As electronic devices advance, they utilize more powerful components and drivers that communicate via high-power, high-frequency signals. Therefore, traditional copper trace designs are not capable of handling this performance. But you can design a controlled impedance PCB with several improvements, such as thicker and wider copper traces to transmit the high-speed signals. Let's get right into it! We have covered these details and much more below.
What is Controlled Impedance
To define controlled impedance, we must look at impedance first. Impedance is the degree to which signal energy flow gets opposed in electric circuits or transmission lines. In other words, it is like the resistance in an electrical circuit.
Regarding PCBs, therefore, controlled impedance is the characteristic impedance (resistance) of PCB transmission lines formed by the signal traces and the associated reference planes.
Signal tracks on a PCB
Controlled impedance is crucial when propagating high-frequency signals on PCB transmission lines because it helps maintain signal integrity.
Factors Affecting Trace Impedance
Trace impedance usually varies between 25 and 125 ohms, depending on the following factors.
- Copper thickness
- Trace width
- Trace geometry
- Prepreg material thickness
- Core thickness
- Signal layer distance from the reference copper plane
- Prepreg and core material dielectric constant
- Presence/absence of copper resist
- Signal passage through vias
Why Control Impedance on PCBs
Electronic devices have become faster and more complicated due to their high switching speeds. They should minimize signal integrity issues while handling high-speed signals. Since they use circuit boards, you should consider their copper traces as transmission lines, not point-to-point connections. Additionally, ensure you learn the importance of impedance matching to reduce signal integrity issues further.
Other reasons why you should control impedance include the following.
To Optimize Performance
Controlled impedance PCBs ensure high-quality device performance by consuming less energy, performing faster, and lasting longer.
The PCB layout/design stage is critical for ensuring the board achieves high-speed, high-frequency signal propagation with little to no degradation.
A high-power RF PCB on a Smith chart for tuning and impedance matching
Managing Electromagnetic Interference
If implemented correctly, impedance control prevents circuit disruptions due to electromagnetic interference.
Reflection energy pulses can disrupt circuits and neighboring components, interrupting energy flow and causing product failure in the worst-case scenario.
Controlling Energy Flow
Controlled impedance is vital when transitioning from lower to higher ohm environments. Why? These transitions can cause energy reflections to look like powerful pulses that disrupt the flow of energy.
Therefore, if your application involves using radio frequencies or high-power digital devices, it is imperative to use controlled impedance PCBs.
Increase the Signal Power
The function of PCB traces is to transfer power signals from the driver to the receiver. However, you can only achieve maximum signal power with matching impedances.
Impedance control features impedance matching, which designs the input or output impedance to maximize power transfer.
A high-tech circuit board
How To Control Impedance
It is possible to avoid the back-and-forth energy reflections between the source/driver and load by implementing an impedance matching design. This design ensures all the energy coupled from the source goes into routing, then from routing, and into the board.
An RF and microwave PCB on a Smith chart for impedance matching
Combining the following techniques will help you achieve a controlled impedance design.
Impedance Matching the Components
When matching the components, check for signal nets that could present signal integrity issues during the design capture stage. This process will help you include any extra termination components before PCB design commences.
With high impedance on the input pins and low impedance on the output pins, you will most likely need termination components to the design to achieve the correct impedance.
Routing the Board To Give the Required Impedance
Controlled impedance routing involves routing the board to feature tracks with a defined impedance. Remember, factors like trace routing dimensions and properties of the board materials affect signal impedance routing.
Multilayer PCB routing workflow using CAD software
Calculating Controlled Impedance in PCBs
The transmission line types that need controlled impedance include the following.
- Single-ended microstrips
- Stripline differential pair
- Single-ended stripline
- Microstrip differential pair
- Coplanar (differential and single-ended)
- Embedded microstrip
Impedance calculations depend on the impedance specifications (target impedance), layout, and board layer buildup.
You can use simple equations to calculate the approximated controlled impedance values. They help obtain nominal values of the impedance trace dimensions and are vital for trace widths & spacings exceeding 15mil.
However, these equations are quite complex, so it is easier to use impedance calculators. Keep in mind that you need variables like the trace thickness, track height, isolation height, and dielectric constant for these online calculators.
