Fresh PCB Concepts: Designing Controlled Impedance PCBs

Ryan_Miller_v1.jpgImagine that you are getting ready to work on a new PCB when the electronics engineer you work with suddenly gives you a controlled impedance requirement for the board. This will be your first experience designing a PCB with controlled impedance traces. Where do you begin? I encourage you to seek out IPC-2141, Design Guide for High-Speed Controlled Impedance Circuit Boards, to help answer that question; in addition, this introduction may also help.

Controlled impedance traces are necessary on some PCBs for high-speed signal transmission. The goal with controlled impedance traces is to design the proper propagation delay into the trace. On boards with an antenna, impedance matching is required for a reliable signal. Other boards may have components that require a specific propagation delay in order to perform calculations.

There are many types of both simple and complex impedance structures. No matter the specification, there is a controlled impedance structure to suit your needs. Fortunately, the simple structures cover many applications. Here is an overview of two of the most popular impedance structures:

The most basic CI structure is a microstrip, otherwise known as the single-ended trace. Most of the time, these traces are specified at 50Ω, but they can have other impedance targets. Most of the engineers who request my assistance with these traces are connecting to antennas. It is generally best practice to reference the next adjacent plane in the stackup. Called the reference plane, it will be the nearest plane to the traces.

Sometimes components and connections require edge-coupled micro strips. These traces are commonly referred to as differential pairs due to the way they operate. This structure has two traces placed side-by-side, with a reference plane. The two traces carry the same signal, but the signals are at different polarities. The device that receives the signal uses the difference between the two lines to perform its function. Some of the common impedance targets for these traces are 90Ω and 100Ω. Like the single-ended traces, the reference plane is typically the closest plane in the stackup.

Both the microstrip and edge-coupled microstrip can be placed on either the external or internal layers. When placed on internal layers, they can be embedded between planes, which will provide some protection from electromagnetic interference (EMI). However, as some components require the external connections from the leads, this is not always an option. Controlled impedance traces are transmission lines just like every other trace on the board. In this respect, they will transmit and receive EMI. In a case where an impedance trace is placed on an external layer, try to keep the trace length as short as possible.

To get the ball rolling on a new design we should start with the stackup. It is much easier to calculate the impedance traces and build them straight into the board; trying to add them in by layer can be a nightmare.

I highly encourage you to talk with your PCB supplier to determine if they can support your planned stackup. Do this well ahead of time. Many times, we plan, we get there—only to have the whole thing fall apart. Why? There are many reasons why the stackup you plan cannot be manufactured by a factory. In addition, many PCB suppliers can provide you with a stackup, complete with dielectric thicknesses and impedance calculations. If you do not have this option, it is not difficult to do yourself. Here are some recommendations:

Learn about the material you plan to use for the PCB. Some applications require the use of an ultra-low loss material, but some do not. Design sustainability into the board by using the appropriate material. Get the datasheet for that material and find the Permittivity value; this will be listed on the datasheet as the Dk, or dielectric constant. The Dk is necessary to calculate the impedances.

Use a good software tool. There are many reliable impedance calculators on the market. Some CAD tools have impedance support already in the environment, meaning only a minimal amount of learning is required to get some work done.

Remember: Changing the dielectric thickness between the controlling plane and the impedance trace will change the impedance value. Assuming all other variables remain the same, increasing the dielectric thickness will increase the impedance value, while—you guessed it—decreasing the dielectric thickness will decrease the impedance value.

There are a few ways the traces can be changed to affect the impedance value, but I will try to keep it simple: Think of the impedance trace as a water pipe. A smaller diameter pipe impedes the water flow more than a larger diameter pipe. Copper traces work just the same in this respect. Decreasing the trace width, or copper weight, will increase the impedance. Again, this is assuming all other variables remain unchanged.

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Finally, be very specific in the controlled impedance specifications you provide. Sometimes engineers use impedance requirements from other boards, which is acceptable providing that the stackup remains the same. Sometimes impedance structures that are no longer necessary for a new design get carried over from an old design and cause confusion. This will generate an engineering question, which may cause a stopping point for the production of the PCB.

Controlled impedance traces are not much different from the other traces on a PCB; all the traces on a PCB are transmission lines. Getting the necessary propagation delay in controlled impedance traces just requires a little more planning, software, and sometimes patience. As always, I recommend working with your PCB supplier as early as possible in the design phase to get it right from the start.

Ryan Miller is a Field Application Engineer at the NCAB Group USA.

