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64 PCB007 MAGAZINE I MARCH 2022 Low-Loss Substrates ese 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. e cumulative effect of the tiny losses in signal energy and associ- ated 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. ere are growing demands for low-loss sub- strates to address high-performance systems, spanning applications from high-end serv- ers 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 sub- strates that can handle demanding applica- tions in data centers and telecom switches, for example. Optimizing the fiber weave effec- tively minimizes micro-variabilities in signal- path characteristics that cause distortions such as signal skew, which ultimately give rise to excessive noise and signaling errors. Attri- butes such as drilling performance and resistance to CAF (conductive anodic filament) formation are also improved. For applications operating at the high- est frequencies in use today, ceramic- filled and PTFE-based materials are achieving the lowest loss factors in the industry. e 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. is absence of any dipole moment negates the oscillations set up in conventional substrate dielectrics due to repeated polarization caused by signal cur- rent. is is manifested as an extremely low dissipation factor (Df ) that helps to reduce signal losses. Today's state-of-the-art low-loss PTFE substrate formulas have Df in the region of 0.0015. On the other hand, the material retains a favorable value of dielectric constant (Dk), which can be about 2.6. ese properties give designers more freedom to optimize the con- ductor trace width and foil-layer thickness than is possible with other low-loss materials, which can help improve both the affordability and reliability of the resulting circuit board. e value of Dk is also extremely consistent over temperature and frequency. In addition, PTFE has generally stable phys- ical characteristics. e entropy change on melting is low, which ensures a high melting point. As a thermoplastic, there is no glass- transition temperature (Tg) and PTFE is also resistant to oxidation or chemical attack. As a result, its properties are consistent over time. Adding a micro-dispersed filler system allows the coefficient of thermal expansion (CTE) to be controlled. Figure 1: Ventec high frequency product solutions.