Tim’s Takeaways: PCB Vias, ‘You Have a Go’

Do you remember the old TV show “Stargate SG-1?” Debuting in 1997, it was a continuation of the 1994 movie “Stargate” and ran for 10 seasons, spawning two additional shows in the Stargate franchise. “Stargate SG-1” was known for its unique blend of sci-fi action and adventure, as well as a generous amount of humor.

The show also showcased a variety of well-done special effects, including creature design, makeup, and amazing starships and battles in space. The signature effect, of course, was the stargate itself, which was described as a wormhole through space. With the exhortation of “SG-1, you have a go” from their commanding officer, the stargate would instantaneously transport an intrepid band of heroes to new and exciting locations each week.

In addition to the general fun factor of the show, there was something else about it that appealed to me on a deeper level as a PCB design professional. It took me a long time before I figured it out, but when I finally did, I was stunned to realize that the stargate is nothing more than a giant via in space!

Okay, that may sound pretty lame, but if you think about the nature of a via, you can see where I’m coming from. Just as the stargate fictitiously conducted people directly from one place to another without the need for intergalactic space travel, a via will conduct a signal directly from one layer of the board to another. Of course, when you start adding time travel, energy weapons, and little gray aliens that aren’t wearing any trousers, the analogy falls apart pretty quickly. But at least we’re on the topic of vias now, which is where we wanted to be all along.

There was a time early in my career as a PCB designer, where we didn’t give much thought to the vias that we used. The signal speed of the designs we were working with was very slow compared to today’s standards, and except for fabrication costs, it didn’t really seem to matter how many holes got punched on the board. But as you know, that level of design technology is a long way behind us now. Vias are now much more than just a hole in the board to connect a signal from one layer to another; they are an integral part of the overall signal integrity of the design.

Take high-speed transmission lines, for example. For the best signal integrity, the traces of a transmission line should be contained on a layer that is sandwiched between two ground plane layers in a stripline configuration. This gives the best impedance control and shielding for the signal, plus the signal return path on the adjacent reference plane will be coupled directly to the signal.

The problem, of course, is that you can rarely contain a trace on just one internal layer. Even if the routing can be done on that one layer, you still need to transition to a surface layer to connect to the components through a via. Back in the day, we would have simply dropped a through-hole via and been done with it, but the high-speed routing requirements of today’s designs need a via solution with a little more finesse.

A through-hole via can act as an antenna and radiate energy, so we’ve come up with some ways to guard against that. Back-drilling the via is one of those ways while using blind vias is another. For high-density interconnect routing on boards with large pin-count ICs, microvias are often the best option. All of this is done to keep the signal path as short as possible through the via, with the added benefit of opening up some more routing channels where the through-hole via used to be. But using vias on a high-speed transmission line opens up yet another can of worms that, interestingly enough, requires the use of even more vias to resolve.

A signal needs a return path, and this is usually accomplished through a ground plane. For a high-speed transmission line, however, the return path becomes even more important and must be clearly defined in order to avoid problems that can disrupt the function of the circuit. The reference planes in a design are looked at very carefully now to make sure that there aren’t any plane splits, slots, and other congested areas that can clog up the signal return paths. Without a clear return path for the signal, the circuit may develop unwanted noise resulting in crosstalk, interference, and even false triggering of the signal.

Therefore, to maintain the best signal integrity, the transmission line needs to be directly adjacent to the ground plane that it is using for its signal return path, and the plane must supply a clear path. When the signal transitions to another layer through a via, however, the return path will be severed unless there are ground stitching vias nearby for the return path to hitch a ride on. These ground vias provide the means for the signal return to continue its path back to the source in what is known as layer paired routing.

There’s still more to how we choose which vias to use and where they should be placed, as well as the need for transmission line signal integrity. For instance, power circuits will need a larger via to handle the current that they are carrying. Without an adequate size, things can get a little toasty for the smaller vias, which is why power pins are usually routed with wider lines and larger diameter vias.

In some cases, vias are also used to transfer heat. Since a circuit board with all its different internal layers and metal planes makes for an excellent heatsink, adding vias under hot components will dissipate that heat throughout the board. This can bring a lot of heat relief to those parts instead of allowing the heat to build up in those areas of the board where the parts are located.

Circuit board test is another important use of a via. To test the connectivity of a fabricated board—as well as the assembly integrity of a fully manufactured board—a variety of tests are conducted on the PCB. These include using test fixtures with probes for every net on the circuit board that contact all the points designated as testpoints simultaneously.

Another method is known as a flying probe test, which is a machine that probes each testpoint individually. The important part of these different testing methods, however, is that vias usually serve as the testpoints which the probes come in contact with.

