Estimated reading time: 5 minutes

Elementary, Mr. Watson: Epic Fails with Design Rules

Let’s go through a little mental exercise. Imagine this scenario with me. The governor of your state declares that he intends to suspend all traffic rules and that absolutely no road laws will be enforced. Imagine the results of such pronouncement on stop signs, red lights, and speed limits if the entire state’s vehicle code becomes a mere suggestion. I bring this up not as a question of the morality of right and wrong but rather to show the results of such a decision, which would be absolute chaos. Just maneuvering a simple intersection could and would most likely result in complete disarray. 

Various sciences, including physics, mathematics, and chemistry, are significantly involved throughout the PCB design process, rules that can sometimes be bent but not broken. However, the rules that designers break and ignore altogether and very often are the design rules. Just like trashing the vehicle code of your state, the absence of design rules has the same ultimate and unfortunate end—chaos in the form of failed designs. 

If I gave a schematic to 10 different PCB designers and asked them to bring back to me a completed layout, we would end up with 10 completely different designs. That is the fascinating thing about PCB design. For us, what we are doing is not just a job; it’s better described as art. Therefore, my point is that every PCB we do will be unique and special. However, with that said, although the design is distinctive, it must be electrically and mechanically accurate to operate in an intended manner. You accomplish that by running the design rules. It’s those rules that bring order to a unique design to make sure it is correct. 

Just so we are clear on terms here. Design rules means any set of parameters that verify the accuracy of a PCB design to the desired design criteria. That would include electrical rule checks (ERC) for the schematic, and design rule checks (DRC) for the printed circuit board. Depending on the various layout software you use, you may have additional checks, including verification on components, bill of material (BOM), or the output documentation. These rules are there for a distinct purpose, and it is imperative to learn how to use them. In addition, use them as gate-keeping items in your design. For example, once you have a finished schematic, you run the ERC to verify that the design is electrically correct. Since it is a gatekeeping item, you do not push your schematic into your PCB until you have a clean ERC report. Moving known problems forward to “fixing things later” does not always go very well because you forget things and issues; unfortunately, it ends up in the final design. 

I think we understand the importance of the design rules for most of us, but I have seen some harmful practices, which, I might add, resulted in some significant epic failures in designs. Here are some of the top bad habits when dealing with design rules. 

Rules Not Setup At All or Incorrectly
Keep in mind that the design rules are the verification of the design electrically and mechanically speaking. Therefore, understanding what your circuit is supposed to do and how it operates is the starting point.

Many times the design rules parameters get set up randomly. I would go as far as to say they often are because they sounded good. However, what type of PCB you are designing will determine the set of rules. For example, the rules for a high-density board are not the same for a high power board. However, the rules will mirror the type of board.  

A subcategory of this bad habit is that the design for manufacturing, testing, assembly, etc., and compliance test requirements often are skipped over entirely. If you know that the design is going through a specific UL compliance, those guidelines drive the setup and running of your design rules. I have seen where design rules templates are used to align with particular compliance requirements. Then, when you want to run the checks based on that compliance requirement, you don’t need to recreate it each time.                  

Rules Changed to Match the Design
This habit is one that I often see when you complete the design and identify the problems in the DRCs, and so to fix the issue, you turn off the rule. That is fundamentally wrong and a misunderstanding of the purpose of the rules. The objective for running the DRCs is not to have a clean report; that is easy to do; just turn the rules off. The actual aim is to take the design through the most stringent checks possible. The guideline is the PCB design conforms to the rules, not the rules to the PCB. Therefore, if there are design violations, you must clean the board. Here’s a great analogy for this: I am driving and my check engine light comes on. To “fix” the problem, I take the light bulb out of my dashboard. 

In addition, I have worked in both the medical equipment industry and with a defense contractor, and if someone ignored known issues and turned rules off, that would be clear grounds for dismissal. 

When Rules Are Checked
In this failure, rules are checked at the “end” rather than throughout the design process.

Knowing and understanding the design rules sets you may have based on your PCB design software is essential, and even more critical is when you run those rules. The PCB design process is linear. Once you finish one step, you move to the next until you have a completed board. Therefore, when you delay running the design rule checks, it causes significant issues. 

First, it causes you to get barraged with all the issues at once. Depending on the complexity of your design, this could be a rather significant list, and I can promise you this is a daunting task. Furthermore, it will show issues that might have occurred at the beginning of your layout, which now, with a completed design, you have a much bigger problem. A typical example of this is wrong component placement, which just compounds the issue when routed. 

The solution is to divide your design rules into what area they are checking and run those rule sets when you finish that step in the process. So, when you finish the component placement, run the design rules for that; when you complete your routing, you then run the rules that apply to that.

By doing it this way, you are checking the design as it is developing. 

In conclusion, there are enough issues and problems in any design to keep you on your toes. You do not need to introduce any more by looking at your DRC rules as mere suggestions. Unless, of course, your company can afford to have multiple runs of PCB designs, which they probably don’t.

John Watson, CID, is a customer success manager at Altium.