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2023

Fresh PCB Concepts: Designing Controlled Impedance PCBs

01-19-2023

Imagine that you are getting ready to work on a new PCB when the electronics engineer you work with suddenly gives you a controlled impedance requirement for the board. This will be your first experience designing a PCB with controlled impedance traces. Where do you begin? I encourage you to seek out IPC-2141, Design Guide for High-Speed Controlled Impedance Circuit Boards, to help answer that question; in addition, this introduction may also help. Controlled impedance traces are necessary on some PCBs for high-speed signal transmission. The goal with controlled impedance traces is to design the proper propagation delay into the trace.

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2022

Fresh PCB Concepts: Copper Coin for Dissipating Heat

11-17-2022

Are you trying to fit more high-power parts on your board, but can’t find the proper heat management technique? With more high-power components there is a need for increased thermal management. When you’ve tried everything else and are still having trouble keeping your parts within proper operating temperature, copper coin is one of the best ways to dissipate heat throughout your board. Nicholas Marks writes this month's installment.

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Fresh PCB Concepts: How Large Does That Pad Really Need to Be?

10-04-2022

Annular ring is the amount of copper left and surrounding the hole after processing. This measurement is taken from the edge of the hole to the edge of the land at the thinnest part. To ensure a robust design, we must design efficient annular ring for PCB interconnections. From Ryan Miller's experience, annular ring design is a misunderstood aspect of printed circuit board (PCB) design. Not because it is complex; it is a dry subject with a few moving parts.

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Fresh PCB Concepts: Part 5—How to Handle Possible Moisture During Shipping, Handling, and Storage

07-21-2022

This is the fifth part in a series titled “What Damage Does the Assembly Process do to a PCB?” In part four of this series, I discussed the effect moisture has on the printed circuit board at soldering temperatures. I explained the material properties of FR-4 laminate and how they are hygroscopic. We also covered an acceptable practice known as dry baking used to force moisture from the product just prior to being exposed to soldering temperatures. I thought it appropriate expand further on part four in this column.

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Fresh PCB Concepts: Sustainability in PCB Design

05-12-2022

In order to meet the increasing demand for smaller, more powerful, and complex electronics, PCB designers are under pressure to create boards that are not only reliable but also sustainable. This can be a challenge, as many of the design choices that maximize performance can be less environmentally friendly. However, with a little extra effort, it is possible to create circuits that are both high-performing and sustainable. In this column, I will explore some of the key factors to consider when designing sustainable PCBs.

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Fresh PCB Concepts: The Right Board for the Flex Job

03-25-2022

We like to say we like the board that is best fit for the job, but what does the right board for your job specifically look like? In this article we will go over the benefits and design types of flexible PCBs. So, what is a flex PCB? A flex board is defined as a bendable board with one or more conductive layers. There are different types of flex boards to fit any situation you may have. IPC defines them in five types, all are different constructions. To put it shortly these constructions are as follows, One layer of flex, double sided, multilayer flex, multilayer rigid-flex, and double or multilayer flex without electrically connected layers.

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Fresh PCB Concepts: What Damage Does the Assembly Process Have on a PCB? Part 4

02-08-2022

In Part 3 of this series, I discussed how phenolic cured laminates can be mechanically weaker than their dicey cured laminate counterparts. I pointed out some of the material properties listed on material data sheets that explain and support this point. Whereas the phenolic systems are better at thermal management, the dicey systems are better under mechanical stress. There is no right or wrong here. The systems just perform differently under different circumstances. Understanding the differences and how they relate to the applied assembly process are important to ensure success. For this post I would like to discuss the effect moisture has on the printed circuit board.

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2021

Fresh PCB Concepts: Does the Assembly Process Damage a PCB? (Part 3—Phenolic Epoxy Systems)

12-16-2021

In part 2 of this series, I explained how the T260 and T288 material datasheet values could be used as an indicator of how durable a laminate system (FR-4) shall be when exposed to heat. The higher the temperature applied, the less time it takes to delaminate the FR-4. Traditional dicey cured epoxy systems do not stand up to lead-free assembly temperatures as well as one would think. The phenolic cured epoxy systems are much better suited and able to withstand the higher temperatures applied with lead-free assembly temperatures for longer periods of time.

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Fresh PCB Concepts: Recommendations for Track Welding and Open Circuit Repair

11-23-2021

Track welding is what some factories may opt for if they find an open circuit. The technique is to attach/stick/weld a thin piece of copper across the broken track. Sounds okay, right? But how reliable is the repair? And how does IPC cover this subject? Well, IPC doesn’t have too much to add on this subject other than mentioning that the customer and supplier should agree whether repairs are acceptable or not. Therefore, if the customer does not advise it is not acceptable, then it is acceptable by default. In my experience, the issue is rarely discussed between the supplier and customer.