To facilitate this testing, a PCB designer will use some specialized features in their CAD tools to flag specific vias as testpoints. With a via marked as a testpoint, the location of that via and the net it is attached to then can be extracted from the CAD database in order to create a testpoint file. This data is then used for building test fixtures and programing the test machines. In some cases, the size and shape of the testpoint vias will even be changed on the layout to visually identify them as a testpoint on the fabricated circuit board.

Additionally, these testpoint vias are usually subjected to additional design rules and constraints than a regular via is. For obvious reasons, they can’t be placed under components, and they need to have specified clearances from each other to ensure that the probes on the bed-of-nails test fixture can easily access them.

Considering all that they are used for and what is required of them, vias are actually subjected to a lot more action and adventure than I would have expected. I wouldn’t put them on the same level of excitement as SG-1, but vias do have a lot of intricacies to them that commands respect. There is a lot to consider when working with them to ensure that your PCB design performs up to specification and is manufacturable the way you intended it to be.

Thankfully, there are people in the design community who are a whole lot smarter than I am about the best way to work with vias. You have the opportunity to learn from them in this edition of Design007 Magazine. I’m looking forward to reading myself what I-Connect007 and its experts have to say about vias when I receive my own copy. Until next time then, keep on designing, and vias, you have a go!

Tim Haag writes technical, thought-leadership content for First Page Sage on his longtime career as a PCB designer and EDA technologist.

This column originally appeared in the November 2020 issue of Design007 Magazine.

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2020

Tim’s Takeaways: PCB Vias, ‘You Have a Go’

11-13-2020

Do you remember the old TV show “Stargate SG-1?” With the exhortation of “SG-1, you have a go” from their commanding officer, the stargate would instantaneously transport an intrepid band of heroes to new and exciting locations each week. Tim Haag details his realization that the stargate is nothing more than a giant via in space!

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Tim’s Takeaways: Thermal Management for PCB Designers—Staying Out of the Fire

09-09-2020

If there’s one thing in life that really feels the pressure of being in the hot seat, it’s the PCBs that we design. But PCB designers often feel a lot of pressure while doing their work, which puts them squarely in the hot seat. Tim Haag shares four techniques in thermal management for PCB designers.

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Tim's Takeaways: Navigating Industry Expectations

05-29-2020

While some expectations are normal—and, well, expected—in the workplace, there are also those that do more harm than good. Tim Haag unpacks negative expectations and shares suggestions for improving communication in the workplace, as well as positive expectations that you can set for yourself.

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Tim’s Takeaways: Working From Home—5 Tips for Newbies

03-24-2020

Due to the COVID-19 outbreak, many people who have worked in an office environment for their entire career have suddenly found themselves shifted to working remotely. At first, this may seem like it isn’t that big of a change, but it may be a bigger deal than you realize. Tim Haag, who has worked from home for over 17 years, shares five tips for making the most of this situation and working successfully from home.

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Tim’s Takeaways: Clearing Up the Buzz

02-14-2020

My first “real” job in the world of electronics was working at a Radio Shack store back in the late ‘70s. It was a step up from flipping burgers, but it didn’t last long. However, there was one notable aspect of that job; I was there during the time that Radio Shack introduced its first personal computer—the TRS-80. Although it is practically unimaginable now, in those days, there wasn’t much in the way of personal computing available for the general consumer.

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2019

Tim's Takeaways: Realizing a Higher Standard for PCB Design

10-09-2019

To the untrained eye, one circuit board may look pretty much like any other, but as we know, there are major differences between them. Not only are they different in purpose and design but also in how they are manufactured for specific industries. If you are designing medical equipment, for instance, you will have to meet many different regulatory requirements from organizations, such as the Food and Drug Administration (FDA), World Health Organization (WHO), and International Electrotechnical Commission (IEC), among others.

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Tim's Takeaways: Clear Communication Takes the Cake

07-10-2019

Whether baking a cake or building a circuit board, it’s all about clear communication. If the person writing the recipe had not made the choice to clearly communicate what their intentions were for baking that cake, I would have been lost. A missing ingredient here or an incorrect oven temperature there and my birthday surprise would have ended up in the garbage in the same way a successfully built circuit board starts with clear communication from the designer. Circuit board manufacturers want to create a perfect PCB for you, but they can only do so to the extent of the instructions that you give them.

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Tim's Takeaways: Rules Keep You from Crossing the Line

06-20-2019

Driving rules are designed to keep drivers between the lines of traffic instead of crossing over those lines into dangerous situations. Similarly, design rules are also intended to keep PCB trace routing between the lines instead of crossing over them as well. But you might be surprised how many people refuse to use the full potential of their DRCs to protect themselves, and in some cases, refuse to use them at all.

