“Bogatin’s Practical Guide to Transmission Line Design and Characterization for Signal Integrity Applications” – A Review

Everything you ever wanted to know about PCB transmission lines – and more – in a digestible format with just the right amount of math to back up the illuminating practical illustrations.


Ed.: Martyn Gaudion is managing director of Polar Instruments Ltd. He began his career at Tektronix in test engineering on high-bandwidth portable oscilloscopes. He joined Polar in 1990, where he was responsible for the design and development of the Toneohm 950, Polar’s multilayer PCB short circuit locator. He became CEO in 2010. He also develops tailored content for the Polar YouTube channel. He reviewed this book for PCD&F.

Hot off the virtual press – a copy of Dr. Eric Bogatin’s new guide to transmission line design appeared in my Artech eBook account.

Do we really need another transmission line book? That’s what Dr. Bogatin asks right at the outset. After reading this new tome from virtual cover to cover, yes we do. This is a thoroughly practical book an peppered with links to Bogatins’s brief informative video explanations which expand and add dynamic content in a way that printed matter alone cannot.

Whether you are a recent graduate who wants a more practical insight to the behavior of transmission lines after doing all the hard work of the pure math side of study, or an experienced electrical engineer moving into the high speed arena – or even a PCB technologist or fabricator wanting an insight into all the mysterious terminology that surrounds the subject – this is a resource book for you. It is equally valuable whether you are dipping into chapters of specific interest, or taking a deep breath and reading from (virtual) cover to cover.

In my day job I spend most of my time helping customers who are new to transmission lines ensure that they document and design them correctly for fabrication, and I confess over the years much is taken as given. By reading Bogatin’s new book I have gained insight into transmission line behavior that is very familiar but I didn’t know the why – and the why makes everything make more sense. 

It is staggering that the electrical behavior of a simple pair of copper traces with a sandwich of dielectric material can generate a book running to 600 pages without loss of interest, but this is exactly what Bogatin does with the subject. Along the way you will find out why you should always think of signal and return paths and not in terms of signal and ground. You will find that while the RF and digital design spaces may run at similar frequencies, the design considerations for both are poles apart. (No pun intended.) You will also discover that simulators and field solvers don’t design circuits – you do – and you best have an idea of what you intend to happen and the expected outcome before reaching for the simulator. Words are important, and Bogatin stresses that though digital and RF and EMC specialists all deal with high-speed signals – and a lot of the jargon is similar – there are often situations where technical terms overlap while their meanings don’t. Bogatin takes an important stance in defining and understanding the terminology to ensure you are understood when working across disciplines.

On measurement – there are many precision tools for measuring high speed signals and time and frequency domain information, all with accuracy beyond your dreams – but as with simulation – Bogatin cautions that unless you understand what you are measuring and how to design your test vehicle, any or all of that expensive equipment can lead you to the wrong answer. Time spent in the measurement section of the book is well invested and will enable you to extract the best possible measurements from whatever TDR/sampling oscilloscope/vector network analyzer you have to hand.

I personally like the examples where Bogatin mixes electronic timescales in nanoseconds with human relatable timescales (days) to bring tangible meaning to his explanations. I also like his informative section on why intuition in the frequency domain does not translate easily (at all?) to the time domain, and that while both are valid and useful you need to work with a degree of selective schizophrenia while working in these domains.

Last but not least, alongside the video links and examples are links to both evaluation versions of commercial tools and useful no cost utilities so you can run the simulations and experiment for yourself.

Martyn Gaudion, June 2020

Bogatin’s Practical Guide to Transmission Line Design and Characterization for Signal Integrity Applications

by Dr. Eric Bogatin

Available from Artech House

The Area Ratio for Odd-Shaped Stencil Apertures

Joey writes:

Dear Dr. Ron,

I have a stencil aperture with an unusual shape. See Figure 1. How do I calculate the area ratio? The stencil thickness is 5 mils. The dimensions of the aperture are also in mils.

Figure 1. Joey’s Stencil Aperture

Joey,

The area ratio is simply the area of the stencil aperture opening divided by the area of the sidewalls. For common aperture geometries such as circles, squares, etc. it is easy to derive formulas. See Figure 2.

Figure 2. Formulas can be developed for common aperture shapes.

