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Digging Deeper Into Dissipation Factor
Dissipation factor, also known as loss tangent, is a printed-circuit-board (PCB) material parameter probably often overlooked when engineers size up their possible choices for PCB materials. But it is a parameter that can tell a great deal about how a material will perform in different applications and environments. And it is a PCB parameter that is certainly worth spending a little time to get to know better.
The last two blogs took a closer look at one of the first PCB material parameters that circuit designers consider—dielectric constant. Although a circuit material’s dissipation factor (Df) is not typically afforded the same time and attention as dielectric constant, a PCB’s Df can provide invaluable insight into a circuit material’s behavior when loss is important, when signal distortion must be minimized, and when signal integrity (SI) must be preserved. High-frequency analog and high-speed digital circuit designers are expecting more than ever from their PCB materials, with those circuit materials transporting RF/microwave signals, high-speed digital signals, and often a combination of different technologies as part of a mixed-signal circuit design, and a circuit material’s Df can indicate how a particular material will handle complex signal routing. As a simple definition, Df is a measure of a dielectric material’s tendency to absorb some of the AC energy from an electromagnetic (EM) field passing through the material.
In some ways, a circuit material’s Df is synonymous with insertion loss, since in most cases lower values for each parameter are desirable. Especially at higher frequencies, dielectric losses can dominate circuit losses that are due to PCB materials; however, there are losses which can be more significant depending upon the circuit thickness and configuration. The other PCB material loss is conductor loss (although such loss can be impacted by a number of factors, including frequency, material dielectric constant, conductor thickness, conductor finish, even the surface roughness of the conductor). Assuming conductor loss is well understood, selecting a PCB material with low Df value will usually ensure that high-frequency circuits fabricated on that material will exhibit minimal losses. Although there is typically some debate as to how important low PCB Df is for logic circuits, but low loss for digital circuits usually translates into higher edge rates.
In evaluating PCB parameters, some circuit designers have found that it makes sense to link dissipation factor with a circuit material’s thermal conductivity, especially when operating at higher power levels, since the higher Df can mean higher dielectric loss for the material and more heat generated at higher power levels, especially for a material with lower thermal conductivity. For higher-power, high-frequency circuits, ideally a circuit material exhibits low Df and high thermal conductivity, so that minimal heat is produced due to dielectric loss and whatever heat is generated can be readily transferred from the PCB material to a heatsink.
In comparing practical materials, RO4350B™hydrocarbon ceramic circuit material from Rogers Corp., with a dielectric constant of 3.48 at 10 GHz in the material’s z axis, offers very high thermal conductivity of 0.62 W/m/K along with a relatively low Df of 0.0037 in the z-axis at 10 GHz. Compared to popular glass or ceramic-filled PTFE circuit materials, with dielectric constant of typically 2.50 at 10 GHz and somewhat lower Df of 0.0019 at 10 GHz, it provides considerably better dissipation of heat, since the thermal conductivity of the PTFE is typically between 0.19 and 0.20 W/m/K. But for those high-frequency circuits that must handle high-power signal levels with effective thermal management, the RT/duroid® 6035HTC circuit material from Rogers Corp. features that rare combination of very low Df (0.0013) and very high thermal conductivity (1.44 W/m/K).
Selecting PCB materials with low Df values can help minimize losses but, of course, there is a tendency for circuit materials with lower dissipation factor to be more expensive than materials with higher dissipation factor values. The tradeoff is essentially price for performance, so that the importance of performance to a particular application will dictate the possible price range for the circuit material.
In sorting through PCB materials by their Df values, it should be noted that temperature effects, such as the heat dissipated by active components and integrated circuits (ICs), can impact a circuit material’s Df value, causing a rise in Df at elevated temperatures. This behavior is another reason for the close ties between a circuit material’s Df and its thermal conductivity. Since active devices can generate heat within their proximity and even the best PCB and cabinet thermal designs will be challenged to fully dissipate all of the heat produced by an active device, the effective Df of a PCB material may be somewhat higher at elevated environmental temperatures and power levels for a given circuit design. For any CAE circuit models that include PCB Df as an input, the effects of temperature on Df should be considered in any circuit simulations in order to achieve a more accurate estimate of dielectric loss for a material over the operating temperature range.
Any PCB material comparison based on Df should take note of the test methods used by a PCB material supplier to determine Df, since different measurement approaches, such as those based on microstrip or stripline test structures, can yield different values of Df for the same circuit material. A number of different test methods are used to determine Df, including with copper-clad as well as unclad dielectric materials. Some methods, such as ASTM-D150 and IPC TM-650 184.108.40.206, fabricate a parallel-plate capacitor on the material and characterize the capacitor, while some approaches, such as IPC TM-650 220.127.116.11C and 18.104.22.168.1, use a stripline resonator and rely on difficult-to build mechanical test fixtures. There is no ideal method for determining Df but in comparing different materials for this important PCB parameter, Df values should be compared for the same characterization methods to be fair.
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.