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48 DESIGN007 MAGAZINE I AUGUST 2018 This is a topic I have written about a few times, most recently in my August 2017 col- umn, and with good reason. The answer, "it depends," typically annoys engineers; fortu- nately, there is a list of items that can help the PCB designer answer the question of when to transition from FR-4 to high-frequency materials. The bottom line is that FR-4 mate rials have been around a long time, and they work well within their formulated range. The same can be said for high-frequency circuit materials, which are also thought of as low-loss mate - rials. However, there is a definite gray area that can muddy the waters when a designer is weighing a possible decision to switch from FR-4 to low-loss materials. High-frequency materials also have inherent properties that can benefit some applications that are not related to high frequencies or low loss. The material property differences between FR-4 and high-frequency materials is critical for the designer to understand when considering the use of either material. It may be a good idea to compare properties of FR-4 to a commonly used high-frequency material. The low-loss material to be consid- ered here is the RO4835 laminate. The FR-4 material that will be examined is a higher-qual- ity, high-Tg FR-4 material. This comparison is mostly meant to trigger a designer's thought process; there are admittedly exceptions with Transitioning from FR-4 to High-Frequency Materials, Revisited Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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