For an unusual shape like yours, it is easiest to simply calculate and divide the areas. From Figure 1, we get that area of the aperture opening as: 40*24+ the area of the two triangles. A little geometry (can you do it?) shows each triangle to have an area of 89 sq mils. So, the total area is 960 + 2*89 = 1138 sq mils. The perimeter is 40+24+16+16+28+12+16+16 = 168 mils, hence the area of the sidewalls is 168*5 = 840 sq mils. Therefore, the area ratio is 1138/840 = 1.355. Experience has shown that an area ratio of > 0.66 is needed for good solder paste transfer efficiency, so this stencil aperture will do well for transfer efficiency.

Careful thought would suggest that the triangular protrusions alone do not have a good area ratio. Calculations show their area ratios to be 0.37. So, the transfer efficiency in this part of the aperture might not be good. However, the area of the rectangle is so great, more than five times that of the triangles, as to alleviate this concern.

Dr. Ron



Autonomous Vehicles Even Farther Out in Time

Folks,

Readers of this blog will remember that I have been a skeptic of self-driving cars emerging in the near term. I am even less sanguine today. A recent article supports my perspective. Humans just do so many things effortlessly that sensors and computers cannot duplicate.

As an example, suppose there are five people at a street corner. These individuals non-verbally communicate intent that other humans easily pick-up on. If they are talking to each other and not facing the road, a human rightly concludes they are not planning on crossing. If they are facing the road and looking at the traffic, a human expects they plan to cross. This intuition is well beyond any AI’s ability to interpret and will be for decades to come.

Figure 1. A human recognizes that these students aren’t planning on crossing the street.

Autonomous vehicles are typically over designed to not cause accidents. Therefore, in some cases, if a pedestrian sticks their hand out into a road to wave at a self-driving car, it will stop. Whereas a human would recognize that the person is just goofing-off or being friendly.

All of this new information makes Elon Musk’s claim that Tesla will have a car on the road in 2022 without a steering wheel hard to accept.

To be fair, self-driving cars in controlled conditions, such as low traffic, well-marked routes, in good weather, will become more common in the decade ahead. However, an autonomous vehicle that can pick me up from my poorly marked 200 foot driveway, off an unmarked country road in Vermont, and then drive me to terminal C at Boston’s Logan airport is many decades away.

So, if you know someone who wants to be a truck driver, I feel that that will continue to be a fruitful career for a long time. In addition, those of us who manufacture electronics can take comfort in the fact that autonomous vehicles will need much more electronics than originally thought.

Cheers,

Dr. Ron

Whither Hong Kong?

The dispute between China and the US over trade, IP protection, human rights, and basically everything else ratcheted up a notch today as President Donald Trump announced the start of the process to revoke Hong Kong’s favored trade status with the US.

“I am directing my administration to begin the process of eliminating policy exemptions that give Hong Kong different and special treatment,” Trump said in a statement.

“My announcement today will affect the full range of agreements that we have with Hong Kong, from our extradition treaty, to our export controls and technologies. We will take action to revoke Hong Kong’s preferential treatment as a separate customs and travel territory from the rest of China.”

Hong Kong is not a major landing spot for manufacturers anymore. There are roughly 30 EMS companies of any size with operations there, per the CIRCUITS ASSEMBLY Directory of EMS Companies. VTech and Wong’s are the only Top 50 EMS companies based there. There are no bare board fabrication operations of any major size.

According to one source of mine, some companies use Hong Kong as a legal way to finish assembly to bypass tariffs on Chinese made goods from the mainland. If so, the president’s action will render that moot.

My question is, what will this mean for the scores of electronics companies that have sourcing operations in Hong Kong? While most of their business is done across borders, Hong Kong offers a more Western feel (and rules) for ex-pats. With the Beijing taking an ever greater interest in the city-state, that is almost certain to change.

I could see companies moving their program management staff elsewhere, if for no other reason than Hong Kong is expensive — maybe the most expensive place in the world for ex-pats. But if so, where would they go? And what will happen to Hong Kong if other industries follow suit?

Thixotropy: An Important Solder Paste Property

Folks,

To the SMT process engineer, the second most important thixotropic material in their lives is solder paste. If solder paste was not thixotropic, it would be difficult to print and would likely slump after printing the paste. What is a thixotropic material? It is a material that has a low viscosity when it is shear stressed and a high viscosity when it is not shear stressed. So, when the solder paste is forced through the stencil aperture by a squeegee, its viscosity plummets and allows it to fill the aperture. See Figure 1.

