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Design007-Aug2019

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58 DESIGN007 MAGAZINE I AUGUST 2019 Most of the engineering programs at uni- versities do not include much information on high-frequency printed circuit board materi- als. There are exceptions, but most trained en- gineers will enter the industry with minimal background on high-frequency circuit materi- als. These materials are the core of the high- frequency or high-speed digital PCB, and un- derstanding the many attributes of these ma- terials can be critical for successful PCB-based application implementation. There are many resources in the industry to assist the engineer for learning about high-frequency materials. Before discussing these resources, a brief dis- cussion on what is typically important for high- frequency materials is a good starting point. Overview High-frequency circuit materials have many properties, and for some applications, certain properties are more important than others. Here is a quick list of material properties that can be important: dielectric constant (Dk), dis- sipation factor (Df), bond strength, coefficient of thermal expansion (CTE), moisture absorp- tion, thermal coefficient of Dk (TCDk), sub- strate thickness control, and thermal conduc- tivity. There are more material properties than those listed here; however, these properties are typically of interest for most high-frequency or high-speed digital applications. The Dk property of the high-frequency mate- rial is usually well-controlled for minimal vari- ation, as opposed to lower-grade circuit ma- terials, which often have large differences in Dk from one piece of material to another. Most high-frequency circuit materials have a doc- umented Dk tolerance, which can be crucial for the designer to consider when trying to ac- count for real-world variations of an applica- tion. Along with Dk variation, a related prop- erty is often overlooked: TCDk. This property High-frequency Material Technical Resources Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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