Signal Integrity and Layout: Learning to Partner

I have a distant cousin who spent his career doing high-speed PCB layout. At a family gathering, when he learned I was in Signal Integrity (SI), he looked at me warily. I was one of those guys. He then shared how we was going crazy routing, re-routing, and again re-routing a high-speed PCB because an SI engineer was giving him constraints that were impossible to meet. Lots of time was wasted, and he was clearly upset. Then I learned this was a typical part of his job. What should have been a simple exchange of familial pleasantries went haywire. Viewed from his perspective, I had to agree that we SI engineers have a way of complicating things and can be difficult to work with. I apologized. Even more, I told him that someday I was going to write an appeal to SI engineers to do a better job of working with their layout engineers. That day has arrived.

If you are resolving signal integrity constraints, we are about to venture into a critical soft skill you will need to succeed at your job. If you are a layout engineer, please share this section with the SI engineers you work with. Partnering with layout is one of the SI skills covered in the soft skills section in my book Signal Integrity, In Practice; these skills are displayed in Figure 1.¹

Figure 1. The soft skills of signal integrity.

If there were ever two jobs at odds with each other, they are signal integrity and layout. The two tasks have grown up together, yet exploited compute power in very different ways. In an odd quirk of history, around the time layout systems became automated (remember hand-taping?), we also entered the “high-speed era.” Just when it seemed the layout task was going to simplify, a new type of “digital” signal arrived: one that was looking far more analog than digital. As signals increased to tens of MHz, an engineering discipline called “signal integrity” emerged. While automated netlists and back-annotation ensured “connectivity integrity,” the layout task had to jump another hurdle. “High-speed” meant it was not enough to confirm the connection was made, SI would require specific things about the way the connection was made. Things like lengths, impedances, couplings, along with a dizzying array of ideas and terminology foreign to mostly everyone at the time.

In the Beginning

In the beginning, there was layout. Layout was clearly the incumbent, meanwhile, SI the new kid on the block. While it is hard to picture in the 2020s, there was a time when SI engineers struggled to find something to do—even to justify our existence. We marketed our skills to design teams, trying to invent language to explain why and how we could be helpful. One of my favorite pictures at the time was a couple SI engineers (although, we were not called that yet) sitting on a street corner holding a cardboard sign that read, “Will Do Analog Simulation for Food.”

During the transition to the high-speed era, I cut my SI teeth at Intel working on the PC.² While I did not realize it at the time, one of the greatest boons in my career was getting paired up with a savvy and seasoned layout guy named Gary. Gary had gray hair, knew his craft well, knew how to be the incumbent, and had a way of looking at me when I wanted him to do something in layout that was edgy, if not unreasonable. In time, SI engineers would gain the upper-hand—and perhaps even abuse it—yet, in the beginning, there wasn’t the slightest chance we would get away with imposing anything on layout that could not be justified. This was particularly true on the PC, where commoditization and cost defined everything we did. So the gifts I was given were two-fold:

I had to learn the complexities and intricacies of the layout task and suit my SI ideas and constraints to these realities.
I had to justify layout complexities I wanted to impose with real data, accompanied by a clear and compelling explanation of why.

If you began working in SI after the year 2000, it’s possible you were not given those gifts. Instead, you simply became a gatekeeper tasked with defining a “good” or “bad” layout—even though you may have little layout experience. Whatever the case, I strongly recommend all SI engineers learn how to do these two things; we might call them the “Golden Rules of SI’s Interactions with Layout.” Your layout engineer is your conduit into manufacturing the product, and as such, is an invaluable asset to help you understand how your ideas fit in the real world.

Guidelines to Working with Layout

I like layout engineers and have found them to be a unique set of individuals. Overall, they are hard-working, practical, focused, experienced, humorous, good communicators, and masterful at negotiating. I like to think SI engineers have these qualities too, but sometimes SI positions get filled with engineers who have theoretical training, yet lack pragmatism —perhaps more hard skills than soft skills. To help bridge the gap between SI and layout, here are seven things you may find helpful when working with a layout engineer.

There is not one thing an SI engineer can ask for that makes the layout task easier. Not one. Feel free to accuse me of hyperbole, but it is helpful if you face this truth sooner than later. SI and layout are intrinsically in tension, so you need to figure out how to work together and focus on the one factor uniting you: a working product delivered on schedule.
Your layout engineer has already been over-constrained by SI guidelines and engineers before you even arrived. This puts your tasks more than just in tension; it puts you at a disadvantage before you say anything.
Layout engineers want the design to work too, and generally care more about schedule than SI. Actually, layout is accustomed to being asked to get the project’s fabrication back on track after others were late on their tasks. So do not be surprised if and when your layout engineer is focused on schedule and presses you for efficient, clear, and actionable interaction.
If you enjoy a challenge, on your next project measure your success by the layout engineer’s opinion of you when the project is done. I do this; it is how I ensure I am succeeding at the two skills listed in the previous section.
If layout asks for guidance on a certain intricacy and you are not sure of the answer, say “I don’t know.” These are powerful words. Then, when possible, go find the answer using simulation, published literature, measurement or all the above. Given technology’s rate of change, some questions do not have answers, and “I don’t know” can be the first step to finding one.
When there are multiple ways to get the job done, let layout choose whatever is most convenient for them. When layout and SI reach an impasse, defer to layout. If that thought unnerves you, offer more data or better data to tip the scales to SI.
Layout needs SI to make clear statements and requests in plain language. Better yet, use layout’s terminology. If you are sensing I am suggesting you learn how to look at the project from layout’s perspective and understand what they are up against, including their tools, clearances, DRCs, tasks, and schedules, you would be correct.

