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Beyond Design: Next-Gen PCBs—Substrate Integrated Waveguides

As PCB transmission frequencies head toward 100GHz and beyond, the current mainstream PCB technology, the copper interconnect, is reaching its performance threshold. Ultimately, it is dielectric loss, copper roughness, and data transfer capacity that are the culprits. However, the biggest performance restriction for PCB interconnects is the size of the conductor. Metallic waveguides, on the other hand, are a better option than traditional transmission lines, but they are bulky, expensive and non-planar in nature. However, recently, substrate integrated waveguides (SIW) structures have emerged as a viable alternative and are ideally suited to the high-speed transmission of electromagnetic waves.

SIWs are planar structures fabricated using two periodic rows of PTH vias or slots connecting top and bottom copper ground planes of a dielectric substrate. In this month’s column, I will review the substrate integrated waveguide and its incorporation with the microstrip transmission line.

Since SIWs are fabricated as part of the multilayer PCB stack, they can be integrated with other planar transmission lines. SIW retain the low loss property of conventional metallic waveguides and are widely used as interconnects in RF and microwave high-frequency circuits to improve bandwidth. However, the signal propagates through the dielectric material rather than through air which slows the signal transmission speed, to about half the speed-of-light, which is still more than adequate for this application.

Transmission lines in the form of microstrip, stripline, coplanar waveguide (CPW), and their derivatives of geometry, have been the backbone of the modern electronic systems for many years. Following the evolution of IC technologies and processing techniques, these fundamental structures have been continuously studied and improved to meet the constantly updated bandwidth and expanded capabilities requirements.

However, the ever-increasing demands for bandwidth and performance, as well as the highly anticipated applications of millimeter-wave (mmWave), have raised the fundamental question of whether classic copper transmission lines are able to cope with the demands for low loss and low dispersion propagation.

Note: mmWave frequencies refer to the electromagnetic spectrum with wavelengths between 1–10 mm representing the frequency range between 30–300GHz. Despite the efforts to evolve and improve the existing transmission line structures, it remains a technological challenge, which necessitates the emergence of a revolutionary concept.

To read this entire column, which appeared in the November 2017 issue of The PCB Design Magazine, click here.