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A Brief Introduction to Impedance Control PCB
Before understanding the impedance control PCB, we first understand what impedance control is.
The conductors in the circuit board will have various signal transmissions. To improve its transmission rate and increase the frequency, the line itself, due to etching, lamination thickness, wire width and other factors, will cause changes in impedance value, causing signal distortion. Therefore, the conductor on the high-speed circuit board’s impedance value should be controlled within a specific range, called “impedance control”.
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The Advantages of Impedance Control PCB
- Impedance Control PCBs allow for higher signal bandwidth and improved signal integrity.
- Increased Impedance control can support high-speed signals, reducing crosstalk.
- Impedance-controlled boards reduce EMI/RFI emissions more efficiently than standard boards.
- Controlled impedance PCBs are designed to eliminate reflections caused by mismatched impedances in the circuit.
Types of Impedance Control
1. Variable impedance control
2. Tunable impedance control
3. Adaptive impedance control
Adaptive impedance control is a type of impedance control that uses sensors to measure the environment and react accordingly. It allows for more robust impedance control, as the system can automatically respond to changing environments and adapt its impedance parameters to achieve optimal performance. Adaptive impedance control can be employed in applications such as tracking objects or navigating unknown terrain, where environmental conditions may change unexpectedly. These are the main types of controlled impedance PCBs.
Introduction to Characteristic Impedance
According to the principle of signal transmission in PCB, signals are the function of time and distance variables, so the signal may change in every part of connecting circuit in PCB. That’s why you need to determine the AC impedance when designing the impedance PCB, which means you need to determine characteristic impedance in PCB, so the characteristic impedance is the ratio of voltage change and the current change of the transmission line, transmission trace characteristic impedance only relative to signal wiring characteristic itself.
In an actual controlled impedance PCB, the electronic resistance value of connect wire itself is less than the distributed impedance of the system, especially in high-frequency PCB circuits. Therefore, the characteristic impedance is mainly decided by the distributed impedance brought by unit-distributed capacitance and unit-distributed inductance of connecting the wire.
Characteristic impedance mainly uses in high-speed signal transmission. The electrical performance provided by the PCB applied to high-speed signal transmission and the high-frequency circuit must be able to prevent reflection during signal transmission PCB must be able to avoid reflection during signal transmission, signal keep complete, reduce transmission loss, and plays a matching role, after that, you can get an entire, reliable, accurate, without interference, noiseless transmission signal.
After you learn the characteristic impedance of PCB, you will know that the impedance, big or small, can not simply understand as the bigger, the better or the smaller, the better. The key is matching.
Usually, only the high-speed PCB will make impedance, and during the impedance trace production process, the trace width tolerance needs to control to 5%-10%. Other trace widths do not need impedance and need to control the tolerance to 20%, so the difficulty of impedance production is higher, some factories can’t make it, but Viasion can make all kinds of controlled impedance PCBs very well.
How to Choose the Stack-up When the PCB Needs Impedance Control?
We must complete the step of taking the stack-up into account for impedance control PCB. The stack-up refers to the arrangement of different layers on a PCB, which can impact impedance control in PCB. Choosing the correct stack-up can help ensure impedance control is achieved without any issues.
The most common stack-ups used for impedance control are 6 and 8 layers. Six layers usually provide enough impedance control, but an eight-layer stack-up is recommended in cases where the impedance is greater than 25 ohms or if the signal speed needs to be fast. To ensure impedance control in PCB, each of these layers should be carefully chosen.
The first two layers typically consist of signal and ground planes. It is followed by four dielectric layers, which act as the impedance control components for the stack-up. The final two layers typically consist of a power and ground plane.
What’s more, when we pay attention to the stack-up during the impedance control design, we should select materials with appropriate impedance values and never ignore the impedance tolerance of the stack-up, then we can get the perfect controlled impedance PCB.
Common Factors Affecting Impedance
The Functions of Impedance Control PCB
A wide range of impedance values can be set through software controls, allowing precise impedance matching between two components. This controlled impedance PCB is commonly used in high-frequency and high-power applications, such as radio frequency amplifiers, satellite communication systems and optical networks.
Impedance control PCBs are designed to balance trace impedance and reduce crosstalk between high-speed signals, thereby improving signal integrity and reducing impedance losses. It makes impedance control boards essential components in any high-speed circuit design.
Impedance control PCBs are designed to reduce losses due to impedance mismatches when signals are sent from one component to another. Impedance mismatch can be caused by various factors, such as mismatched impedance values between two components, impedance variation caused by changes in temperature, or impedance drift due to current flow. When impedance mismatches occur, a large amount of signal energy is lost in reflections and noise.
Impedance control PCBs are designed to eliminate impedance mismatch by introducing impedance-matching structures such as transmission lines, vias, and plating structures with specific impedance profiles. The impedance profile is set to match the impedance value between two components, thus reducing impedance mismatch and preventing signal loss.
Common Problems and Causes of Impedance Control PCB
- For the problem of control parameters for impedance control PCB, you can achieve the control requirements through mutual adjustment during the PCB production.
- After the lamination process of controlled impedance PCB production, you need to make a microsection to analyze the board. If the dielectric thickness is reduced, you can adjust the trace to smaller to achieve the impedance requirement; If the dielectric thickness is slightly thick, you can increase the copper thickness to turn down the impedance value to achieve the impedance requirement.
- In the impedance test, the most substantial possibility is that the engineering and test strip design has problems if the theory and practice values differ.
If the impedance value is exorbitant ( impedance value exorbitant divided into inner layer exorbitant and outer layer excessive), the main reason as below:
- The trace width is a little small, resulting in the PCB impedance value heightening.
- The copper thickness of the PCB is a little thin, resulting in the PCB impedance value heightening.
