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36 The PCB Design Magazine • April 2015 by John Coonrod RoGeRs CoRPoRATioN LIGHTNING SPEED LAMINATES RF Power Capabilities of High-Frequency PCBs coulmn I often hear this question: "How much RF power can be applied to a high-frequency PCB?" My answer sometimes surprises engineers. I tell them that they can put as much RF power into the PCB as they want, with the assump- tion that the PCB does not exceed its maximum operating temperature (MOT). MOT refers to the maximum temperature to which a circuit can be exposed without degradation of critical properties. The actual RF power limit of a PCB is based on the MOT of the circuit, and that is dependent on the circuit material, the PCB con- struction and fabrication process. The relative thermal index (RTI) is a rating given to UL-rated circuit materials for the maxi- mum temperature to which the raw material can be exposed indefinitely without degrada- tion in material properties. But when the raw material is made into a circuit, MOT is the rat- ing that is most applicable to the power-han- dling capability of a circuit. The MOT is always less than a circuit material's RTI. When review- ing the maximum RF power-handling capabil- ity of a PCB, MOT is used as the maximum tem- perature of which a circuit can be exposed over long periods of time. For example, a circuit with a heat rise of +70°C above an ambient of +25°C must en- dure a temperature of +95°C indefinitely. The RF power which creates this heat rise is accept- able if the circuit has a MOT rating of +105°C. But if the circuit's heat rise is greater than +80°C above ambient, the applied RF power level that created the heat rise would not be acceptable. When considering circuit heating due to ap- plied RF power, modeling the heat rise of high- frequency PCBs can be difficult. Many variables influence heat rise, and they must be taken into account. Insertion loss is the total RF loss of a high-frequency PCB and is equal to the sum- mation of conductor loss, dielectric loss, radia- tion loss and leakage loss. Insertion loss is the cause of the heat generated when RF power is applied. A circuit with a high level of insertion loss will generate more heat than a circuit with lower insertion loss, when considering the same

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