Impedance control is an important consideration when looking for components to use in a Printed Circuit Board (PCB). That is also why consumers and electronic companies are always exploring options to get the best impedance matching in PCB.
In this article, we will walk you through the basic concept of impedance control as it pertains to circuit board design. You will also learn about the best ways of calculating impedance control in a circuit board, as well as how to control it and the best manufacturers/factories you can rely on.
What Is Impedance Control in PCB?
The first thing we want to consider is the meaning of impedance control. An understanding of the concept will make it simpler for you to realize the importance of using it as a control measure in your circuit board.
Impedance control refers to the reactance and resistance that a circuit board is expected to attain so that the design will be perfected.
Also, impedance control involves the propagation of signals in a Printed Circuit Board (PCB) without distortions. The control measure must be applied because it helps to provide a repeatable high-frequency performance of the circuit board it is used on.
How to Determine Impedance Control of a Printed Circuit Board
Do you want to determine or find out if your circuit board has the right impedance control? It is simple if you know what to look for.
The first thing we want you to understand is that the impedance of Printed Circuit Boards (PCBs) is determined by the materials used in designing the circuit board. It is also determined by the physical dimensions of the circuit board. Above all, the impedance control of a Printed Circuit Board (PCB) is measured in Ohms (Ω).
That said, let us now walk you through the other important factors that contribute to the evaluation/determining of the impedance control of a circuit board.
1. Trace Impedance
The first thing you must have in mind is that the impedance of the trace is one of the most important elements of the circuit board.
For that reason, you should be aiming for trace impedance control of 25 and 125 Ohms.
2. Targeted Impedance Value
The second way of determining the impedance of a circuit board is to figure out and aim to meet the targeted impedance value.
What this means is that the PCB manufacturer/designer must ensure that the stack-up and choice of trace used in the circuit board are designed to meet the targeted impedance value. It also makes sense that a certain tolerance is maintained, especially when it involves high-frequency signals.
3. Solder Resistance
The presence of solder resistance on the circuit board can also be another factor that influences the control of impedance on the Printed Circuit Board (PCB).
Aside from those, there are many other ways of determining the impedance control of a circuit board. They include:
- The width and thickness of copper traces
- The distance of the impedance control from the reference copper plane.
- The method of passing the signal through vias
- Prepreg and core material dielectric constant
- Prepreg material thickness of core thickness on each side of the trace
Service Levels for Impedance Control
You now know some of the basics of impedance control PCB. Now, we will look at how the same control can be used for facilitating control and propagation of signals in a circuit board.
Ideally, there are 3 service levels/methods of facilitating impedance matching in PCB. They have been explained below so you will have a better idea of how they work in a circuit board.
1. Impedance Control
this is the ideal concentration and basic of impedance control in the Printed Circuit Board (PCB).
If this is what you want, it implies that the PCB manufacturer/designer will have to continually make efforts to design the circuit board until the required/expected impedance control is attained.
2. No Impedance Control
For some reason, you want to use the second level of service, which entails the removal of impedance control.
In such an instance, additional efforts will not be made to hit the desired/expected impedance level because the tolerance of the impedance is already loose/low.
Since additional design elements wouldn’t be required, it means that the PCB manufacturer will expedite action in the manufacturing of your Printed Circuit Board (PCB).
3. Impedance Watching
Last but not least is Impedance Watching. It is considered one of the most tactical because of the adjustments that need to be made.
Ideally, the designing of a circuit board using Impedance Watching entails that the PCB designer/manufacturer will have to outline the traces of the impedance control on the circuit board. After that is done, the next step is for the provider to accordingly adjust the trace width and the dielectric height. That way, there will be uniformity in the design.
But, that is not all there is to it. After the necessary adjustments, the PCB manufacturer will need to approve of the specifications you made for the circuit board. It is only after the approval that the quality impedance control factory will start working on the design.
How to Control Impedance of PCB
Do you want to know the best ways to control impedance in a Printed Circuit Board (PCB)? It is not rocket science because it is something you can do with the help of your PCB manufacturer.
Here are some of the excellent strategies you can always use to control the level of impedance in your circuit board:
The first step is to have an accurate measurement of the impedance level that is to be used in the circuit board. The reason is that a measured impedance level has its way of regulating the overall stability of the circuit board.
For example, if you were working on a TV cable or satellite, the first thing you want to do is to set the coaxial cable of the TV to act as a conductor while the antenna will work as the source. When that happens, it will be easier to measure and control the electrical impedance of the cable by evaluating the characteristics of other important elements, including:
- Conductors and
2. Matching impedance
Truth be told – nothing can be achieved if there was no way to ensure that the impedance of the circuit board matches. That brings us to the second approach to controlling the impedance of a circuit board.
