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Military Microwaves Supplement
Handling high power in an RF/microwave printed-circuit board (PCB) requires not only effective circuit-design techniques, but PCB material capable of “getting the heat out.” High-power handling for a PCB material is synonymous with low loss and higher thermal conductivity. But in choosing a circuit material for high-power applications, such as power amplifiers and power combiners/dividers, many other PCB parameters come into play. This includes the maximum operating temperature (MOT) for a given material, which essentially describes a danger temperature above which performance and reliability problems can be aggravated.
What makes one circuit material better suited for high-power applications than another? Why might a PCB material such as RT/duroid® 6035HTC from Rogers Corp. be a better match for a high-power RF/microwave circuit than RO4350B™ or RO4360G2™ circuit materials? Understanding how key circuit-material parameters relate to high-power applications can go a long way towards picking the best PCB materials for high-power circuits.
High-power PCBs inevitably generate heat, due to nonideal energy flow through a PCB’s copper conductors and its dielectric materials. The power, and resulting heat, may come from signals fed to a PCB from an external source to the input of the circuit or from a heat source, such as a power amplifier, on the PCB itself. Typically the goal is to flow the energy from the source on the PCB or from the input to the output of the circuit with as little heat generated as possible. Heat produced on a PCB results from a number of PCB material characteristics, including conductor losses, dielectric losses, dielectric constant (Dk), and dissipation factor (Df), and the high-power/thermal characteristics of different PCB materials can be quickly compared by means of their thermal conductivity (TC), a parameter that describes how effectively heat is transferred through a material as a function of applied or generated power. It is described in terms of watts of power per meter of material per degree Kelvin (W/m/K).
One of the first parameters to examine when comparing different PCB materials for high-power use is the TC, with higher values indicating efficient energy transfer and more effective capabilities to minimize heat rises in PCBs at higher power levels. A PCB material consists of circuit traces and ground planes formed of copper metal, with extremely high TC for good heat flow, and dielectric materials with much lower TC values. With a thermal conductivity of about 400 W/m/K, copper moves thermal energy very quickly from one location to another. Dielectric substrate materials have much lower TC values so that heat can build at interfaces between the copper and dielectric materials.
TC values for circuit materials tend to be more indicative of materials that act as thermal insulators rather than as thermal conductors, which makes the task of removing heat from high-power circuits all the more challenging. The TC for low-cost FR-4 circuit materials, for example, is about 0.20 W/m/K. For lower rises in circuit temperature at higher power levels, a PCB material with higher TC value is required. RO4350B laminate has a considerably higher TC, at 0.62 W/m/K, while RO4360G2 laminate provides typical TC value of 0.80 W/m/K. Of course, for high-power RF/microwave applications where heat rises must be minimized, Rogers RT/duroid 6035HTC circuit material, with a TC value of 1.44 W/m/K, handles high power levels with minimal temperature rises. Rogers RT/duroid 6035HTC laminate is a fluoropolymer composite PCB material with a unique filler system for high thermal conductivity; it is available with reverse-treated, electrodeposited copper foil. This laminate features a low dissipation factor (loss tangent) of 0.0013 at 10 GHz, compared to dissipation factors of 0.0037 for RO4350B laminate and 0.0038 for RO4360G2 laminate.
In addition to using materials that can handle power and heat with low loss, PCBs must be fabricated using conductive and thermal materials with closely matched coefficient of thermal expansion (CTE), so that any expansion or contraction of the materials due to power/temperature effects will occur at the same rate to minimize stress on the material interfaces, such as for plated through holes (PTHs) used to channel heat through a PCB.
But TC is just one parameter for comparing different PCB materials for high-power RF/microwave applications. While Dk is not one of the more critical material parameters when sorting through PCB material choices for high-power applications, circuit materials with lower Dk values yield circuits with wider conductor widths for a desired frequency and impedance. This results in less loss through those circuits (and less heat generated at higher power levels).
Another important material characteristic to consider for high-power applications is loss. A PCB with higher loss will generate more heat from the RF/microwave power it handles. The insertion loss from a circuit board consists of a number of different types of PCB-based losses, including conductor loss, dielectric loss, and radiation loss [which is typically dependent upon the type of transmission line, such as microstrip versus coplanar-waveguide (CPW) circuits]. Conductor losses can be affected by a number of different PCB characteristics, such as circuit material thickness, frequency, dielectric constant (Dk), conductor surface roughness, and even the finish on the PCB’s plated metal surfaces.
Dielectric losses are usually associated with a PCB’s dissipation factor, with lower values indicating lower loss (and less heat generated at higher power levels). In general, a PCB material suited for higher-power applications should have high thermal conductivity and low dissipation factor, so that less heat is generated for a given RF/microwave power level. Another key material parameter to consider for higher-power PCBs is maximum operating temperature (MOT), which is the maximum temperature that the circuit can handle over an indefinite period of time.
Regardless of PCB material, the heat flow through higher-power RF/microwave circuits can be improved through the use of PTHs and PTH “viafarms” to flow heat from a heat source mounted on the PCB to heat-dissipating ground planes and heat sinks. In terms of a PCB material choice for higher RF/microwave power levels, optimal materials would include low Dk, low Df, high TC, and smooth copper surfaces.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.
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