will vary quite a bit from tool to tool, but their purpose is always the same, to help you to design a better circuit board. Here are some ideas on how to set up and work with these rules. First, let’s take a brief visit to the history books to find out more about the origins of PCB layout rules and constraints in the CAD tools you use today.
The Evolving Role of PCB Layout Design Rules
A circuit board can’t function or be assembled if basic electrical and design for manufacturing rules aren’t followed. The PCB design tools that we use today will guide us in following those rules, but it wasn’t always that way. The primary goal was to prevent individual net objects from touching each other and creating a direct short on the board, but other than that, the rules were very minimal:
- Typically there was only one default trace width and clearance rule to set.
- Designers were responsible for manually changing the width of their traces for power and ground nets, controlled impedance routing, or any other net requirements.
- Designers had to visually confirm any clearance spacings that differed from the default value. (There were times that I would hold a ruler up to the screen to compare clearances.)
- Designers would either reset the value, manipulate the grid, or draw spacings around the part on a graphical layer for a visual reference for components that required greater clearances than the default value. In some cases, we even changed the pads’ size on the parts we were working with and then altered their Gerber D-codes to shrink them back down again for manufacturing.
- Some early CAD systems with proprietary hardware alerted designers to an online error by beeping. This beeping could be so annoying that designers were known to cut the speaker wires, keeping everyone around them from hearing their errors.
Thankfully the PCB design rules improved over time, and working with them more comfortable. Enhanced constraints give designers better control over multiple trace widths and spacings and multiple placement clearances for different components. Additional rules were added for non-net objects such as silkscreen text and elements, tooling and mounting holes, and other features on the board.
Another significant improvement came with the ability to check the height of a component to other board features. Initially, this was a simple check to the part’s outline in what is known as 2 ½ D checking. With STEP models and 3D capabilities of the CAD systems, the designer can see how the contours of a complicated part, like a connector, will fit together with other board objects. These advancements have given the designer the ability to catch placement errors during the board’s layout without waiting for a prototype.
Now that we’ve seen these design rules’ progression over time, we’ll look next at how you can use them to their fullest capabilities.
Making PCB Layout Rules and Constraints Work for You
While design rules and constraints have traditionally belonged to the PCB layout, this has been slowly changing over the years. Now, many CAD tools have utilities set up within their schematic capture systems to input the rules. Utilities allow the design engineers to specify trace widths and spacings for sensitive signals and designate net classes for critical groups of nets. All of this helps improve the corroboration and efficiency of the design group during the development of the printed circuit board.
The method of inputting design rules and constraints has grown quite a bit as well over the years. What used to take the layout team a lot of work by individually entering values through pulldown menus and command lines is now handled with a spreadsheet type management system. In the picture below, you can see an example of the Constraint Manager used in Cadence’s OrCAD PCB Designer tools. The individual values can be entered or copied into each cell of the spreadsheet, and several mechanisms exist to manipulate the data further once it is in place. The designer can even save a copy of the rules and import them into a different design’s constraint manager later on.
The design rules not only set up the trace widths and spacings as we have described, but they also allow the designer to set up more precise engineering controls. These include impedance values, minimum and maximum propagation delays, differential pairs, and a host of other settings. The designer can also use the constraints to set up trace lengths, matched lengths, routing topologies, preferred vias, same net spacing, and much more. The key is to fully use these rules and constraints to get the maximum benefit from them:
- Make sure that any rules entered into the schematic transfer over correctly into the layout.
- Add rules that weren’t initially included in the schematic, which often happens with power and ground net values or component classes.
- Add the mechanical and manufacturing clearance rules.
- Add rules for testpoint clearances to govern their distances to each other, the board edge, and other components.
Many rules can be entered into these systems, but the more you include, the less you must remember to check for later. The trick is to know which rules need to be entered, and we have some ideas that can help you with that.
An example of managing PCB design rules by using the Constraint Manager in OrCAD
Research the Rules for Your PCB Before You Start Layout
The first place to start when preparing the rules that need to be entered into your PCB design is your own company. Design groups typically have company standards that they work to that should be part of your routine procedures. Another useful resource is industry standards such as the IPC guidelines for circuit board layout. The best resource is the PCB contract manufacturer that will be building your circuit board. They will give you the information you need concerning how the board layer stackup will need to be configured and what design rules will be required for optimum fabrication and assembly.
At VSE, we have been helping our customers with circuit board design rules for over 30 years. Our engineering team not only knows what trace widths and spacings should be used for the type of board that you are having built, but they can also assist you with other design-related questions and concerns as well. We can help you set up your design to be manufactured as efficiently as possible for the best possible performance.
