I have a distant cousin who spent his career doing high-speed
PCB layout. At a family gathering we
were learning what each other did for a living, and when he learned I’m in
Signal Integrity he looked at me warily.
I was one of “those” guys. He
then shared how we was going crazy routing, re-routing, and re-routing a
high-speed PCB because an SI Engineer was giving him constraints that were
impossible to meet. Lots of time wasted,
and he was clearly upset. Then I learned
this was a “normal” part of his job. What
should have been a simple exchange of familial pleasantries went haywire. Viewed from his eyes, 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
If you are a Signal Integrity Engineer, we are about to
venture into one of the critical “soft skills” you’ll need
to succeed at your job. If you are a
Layout Engineer, please forward this article to the SI Engineers you work with.
If there were ever two jobs at odds with each other, it is
the Signal Integrity (SI) Engineer and the Layout Engineer. 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, back-annotation,
and such ensured “Connectivity Integrity”, the Layout task had to jump another
hurdle. “High-speed” meant it wasn’t
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
…there was Layout. Layout was clearly
the incumbent and SI the new kid on the block.
While it’s hard to picture in 2020, 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 weren’t called that yet) sitting on a street
corner holding a cardboard sign “will do analog simulation for food”. Really.
During the transition to the high-speed era, I
cut my SI teeth at Intel working on the PC.
While I didn’t 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 grey 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’d get away with imposing anything on layout that couldn’t
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:
A. I had to learn the
complexities and intricacies of the layout task, and fit my SI ideas and
constraints around that.
B. 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 those two things (A and B, above); we might call them
the “Golden Rules of SI’s Interaction 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.
On Working With your
Layout Engineer, 7 Axioms and GuidelinesI 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; in
short, more hard
skills than soft skills. To help bridge the gap between SI and Layout,
here are 7 things you should find helpful when working with your Layout
1. 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 in the Comments section below, but
it’s 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
2. 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
3. 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 don’t be
surprised if and when your layout engineer is focused on schedule and presses
you for efficient, clear, and actionable interaction.
4. 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’s how I ensure I’m succeeding at the two skills listed in
the previous section.
5. If Layout asks for
guidance on a certain intricacy and you’re 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 don’t have answers and “I don’t know” can be the first step in
6. 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 or better data to tip the scales to SI.
7. Layout needs SI to
make clear statements and requests in plain English. Better yet, using Layout’s terminology. If you are sensing I’m suggesting you learn
how to look at the project from layout’s perspective and understand what
they’re up against including their tools, clearances, DRCs, tasks and
schedules, you’re right.
These are some helpful ideas from my perspective as an SI
Engineer, but here are some thoughts directly from the perspective of layout
Thought from Layout
EngineersTo help SI Engineers improve our 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
talked about the habits of good SI Engineers, and what SI could do to improve
how we interact with layout. Here’s what
they told me:
a. 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
b. It’s important to
get to know the person you’re 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
becomes 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.” See Figure 1. 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.”
Figure 1: Lunch with
a Layout Engineer. Lany has 34 years of
c. 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 5 different
SI engineers and get 5 different answers”, which is frustrating. In fact, it’s 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 would be frustrating? So a single
source to interact with is helpful. It’s
also helpful when hardware engineering gets involved to understand the issues, mediate
the negotiation, take responsibility and make decisions.
d. Timely guidance is
imperative. SI needs to understand that
layout performs “hundreds of times” what seems to us 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 “getting
SI to participate can be difficult”.
Let’s change that. Design re-use
is another way to ensure constraints are clear and on-time. In other words, if the interface has already been
simulated, constrained, manufactured and tested to work well, do not be afraid
to re-use the same constraints – even cut and paste the layout when possible. For a diagram showing concurrent design and
implementation, see Figure 2. Note that
SI’s “constraints” and Schematic’s “netlist” are both needed for routing to succeed.
Figure 2: Product Development Cycle. SI (red) is concurrent
with Schematic (green) and Layout (blue).
e. In general,
layout’s interaction with SI has been positive and manageable. This 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, as already stated,
these successful layout engineers have learned to be good negotiators, to
explain trade-offs, and provide data on what doesn’t work and why. In some ways, that’s engineering’s normal
give and take of negotiating dozens of conflicting variables and constraints. However layout’s experience differs
substantially based on the attitude of the SI Engineer, and sadly some of us
have not yet learned how to partner.
f. Layout provides
valuable input to the design and engineering task. Perhaps you’ve noticed I have consistently
used the term “Layout Engineer”? Interestingly,
in speaking with these experienced layout engineers, the terminology they
consistently use is “layout people”, “designers”, or “layout person”. Why is that?
It may be the perception imparted by engineering degrees, which they
told me many layout engineers do not possess.
Instead many have learned their craft “on the job”, gaining practical
skills and experience we might lack. In
my dealings with layout I have found them well-skilled at “engineering” in
every sense of the word – typically providing essential insight and information
I need to succeed with my SI task. If
you looked at their job title you would see “CAD Engineer” or “PCB Design
Engineer”. Titles aside, at times we
have under-valued layout. I believe SI
Engineers would do well to esteem the skills of Layout Engineers and ensure we
do not waste their time as we would not want them to waste ours.
Working on a TeamOne way or another, design teams 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
we cooperate and fit our tasks together makes all the difference. While “schedule” can seem like an
arch-nemesis, one layout engineer pointed out that wasting time and cost
affects “not the company’s money, but our money”.
Multi-disciplinary collaboration has yielded tremendous
progress throughout our industry. I remember
when ICs started to arrange their pinouts for PCB route-ability, and now that
is commonplace. And how about PCI
Express allowing you to connect p to n and n to p if that made layout easier
and save vias? Furthermore, multi-lane
serial links that de-skew at the PCS (Physical Coding Sublayer) level without
requiring length-matching on the PCB was a giant step beyond DDRx matching
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 us, as SI Engineers, find practical ways to improve the
way we fit into the larger project team – particularly focusing on our
interaction with layout.
In ConclusionA look at Signal
Integrity, In Practice 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 come back to this article for a few fresh ideas.