Impedance Control (EImpedance Controlling) is essential in high-speed PCB designs to ensure accurate signal transmission. Signal transmission speed can be increased by raising the signal frequency. However, if factors such as etching, stack thickness, and wire width are not properly managed, the impedance will fluctuate, causing signal distortion. Thus, the impedance of conductors in high-speed circuit boards must be controlled within a specific range, which is known as “impedance control.”
The impedance of PCB traces depends on various factors such as inductive and capacitive inductance, resistance, and conductance. The primary factors affecting impedance include:
- Width of the copper trace
- Thickness of the copper trace
- Dielectric constant of the medium
- Thickness of the medium
- Thickness of the pad
- Path of the ground line
- Traces surrounding the main trace
The impedance of PCB traces typically ranges from 25 to 120 ohms.
In real-world applications, PCB transmission lines consist of wire traces, one or more reference layers, and insulating materials. These traces and reference layers together define the controlled impedance. Multi-layer PCBs are often used, and the impedance is determined by the physical structure and the electronic properties of the insulating material, including:
- Width and thickness of the signal trace
- Height of the core or pre-filled material on both sides of the trace
- Configuration of the traces and reference layers
- Dielectric constant of the core and pre-filled materials
There are two primary types of PCB transmission lines: Microstrip and Stripline.
Microstrip is a transmission line used in PCB technology to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric substrate. Components such as antennas, couplers, filters, and power dividers are formed using microstrip technology, with the entire device pattern etched onto the substrate.
Cross-section of microstrip geometry: Conductor (A) separated from ground plane (D) by dielectric substrate (C), with air as the upper dielectric (B).
Microstrip is cost-effective compared to traditional waveguide technology, as it is lighter and more compact. It was developed by ITT Laboratories as an alternative to stripline, first introduced in the 1952 IRE Proceedings by Grieg and Engelmann.
However, microstrip has some limitations compared to waveguide technology, including lower power handling capacity and higher signal losses. Moreover, microstrip is open to the environment and susceptible to cross-talk and unintentional radiation.
For the lowest cost, microstrip can be built on standard FR-4 substrates. However, due to high dielectric losses and inconsistent dielectric constants at microwave frequencies, an alumina substrate is often preferred.
Microstrip transmission lines are also integrated into monolithic microwave circuits and are used in high-speed digital PCB designs to minimize signal distortion, cross-talk, and radiation.
Microstrip is one of several planar transmission line types. Others include stripline and coplanar waveguide, and all these can be integrated on the same substrate.
A differential microstrip, consisting of two balanced signal microstrip lines, is commonly used in high-speed signals such as DDR2 SDRAM clocks, USB Hi-Speed data lines, PCI Express data lines, and LVDS data lines. Differential pairs are supported by most PCB design tools.
Stripline is another form of planar transmission line, invented by Robert M. Barrett at the Air Force Cambridge Research Centre in the 1950s. It is the earliest form of transmission line using flat conductors.
In stripline, a flat metal strip is sandwiched between two parallel ground planes, with the insulating material of the substrate acting as the dielectric. The width of the strip, the thickness of the substrate, and the substrate’s relative permittivity determine the characteristic impedance. The central conductor may not be symmetrically placed between the ground planes, and the dielectric material may vary above and below the conductor.
The cross-section diagram of a stripline geometry. The central conductor (A) is sandwiched between two ground planes (B and D), and the structure is supported by a dielectric material (C).
To prevent the propagation of unwanted modes, the two ground planes must be electrically connected. This is typically achieved by a series of vias running parallel to the strip on both sides of the structure.
Similar to coaxial cables, striplines are non-dispersive and have no cutoff frequency. They offer better isolation between adjacent traces compared to microstrips. Striplines also provide superior immunity to radiated RF emissions, but at the cost of slower signal propagation speeds when compared to microstrip lines. The effective permittivity of striplines is equal to the relative permittivity of the dielectric substrate, as wave propagation occurs only within the substrate. Consequently, striplines exhibit a higher effective permittivity than microstrips, resulting in reduced wave propagation speed.
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