These are some helpful ideas from my perspective as an SI engineer, but following are some thoughts directly from the perspective of layout engineers.

Advice from Layout Engineers

To help SI improve interaction with layout, I interviewed experienced layout engineers who I have found to be masterful at both layout and managing interaction with SI. Combined, they represent well over 100 years of layout experience. We discussed the habits of good SI engineers, and what SI could do to improve interaction with layout. Below is a summary of the main takeaways from our conversations:

Multi-disciplinary Negotiation: As PCBs get more complex, the design process becomes increasingly a negotiation. All perspectives need to explain what they need and why, and know how to work with other perspectives across design tasks. This applies not just to SI and layout, but also to mechanical, power, and so on. Engineering is multi-disciplinary, and layout brings the perspective of what is reasonable and manufacturable.
Relationship: It is important to get to know the person you are working with, and how to interface with them. Some want and need more explanation; others readily accept what you say. Once the relationship is established and respected, communication and progress become simpler. One layout engineer suggested “Go to lunch with your layout guy to form a good working relationship. Most of the time, lunch will crumble any unwanted wall. Start the conversation at the beginning of the project, and then everything will flow.” A good working relationship can be more difficult to achieve with remote design teams and functions. If you are in that situation and get a chance to meet distant team members in person, do it. Another layout engineer stressed: “If you meet someone in person, they are easier to work with.”
Single SI Source: When asked to describe what a positive interaction with SI looks like they replied, “we need specific information about what needs to be done” and the best engineers to work with are “the ones who can explain why.” They said a single SI person is helpful because “I can go to five different SI engineers and get five different answers,” which is frustrating. In fact, it is a normal experience for IC guidelines, SI engineer A, and SI engineer B to specify contradicting layout rules. Imagine using a model or simulator that gave you a different result every time you used it—wouldn’t that be frustrating? So, a single source to interact with is helpful. Another layout engineer expressed frustration at the typical task of negotiating physical solutions between PI and SI engineers, who often provide contradictory requirements. As such, it is helpful when hardware engineering gets involved to understand the issues, mediate the negotiation, take responsibility, and make decisions.
Timeliness: Timely guidance is imperative. SI needs to understand that layout implements “hundreds of times” what seems to be a single constraint. Not only do constraints need to be practical and reasonable, but changing a constraint after layout has begun forces a significant amount of re-work. Again, layout expressed untimely and changing constraints as a “normal” problem. To mitigate this, “Placement Reviews” before routing starts can be helpful. But, according to layout engineers, “getting SI to participate can be difficult.” Let’s change that. For a flowchart showing timely design-cycle SI-Layout-Schematic interactions, see "An Optimized Methodology for High-Speed Design," (pp. 10-13).³
Partnering: In general, layout’s interaction with SI has been positive and manageable. That was good to hear. Yet, when asked if SI asks them to do things that are unreasonable, if not impossible, they did not hesitate to answer, “all the time.” As such, these successful layout engineers have learned to be good negotiators, to explain trade-offs, and provide data on what does not work and why. This is engineering’s normal give and take of negotiating dozens of conflicting variables and constraints. However, layout advised that projects differ substantially based on the attitude of the SI engineer. Sadly, some SI engineers have not yet learned how to partner—a skill I view as imperative if you want to succeed at SI.

Working as a Team

One way or another, design teams must learn to work through the challenges. As one layout engineer said, “The project doesn’t die, that’s not an option.” From the perspective of job satisfaction, how SI and layout work together makes all the difference. Indeed, every shipping product has found a way to reconcile all the disciplines required for its inception. We all do it every day. My purpose here is to help those working in SI find practical ways to improve the way we fit into the larger project team—particularly focusing on our interaction with layout.

Conclusion

A look at Signal Integrity, In Practice¹ wouldn’t be complete without pausing to consider those we work with and influence the most: layout engineers. How can our crafts better complement each other? How can we enhance our cooperation to ensure everyone’s interests are addressed? As you endeavor to do your job well, you will ask yourself these questions again and again. Thankfully, there are answers. Someday, if those answers are feeling elusive, my hope is that you can return here for motivation and fresh ideas.