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Tim's Takeaways: I Think I’ll Go for a Walk

04-08-2019

Many years ago, my boss at a PCB design service bureau had his own unique way of encouraging us to take a break. He would come through the design bay and call out in his deep baritone voice, “DARTS!” and we would all follow him into the break area for a quick game. In addition to the benefits of taking a break, forcing our eyes to focus in and out as we threw a dart was a great way to relieve us all from the eye strain of older CRT monitors.

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Tim's Takeaways: A Job Worth Doing

02-28-2019

I get it. We PCB designers are made of the kind of tough stuff where we will work ourselves to death if given the chance. But in our all of our efforts, are we really doing it right, or could we somehow be doing it better? Let’s take a moment to consider some other ways that we might help ourselves to improve.

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2018

Tim's Takeaways: Contract Positions—Go the Extra Mile

10-10-2018

For newbies just entering the industry or experienced designers who have always worked for a corporation, the transition to contractor can be a real culture shock. The allure of working from home and setting your own hours can quickly be replaced by the realities of chasing jobs and wondering where your next payday will come from. However, there are some wonderful aspects of working as a contractor that can make it very worthwhile.

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Tim's Takeaways: Where Have All the Designers Gone (and Who Will be Taking Their Place)?

08-17-2018

We have a lot to pass on to the new designers. We must stress the importance of understanding of the roots of our industry and why this design knowledge is important. I have worked with many designers who don’t understand anything about the output of their design files. They go through a procedure, hit a series of commands, and presto: The design files are all wrapped up in a neat little zip file ready to go out to the manufacturer. That’s all well and good, until something breaks or a manufacturer has a specific question. It would be a great thing to make sure that the designers of tomorrow understand what a Gerber file and an aperture list really is.

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Tim's Takeaways: Hiring the Right PCB Designer

06-04-2018

Like the rest of you, I’ve had times of unemployment, when your daily job is looking for work. You find yourself writing and then rewriting your resume, searching online forums and job search sites, and applying to every job that you can find. I’ve also hired people, and I know what hiring managers face. But hiring managers may be hurting their companies by drawing up a list of expectations so tight that highly qualified people may be slipping between the cracks.

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Will Cool Technology Attract the Next Generation of PCB Designers?

04-17-2018

If I had the opportunity to design some boards that went into medical detection equipment like my new blood pressure cuff, I would be extremely motivated to do that. Maybe what we should be focusing on is not just playing with the new toys, but showing the younger generation different ways to think about how they can improve upon these new toys.

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Customer Support: What do PCB Designers Really Want?

03-19-2018

First, let’s throw a leash around the elephant in the room. That’s my way of saying, “Here are some things that designers want, but we in the support business just can’t give it to them.” The first one that comes to mind: Customers have asked, manipulated, and even tricked me in their attempts to get free software.

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Tim's Takeaways: Good Support Isn’t Just for Customers

03-06-2018

I have been working in PCB CAD tools customer support for years and years, and it isn’t that often that the tables are turned and I have someone who is supporting me. I’ve got to say, it was a pleasure being the recipient of some quality support.

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2017

True Design Efficiency: Think Before You Click

10-09-2017

At the captive shops that I’ve worked with, where the designers were more involved in the entire design cycle and had better access to the corporate libraries, staff engineers, etc., the story was often the same. Some designers would jump into the deep end of the pool of design without any thought to drowning while others would be so busy lacing up their life preservers of preparation that they would take too long getting out of the shallows and into the depth of their design. So, what’s the best approach here?

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Tim's Takeaways: It Really Wasn’t My Fault

09-07-2017

I once received verbal instructions from an engineer who directed me to make a certain change. I didn’t think anything of it. Many months later, this same engineer told me that there were troubles with the board and all its successive versions because of the change that I had made. He ended up making it right in the end. But in hindsight, what could I have done to save myself a couple of months of suspense and worry?

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Tim's Takeaways: Stepping into the Great Unknown

08-16-2017

Many years ago, I was given the opportunity to switch my career path from senior circuit board designer to CAD systems administrator. I wasn’t certain that I wanted to give up the comfort of being a designer; after all, I had been one for a long time. But I knew that this transition would help my overall knowledge base of everything CAD-related, as well as better position me in my quest for a management position. So, I pulled the trigger and accepted the new job even though the idea of stepping into the great unknown like that was very intimidating.

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Tim's Takeaways: Design Tools of Tomorrow--A Real 'Marvel'

04-05-2017

Imagine if you could interact with your design as a hologram floating in front of you the way Tony Stark did in the movie "Iron Man." Wouldn’t it be amazing if you could pick a section on your holographic design with your hands and expand it to the point where you could peer into it, spin it around, and manipulate it as you desired? Want to push a trace down to a different layer? Just give it a nudge in the right direction and the holographic display changes it to the next layer. Don’t like the way a certain area fill looks? Then just grab it with your fingers and pull it out and throw it into the virtual garbage can.