The idea here is that the PCB manufacturer/designer needs to ensure that the signal on the circuit board smoothly moves from the source of a message to the load.
Also, we must emphasize that the reason for looking into this is that if the input impedance and output impedance match, then the same should be extended to ensuring that the load impedance and reference track must also match.
finally, the circuit board in question must be subjected to rigorous tests because that is one of the excellent ways of finding out potential downsides to the board’s design.
Impedance Control Verification
Now, we must consider another important aspect of impedance control in Printed Circuit Boards (PCBs). We are talking about how to verify or confirm the validation of the impedance control.
Usually, the importance of impedance control verification or measurement is meant to uncover some of the loopholes that might limit the viability of the circuit board.
There are many methods of measuring impedance control in a circuit board, but we will discuss some of the popularized ones so you will have an idea about how they work.
Use High-Quality Cables and Probes
The first method of verifying or measuring impedance control of a circuit board is to use high-quality cables and probes. The idea is that if the probes or cables are used, they will simplify the process of evaluating the circuit board.
Besides, you might want to resort to using gated cursors because they play an important role in obtaining the mean value of the board.
Third, you might be interested to use the third option of using calibration quality adapters, especially when there is an urgent need to use RF adapters.
The second method is to use test coupons. There is no doubt that test coupons play an important role in the evaluation and measurement of the impedance control of a circuit board.
The reason for that is not unconnected to the fact that test coupons are typically fabricated at the edges of the circuit board.
Because of that, the coupons have become a veritable method of facilitating inspection of internal structures, layer alignment, and electrical connectivity of the circuit board.
Time-Domain Reflectometer (TDR) Can Help
The third option you want to explore is the Time-Domain Reflectometer (TDR). It is primarily used for measuring the impedance level of a Printed Circuit Board (PCB) by sending signals along a line and measuring the part of a message in the signal to confirm if the impedance matched or not.
Mistakes to Avoid When Designing a Circuit Board for Impedance Control
We would like to point out that some mistakes tend to be made if the PCB manufacturer is not careful. These mistakes can be avoided if the manufacturer/designer is certain of the potential causes so it will be easier to address them.
For this article and not to make things complicated for you; we have sub-divided these mistakes into different parts. That way, you will have a general overview of each of the potential places that the mistakes can be reflected in the design of the circuit board.
Mistakes to Avoid During Stack-Up
The first mistake that PCB manufacturers make is usually when the stack-up of the circuit board is being designed.
We have taken the time to evaluate some of the potential mistakes that can be avoided during the stack-up of the circuit board, especially as it pertains to impedance control PCB.
The first mistake is made in the spacing of the impedance traces. In this case, the PCB manufacturer must be careful because not spacing the impedance trace well enough can reduce the movement of signals on the circuit board.
To that end, the spacing between the two traces of the circuit board must not be more than twice the width of the traces.
Also, the PCB manufacturer must make conscious efforts to ensure that the width of the trace is not two times more than the dielectric thickness between the nearest reference layer and the target signal layer.
The major mistake that must be 3 different types of prepregs must not be used in a stack-up of the circuit board.
Mistakes To Avoid During Routing
Some mistakes must be avoided during the routing of the Printed Circuit Board (PCB).
The ideal length that must be used to prevent mismatching is to maintain a length match of +/-5 mils of the different pairs.
Traces Crossing Split Planes
Traces can be prevented from crossing the split planes by ensuring that they are not routed across a split plane.
How Is PCB Impedance Calculated?
There are different methods used for the calculation of PCB impedance. Some of them have been explained below:
Use an Online Calculator
The quickest way to calculate the impedance of a circuit board is to use an online calculator provided by your PCB manufacturer. You can easily locate that on the manufacturer’s website.
The second option you have is to use the Smith Chart. The work of the chart is to graphically highlight and determine the coefficient relationship between the load and source impedances on the circuit board.
Using computer simulation is another excellent way of figuring out the impedance of a circuit board. It works by enabling automatic modification of the relevant parameters that will be used for selecting the best design to use for the circuit board.
However, using computer simulation as a PCB impedance calculator has a notable downside, which is the higher cost of using the software.
Impedance control helps you to have an idea of the quality of your PCB design while making it easier for you to find out potential mistakes and errors.
You can select the best PCB manufacturer that will offer you excellent work at re