Figure 1. The viscosity of solder paste dramatically decreases as it is forced through the stencil aperatures.

When the stencil is removed, the resulting solder paste deposit experiences no shear stress so the deposit maintains the shape of a “brick.” See Figure 2. So thixotropy is a very helpful property of solder pastes.

Figure 2. After printing, the solder paste viscosity is high, enabling the depost to maintain the brick shape. Figure courtesy of Ron Lasky, Jim Hall, and Phil Zarrow.

If solder paste was dilatant, it would be a disaster. These materials are the opposite of thixotropic materials. They have a low viscosity when not shear stressed and a high viscosity when shear stressed. So they could not be forced through the stencil aperture and, if they could, they would flow all over the board. Cornstarch and water is an example of a dilatant material.

Oh, yes, what is the most important thixotropic material to the SMT process engineer? Their blood. When getting up from lying down, our heart automatically makes a strong “pump” to rush the flow of blood to our head. Since blood is thixotropic, it shear thins and makes it easier for our heart to get the needed blood up to our head. If blood was not thixotropic, we might faint every time we rise from reclining!

Cheers,

Dr. Ron

Zarrow and Hall’s “Board Talk” Becomes a Book

Folks,

There are a few good books that relate to electronics assembly. Ray Prasad’s Surface Mount Technology: Principles and Practice comes to mind. However, few (none?) teach the skills that need to be developed to become an electronics assembly process engineer, so Jim Hall and I collaborated on Handbook of Electronic Assembly and Guide to SMTA Certification a few years ago.

There was still a gap, however. No book existed that discussed troubleshooting everyday assembly defects and challenges. My good friends Phil Zarrow and Jim Hall have addressed this information in their Circuit Insight radio show Board Talk. All that was needed was a little encouragement to assemble it in book form. This task has now been accomplished!

Phil and Jim’s Troubleshooting Electronics Assembly is certainly one of the most useful books available for everyday SMT and though-hole assembly challenges.

Phil and Jim’s Book Can Help with Everyday Assembly Challenges

Check it out.

Cheers,

Dr. Ron

‘The Era of Offshoring US Jobs Is Over’ … or Is It?

That’s what the US Trade Representative says in an editorial in the New York Times today. And he gets the drivers right, mostly. But the results? “The United States lost five million manufacturing jobs. That, in turn, devastated towns and contributed to the breakdown of families, an opioid epidemic and despair.”

That’s just a crazy extrapolation. The US was at 3.1% unemployment prior to Covid-19.

Repeat after me: There. Were. More. Jobs. Than. Qualified. Workers.

For two decades, the no. 1 complaint I’ve heard from US business owners is the lack of manufacturing talent. Even in times of higher unemployment rates (the last two months notwithstanding), managers consistently noted the lack of basic communication and math skills among the workers available.

In his op-ed, USTR Robert Lighthizer adds, “If you want certainty, bring your plants back to America.”

It’s not that simple. You need the whole supply chain. And you need an end-market. The US, at 327 million people, isn’t big enough to sustain a company of any real size; those firms must be able to sell into other (larger) markets too.

And all those other big markets (China, Brazil, EU, etc.) have their own “make local” requirements and incentives.

I wish Lighthizer were right. But I’ll say it again: The US does not have enough worker talent to handle manufacturing at the cost necessary to satisfy the US market.

Covid-19 is Creating a Perfect Storm for Manufacturing

By Rafael Gomez, Director Product Strategy, Bright Machines

The pandemic’s economic impact started as a supply chain shutdown in Wuhan, China, but rapidly became a three-tier global disruption. As the virus spread, worldwide supply chain was interrupted, followed by an unprecedented shift in product demand and most recently by mandated factory shutdowns imposed on non-essential product manufacturing lines.

Let’s discuss the impact of these disruptions and explore how we can mitigate these forces that threaten to destabilize manufacturing.

Disruption #1 – Manufacturing and the supply chain

The first disruption to manufacturing and the associated supply chain was in China. This was due to the outbreak of novel coronavirus (Covid-19) forced workers in that county to stay home rather than return to work after the Chinese New Year holidays. The resulting impact was that a significant amount of the world’s manufacturing capacity was essentially shut down for an extended period, more than two weeks in most of China, and much longer in Wuhan.

