PCB Impedance Control: Ensuring Signal Integrity in High-Frequency Circuits

Impedance measures the opposition of an electric circuit when alternating current is applied to it. It is the combination of the capacitance and the induction of an electric circuit at high frequency. Impedance is measured in Ohms, similar to resistance. If the impedances are different, reflections and attenuation occur that deteriorate the signal.

For high-frequency analog or digital circuits, it is essential to protect the signals that propagate on the PCB from being damaged. In fact, signals above 100 Mhz are impacted by trace impedance which, if not properly taken into account, can cause unexpected errors that are especially difficult to analyze. Luckily, impedance control allows designers and PCB manufacturers to manage the phenomenon.

What is Impedance Control and Signal Matching

Impedance control matches PCB trace dimensions and locations with the properties of the substrate material to keep the signal noise-free and un-attenuated during the transmission phase.  So, Printed Circuit Board (PCB) traces can no longer be treated as a simple point-to-point connection. Traces need to be considered as transmission lines and impedance matching becomes necessary to lessen or eliminate the impact on Signal Integrity. By following good design practices and approaches, many potential signal integrity issues can be averted or mitigated.

So, here we’ll talk about the importance of impedance control, the causes of signal integrity issues, and ways to avoid them.

Factors Determining Transmission Line Impedance:

Usually, trace impedance is between 25 and 125 Ohms and depends on the following factors:

● The real part of the dielectric constant: Dielectric thickness is directly proportional to impedance. The thicker the dielectric, the higher the impedance.

● Loss tangent and dispersion: Loss tangent in PCB design measures the dielectric material's energy loss as heat when a signal passes through it. It impacts signal integrity, especially at high frequencies, causing signal attenuation. Lower-loss tangent materials are preferred for high-speed and RF designs to reduce signal degradation.

● Distance between the trace and the nearby reference plane:  The distance between two traces is inversely proportional to the impedance. Proper spacing is critical for maintaining controlled impedance, and ensuring signal integrity in high-speed and RF circuits.

● Copper trace thickness and roughness: Copper foil thickness is inversely proportional to impedance. The thicker the copper, the lower the impedance. Copper thickness can be controlled through pattern electroplating or selecting base material copper foil with the appropriate thickness.

● Trace width: Trace width is inversely proportional to impedance. Thinner trace widths come with higher impedance, while wider trace widths come with lower impedance. Controlling trace width within a tolerance of +/- 10% is necessary for better impedance control. To ensure trace width accuracy, engineering compensation is performed on the photomasks based on etch undercut, lithographic errors, and pattern transfer errors.

The board designer must therefore make sure that for high-frequency signals, their choice of trace and stackup makes it possible to achieve a target impedance value, with a certain tolerance. The most advanced electronics CAD design tools calculate this automatically.

Why is Impedance Matching Required?

The function of a PCB trace is to transfer the signal power from the driver device to the receiving device. Power needs to be propagated throughout the length of the trace. However, maximum signal power can only be achieved with matching impedance on the PCB. According to the maximum power transfer theorem, when the impedance of two devices matches, maximum power flow can be noted across the devices. If special care is not taken in the PCB layout stage, then high-frequency signals will definitely degrade as they propagate from the driver to the receiver.

A misconception is that the clock speed of the circuit determines whether the circuit is operating at high speed. But the high-speed signals are related to the rise/fall time of a clock signal, how fast a signal changes from one state to another (Logic 0 and Logic 1).

How to Calculate the PCB Trace Impedance?

As mentioned above there are a lot of factors involved in determining PCB trace impedance. Transmission lines in PCB design may have some different formulas, so precise values for impedance matching are not easy to get with open-source software. The best thing is to contact the manufacturer and calculate the impedance from their calibrated impedance calculator tool.

The JLCPCB impedance calculator tool is an online resource provided by JLCPCB to help designers calculate the impedance of PCB traces. This tool is essential for designing high-frequency circuits where controlled impedance is critical for maintaining signal integrity.

Key Features:

1. Trace Type Selection: Choose between microstrip, stripline, or differential pairs.

2. Input Parameters: Enter the trace width, trace thickness, dielectric constant, and the distance between the trace and the reference plane.

3. Instant Calculations: The tool provides real-time calculations for characteristic impedance based on the input parameters.

4. Material Properties: It allows you to select different PCB materials, which affect the dielectric constant and the impedance.

