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56 DESIGN007 MAGAZINE I OCTOBER 2020 Why Do High-Frequency Materials Have Different Dk Values? Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION When an engineer researches high-fre- quency circuit materials, they will notice there are many offerings of what appears to be the same material type but with different Dk val- ues. One might ask, "Why are so many dif- ferent Dk values desired?" The answer is not straightforward, but there are very good rea- sons for these different materials. This column will give a quick overview of the need for these materials with different Dk values, as related to different high-frequency applications. Insertion loss is often considered with many high-frequency applications and is made up of four components: conductor loss, dielectric loss, leakage loss, and radiation loss. The con- ductor loss is related to the circuit conductor; often, the major concern is the surface rough- ness at the copper-substrate interface. Dielectric loss is mostly related to the dis- sipation factor of the circuit material. Leakage loss is associated with the volume resistivity of the dielectric mate- rial, which is between the copper planes. Typi- cally, the volume resistivity is very high for high-frequency circuit materials, and due to that good property, leakage loss is not a con- cern. Although, some high-power applications can have an issue with leakage loss. The last component of insertion loss is radiation loss, and that is how much loss is radiated off the circuit. Radiation loss is generally not desired because it is an increase of the signal loss, but there is an additional concern with the radiated energy having the potential to corrupt neigh- boring circuitry. There are many different items that can cause radiation loss to vary, includ- ing circuit design, substrate thickness, PCB fabrication anomalies, the Dk of the substrate, and frequency. In general, a high-frequency circuit material with a low Dk can have increased radiation

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