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60 The PCB Design Magazine • August 2017 Another issue is related to PCB fabrication processing. Some high-frequency materials are PTFE-based, and those type of materials can re- quire very different processing parameters as compared to FR-4 materials. There are other motivating factors for pro- gressing on from FR-4. Due to the demands of the high-frequency industry, most high-fre- quency materials are formulated to have tighter Dk tolerance than FR-4 as well as offering tighter thickness control of the substrate. These tight- ly controlled laminate properties can be very beneficial for building a controlled impedance board with a narrow specification for imped- ance. Additionally, high-frequency materials are typically formulated to have very low mois- ture absorption and for certain applications that property may be more important than the benefit of the materials' lower Df. Lastly, high- frequency materials are typically formulated to have low thermal coefficient of Dk (TCDk), and this property is a measure of how much the Dk will change with a change in temperature. For certain applications, TCDk can be more critical than loss and this is another reason for using a high-frequency laminate. Fortunately, new PCB materials are con- stantly being developed, and many of these bridge the gap between FR-4 and high-frequen- cy materials. For instance, Rogers Kappa 438 has many attributes which are similar to FR-4, while offering many benefits associated with high-frequency materials. This material has a Df value of 0.005; it is not as low as the Df of true low-loss materials, but it is obviously a signifi- cant improvement in loss as compared to the mid-loss or high-loss FR-4 materials. The loss benefit can be important. Howev- er, Kappa 438 has been formulated to have the same Dk as many commonly used FR-4 mate- rials. The Dk for this material is 4.38 and that means it can be used without significant modi- fication to the circuit design, when replacing FR-4. Additionally, the Dk tolerance is much tighter than most FR-4 materials, at ± 0.05. The moisture absorption is low, TCDk is low, and CTE is in a range where it can be used for high layer count PCB constructions and it can use the same processes as FR-4. Of course, when changing to any different material for a PCB construction, it is recom- mended that the PCB fabricator optimize the process for the material in each particular build. But the good news is that, if you must make the transition from FR-4 to a high-frequency, low-loss material, there is a variety of available materials for the technologist to choose from. PCBDESIGN John Coonrod is the technical mar- keting manager at Rogers Corpora- tion. TRANSITIONING FROM FR-4 TO HIGH-FREQUENCY MATERIALS The recent discoveries of topological materi- als—a new class of relativistic quantum materi- als—hold great promise for use in energy saving electronics. Researchers in the Louisi- ana Consortium for Neutron Scattering, or LaCNS, recently reported the first observation of this topological behavior in a magnet. The phrase "topological ma- terials" refers to materials where the current carry- ing electrons act as if they have no mass similar to the properties of photons, the particles that make up light. Amazingly, these electronic states are ro- bust and immune to defects and disorder because they are protected from scattering by symmetry. The magnetism brings with it an important symmetry breaking property—time reversal sym- metry, or TRS, breaking where the ability to run time backward would no longer return the system back to its starting conditions. Scientists Discover New Magnet with Nearly Massless Charge Carriers

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