What is impedance control and how to perform impedance control on PCBs

In the design of modern electronic devices, PCBs play a crucial role. The performance of PCBs directly affects the stability, reliability and transmission efficiency of the entire electronic system. Among them, impedance control is an important part of PCB design. Since modern digital circuits have shorter signal transmission times and higher clock rates, PCB traces are no longer simple connections, but transmission lines correspondingly. PCB impedance control refers to controlling the transmission speed and impedance matching of signals on the PCB to ensure the quality and stability of signal transmission.

In practical situations, it’s necessary to control the trace impedance when the digital marginal velocity is higher than 1ns or the analog frequency exceeds 300MHz. One of the key parameters of PCB trace is its characteristic impedance (i.e. the ratio of voltage to current when the wave is transmitted along the signal transmission line). The characteristic impedance of wires on PCBs is an important indicator of PCB design. Especially in the high frequency PCB design, it’s necessary to consider whether the characteristic impedance of the wire is consistent with or match the required characteristic impedance of the device or signal. This involves 2 concepts : impedance control and impedance matching. This article focuses on the issues of impedance control and stack design.

There’re various signals transmitted in the conductors of the PCB. In order to improve its transmission rate, its frequency must be increased. The impedance value of the circuit itself varies due to factors such as etching, layer thickness and wire width, etc., causing signal distortion. Therefore, the impedance value of conductors on high-speed PCBs should be controlled within a certain range, which is called "impedance control".

The impedance of PCB traces is determined by their inductive and capacitive inductance, resistance and conductivity coefficient. The factors that affect the impedance of PCB wiring mainly include the width and thickness of the copper wire, the dielectric constant and thickness of the medium, the thickness of the solder pad, the path of the ground wire, and the wiring around the wiring, etc. The range of PCB impedance is 25 to 120 ohms.

In practice, PCB transmission lines typically consist of a wire trace, one or more reference layers, and insulation materials. The trace and layer constitute the control impedance. PCBs often adopt multi-layer structures, and impedance control can also be constructed in various ways. However, regardless of the method used, the impedance value will be determined by its physical structure and the electronic properties of the insulating material:

A microstrip is a strip of wire which refers to a transmission line with only one side having a reference plane. The top and sides are exposed to air (or be coated) and are located on the surface of an insulation constant Er PCB, with the power or ground layer as a reference. As shown in the following figure:

Note: In actual PCB manufacturing, the PCB factory usually coats a layer of green ink on the surface of the PCB. Therefore, in actual impedance calculations, the model shown in the following figure is usually used  for surface microstrip lines.

A stripline is a strip of wire placed between 2 reference planes, as shown in the following figure. The dielectric constants of the dielectric represented by H1 and H2 can be different.

The above 2 cases are just a typical demonstration of microstrip and striplines, usually used for the learning of embedded IoT intelligent hardware and other systems. There’re many specific types of microstrip and striplines, such as coated microstrip, which are related to the specific stacking structure of PCBs.

The equation used to calculate characteristic impedance requires complex mathematical calculations, usually using field solving methods, including boundary element analysis. Therefore, with using specialized impedance calculation software SI9000, all we need to do is control the parameters of characteristic impedance:

The dielectric constant Er of the insulation layer, the wiring width W1 and W2 (trapezoidal), the wiring thickness T, and the insulation layer thickness H.

W – designed line width

A – etch loss (see table above)

The reason for the inconsistent width between the top and bottom of the line is that during manufacturing process of PCBs, corrosion occurs from top to bottom, resulting in a trapezoidal shape of the corroded line.

There’s a corresponding relationship between the line thickness T and the copper thickness of this layer, as follows:

* Due to the small influence of solder mask thickness on impedance, it’s assumed to be a constant value of 0.5mil.

We can achieve impedance control by controlling these parameters. Taking Anwei’s bottom PCB as an example, we’ll explain the steps of impedance control and the use of SI9000 :

The second layer is the ground plane, the fifth layer is the power plane, and the remaining layers are the signal layers.

Explanation: The dielectric between the intermediate layers is FR-4, with a dielectric constant of 4.2; The top and bottom layers are bare layers that come into direct contact with air, and the dielectric constant of the air is 1.

PCB designers can fully utilize the hierarchical structure of PCBs to achieve impedance control. By placing different signal layers in different layer positions, interlayer capacitance and inductance can be effectively controlled. Generally speaking, the inner layer uses high impedance materials and the outer layer uses low impedance materials to reduce the impact of reflection and crosstalk.

Differential signal transmission lines can provide better anti-interference ability and lower crosstalk risk. Differential signal transmission lines are a pair of parallel wires with opposite voltages but equal sizes, which can provide better signal integrity and anti-interference ability. The impedance of differential signal transmission lines is usually controlled by the selection of line spacing, width and ground plane.

Geometric parameters such as PCB line width, spacing and layout can also be used to control impedance. For common microstrip lines, thicker line width and larger spacing can reduce impedance. For coaxial lines, smaller inner line diameters and larger outer line radii can increase impedance. The selection of wiring geometry requires optimization based on specific impedance requirements and signal frequency.

The dielectric constant of PCB materials also affects impedance. Choosing materials with stable dielectric properties is part of impedance control. In high-frequency and high-speed applications, commonly used materials include FR-4 (glass fiber reinforced board), PTFE (polytetrafluoroethylene), and RF (radio frequency) laminates.

Before PCB design, using simulation and design tools can help designers quickly and accurately verify and optimize impedance. These tools can simulate circuit behavior, signal transmission losses and electromagnetic interactions to determine the optimal PCB design parameters. Some common simulation tools include CST Studio Suite, HyperLynx and ADS. 

PCB impedance control plays a crucial role in high-speed digital and analog circuits. Through reasonable hierarchical design, the use of differential signal transmission lines, control of wiring geometry, selection of appropriate PCB materials, and the use of simulation and design tools, precise impedance control can be achieved ,thereby improving circuit performance and signal integrity.