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Tim's Takeaways: 'Sparks' to the Rescue in RF Design

01-03-2017

Just like the early days of radio where Sparks the radio specialist was in demand to get the job done, we now need RF specialists to work together with electrical engineers to create the intricate designs required for RF circuits. You are now Sparks, the go-to specialist who will take care of RF design business.

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2016

The Basics of Hybrid Design, Part 3

06-16-2016

The world of hybrid design is growing, and we have lots of hybrid-specific functionality built into our software that helps designers meet and conquer the unique hybrid design requirements that they are faced with. And yet many designers out there (and I used to be one of them) have no idea what is meant when people start talking about hybrid design.

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The Basics of Hybrid Design, Part 2

05-16-2016

In the first part of this series, we discussed the basics of hybrid design from the PCB designer’s perspective, and here we will continue that discussion.

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The Principles of Hybrid Design, Part 1

04-25-2016

What exactly is a hybrid design? We are seeing more and more of our customers exploring the world of hybrid design, and we are getting new customers for whom hybrid design is their sole focus. The world of hybrid design is growing and we have lots of hybrid-specific functionality built into our software that helps designers conquer the unique hybrid design requirements.

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2015

Tim's Takeaways: The Utility Belt

05-12-2015

The utility belt is a great thing to have. Batman would be long dead without his, and Tim “The Tool Man” Taylor would be useless without his. But for a circuit board designer, a utility belt is equally important. All of us at one time or another will have questions about the CAD system we use, and one essential tool to have in your utility belt is a list of people you can go to for help. At the top of this list should be your CAD system’s friendly customer support staff (like me).

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DFM: The PCB Designer as Arbitrator

04-08-2015

Design engineering is usually a combination of electrical and mechanical engineers. Although these two groups can have their own dramatic conflicts between each other, they will usually end up working together because they ultimately serve each other’s needs. But the manufacturing engineering requirements usually come from a completely different department or from an outside manufacturing vendor.

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2014

Like it or Not, You're a Role Model

12-24-2014

"During the years that I built my skills as a circuit board designer, many people helped shape my character. Some were impulsively brilliant at laying out a board, while others were steady and consistent in their approach to work, dotting every 'i' and crossing every 't.' But they were all patient with me, answering my questions, showing me the ropes, and setting good examples for me to follow," says Columnist Tim Haag.

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Blink and You Will Miss It

11-05-2014

Tim Haag writes, "Friedrich Nietzsche said, 'That which does not kill us makes us stronger.' Well, that adage certainly proved to be true in my situation. If I hadn't been ripped from my secure position and forced to contract for a short season, who knows how my future would have eventually unfolded. And if it hadn't been for that brief season of hardship, would I have had the strength and flexibility to succeed later on?"

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Tim's Takeaways: Blink and You Will Miss It

11-05-2014

Tim Haag writes, "Friedrich Nietzsche said, 'That which does not kill us makes us stronger.' Well, that adage certainly proved to be true in my situation. If I hadn't been ripped from my secure position and forced to contract for a short season, who knows how my future would have eventually unfolded. And if it hadn't been for that brief season of hardship, would I have had the strength and flexibility to succeed later on?"

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There Are No Stupid Questions

09-10-2014

Many of us who have been designing boards for years have had to deal with annoying questions from "the kids." You know who I mean: The rookies, newbies, greenhorns, or puppies just starting out in their design careers. We've all had to answer questions like, "Why is library development so important?" or "Why is solder mask green?"

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Tim's Takeaways: There Are No Stupid Questions

09-10-2014

Many of us who have been designing boards for years have had to deal with annoying questions from "the kids." You know who I mean: The rookies, newbies, greenhorns, or puppies just starting out in their design careers. We've all had to answer questions like, "Why is library development so important?" or "Why is solder mask green?"

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Design Rule Checks - For Your Protection

07-09-2014

Columnist Tim Haag writes, "I have designed multitudes of PCBs over the years, but I have a confession to make: It can be hard for me to run that final design rule check. I know that it is important, but at the end of a long design cycle, I just want to be done. I don't want to redo anything, and I sure don't want to look at my own errors. Do any of you feel that way?"

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Tim's Takeaways: Design Rule Checks - For Your Protection

07-09-2014

Columnist Tim Haag writes, "I have designed multitudes of PCBs over the years, but I have a confession to make: It can be hard for me to run that final design rule check. I know that it is important, but at the end of a long design cycle, I just want to be done. I don't want to redo anything, and I sure don't want to look at my own errors. Do any of you feel that way?"

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Customer Support: Not Just for Customers Anymore

06-04-2014

Columnist Tim Haag writes, "In my role as the customer support manager, I have seen plenty of examples of customer support. But my point here is not to focus on customer support as a function of a support technician. Instead, I want to explore the concept of how we should all strive to provide the best level of customer support in our jobs, no matter what we do."

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