- The dielectric layer thickness is too thick, and the outer layer solder mask is too thick, resulting in the controlled impedance PCB impedance value heightening.
Considerations for Impedance Control PCB Design
Carrying out the design of impedance control PCB should control the following impedance parameters better. Also, selecting suitable materials can aid in the design process. Besides, remember to design efficient power distribution systems, trace geometries, and solder mask layers to ensure impedance control.
- Capacitance
- Inductance
- Impedance level
In addition to impedance parameters, engineers must also consider the physical characteristics of their impedance control PCBs. After all, no one can be sure that the size and shape of the PCB have little impact on the performance of the impedance control circuit. Additionally, the impedance control PCB’s mechanical strength and thermal conductivity should be considered to ensure that it can withstand the mounting forces and thermal load it may experience.
Designing controlled impedance PCBs requires careful attention to detail and an understanding how impedance parameters work.
Manufacturers should use advanced testing and inspection methods to ensure impedance control PCBs meet the desired specifications. It includes impedance measurements, cross-section analysis, thermal imaging, electrical tests (resistance, insulation resistance), visual inspections (for cracks and other defects), solderability testing and more. These tests can help identify potential issues with controlled impedance PCBs and ensure they are up to standard.
With the proper design techniques and testing methods, impedance control PCBs can provide reliable performance for many applications.
Tips for Optimizing Impedance Control on PCBs
The first step is to ensure that the impedance of the wires used in the PCB design is correct. Impedance control can be achieved through careful trace width and spacing calculations, as well as by controlling the PCB material’s dielectric thickness and relative permittivity.
Additionally, Impedance Control can be further improved by addressing the effects of vias on signal integrity in the design. It can be done by using multiple vias instead of single buried or blind vias to create parallel paths for signals, optimizing via size and spacing for high-speed connections.
Lastly, Impedance Control can be further enhanced by ensuring that signal traces are separated from each other with proper shielding layers.
Following these tips, Impedance Control on PCBs can be significantly improved and optimized to ensure a reliable connection between components. In addition, it helps in reducing the risk of signal integrity issues and allows for enhanced performance.
If you have doubts about the PCB impedance matching, please contact us freely. Our engineer will give you advise about your controlled impedance PCB manufacturing.
What Should Be Offered to the PCB Manufacturer for Impedance Control PCB?
- Mark which trace needs to make impedance control in PCB, send by word or picture.
- The calculated impedance in PCB and pressing lamination need to provide the trace width and trace spacing so that the factory can make exceptional control during production and ensure that the finished product meets the impedance value.
- If the customer wants to test the impedance independently, they need to note that it should provide an impedance bar. The customer can test the impedance bar by an impedance tester, and if the customer is proficient in impedance, they can design impedance where there is space in the board for later testing. By calculating the PCB trace impedance on the bar, customers can assure if the controlled impedance meet the design.
Impedance Test
The primary impedance test method is TDR ( Time domain reflection method ). The basic principle is the instrument launches a pulse signal, passes through the test piece of the PCB and then folds back, measures the characteristic impedance value change of signal launch and return, and after computer analysis, can get the characteristic impedance.
After testing the impedance, we will provide a test report for impedance control. See the example for one of our impedance test reports:
| No. | L1:50±5Ω Trace Width:0.19mm | L3:50±5Ω Trace Width:0.076mm | L3:100±10Ω Differential impedance/Trace Width:0.076mm | L3:90±9Ω Differential impedance/Trace Width:0.079mm | L6:50±5Ω Trace Width:0.76mm | L6:100±10Ω Differential impedance/Trace Width:0.076mm | L6:90±9Ω Differential impedance/Trace Width:0.079mm | L8:50±5Ω Trace Width:0.19mm |
| 1 | 47.4 | 50.7 | 102.1 | 96.5 | 49.8 | 104.9 | 94.4 | 46.9 |
| 2 | 46.8 | 47.8 | 98 | 92.6 | 46.5 | 98.5 | 88.3 | 47.2 |
| 3 | 46.6 | 49.4 | 103.3 | 95.4 | 51.2 | 101.8 | 95.1 | 45.9 |
| 4 | 48.4 | 49.3 | 102.9 | 94.8 | 50.5 | 102.7 | 95.6 | 47.2 |
| 5 | 46.4 | 49.5 | 102.9 | 93.8 | 50.6 | 102.5 | 95.6 | 45.1 |
| 6 | 48.3 | 50.3 | 99.6 | 90.6 | 48.6 | 99.5 | 88.9 | 49.2 |
| 7 | 47.9 | 48.9 | 101.3 | 91.5 | 49.5 | 98.6 | 92.1 | 48.6 |
| 8 | 46.8 | 47.8 | 98.9 | 92.5 | 47.3 | 97.8 | 90.8 | 47.9 |
| 9 | 49.2 | 50.1 | 98.3 | 90.8 | 50.4 | 102.3 | 91.6 | 50.3 |
| 10 | 48.7 | 47.6 | 102.1 | 93.1 | 50.1 | 102.1 | 90.7 | 50.1 |
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Impedance Control PCB is mainly applied to communication systems, radar systems, hard disk drives, RFID systems, and other high-speed digital electronics.
Yes, Impedance Control PCBs are generally more expensive than traditional FR-4 boards.
During controlled impedance PCB engineering and production, we will add test bar in very panel of the circuit board. In the test bar, all the PCB trace impedance will be exist. So we can test the impedance traces on these controlled impedance bars.
The lead time for Impedance Control PCB manufacturing depends on various factors, including the complexity and size of the project, how many layers are needed, and the materials used.
Yes, we can produce controlled impedance PCBs with 3% impedance tolerance.
