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MAY 2023 I DESIGN007 MAGAZINE 21 bonds in the material. A high Tg guards against barrel cracking and pad fracture during reflow. Standard FR-4 has a Tg of 135-170 ° C, whereas the high-speed materials are generally well over 200 ° C. Decomposition temperature (Td) is the temperature at which the material chemically decomposes. is is the maximum limit or the point of no return. Most materials have a Td of 320 ° C, so it is not an issue. e coefficient of thermal expansion (CTE) in the Z direc- tion is the rate of expansion as the material heats up. e CTE should be as low as possi- ble (<70 ppm). ere are many other proper- ties of materials to consider, but the material manufacturers know the right combination for high-speed design. Fortunately, a material with a low Dk has a low Df, a high Tg, and a high Td, which is exactly what is needed for high-speed design. So, considering the Dk and Df alone can get you in the ballpark for the right mate- rial for your design. With so many materials to choose from, which is the best for your specific product? Low cost generally means low quality. But the price of poor yields drives up the final material cost. Dielectric material selection is usually driven by the frequency and rise time of the digital signal, with lower values of loss most suitable for high-frequency applications. ese materi- als generally exhibit lower values of dielectric constant, resulting in faster signal propagation. Table 1 shows the loss profile ranges of dielec- tric materials. Figure 2 depicts the profile for dielectric materials with a Df < 0.005. e iCD materials Table 1: Loss profile ranges Figure 2: Loss profile for ultra-low loss dielectric materials. (Source: iCD Materials Planner)