The Pulse: Simulating Stackup and Signal Integrity

Simulation and modelling are long-time friends of the engineer. I recently read The Works of Isambard Kingdom Brunel [1] (Pugsley). The math and modelling that accompanied his civil engineering works almost 200 years ago are quite humbling—even more so given that all his calculations were made by hand, devoid of the luxury of high-speed computing that bathes the engineer of this century in an abundance of tools.

But the purpose of modelling then was the same as now—to reduce the number of prototypes, to predict safety margins for structural loads, and, in Brunel’s case as an engineer, he also had a head for marketing as his mathematical and engineering abilities allowed him to build bridges with fewer materials and shallower curves than had his peers doubting their longevity. However, here we are 200 years later, and high-speed trains and traffic are still using his elegantly designed structures with safety.

Gaudion_Fig1_cap.jpgJust as with modern engineering challenges, the materials in his hands were not ideal, wrought iron was relatively new, and wooden structures, though well understood, were still natural materials and subject to the ravages of damp and decay over the life of the structure. Brunel had to understand both the math, and the limitations and inherent variability of the materials he deployed. This was especially the case in tunnelling operations where, despite geological surveys, the nature of the material being tunnelled was not always as predictable as expected.

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Electronic designs in which the architecture deploys the PCB are also subject to variation, and just like Brunel, when designing a PCB in today’s computer simulated environment, it helps to have a thorough understanding of the foundation you are using to build your design. The unsung hero of electronic designs—the PCB—provides the mechanical, thermal, and electrical platform on which 90% of electronic designs are realised.

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Simulation can only be as good as the source data, so the better understanding you have of the materials in your stack, the better the understanding of the effect of the PCB production process on those materials. This puts you a step ahead when you are modelling how the finished product will behave. Modeling stackups involves a mix of disciplines, the simulation tool needs to know the material types, the glass/resin ratios, and the target copper density of the finished design to predict how the materials will press and cure into the final finished height. Locations of drill ends are needed so that plating thickness can be accounted for.

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Understanding Materials
Returning to the civil engineering analogy, when tunnelling it’s a matter of life or death, so it is vital to understand the strata you are working in. With PCBs, careful consideration of the substrate materials can yield benefits. Working with spread glass reinforcement as opposed to traditional open weave glass can yield signal integrity benefits as the transmission line runs in a “more” homogeneous medium. But you need to keep in mind the mechanical and fluid flow considerations, as the flat glass—while better for laser drilling and signal integrity— provides a barrier to resin flow across the reinforcement, so you need to ensure adequate resin content to avoid glass stop, or delamination.

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Copper Characteristics

Moving from the dielectric medium to the copper plane and signal layers, new designers will often think they can use datasheet values for copper roughness. This is an understandable position if they don’t have a knowledge of the PCB fabrication process, but a glance at Figure 4 shows that the PCB fabrication process adds plated copper to the surfaces containing the drill ends to connect the vias in the PTH process. Therefore, the roughness needed for simulation is that of the plated layer and not of the base material. Simulation can only be as good as the input data fed to the simulator. The data does exist, but it is the job of the designer to know where to look, and when to take that data from a material data sheet, and when it needs to come from the fabricator.

Once the correct input data is available, then even the roughness may be fed into the simulator and the effects on signal integrity modelled with ease.

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Figure 7 presents the classic modelling and simulation dilemma. The Huray model is an excellent way of predicting the effect of copper roughness on signal losses along a transmission line, something which is inherently challenging. However, to “feed” the Huray model with good input data requires an SEM image of the finished plated or unplated copper surface. Not everyone has a scanning electron microscope on hand, and so a compromise in estimating the parameters is needed for day to day as opposed to laboratory use. Fortunately, Bert Simonovich of Lamsim Enterprises has provided a method which translates the matte and drum side roughness into Huray input parameters which are “good enough” to make a very usable model for predicting roughness impacted insertion loss.

Simulation Models
Whenever simulating, keep in mind the words of Polar’s favourite eminent British statistician, George Edward Pelham, “All models are wrong—but some are useful.” This is a piece of advice that engineers and fabricators need to take to heart; while the mathematical models we use for modelling save time, make better product, and increase yields, we only get the best from them by applying them with a good understanding of the input data and the limitations of the base materials used, and the manufacturing processes of pressing, plating, curing, and chemical adhesion promotion treatments applied to the finished article.

