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PCBD-Apr2015

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38 The PCB Design Magazine • April 2015 amount of applied RF power. Insertion loss can also be difficult to model, because there are sub- components that make up insertion loss. Typi- cally, the major contributors to insertion loss are dielectric loss and conductor loss. Dielectric loss is related to the dissipation factor (Df) and the tangent delta (tanδ) of the material. A material with higher Df causes higher dielectric loss, which in turn can cause higher insertion loss and high temperature rise with applied RF power. Con - ductor loss is far more compli- cated than dielectric loss, with several components making up conductor loss. In general, a cir- cuit using copper with a rough surface will have more conduc- tor losses than a circuit using smooth copper. Additionally, there is a circuit thickness rela- tionship, and a thinner circuit will be more prone to conduc- tor loss variables than a thicker circuit. The thicker circuit is more dominated by dielectric loss. One major consideration for understanding RF power capabili- ties of a high-frequency PCB is to understand the impact of insertion loss. Gener- ally, a circuit material and design will be chosen to minimize insertion loss, but there are trad- eoffs and other issues to consider. All circuit materials exhibit a property known as thermal conductivity: the measure of the ability to pass heat energy through that ma- terial. An extremely good thermal conductor is copper, which has a thermal conductivity value of 400 W/m/K. However, most substrates used for high-frequency PCBs have thermal conduc- tivity values that are considered a thermal in- sulator or a very poor thermal conductor. Most high-frequency circuit materials have thermal conductivity values in the range of 0.2–0.4 W/m/K. A value of 0.5 W/m/K or higher is con- sidered good for thermal conductivity for a PCB dielectric material. Now, let's consider at quick tradeoff. There are some extremely low-loss PTFE materials which can be designed so the circuit will have minimal insertion loss. This means the circuit will generate less heat when RF power is applied and a designer may assume that higher power levels could be applied. However, many PTFE materials have very low thermal conductivity and even though there is less heat generated, the heat cannot efficiently get out of the circuit, so the circuit may heat more than expected. Another tradeoff to consid - er is the thickness of the cir- cuit. As an example, a double- sided circuit, which is a simple microstrip circuit bonded to a heat sink, will stay cooler if the circuit is thin, as opposed to thick, when using the same materials and same applied RF power. The thinner circuit has a shorter heat flow path from where the heat is generated at the signal conductor, through the dielectric and to the ground plane below which is attached to the heat sink. There are several addition- al tradeoffs to consider, but a quick summary would show that the optimum circuit would use a material with low dielectric loss and smooth copper, which gives low insertion loss and generates less heat. Additionally, the opti- mum material would have high thermal con- ductivity and would be relatively thin. A few high-frequency materials meet these criteria. When working with RF and microwave designs, consulting your materials provider can save you time and money. These companies have plenty of information about thermal con- ductivity, insertion loss, heat flow, overall ther- mal management, and much more. PCBDESIGN John Coonrod is a senior market development engineer for Rogers Corporation. To read past columns, or to reach Coonrod, click here. RF PoWER CAPABILITIES oF HIGH-FREqUENCy PCBS continues one major consideration for understanding rF power capabilities of a high-frequency PCB is to understand the impact of insertion loss. Generally, a circuit material and design will be chosen to minimize insertion loss, but there are tradeoffs and other issues to consider. " " Lightning speed laminates

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