A Beginners Guide to Impedance Control in PCB Design

When a digital signal is transmitted from one point to another, it causes a change in the state of the signal line. This change can be understood as an electromagnetic wave moving through the circuit. Reflection occurs when this wave encounters a boundary between different mediums. At this boundary, some of the wave's energy continues as a signal, while the rest is reflected. This process repeats until the energy is either absorbed by the circuit or dissipated into the environment.

For electrical engineers, this boundary is typically defined by a change in electrical impedance. In PCB design, reflections happen when a signal encounters an impedance mismatch along a trace. This mismatch causes part of the signal to reflect back towards its source, leading to signal integrity issues such as distortion, noise, and data errors, especially in high-speed digital or RF circuits.

1.    Impedance of a Circuit:

In circuits with resistors, inductors, and capacitors, the total equivalent resistance that hinders the flow of current in the circuit is called impedance. Impedance is composed of resistive and reactive elements. Resistors dissipate a circuit’s energy as heat. The recoverable energy in a circuit exists in the electromagnetic fields that permeate and surround conductors, inductors, and capacitors.

Impedance is commonly represented by the symbol "Z" measured in ohms (Ω) and is a complex number, with the real part known as resistance and the imaginary part known as reactance. Impedance is the combined effect of resistance, inductance, and capacitance in AC circuits. The impedance of a specific circuit is not constant; its value is determined jointly by AC frequency, resistance (R), inductance (L), and capacitance (C), and therefore varies with changes in frequency.

2.    What is Impedance Matching?

Impedance matching is a way to ensure compatibility between a signal source or transmission line and its load. It can be classified into low-frequency and high-frequency matching. In low-frequency circuits, where the wavelength is relatively long compared to the transmission line, reflections can be disregarded. However, in high-frequency circuits, with shorter wavelengths comparable to the length of the transmission line, reflected signals superimposed on the original signal can alter its shape and affect signal quality.

Signal Reflections:

The behaviour of circuits at high frequencies change due to parasitic effects like fringe capacitance and inductance. PCB signal traces also behave like transmission lines, and every point along the signal trace has impedance.

As a result, the original signal becomes distorted, and what was intended to be transmitted from the transmitting end may change by the time it reaches the receiving end. Therefore, to achieve distortion-free signal transmission, PCB signal traces must maintain consistent impedance.

3.    Commonly used Impedance Lines in PCB Design:

Impedance matching effectively reduces or eliminates high-frequency signal reflections. Commonly used impedance lines can be classified into the following four types:

⦁ Single Ended Impedance Line:

The single-ended impedance line refers to the impedance of a single trace on a PCB and is typically categorized into two main types: Microstrip and Stripline.

⦁ Microstrip Line: A signal trace on the outer layer of a PCB, with a ground plane directly below it on an inner layer. The impedance is controlled by the width of the trace, the thickness of the dielectric (insulating material) between the trace and the ground plane, and the dielectric constant of the material.

⦁ Stripline: A signal trace sandwiched between two ground planes, typically on inner layers of the PCB. The impedance is controlled by the trace width, the dielectric thickness above and below the trace, and the dielectric constant.

⦁ Differential Pair Impedance Line:

Two parallel traces carrying equal and opposite signals, typically used for high-speed data transmission. The impedance is controlled by the trace width, spacing between the traces, and the dielectric properties of the material.  It has a normalized impedance of 90–110 ohms.

⦁ Coplanar Single Ended/Differential Pair Impedance Line:

A signal trace on an outer layer with ground planes on either side of the trace, typically on the same layer. The impedance is determined by the trace width, the spacing between the trace and the adjacent ground planes, the thickness of the dielectric below the trace, and the dielectric constant. It has normalized impedance of 50 ohms (single-ended), 90–100 ohms (differential pair).

4.    Impedance Matching PCB Design Considerations:

Impedance Calculation and Measurement of a PCB Track:

For orders requiring impedance control, it is essential to provide your impedance requirements in the form of a table or diagram, along with the compressed PCB files. The general value of impedance of tracks are listed in the figure below with trace width, gap and layer information.

Using JLCPCB Impedance Calculator:

Open JLCPCB's "Impedance Calculator" and input the impedance values while selecting the corresponding layer stack-up and other relevant parameters like board thickness. Design the corresponding line width and spacing in your engineering data.

Important Manufacturer’s Note: For orders with "Impedance Control" selected as "Yes", JLCPCB will control the impedance within a tolerance of ±10%. If you choose "No", we will not control the impedance, but we will ensure that the line width and spacing are within a tolerance of +/-20%. Impedance control is not yet available for double-sided boards.

Conclusion:

Impedance matching is a critical aspect of high-speed PCB design, ensuring optimal signal transmission and preserving signal integrity. By carefully considering impedance values, line widths, spacing, dielectric properties, and reference layers, designers can effectively minimize signal reflections and distortion. Implementing impedance-controlled lines and utilizing tools like JLCPCB's Impedance Calculator can streamline the design process and help achieve the desired impedance values. With proper impedance matching techniques, designers can enhance the performance and reliability of high-speed PCBs, enabling seamless transmission of electronic signals in modern electronic systems.