This manufacturing and supply chain shutdown turned out to be just the start.  As the virus spread, manufacturing shutdowns rapidly spread throughout Europe and the US. We are now faced with the challenge to scale additional capacity or rapidly move production from one facility to another, neither of which are feasible in the manufacturing industry.

Disruption #2 –Demand volatility

Just as China’s factories started to come back online it became abundantly clear that the challenges were global and that certain products like PPE (Personal Protective Equipment) and medical devices were in unprecedented demand in terms of volumes and urgency. Meanwhile, workers, who are themselves consumers, were staying home and not shopping, sending economic shockwaves around the world, resulting in a dramatic downturn in market demand for non-essential or discretionary products. Add government and administrative intervention, including the loosening of FDA regulations and the use of the Defense Production Act in the USA, and it’s easy to see how the manufacturing industry was suddenly forced to deal with the unprecedented reactionary shift in market demand.

Disruption #3 – Workplace challenges

The third disruption came in the form of government directives to shelter in place and enforcement of workplace social distancing (including new OSHA guidelines).  Furthermore, non-essential factories have been shut down for an extended period. Once factories reopen, manufacturing plants will need to adhere to new and complex regulations. For example, when factories re-opened in China, they were mandated toto demonstrate ten-day supply of face masks for each worker. For example, a factory of 500 operators would need 10,000 masks to be authorized to continue operations. For many factories, an ongoing supply of PPEs in short supply and can be challenging and costly to obtain.

Once manufacturing companies receive authorization to restart operation, workplace social distancing on the factory floor will impact every discrete manufacturing function Traditionally, manual assembly lines are designed with minimum operator to operator spacing to facilitate the passing of product between stations and to minimize required floor space. With the new OSHA directives, these manual lines will need to be redesigned to increase operator spacing.  factories have met these challenges in creative style, like running extra shifts to redeploy staff and keep them distanced.

The data-haves and data-have-nots

Manufacturers that have embraced digital transformation, and the associated software-controlled automation, are best equipped to succeed in light of these disruptions. Real-time data drives visibility, which allows these “digital haves” to see the impacts of disruption sooner. Meanwhile, smart automation provides tools to adapt and adjust course quickly. Not only are these companies able to adapt production to meet increased demand or comply with new regulations, they are able to rise to the challenge of manufacturing the machines, devices, and consumables needed to help fight the virus, perhaps offsetting the loss of orders for ‘non-essential’ products.

The Future is agile and resilient

This perfect storm of disruption has exposed limitations of traditional manufacturing ecosystems and their associated supply chains. It has become clear that manufacturers need to move away from traditional analogue operational models, where production takes significant and costly time to set up on a line and requires constant tweaking or adjustment by experts with tribal knowledge of manufacturing processes.

To minimize the impact of economic disruption, manufacturers need to operate in a new paradigm.  This new version of manufacturing is fully data-enabled and software-driven to deliver an automated solution that provides the resilience to cope with disruption and the agility to react and adapt when that inevitable disruption occurs.

Considering previous viral outbreaks and natural disasters, Covid-19 isn’t the first global event to disrupt manufacturing and the supply chain, and it certainly won’t be the last. One key learning from this unprecedented event is that companies that have embraced digital transformation of manufacturing are the most robustly equipped to survive this economic disruption. These forward-thinking manufacturers will surely reap the prosperous benefits of their proactive digital transformation.

https://www.brightmachines.com/blog/

Make in India?

A colleague asks whether companies are looking at India as a country they can source electronics goods.

Good question. I would say that right now it’s not high on the list. It has a long way to go to develop the infrastructure and mass of supply chain companies dedicated to electronics (component manufacturers, laminate suppliers, chemistry suppliers, etc.). 

But … the bloom is way, way off the rose in China. China is less attractive from a labor rate perspective, and coupled with the tariffs, firms were already looking at alternatives even prior to Covid. See below for the year-over-year changes in electronics imports to the US from certain nations:

US electronics imports from selected nations, 2017-19

India’s electronics imports to the US grew 20%+ year-over-year in back-to-back years. Granted, it was starting from a low base: imports in 2016 (the base year) were just $754 million, and so even with the increase the total is just over $1.1 billion. Vietnam, another big gainer, is at now at $22.7 billion. China, even with the dip in 2019, was at $170 billion.

I do think US companies will to a greater degree be looking at nations outside China as potential manufacturing centers. India’s massive population continues to make it attractive of course. Now it needs to attract a few more assembly companies, which in turn will drive the suppliers to locate there.