References

  1. The Works of Isambard Kingdom Brunel, edited by Sir Alfred Pugsley, Institute of Civil Engineers, London/University of Bristol.

 This column originally appeared in the April 2021 issue of Design007 Magazine.

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2021

The Pulse: Simulating Stackup and Signal Integrity

04-22-2021

Civil engineer Isambard Kingdom Brunel set a high bar for simulation and modelling—to reduce the number of prototypes and predict the safety margins for structural loads.

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2020

The Pulse: Don’t Ignore DC Trace Resistance

12-16-2020

Time flies! But the laws of physics don’t. Martyn Gaudion focuses on how important it is becoming to take DC trace resistance into account when measuring and specifying thin copper traces.

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The Pulse: Application Notes—Advice for Authors

07-27-2020

Application notes are the key to shedding light on new topics or new products and software tools in an easily digestible form. As both a consumer and an author many application notes, Martyn Gaudion explores various types and how to approach them.

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The Pulse: Communicating Materials From Design to PCB Fabrication

05-12-2020

Designer and fabricator communication—especially for high-speed PCBs—should be a bidirectional “thing.” It is so easy for a designer to say, “Just build this,” and hand over a challenging design to a fabricator who could have performed better with some preliminary conversation or dialog before placing the order. Martyn Gaudion explores communicating materials from PCB design to fabrication.

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2019

The Pulse: Modelled, Measured, Mindful—Closing the SI Loop

07-18-2019

In this woolly world where high-speed signals enter a transmission line with a well-defined shape and emerge at the receiving end eroded and distorted—and at the limits of interpretation by the receiver—it is well worth running simulation to look at the various levers that can be figuratively pulled to help the pulse arrive in a reasonable shape. At speeds up to 2 or 3 GHz, it usually suffices to ensure the transmission line impedance matches the driver and receiver. And a field solver makes light work of the calculation. But push the frequency higher, and other factors come into play.

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2018

The Pulse: The Rough Road to Revelation

03-07-2018

Several years ago, an unsuspecting French yachtsman moored his yacht to the railings of the local harbour. For a very nervous full tide cycle, he awaited to see if the cleats would pull out of the glass fiber hull. Fortunately, the glass held. A yachtsman at high tide isn’t too worried about whether the seabed is rough or smooth, but at low tide, the concern about a sandy or rocky seabed is altogether different. With PCBs, the move to low-loss laminates exposes a similar situation.

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2017

The Pulse: Tangential Thoughts--Loss Tangent Values

12-06-2017

Numbers are fascinating things, and the way they are presented can influence our thinking far more than we would like to admit, with $15.99 seeming like a much better deal than $16. Likewise, a salary of $60,000 sounds better than one of $0.061 million, even though the latter is a larger number. Our brain has been programmed to suppress the importance of numbers to the right of the decimal point. Such is the case with the loss tangent of materials. It is a tiny number and so to our minds looks insignificant, but it has a directly proportional effect on the energy loss suffered by a dielectric.

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2016

Vias, Modeling, and Signal Integrity

12-05-2016

Remember that good modeling can’t fix a bad design. The model can tell you where a design is weak, but if you have committed your design to product, the model can only tell you how it behaves. Some less experienced designers seem to think a better model will fix something that doesn’t work; it won’t. It will only reassure you that the design was bad in the first place.

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2015

Impedance Control, Revisited

06-10-2015

The positives for new fabricators and designers lie in the fact that, even though impedance control may be new to them, there is a wealth of information available. Some of this information is common sense and some is a little counterintuitive. So, this month I’d like to go back to the fundamentals, and even if you are an experienced hand at the subject, it can be worth revisiting the basics from time to time.

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I3: Incident, Instantaneous, Impedance

03-11-2015

In my December 2013 column, I discussed “rooting out the root cause” and how sometimes, the real root cause is hidden when digging for the solution to a problem. In that column, I described how sometimes in an attempt to better correlate measured impedance with modelled impedance, fabricators were tempted to “goal seek” the dielectric constant to reduce the gap between predicted and measured impedance.

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2014

Tolerant of Tolerance?

