Connect the Dots: A Proactive Approach to Controlled Impedance

Most can agree that controlling impedance is critical to signal integrity and board performance in devices powering high-speed digital applications, telecommunications, or RF communications. How to do so is another matter. It is common practice to include impedance-related notes with a PCB design and rely on the manufacturer to determine the proper trace parameters. This inherently passive methodology often leads to delays, cost overruns, and even batches of useless boards.

PCB trace impedance is determined by its inductive and capacitive reactance, resistance, and conductance, usually ranging from 25–125 ohms. Factors dictating impedance include:

  • The distance from other copper features
  • The width and thickness of the copper signal trace
  • The thickness of material on either side of the copper trace
  • The dielectric constant of board material

You can save time, money, and effort if you are aware of the impedance math when you sit down to design your board. Gain this awareness by using a good impedance calculator, and you can build the right tolerances into your design. Impedance testing becomes a double-check of your work instead of the tool you rely on to tell you if your documentation is correct. Documenting impedance requirements properly is more onerous than most people realize. Though it seems simple (e.g., state your target impedance, trace requirements, and material tolerances), PCB documentation is a details game that often leaves knowledge gaps for your manufacturer.

For example, picture a design for a four-layer board with two signal layers and two planes and a seemingly complete set of drawing notes. Now, let’s say the documentation doesn’t specify if both signal layers and trace widths require impedance control. In this case, the board manufacturer makes assumptions and heads for production or kicks it back to the designer for clarification. One scenario slows you down, and the other risks manufacture of boards that may not work properly.

To read this entire column, which appeared in the September 2019 issue of Design007 Magazine, click here.

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2019

Connect the Dots: A Proactive Approach to Controlled Impedance

10-31-2019

You can save time, money, and effort if you are aware of the impedance math when you sit down to design your board. Gain this awareness by using a good impedance calculator, and you can build the right tolerances into your design. Impedance testing becomes a double-check of your work instead of the tool you rely on to tell you if your documentation is correct. Documenting impedance requirements properly is more onerous than most people realize. Though it seems simple, PCB documentation is a details game that often leaves knowledge gaps for your manufacturer.

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Connecting the Dots: Managing Global Supply Chain Uncertainty

10-03-2019

We are well into the second year of tariff-centric trade policy, and one thing appears certain—uncertainty is here to stay. Though most of the media focus has been on cars and steel or consumer prices and corporate profits, the enduring challenge for both the electronics and PCB industries has been maintaining reliable global supply chains.

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Connect the Dots: MakeHarvard 2019: Bigger and Better!

09-04-2019

Sunstone Circuits was eager to return to MakeHarvard as a sponsor and creator of a competition category this year, also serving as both mentors and competition judges. If you were there, you saw us—we were hard to miss in our bright orange vests. As mentors, we were out and about helping students and answering questions.

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Connect the Dots: Five Best Practices to Ensure Manufacturability

08-01-2019

When you send your design for manufacturing, your partner does not know what type of device the board will be part of nor the conditions in which it will have to perform. It’s common for harsh environments or exposure to mess up a board’s performance. If you call out materials that will not tolerate the end-product’s operating environment, bad things can happen—such as a smoking board, for example. Be sure your board can tolerate thermal stress or solder joints risk breaking and damaging components.

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Connect the Dots: The Future of PCB Manufacturing Doesn't Belong to Robots, but to the Users

07-09-2019

Is the world ready for the consequences of rapid automation? Will the use of robots displace entire categories of workers? Can artificial intelligence really “think”? How will manufacturing, including PCB manufacturing, be affected by all of these smart robots? These questions actually come from a pamphlet published in 1955: "The Age of Automation: Its Effects on Human Welfare."

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Connect the Dots: Accurate Gerber Files Are Mission-Critical for Smooth PCB Manufacturing

05-30-2019

Gerber files can reveal design issues ahead of the quote process and ensure your manufacturer has everything needed to produce your boards correctly. After consulting with Engineering Support Specialist Eric Haugen, we explored some best practices for making sure that Gerber files are accurate.

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Connect the Dots: Preparing for Tomorrow’s Technology Today

05-16-2019

At a recent Sunstone Circuits planning summit, Matt Stevenson, VP of sales and marketing, and I led a wide-ranging discussion about emerging technologies and how they will impact PCB manufacturing. The following is an abridged transcript of this conversation.

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Connect the Dots: Exploding PCBs: Don’t Lose Track of Voltage in Your Design

04-01-2019

Managing split planes? Your CAM tool will not do it for you. We see this almost every day—not exploding PCBs, which pretty rare—but rather problems created by having more than one voltage on a power plane layer. From where we sit, this is one of the more insidious and costly challenges facing PCB designers.

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Connect the Dots: Selecting the Right Board Thickness—A PCB Designer’s Balancing Act

02-21-2019

Choosing wisely is critical for PCB quality and performance, but it can be tricky depending on size constraints, functional requirements, and environmental factors. While we sometimes have a general idea about assembly requirements or how the board will be used, there can still be a lot of unanswered questions as we begin the manufacturing process. After all, there’s a big difference between a PCB going into a drone and a PCB that will be part of a submersible drone and needs to be the size of a tennis ball, withstand intense heat or cold, and function forty fathoms below the surface.

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Connect the Dots: Key Guidelines for Clean Schematic Designs

01-08-2019

The smoke has not even cleared, but they already know what the problem is. It was supposed to be a celebratory moment for the team, plugging in the first board from the initial shipment. Instead, the room is as silent as a morgue. The engineer knows that they put the capacitor in place. However, on closer examination, it is not close enough to the IC pad to make a difference. Everyone turns to the PCB designer. “But I put it right where the schematic said to!” they say. The fix is easily implemented. It takes 15 minutes to produce a new design. Unfortunately, the break room already has an ample supply of coasters, and that’s all this batch of boards can now be used for.

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2018

Connect the Dots: Six Tips to Ensure Parts Fit on Your Board

12-12-2018

One of the most frustrating mismatches with alternative through-hole parts occurs when the land pattern matches, but the pin size is off. If hole sizes are too tight, pins may not fit through the holes, or if they do go into the holes, they may not solder well. Solder will need to flow through the gap between the pin and the hole barrel. If there is not enough space to allow enough solder mass to flow through the hole, the circuit board will absorb heat from the molten solder and cause the solder to solidify partway up the hole. This is called a cold solder joint and can result in a premature failure of your circuit.

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Connect the Dots: New Landing Design to Reduce Thermal Pad Failure

11-16-2018

You’ve finally finished your design. All the traces are correct and the IC landings are to the manufacturer’s specifications. A short run of test boards performs perfectly. For best results, you select a reputable domestic board house for production and a quality assembly shop to do the soldering. When the finished boards arrive, everything looks great. You’re in high spirits and congratulate yourself on a job well done. Then the reports start coming in.

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