Select the type of transmission line (e.g., microstrip or stripline), and enter the PCB material's dielectric constant. Input the trace width, thickness, and distance to the reference plane. The tool will calculate and display the impedance value. This tool is widely used in PCB design to ensure the impedance matches the required specifications, particularly in high-speed or RF circuits. You can access the JLCPCB impedance calculator through the website.

Which Calculations an Impedance Control method perform on PCB?

The longer the trace or the higher the frequency, the more adaptation in Impedance is required. Any lack of rigor at this stage can increase the switching time for an electronic device or circuit and cause unexpected errors. Uncontrolled impedance is difficult to analyze once the components are mounted on the circuit. Components have different tolerance capacities depending on their batch. An impedance control method performs the critical length calculations depending on signal speed.

This is why trace impedances and their tolerances must be checked early on in the PCB design. Designers must work hand in hand with the manufacturer to guarantee the compliance of component values.

Methods to Achieve Impedance Matching?

Well-controlled impedance means that the trace impedance is constant at every point along the path on the PCB. This means that wherever the trace travels, even if it changes layers, the impedance should be the same throughout the part, from the source to the destination.

There are a few important design criteria that we need to consider. Keep in mind that many of the problems relating to reflections relating to EMI can be prevented by good PCB design techniques:

1) Material With Low Dielectric Constant:

In the past, typically FR4 was specified. But, with high-speed designs, the use of the correct laminate is critical. The use of a material with a lower dielectric constant (Dk) is advisable and preferred. This not only ensures the best signal performance, but will also minimize any signal distortion or phase jitter of the signal. So, you would need to select the material that is the most suitable for your application and specify this in your manufacturing notes.

2) Signal Return Paths:

Including power planes that can supply a signal return path below each signal path is an essential step in controlling impedance. By avoiding discontinuities (such as a split or blowout in the power plane, underneath any critical routing), the return path current flowing through the plane will seek to follow the same physical path as the route on the signal layer.

3) Layer Stack up:

A layer stack-up refers to the arrangement of various layers in a PCB, including signal layers, ground planes, power planes, and dielectric materials. The stack-up plays a significant role in controlling the impedance of signal traces, which is critical for impedance matching. Proper layer arrangement is key; signal layers are typically placed adjacent to reference planes (either ground or power planes), which helps stabilize and control the impedance.

Controlled impedance traces, such as microstrip or stripline, are designed with precise impedance control by adjusting trace width, layer stack-up, and dielectric properties.

Careful attention needs to be paid to:

● Trace lines should be kept as short as possible and reduce lengths wherever possible.

● The choice of decoupling capacitors and quantity needed and routing loops.

● Routing stubs and discontinuities, degrade of the signal quality, should be avoided.

● For differential pair routing, try and ensure that the signal pairs have the same length.

● Ground reference plane and current return paths.

● Inductances caused by poor component packaging.

● Consider using immersion silver as a surface finish rather than ENIG. Nickel content in ENIG is very lossy and due to the skin effect, it is not very good for high-speed designs.

Impedance Control Verification

Impedance control can be verified after the PCB is manufactured by using test coupons (a test coupon is a PCB used to test the quality of a PCB fabrication process. Test coupons are fabricated on the same panel as the PCBs, typically at the edges and ensure proper layer alignment, electrical connectivity, and cross-section). Then, by using a Time-Domain Reflectometer (TDR) the impedance can be tested. Subsequently, a report will be generated to indicate if the characteristic impedance was achieved on your PCB.

The overall performance and EMC behavior of electronic equipment are not only determined by the design of the circuitry and geometry of the layout but also by the power distribution network.

Conclusion

The impedance signals experienced on an actual PCB layout may differ much from the ideal value you calculated from a schematic. The main reason for this is the substrate's existence and the board's tracing layout. This results in consequences like crosstalk, which deviates from desirable impedance levels. Power integrity issues like ringing may occur when circuits switch at high frequencies. At higher frequencies, your power delivery network's impedance will also diverge from the optimum capacitive behavior, which might cause issues with signal integrity and power integrity.

By understanding the causal factors for impedance mismatch and acquiring the knowledge of design practices that can mitigate or remove impedance issues, the PCB designer can create a truly engineered solution. A robust design that can be manufactured into a reliable and high performing printed circuit board.