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Fresh PCB Concepts: High-tech PCBs from Design to Volume Production

10-25-2021

Today I will talk about producing high-tech PCBs from design to volume production, since it is within this area that customers often do not reach their initial targets regarding time, cost and performance.

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Fresh PCB Concepts: HDI PCBs for New Kids in the Industry

09-30-2021

I have been working with NCAB Group for three years now. When I began I had no experience with PCBs and since have learned so much working with our customers’ PCB designs. I would like to share the basic fundamentals and design features of my favorite type of PCB, high-density interconnect, or HDI.

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Fresh PCB Concepts: Holy Cow, What a Lead Time

08-19-2021

It's summertime and we are adapting to our second COVID summer. However, things are most definitely a bit different these days. Recently I got an order, just like every day. The designer had spent the last three weeks completing the design and got it approved for order placement. The next step in the flow is when the purchaser spends a few days to get prices right, and then the order can be placed.

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Fresh PCB Concepts: Does the Assembly Process Damage a PCB? (Part 2—Time to Delamination)

08-12-2021

In part 1 of this series published back in April, I commented upon the effect the assembly process has upon a printed circuit board. There is another gauge that can be used to help a designer or contract manufacturer understand this point and that is the time to delamination test. These are referred to as either the T260 or T288 tests.

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Fresh PCB Concepts: Auditing a Factory—When Everyone Wins in the PCB Supply Chain

07-25-2021

One of the most important activities when it comes to maintaining the best quality of PCBs is to constantly be evaluating partner factories. An important tool for securing this is auditing those factories on a consistent basis. When the factory develops and the bar gets pushed a little higher each time, we’re in a situation where we all benefit.

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Fresh PCB Concepts: HDI Microvia Features in Illustrations

06-17-2021

Ruben Contreras explains microvias and discusses aspect ratios with microvias. This is important to know when designing an HDI PCB because the different types vary in complexity. And the more complex, the more this affects the cost.

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Fresh PCB Concepts: Don't Forget AABUS

04-28-2021

The most important thing is to know a standard and how to use it. Here is all you need to know about AABUS, what it means, how to handle it, and basically a list of issues that needs AABUS.

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Fresh PCB Concepts: Does the Assembly Process Damage a PCB? (Part 1—Soldering)

04-14-2021

Every time a printed circuit board is exposed to soldering temperatures it is damaged. This is the case not only for lead-free soldering applications but also for eutectic soldering consisting of tin-lead.

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2020

Fresh PCB Concepts: How 5G is Influencing PCB Technology Trends

12-17-2020

We have all heard about the Internet of Things (IoT) and artificial intelligence (AI). In combination with the increased data transfer rates available through 5G, they can open up a whole new level of connectivity and communication between devices and things.

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Fresh PCB Concepts: Finding and Qualifying a Long-Term Partner

11-05-2020

Finding the right factories is not an easy task. Anyone can take customers’ files and send them to whichever factory is available. But what guarantees does the customer have that the factory used is reliable in producing the design? Ruben Contreras details how to find and qualify a long-term partner.

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Fresh PCB Concepts: 4 Characteristics to Consider When Selecting PCB Base Materials

10-08-2020

Selecting the correct material is critical if you want your circuit board to survive the assembly process or come out of the assembly process in good condition. Jeffrey Beauchamp explains the four main characteristics from the IPC-4101 material specification that are critical in finding the performance of your base material.

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Fresh PCB Concepts: Advantages of Application-Engineered PCBs

09-17-2020

When working with your PCB supplier, do you have a dedicated engineering resource to help with the design of your PCB before fabrication? How about a resource that has experience and knowledge about the different applications for PCBs? Jeffrey Beauchamp explains how this is one of the most important and valuable factors when producing high-reliability PCBs, as well as what—or who—this resource could be.

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Fresh PCB Concepts: How Do You Calculate Finished Copper?

08-13-2020

How do you calculate finished copper on a PCB? This may sound simple, but Ruben Contreras has seen copper thickness called out either on the drawing or the specification, which can lead to additional EQs and, in some cases, additional costs. In this column, he explains the unintentional results that can come from misunderstanding what was requested.

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Fresh PCB Concepts: Pros and Cons of the 6 Most Common Surface Finishes

07-23-2020

There are only two different types of surface finishes for PCBs: organic and metal. Harry Kennedy describes the pros and cons of the six most common finishes on the market.

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Fresh PCB Concepts: 7 Options for Via Treatment

06-18-2020

In some cases, it's acceptable to have via holes that are completely exposed in a PCB design. But there are many others where the hole should either be covered and/or tented, or in most cases, plugged. Jeffrey Beauchamp shares seven different via hole protection types based on IPC-4761.

