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66 PCB007 MAGAZINE I MARCH 2022 PTFE in Practice Currently, PTFE is the best material tech- nology we have today for applications that require low loss at high frequencies. It is the base for commercial low-loss materials such as Ventec's tec-speed 30 series (Figure 1). ese are used in applications such as anten- nas for 77-81 GHz automotive radar, traffic detection radar, low-noise power amplifiers and antennas for wireless communications, and satellite communication systems. However, working with PTFE brings its own challenges that must be overcome. Because relatively little energy is gained on melting, the material does not flow, even when molten, such that processing is usually achieved by sinter- ing. Moreover, attaching copper foil to PTFE is difficult owing to the extremely low surface energy. is low surface energy, of course, is the property that gives PTFE its excellent non- stick characteristics in lubricants and cook- ware. Pure PTFE is also extremely challenging as far as processes like cutting and machining are concerned. Blending PTFE with ceramic fill- ers improves the mechanical stability and opti- mizes the overall performance. e composi- tion and ceramic particle size can be adjusted to optimize manufacturability and in-applica- tion performance of the resulting substrate. To assist with the adhesion of copper foil, a certain degree of roughness is desired in the bonding surface of the foil. On the other hand, the impact of the skin effect at high signal fre- quencies must be considered. At 10 MHz, the skin depth is about 21 microns whereas the standard copper foil has overall thickness up to 35 microns and a pro- file depth (surface rough- ness) of up to 10 microns. In this case, the skin depth is greater than the pro- file and the current path can be relatively unaf- fected. At 100 MHz, where the skin depth is only 6.6 microns, the current-carrying chan- nel is affected by the peaks and troughs of the surface profile and suffers from increased path length and resistance (Figure 2). Low-profile (5.0-9.9 microns) and very low profile (sub-5 micron) copper foils can help alleviate problems caused by the skin effect. Most recently, almost-no-profile (ANP) cop- per has been developed. While these are cer- tainly effective in improving high-frequency signal performance, attaching very low- profile foils to PTFE substrate remains extremely challenging and requires proprie- tary know-how. The Future for Low-Loss Substrates Today's advanced low-Df, low-Dk PTFE materials are the best low-loss formulas the industry has and will likely remain so for some time. With further development, they will undoubtedly become more cost-effective and easily used in a wider variety of applica- tions, particularly as more and more of these are expected to push toward higher frequen- cies striving to match those market demands for more features, more data, and faster per- formance. PCB007 Alun Morgan is a technology ambassador at Ventec International Group. Figure 2. Skin effect in copper foil at 100 MHz.