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PCBD-Nov2017

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56 The PCB Design Magazine • November 2017 As PCB transmission frequencies head to- ward 100GHz and beyond, the current main- stream PCB technology, the copper intercon- nect, is reaching its performance threshold. Ul- timately, 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 conduc- tor. 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 electro- magnetic waves. SIW are planar structures fabricated using two periodic rows of PTH vias or slots connect- ing top and bottom copper ground planes of a dielectric substrate as shown in Figure 1 (left). In this month's column, I will review the sub- strate integrated waveguide and its incorpora- tion with the microstrip transmission line. Since SIWs are fabricated as part of the mul- tilayer PCB stack, they can be integrated with other planar transmission lines. SIW retain the low loss property of conventional metallic wave- guides and are widely used as interconnects in RF and microwave high-frequency circuits to improve bandwidth. However, the signal propa- gates 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 struc - tures have been continuously studied and im- proved to meet the constantly updated band- width 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 transmis- sion line structures, it remains a technological challenge, which necessitates the emergence of a revolutionary concept. by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA Next-Gen PCBs—Substrate Integrated Waveguides BEYOND DESIGN Figure 1: The SIW (left) has similar properties to the metallic waveguide (right).

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