Developments in Low-Loss Substrates for High-Frequency Applications


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The electronics industry as we know it today can trace its birth to the creation of the first integrated circuit in 1958, although conception occurred 10 years earlier with the invention of the transistor. That first IC contained a single transistor and four passive components. To say things have come a long way since then is a huge understatement.

Only one thing may have matched the meteoric pace of progress in our industry—market expectation. Exponential advancement has become the norm, and this is now achieved through an aggregation of improvements, rather than a large leap in one aspect such as chip lithography (Moore’s Law), or processor frequency scaling (Dennard).

It’s a small step from acknowledging this reality to adopting a holistic view that acknowledges the contribution each aspect of the system can make toward the overall performance and that seeks to optimize the interactions between them. Accordingly, in cutting-edge applications, we no longer have the luxury of treating the PCB as merely a medium for mounting and connecting components. At high signal speeds in particular, the properties of the substrate, copper foil, and trace geometries govern whether the system can deliver the required performance.

Many within the industry already understand that the PCB has become a high-tech component in itself, particularly those departments working on applications in automotive radar, 5G, and satellite communications at multi-gigahertz frequencies. 

Low-Loss Substrates

These applications are operating close to the limits of the capabilities typical materials can offer. Resistive loss mechanisms, including the skin effect in copper conductors and dielectric losses due to the molecular dipole moment in the insulating substrate need to be understood and carefully managed. The cumulative effect of the tiny losses in signal energy and associated thermal dissipation incurred with every signaling transition becomes appreciable. If not properly addressed, these losses demand more powerful transmitters, more sensitive receivers, and extra thermal management than are practicable within the typical constraints on power, as well as size, weight, and cost that usually prevail. 

There are growing demands for low-loss substrates to address high-performance systems, spanning applications from high-end servers and telecom infrastructure all the way to mmWave 5G, satellite, and radar applications. 

By enhancing aspects of PCB laminates, it has been possible to produce low-loss substrates that can handle demanding applications in data centers and telecom switches, for example. Optimizing the fiber weave effectively minimizes micro-variabilities in signal-path characteristics that cause distortions such as signal skew, which ultimately give rise to excessive noise and signaling errors. Attributes such as drilling performance and resistance to CAF (conductive anodic filament) formation are also improved.

For applications operating at the highest frequencies in use today, ceramic-filled and PTFE-based materials are achieving the lowest loss factors in the industry. The molecular structure of PTFE (polytetrafluoroethylene) arranges fluorine atoms as spirals around the carbon backbone to create a rod-like stiff cylindrical shape that has no dipole moment. This absence of any dipole moment negates the oscillations set up in conventional substrate dielectrics due to repeated polarization caused by signal current. This is manifested as an extremely low dissipation factor (Df) that helps to reduce signal losses. 

To read this entire article, which appeared in the March 2022 issue of PCB007 Magazine, click here.

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