PCB Layout Check Design Rules: What’s The Difference Between DRCs & MRCs?

December 25, 2017 Altium Designer

Picture of brush full of fur with dog in the background

There are a few things that I absolutely want answers to before my time on earth is done. First of all, why is it that my dog sheds more in December than he does in June? Doesn’t he need all that fur for the winter? Second, why is it that people who are great drivers on the road aren’t able to push their shopping cart intelligently through the supermarket? It seems like they go out of their way to head in the opposite direction as everyone else, or stop completely in the middle of the aisle causing a major traffic jam between the produce and dairy areas. 

I know that I am more than likely never going to get answers to those questions, but there are some questions out there that we can work with. For instance, what is the difference between DRCs and MRCs in the world of PCB design? DRCs are “Design Rule Checks”, while MRCs are “Manufacturing Rule Checks” and yes, there is a difference between the two. Unless you try to consciously place your shopping cart directly in the middle of any aisle you’re in, you’re going to want to know the difference and how to work with both to optimize your design. 

PCB Layout Check Design Rules

As annoying as rules can be at times, we all know that life would be pretty dangerous without them. People would drive too fast, houses would crumble because they were built without a proper foundation, and the neighbor’s pet alligator would terrify our kids. Designing a printed circuit board without rules is also an accident waiting to happen. To help protect us, DRCs are built into our CAD systems to ensure that we stay within the acceptable boundaries of our design.

Today’s layout tools continue to add more and more of these rules to their checking routines. Usually, all of these rules are offered in one design rule menu and are broken up into separate categories for the convenience of the designer. Using all of the design rule checks that are available in your PCB layout tools is the best way to ensure that you have created an error-free design. A PCB design that is laid out without design rules is an accident waiting to happen. Don’t allow yourself to be part of that accident. Use those DRCs and MRCs in the way they were intended to be used. 

Close up picture of  board

DRCs and MRCs will keep you on track as you design your PCB

You Need Rules, but How Do You Distinguish DRCs from MRCs

DRCs are usually considered to be violations on the PCB that will affect its electrical functions as opposed to how the board is manufactured. Trace widths, the number of spokes in a thermal relief pad, and other electrical constraints can cause the board to not work as it should if they are an incorrect size. These objects are among those DRC items that usually don’t have any effect the manufacturing of the board. On the other hand, there are other DRC items such as object clearances between electrical conductors (traces, component pads, and area fills), that affect both the electrical functions and the manufacturing of the board. Traditionally these have been considered as DRCs. 

DRCs should be run online while the designer is laying out the PCB. This will prevent parts from being placed too close to each other, and traces from shorting out to each other. At any time during the design, a batch DRC pass can be run as well. This will catch any DRC errors that were ignored during design, or if the online DRC was not used. This makes the DRC as if a food market had designated arrows and roads for directing cart traffic, and had observable rules and notifications for intersections. Can you imagine? Red light before the bakery! 

Critical PCB Design Adjustments with MRCs

Traffic rules for shopping the aisles is one thing, but we all know the real time waster when you go shopping is spending forever waiting in the four checkout lines available out of the 20 checkout lines possible. If checkout had common rules and violations, that might be like an MRC. It used to be that MRCs lived only in the realm of the manufacturers. When I first started designing printed circuit boards, traditional DRCs were the only checks that our layout tools could run. We would then turn the design over to the manufacturer and they would run MRCs with their own specialized tools.  

It was very frustrating to have a design rejected and returned because of problems that we couldn’t check for with our layout tools. Items such as silkscreen to silkscreen clearances, or detecting solder mask slivers were not checks that early layout tools could perform. Fortunately the MRCs that used to be available in specialized tools only are now being included as part of the entire DRC package in today’s layout tools. PCB designers are able to catch MRC errors along with their DRC errors and correct them at the same time. Unfortunately, we are still working on having common rules for speeding up checkout, and I am very far out from finding out my dog’s shedding schedule.

Picture of the Design Rule Checker menu in Altium

You will find a complete set of DRCs and MRCs in your PCB layout check design rules menu

I said at the beginning that there was a difference between DRCs and MRCs, and traditionally that is true. The fact is though that both sets of rules are needed together to create an electrically correct PCB that will go through manufacturing smoothly and without errors. Rules for items such as signal integrity and high speed are required as design technology continues to grow.  

The jury’s still out on why my dog sheds more, but I can definitely imagine a world where going shopping doesn’t take me two hours. PCB design software, like Altium Designer, has a full suite of the DRCs and MRCs that we have been talking about already built into it. These checks will help you to deliver a clean design to your manufacturer the first time.

Would you like to find out more about how Altium can help your next PCB design with its advanced DRCs? Talk to an expert at Altium.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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