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Fresh PCB Concepts: Why Design and Produce PCBs Beyond Industry Specification?

05-07-2020

While questions may be an annoyance, especially when you finally have a board designed and are ready to have it built for a product, Ruben Contreras explains the importance of asking these questions and requiring specifications.

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Fresh PCB Concepts: Can Better Guidelines on Cosmetic Failures ‘Save’ Functioning PCBs?

05-05-2020

Every year, fully functional PCBs are scrapped due to cosmetic “failures” that are not approved. Is this right, or do we need to make an even more precise set of rules on how to handle this? Jan Pedersen shares his thoughts on the issue.

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Fresh PCB Concepts: What You Should Know About Your Board’s Solder Mask

04-09-2020

It’s a weird time we’re all in, and for most designers, work has slowed down or even stopped. While work might have slowed, now is a good time to review some of your past PCB projects to see if you can improve functionality and reduce field failures. Harry Kennedy explains how one of the simplest ways to do that is to start from the top: solder mask.

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2019

Fresh PCB Concepts: Designing a PCB for Telecom Applications

12-12-2019

Jeff Beauchamp and Harry Kennedy discuss PCBs for telecommunication applications, including key factors to consider, such as design and material considerations. They also recommend involving your PCB supplier at the time of design to help ensure manufacturability at the lowest possible cost.

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Fresh PCB Concepts: The Current Material Situation

11-11-2019

We have all heard about the component crisis in the circuit board industry, and maybe you heard about the CCL shortage, but how many are aware of the bare board material shortage? Ruben Contreras explains the current material situation and tips to address this issue.

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Fresh PCB Concepts: Getting It Right From the Start

10-23-2019

When faced with critical time-to-market situations, it is all too easy to say, “It doesn’t matter because this is just the prototype; we can fix this later.” However, if the design is perfected from the beginning, cost savings can be applied, and manufacturability can be ensured. Perhaps most importantly, the design can be adapted with reliability in mind, leaving a seamless transition from prototype to production. How do we get it right from the start?

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Fresh PCB Concepts: Why Material Selection Matters

10-02-2019

When you’re designing a PCB, it’s standard to call out FR-4 material, but you could be holding yourself back or even exposing your board to risk by not knowing more about PCB materials. Let’s take a small look into why. What Is FR-4, exactly? Harry Kennedy of NCAB explains.

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Fresh PCB Concepts: Qualities of Medically Applied PCBs

08-26-2019

In this inaugural column from NCAB Group, Alifiya Arastu discusses details of PCBs used in medical applications, highlighting some of the differences in terms of demands and how the design must be handled.

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Fresh PBC Concepts: What Is Reliability Without Traceability?

06-27-2019

High reliability and compliance are hot topics at conferences all over the world. If you are a supplier to industries like defense, automotive, medical, and aerospace/space, high-reliability and regulatory compliance are strict demands for electronic device manufacturers. This column discusses how high-reliability demands enforce the need for traceability, and at what level the traceability should be.

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Fresh PCB Concepts: My Flexible Story—Flex Circuit Development Through the Decades

04-30-2019

Senior Technical Advisor Jan Pedersen is celebrating 26 years at Elmatica. In this column, he shares his thoughts from his long experience in this exciting industry, and talks about those things that have changed a lot in the past few decades, and the others that haven't.

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Fresh PCB Concepts: Technology’s Future Comes Together—A Great Slogan for Us All!

02-13-2019

“Technology’s Future Comes Together” was the theme of this year's IPC APEX EXPO, which is quite suitable during these changing times. I guess we all need to come together, especially the automotive industry.

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Fresh PCB Concepts: PCB Standards for Medical Device Applications—A Hard Nut to Crack!

02-04-2019

With digitalization, AI, and IoT, the traceability and transparency to how a PCB is produced will be even more important. We must rule out the PCBs that follow the standards to the ones that do not. The day will come when you or someone you know might need a medical device, and you want to make sure it does its job correctly.

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2018

Fresh PCB Concepts: The Solution to the UL Challenge—Industrial Awareness

08-28-2018

Writes Jan Pedersen: The solder-limit subject has been a "hot potato" for a quite some time, with many discussions around the new requirement from Underwriters Laboratories (UL) that UL’s Emma Hudson brought to attention in early 2018.

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Fresh PCB Concepts: The Velocity of Technology— What Does It Really Mean?

07-02-2018

Jan Pedersen: Driving a car is probably one of the areas where the user comes in direct touch with the technology development. And we understand the speed when we see how fast we get new versions of smartphones and other gadgets. But in what direction are we going?

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