When I was a kid, my parents always told me that our integrity is defined by what we do when no one is watching. Although, not publicized it is easy to spot someone with integrity by their actions. Especially, when confronted with challenges. I often reflect on this when designing PCBs to achieve the best signal integrity. Maintaining signal quality on your boards is never easy, but is especially challenging when high-speed signals are involved as board parameters, such as impedance change due to their frequency dependence.
That’s why it’s important to pay attention to impedance control during the PCB design stage. Impedance control involves specifying the required impedance for traces and transmission lines on your PCB. This is particularly important for higher speed signals and can be affected by your substrate material, copper widths, and routing. Even after you’ve laid out your board and selected your materials, you’ll need to work with your contract manufacturer (CM) to actually achieve an optimal design.
What is Impedance Control and Why is It Important?
As I’m sure you remember from school, impedance is a combination of resistance and reactance in a circuit. Sometimes, you need to specify that impedance for a net or trace in order to preserve signal integrity. Impedance control and matching is especially important for faster and longer trace runs.
What exactly happens if your net needs to be impedance controlled but isn’t? When transmitting a signal, you want the termination impedance to match the source impedance and transmission impedance. This will allow the maximum amount of signal power to sent to the end of the line. If the transmission or termination impedances aren’t matched, some of the signal will be reflected back into the trace and distort the incoming signal. Some communication protocols can tolerate more interference and deterioration than others, so you’ll need to know the specifications for your specific signal.
If your traces are relatively short or low-frequency, then you may not need to worry about impedance control. The rule of thumb for “relatively short” is, if it takes more than 1⁄4 of the rise time of a signal for it to reach the end of the trace, it needs to be impedance controlled. When it comes to frequency, if it’s over 200 MHz for boards of general size, then it’s time to start considering impedance matching.
What Affects Impedance Matching?
Now we know the importance of impedance control, but how do we go about actually altering the impedance of a trace? The two variables you can change for impedance control are: (1) the substrate material and layout, and (2) the width and routing of your copper.
Back when I used to nab fruit roll-ups, I was only ever interested in the strawberry flavor. This taught me that material matters, and PCB design is no different. Various substrates have differing dielectric properties that affect a run’s impedance. Things like your laminate, core, and prepreg, along with the thickness of each, can change the impedance of a trace. If you’re designing a high speed board, you’ll need to research exactly what kind of materials are recommended for your frequency range.
The most important task is keeping your substrate uniform. This means that if you’re using woven fiberglass with resin, you need to make sure it’s tightly woven so that the dielectric constant remains regular over its entire area. You will also want to keep an eye on specific material properties like the Coefficient of Thermal Expansion (CTE). When your PCB heats up during operation, your materials will expand. Dielectric thickness directly affects impedance, and this increase in height might throw your board out of spec.
The most obvious contributor to impedance control is the trace itself. Generally, the trace’s width will change in conjunction with dielectric thickness in order to achieve the correct impedance. It’s not just the amount of copper that matters, however; where your trace is routed is also very important. The first thing to check is that your trace does not cross over any gaps in an adjacent layer. A gap in the substrate plane represents a significant difference in dielectric constant over that area, which will interfere with impedance control.
You will need a solid power or ground plane on at least one side of your signal layer. Single-ended traces still have a return path back to their origin that attempts to follow directly underneath the run. A solid plane will allow this current loop to follow the original trace, like a differential pair, and reduce its effects on impedance. Speaking of differential pairs, confirm they are routed close together and match the lengths as closely as possible.
Manufacturing for Impedance Control
After you’ve designed the perfect PCB, you can send it off to your CM with full confidence that your design intent will be implemented, right? Maybe. Some manufacturers will help you with impedance control, but only if you give them the right information. You will still need to keep an eye on materials to ensure that everything comes out within tolerance.
Many manufacturers allow you to do one of two things: specify the dielectric materials, thickness, and copper widths, or specify the desired impedance. If you want to do all of the calculations yourself, I’m sure your CM won’t complain. Just make sure you do them right. Otherwise, you can let your CM know what impedance you want, and they will alter the substrate and traces to hit your number. Just make sure to double check it after they make their changes to ensure nothing else is affected.
When specifying materials, remember that some things come in standard sizes, and all inputs have their own tolerances. For instance, FR4’s dielectric constant can vary significantly in a single batch. If you need something more reliable, you should look into higher high-speed specific materials. It’s easier to change the thickness of some layers or materials than others. Be sure to check communicate with your CM and understand their strengths and limitations before you start designing something that can’t be built.
Whether you’re dealing with delicious fruity snacks or circuit boards, integrity matters. Impedance control for long or high-speed traces will ensure that your signal arrives intact with minimal distortion. The main two knobs you can turn to achieve this are dielectric material and thickness, and copper width and routing. Your primary goal when it comes to substrates is to keep them as uniform as possible. For your traces, make sure that you route them over a solid plane on at least on side, avoid gaps, and keep lengths similar for differential pairs.
|Tempo‘s Custom PCB Manufacturing Service
Last but not least, you’ll need to actually develop your PCB. Make sure you choose a CM that will communicate with you through the process and offer helpful suggestions. Tempo Automation is the PCB industry leader in rapidly building custom boards and meeting precise design specifications. We’ve been doing this for a long time and have the integrity on and off the board to get the job done right.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.