03-30-2014

Wouldn’t life be great if everything fit together perfectly? There would be no need for tolerance. However, for that to be the case, everything would need to be ideal and without variation...

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2013

Rooting Out the Root Cause

08-31-2013

When your measured trace impedance is significantly different from the calculated/modeled trace impedance, be careful before jumping to conclusions.

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Changing, Yet Changeless

01-16-2013

Like the whack-a-mole game where the moles keep popping up at random after being knocked back into their holes, the same old questions about technical hurdles surrounding signal integrity continue to surface as technology advances.

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2012

Repeatability, Reproducibility and Rising Frequency: The R3 Predicament

08-29-2012

One of the more popular editions of The Pulse in 2011 was the article "Transmission Lines - a Voyage From DC." Starting again from DC and working through the frequency bands, Martyn Gaudion looks at what is realistic to achieve and where economic compromises may need to be made.

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2011

Transmission Lines – a Voyage From Dc – No, Not Washington ...Part 2

08-01-2011

In the second part of this two-part article we continue on our voyage through a transmission line from DC onwards and upwards through the frequency spectrum, step by step exploring the characteristics from very low to ultra high frequencies.

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Transmission Lines – a Voyage From DC – No, Not Washington, Part 1

07-01-2011

In this two-part article I'd like to join you on a voyage through a transmission line from DC onwards and upwards through the frequency spectrum. In Part 1 we trace the impedance from infinity at DC to the GHz region where it reaches the steady state value of its characteristic impedance.

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Crosshatching Compromise

06-16-2011

Sometimes engineering results in some uncomfortable compromises; this is often the case with PCBs as the mathematical methods used by the modelling tools are based on "ideal" physical properties of materials rather than the actual physical materials in use.

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Correlation, Communication, Calibration

05-31-2011

At ElectroTest Expo at Bletchley Park, UK, Martyn Gaudion noticed the extent to which some technologies change, while the overall concepts do not. Prospective customers still ask exactly the same questions as they did 50 years ago: “What’s the bandwidth? Will it work in my application? How accurate?” Followed by the predictable, “How much does it cost?”

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When Is a 10ghz Transmission Line Not a 10ghz Transmission Line?

03-13-2011

'Just as in life, in electronics the only certainty is uncertainty.' -- John Allen Paulos

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Regional Differences – a Voyage of Glass Reinforcement

01-13-2011

Why bulk Er is not the same as local Er

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2010

The Pulse: Laminates Losses and Line Length, Part II

12-20-2010

In the last edition of "The Pulse," we began a discussion on how a modern field solver can help choose the most cost-effective material for a high-frequency application. Last month we looked briefly at the effects of line length and dielectric losses and this month we focus on copper losses; all three are primary drivers for losses.

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The Pulse: Laminates Losses and Line Length, Part I

12-01-2010

The EE creating the "platform spec" and the PCB fabricator responsible for its realisation face an array of materials with a mix of choices: From ease of processing to reliability requirements and signal integrity. For then next two months, "The Pulse" will focus on signal integrity, describing how to use field solvers to select the best materials when trading cost versus SI performance.

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Signal Integrity – the ‘S’ Words

10-01-2010

Three words, or rather, phrases are in the process of entering the vernacular of the PCB industry, albeit one phrase is already familiar, but taking on a different meaning. All start with S and all relate in one way or another to signal integrity.

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All Set to Measure Differential Insertion Loss?

09-13-2010

This column discusses the gradual adaptation necessary for PCB fabricators as more and more silicon families drive the industry toward the requirement for in house measurement of insertion loss.

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Zen and the Art of Accurate Impedance Measurement* – With Apologies to Prisi

08-12-2010

In his 1974 philosophical novel "Zen and the art of Motorcycle maintenance” Robert M. Prisig contrasts his regular and ongoing daily approach to motorcycle maintenance with his friend's alternate view of leaving well alone between annual service center based maintenance. What has this got to do with accurate impedance measurement you may ask? Please read on to discover more…

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New Column: The Pulse

07-14-2010

Polar Instruments CEO Martyn Gaudion will be exploring a number of themes. A major SI topic that is set to grow is the emergence of new silicon families designed to push traditional materials into the multi-gigahertz arena. These new chipsets lift transmission speeds up to a point where signal losses rather than reflections become the predominant concern from an SI perspective.

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