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Elementary, Mr. Watson: Rein in Your Design Constraints

I'd like to reminisce about the days of old when PCB design was a more straightforward process. Now and then, I find it grounding to reflect on where we started and how far we've come.

I remember the long hours spent at the light table, carefully laying down black tape to shape each trace, cutting and aligning pads with surgical precision on sheets of Mylar. I often went home with nicks on my fingers from the X-Acto knives and bits of tape all over me. It was as much an art form as it was an engineering task—tactile and methodical, requiring the patience of a sculptor.

Those days are long gone.

A lot has changed in PCB design over the years. One of those significant areas of change is the area of design constraints, the theme for this issue. Not too long ago, your projects were guided by design rule checks (DRC). In the traditional PCB design workflow, usually at the end of your design, you would then set up the design’s rules and checks and see how "close" you got.

The DRC acted as a safety net, typically running after the layout was completed to catch violations such as trace spacing, component clearance, or minimum via size. These were reactive checks applied late in the process.

However, today, we have shifted away from a reactive system to a proactive system called design rule constraints, which are set up and defined before and during the design process. These constraints, as the name implies, constrain what you can do in the design.

Design constraints offer many significant advantages, including helping designers catch problems early in the process. While you're working on a design, the software checks your work as you go. If something goes awry or you violate the rules, perhaps placing two lines too close together, the program will prevent you from routing those two traces at all.

Another great benefit of design constraints is that they help facilitate teamwork. If one designer is working on one section of the board and someone else is working on another, constraints make sure they both follow the same rules. That keeps the design neat, clear, and easy to understand. It also makes the finished board more likely to work correctly the first time.

Design constraints also help you save time. Since you don't have to stop and fix as many mistakes at the end, your work gets done faster. You can complete the design more quickly and proceed to the next step, such as building or testing. This is particularly helpful when working on a large project or when you’re nearing a deadline.

Finally, using constraints makes it easier to reuse parts of your design in future projects. Once you know that these constraints are effective, you can reuse them in the future. This will save time and help maintain high quality in your next designs.

EDA Software Outpaces Fab Capabilities
Over the past decade, our wonderful ECAD software platforms have made giant leaps forward. Today's ECAD tools offer advanced features, including high-speed simulation, automated routing, AI-driven optimization, and real-time collaboration on cloud platforms. With today’s software, designers can now create highly complex PCB layouts with a level of precision and efficiency that was unimaginable just a few years ago.

For example, one popular ECAD software (which I won't name here) can operate at a precision of 0.001 mil (one thousandth of a mil) for a PCB trace, which is 2,000 to 4,000 times smaller than the thickness of a human hair.

As a result, EDA software has been outpacing the capabilities of the fabrication processes for some time now.

Fabricators face challenges such as minimum trace widths, layer alignment tolerances, and material limitations that don't always match the extreme design possibilities ECAD tools offer. Manufacturers are limited by physics. For example, while ECAD software can generate those ultra-fine trace routes and very dense component placement, many fabrication shops may not have the equipment or processes to reliably produce boards at the level of complexity or precision that the ECAD programs can achieve.

This gap means designers must balance pushing the limits of software with realistic expectations of what fabricators can achieve. It also encourages closer communication between design and fabrication teams to ensure manufacturability. In some cases, fabrication plants are working hard to upgrade their technology, adopting finer lithography and inspection tools; however, these investments require time and money.

Just Because You Can, Doesn’t Mean You Should
Can the PCB designer use high levels of precision in the design process? Yes, they could, theoretically. But in this case, it's not just about whether you can—it's about whether you should. Not every design demands tight constraints.

For those fabricators who dare to take on complex PCB designs, there are quite a few challenges. First, complicated designs usually cost more because specialized machines and additional work are required. They can also take longer to finish because the process is more careful and slower. Because the design is complex, more boards may contain errors, resulting in a lower yield. If the boards don't work well after they are made, it can make customers unhappy and hurt the fabricator's reputation. Additionally, working on intricate designs can consume a significant amount of the factory's time and space, making it more challenging to perform other tasks. 
The bottom line on the design side, as everywhere else, is profit. When a PCB design is too complex to implement, it can be costly for the designer. Higher production costs and delays can eat into profits. Fixing mistakes or redesigning also adds extra expenses. If the product is late or has problems, it might lose sales. All of this means less money earned and more money spent.

So, here's the $64,000 question: Why do designers even try to push these limits? (For you younger folks, “The $64,000 Question” was a TV show in the 1950s.) From what I've seen, there comes a point where PCB designs become over-engineered and cross into the territory of over-constraint. This often occurs because the design specifications are unclear or the goals aren't fully understood. When specifications get stricter, designers may feel pressured to meet tough requirements under tight deadlines, which can lead them to set overly aggressive constraints without carefully considering how those rules affect the board's performance, manufacturability, or cost.

Another key reason is experience, or the lack of it. Designers unfamiliar with the limitations of the fabricator may rely on general rules or outdated methods without fully understanding the practical implications of those constraints. Less experienced designers are more likely to over-constrain because they are not yet aware of better options, or other ways to optimize the design.

So, when working with PCB design constraints, proceed with cautious planning and effective communication with your fabricators. It's essential to thoroughly review the design, identify any areas that may exceed current manufacturing capabilities, and discuss possible adjustments with the design team. Establishing clear guidelines for tolerances, materials, and processes can prevent costly errors and delays. By approaching the project with a problem-solving mindset and leveraging early collaboration, fabricators can better manage risks and deliver high-quality results—even with the most advanced designs.

This column originally appeared in the July 2025 issue of Design007 Magazine.