But these impedance calculators give rough approximations or estimations. The PCB manufacturer should do precise impedance calculations before constructing the conductive layer.
PCB Impedance Specification
PCB contract manufacturers can deliver board layer stack-up information, which you can use to calculate the trace height and thickness using software or by hand.
Remember to indicate the stack-up that suits your needs by starting with a reasonable and practical trace copper thickness for routing, spacing, and manufacturability.
Calculate the dielectric thickness, as well, to determine the dielectric material with the specific core dielectric constant for the target impedance.
Lastly, pick the best prepreg sheet option and right core thickness, then recalculate the trace dimensions. Using design software, check if the signal quality (integrity) is okay, then stimulate the critical lines.
An electrical engineer designing a PCB using CAD software
Remember, checking for signal integrity issues requires the driver model, trace & via dimensions, stack up specs, and vias. Rectify these parameters accordingly then you'll get the accurate trace dimensions and board layer stack-up information for the impedance target.
Send this information to the manufacturer by drawing designs with a similar copper thickness. Remember to include notes that specify the dielectric properties (constant and material).
Controlled Impedance PCB Design
The first step in the design process is schematic capture, which lays out a conceptual circuit. Each circuit symbol should have a footprint (the physical package of the component).
After assigning the footprint, generate a netlist (a flat representation of the schematic). This netlist is machine-readable and gives a unique ID to every footprint's pin.
With the schematic, the CAD program will create a rundown of the node connections, enabling you to design the layout.
PCB design using CAD software
Although the board should be as small as possible, it should have wide traces to handle the high-speed circuit. To achieve this, you should design it with multiple layers.
After layering, place the components, starting with the physically constrained ones. For instance, if the board has switches or LEDs, position them to line up with the cut-out holes.
Next, position the large ICs and complicated components, then place the other supporting parts. Lastly, design the copper layer to connect the pins and pads.
PCB layout routing
5 Design Rules for Controlled Impedance PCBs
As a PCB designer, you should follow these rules before sending the controlled impedance board design to the board fabricator.
Differentiate Between CI and Other Traces
Distinguish between the controlled impedance traces and the rest by explicitly labeling the ones requiring CI. This differentiation gives the manufacturer an easier time when fabricating the copper layer.
For instance, if you need several 4mil trace widths and one or two should have a 50-ohm impedance, change their trace widths to 3.9 mils or 4.1 mils.
Also, remember to indicate if the signals belong to differential pairs or are single-ended (SE). You can provide this information in the following manner.
Traces with no impedance requirements should not have any of the above trace widths.
Maintain Differential Pair Routing Symmetry
High-speed differential pair traces should have a parallel route with a constant space between them. The trace width and spacing parameters will help calculate the particular differential impedance.
The process of PCB layout routing during the design process
Coupling/Bypass Capacitor, Via, and Component Placement
Never place vias or components between differential pairs even if the signals get routed around them symmetrically. These elements create impedance value discontinuities that can affect signal integrity.
With coupling/bypass capacitors, position them symmetrically to minimize the extent of signal discontinuity.
Trace Length Matching
Balancing the trace lengths avoids propagation delay between several signals if they travel at the same speed and should reach their destination simultaneously. You can use serpentine traces if some tracks are shorter.
Traces on a PCB. Note the winding paths.
Maintain Adequate Spacing
It is imperative to place adequate spacing between controlled impedance traces. The general rule of thumb is implementing 3W (or 2W minimum), where W is the CI track width.
However, the minimum tracing between high-frequency signals should be 5W with a 30-mil minimum distance from other signal types. This spacing should increase to 50 mils in periodic signals like clocks for better isolation.
Controlled Impedance PCB Applications
- Video signal processors
- Digital and analog telecommunications
- Mobile phones, computers, and tablets
- Motor control modules
- Digital camera
- Video game consoles
- TVs and web boxes
In conclusion, controlled impedance PCBs are essential in modern electrical devices because they can handle high-power, high-frequency signals. We hope this article has been insightful and will help you create better CI PCB designs. If you have any questions or comments, contact us for further details.