RF/microwave power applied to a printed-circuit board (PCB) will generate heat. A key to designing a practical circuit on a given PCB material is to understand how different circuit material properties can impact the heating patterns on an RF/microwave PCB, and to work within the limits of a high-frequency circuit material.
For any RF/microwave PCB, the heat generated by a circuit element, such as a high-power transistor, will follow a general heat-flow model, flowing from the source of the heat to a heat sink or cooler part of the PCB. The properties of the circuit material will have a great deal to do with how the heat flows across the PCB and how heating patterns develop.
The heating patterns on a PCB are related to the current density patterns on that PCB, and the way that current is transformed to heat within a PCB might be best described by the PCB material’s thermal conductivity (TC). The TC, which is typically measured in watts of power per meter of material per degree Kelvin (W/m/K), is often used to compare different rates of energy loss as heat through different materials. Quite simply, higher TC values mean better heat flow through a material. Since PCBs are comprised of different materials, such as copper conductors with very high TC value and dielectric materials with very low TC values, heat flow through a PCB will occur at different rates, resulting in heating patterns such as at where the edges of conductors meet dielectric substrates.
High TC values usually denote materials engineered for high-power applications, where heat flows more efficiently through the material. PCB materials with lower values of TC will exhibit less heat flow through the material, with an increase in circuit temperature.
As a comparison, copper is a good electrical conductor and a good thermal conductor, with an extremely high TC value (typically 400 W/m/K). Heat energy will flow quickly and easily along copper conductors according to standard heat-flow models, with thermal energy moving from an area of higher temperature to an area of lower temperature, such as a heat sink. The dielectric materials used in PCBs have very low TC values and behave more like thermal insulators. For example, polytetrafluoroethylene (PTFE) circuit materials provide good dielectric properties but have a much lower value of TC (typically 0.20 W/m/K), so that much less thermal energy will flow through the material compared to copper. For a circuit material to generate less heat for the same power, it must exhibit a higher value of TC, such as RT/duroid® 6035HTC circuit material from Rogers Corp., with a typical TC value of 1.44 W/m/K.
Dielectric constant (Dk) is another circuit material parameter that can impact the heating patterns through a PCB. Dk is an important circuit material parameter in terms of determining the dimensions of different circuit structures. It also has a great deal to do with the heat flow across a high-frequency PCB, since a material’s Dk helps determine circuit dimensions for a given characteristic impedance. For typical 50-? microstrip circuit designs, for example, PCB materials with lower Dk values support the use of wider conductors for a desired signal frequency compared to circuit materials with higher Dk values.
Additional material parameters that can impact the heating patterns across a PCB include dissipation factor (and how it relates to the loss of the material), copper conductor surface, and even the thickness of the PCB material. As noted earlier, the dielectric material content of a PCB is more like a thermal insulator than a thermal conductor, so a thinner circuit material offers a shorter heat-flow path than a thicker circuit material, resulting in less heat buildup in the dielectric material.
To minimize heat at higher power levels, a PCB material with smaller dissipation factor (Df) is better since this low dielectric loss translates to RF/microwave circuits with lower insertion loss. Lower circuit insertion loss means less heat produced at higher RF/microwave power levels. Copper conductor surface roughness is also related to circuit material loss, with smoother copper conductors usually yielding lower conductor loss, and lower heat produced for a given amount of RF/microwave power handled by a circuit. Copper conductor roughness tends to yield frequency-dependent effects, with some differences in heating at different frequencies.
One additional circuit material parameter, a circuit’s maximum operating temperature (MOT), can be useful for comparing PCB materials intended for high-power use. This is a measure of the maximum temperature at which a circuit can be operated for an indefinite period of time.
To better understand the thermal behavior of typical PCB materials, heat rise testing was performed on a number of different microwave PCB materials, using 50-? microstrip transmission lines to conduct high-power test signals and measure the resulting temperature rises through the PCB materials. Differences in the materials included many of the key material parameters already noted, such as dielectric thickness, copper conductor surface roughness, dielectric constant, insertion loss, and TC.
A worst-case example involved transmission lines fabricated on FR-4 circuit material with Dk value of 4.25, Df of 0.0200, low TC of 0.25 W/m/K, and relatively high insertion loss of 0.37 dB/in. of microstrip transmission line at 3.4 GHz. A best-case example involved RT/duroid 6035HTC circuit material with Dk of 3.60, low Df of 0.0013, much higher TC of 1.44 W/m/K, and low insertion loss of 0.11 dB/in. For 85-W test signals applied at 3.4 GHz, the difference in heat rise for the two materials was startling. For the worst-case material, the temperature rise was +74°C; for the best-case material, with the same test signal, a temperature rise of only +14°C occurred.
Ideally, the heat flow through a PCB construction will follow established heat flow models and the heat produced by an applied source or an on-circuit active device will channel from an area of high temperature to an area of much lower temperature. The various PCB material parameters, such as TC and Df, help provide some insight into how various heating patterns will develop across different PCB materials at higher RF/microwave power levels, and how those PCB material parameters can be consulted when choosing a circuit material for higher power levels.
Note: This blog is based on a Microwave Applications Seminar (MicroApps) presentation, “The Impact of Circuit Material Properties on Microwave PCB’s Heating Patterns,” being presented June 4 @ 9:35 a.m. by the author during IMS2014 (www.ims2014.org) Conference and Exhibition in Tampa, FL .
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.