The Pulse: PCB Design Education—What ‘They’ Don’t Tell You

Lots of people are far more qualified than I am to talk about PCB design, but what fascinates me about both PCB and electronic design is the range of disciplines you need to be aware of, especially when moving into high speed/high power/high reliability arenas. For a new designer entering this space for the first time, it can be quite an eye-opener (no wordplay intended) to discover just how many different disciplines are involved in turning a good design into a fit-for-purpose PCB.

Specialist in How Many Disciplines?
One of the changes that has engulfed all industries in the past 50 years is the increasing level of specialisation required. Gone are the days when an engineer could have an idea, design the electronics, lay the board out, and maybe even build and etch the PCB themselves. When Polar was founded in the mid-1970s, that’s just what could happen. And while this is still possible for the most basic of designs, a PCB designer now needs to at least be aware of a whole spectrum of specialist disciplines which will entail dealing with other companies and industry professionals, to determine where, and where not, external advice is needed for a specific design.

Material Choices
The proliferation of materials in the past 20 years means that a new designer is faced with a bewildering choice of materials for any design; the temptation to “mix and match” to ensure best performance/value for money trade-off means a designer will often need to speak to a base materials specialist for OEM work, or their PCB supplier, or a PCB value-added reseller. Many of these companies now employ former fabrication staff with extensive materials knowledge. Stretched supply chains mean these specialist intermediaries can help especially on low volume or prototype orders.

What to Ask?
I often encounter designers who are reluctant to ask about fabrication and materials as they only feel comfortable asking “when they know what to ask.” I also see some new designers who try to infer everything off a material datasheet, and whilst this gets part of the way to the answer, the datasheet is just that—information about a raw material that the fabricator “processes” into a finished product. Just like baking a loaf of bread, the raw ingredients for a PCB are not the same as the finished product. Pretreatment for adhesion and pressing impact the raw material in a way which will be fabricator dependant and will differ from the raw material specs. Not quite as extreme as trying to make a sandwich out of a bag of raw flour, but you get the picture. If you don’t know what to ask, there are plenty of YouTube resources that cover PCB topics, along with the networks related to industry bodies like IPC and EIPC. These organizations aim to bring together networks of specialists with disciplines from chemistry through metallurgy, drilling, high-speed signalling, reliability, and thermal capability.

More and More
As technology pushes higher speed, more density and more durability, and legislation restricts the use of some materials, there seems to be ever more to contend with from a designer’s perspective. As I mentioned, the only way to deal with this is to discuss requirements for each design with the appropriate source of knowledge from each discipline. The key for a new designer is to be aware that, depending on the design, non-electronic parameters interplay with the design outcome. It is quite fascinating to me that so many disparate disciplines need to work in harmony to produce the most fit-for-purpose design at the most economical cost.

Sometimes the Laws of Physics Work in Your Favour
Fortunately, sometimes technology drives in a direction to make life easier; the drive to ever-shrinking designs means that often high-speed traces can be kept quite short—so short that some parameters which need careful treatment on a larger scale can be of low enough impact to be ignored. Insertion loss is one case where loss is directly proportional to trace length, so keeping traces short can mean you can perhaps build on a lower grade base material. If the traces are very short compared with the highest frequency content, maybe characteristic impedance is not an issue there either.

A Sticky Problem
On the highest-speed interconnects, it is well known that copper roughness impacts the signal losses, and complex modelling is needed to calculate the impact, but the roughness is a “necessary evil” to keep the board from delaminating in the assembly process. Here, the chemists are working their magic to provide treatments and processes that chemically bond the copper to the dielectric, and emerging processes for bonding smooth copper could ease one task from the list of design considerations over the coming years.

Conclusion
A new PCB designer, apart from knowledge of layout—which is after all the job description—needs to be aware and ready to discuss a range of peripheral influencers on the PCB’s characteristics, many of which would seem to the casual observer to have no connection whatsoever to electronic interconnect. However, from an industry which measures time in inches, and thickness in ounces, that surely should come as no surprise.

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

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2021

The Pulse: PCB Design Education—What ‘They’ Don’t Tell You

08-17-2021

For a new designer entering this space for the first time it can be quite an eye opener (no wordplay intended) to discover just how many different disciplines are involved in turning a good design into a fit for purpose PCB.

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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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