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.
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.
3G, 4G, and now 5G?! These mobile network terms are a big part of the way companies differentiate various phones and tablets. It was easy in the beginning…1G meant analog cell phones and 2G meant digital phones. 3G, third-generation, networks were supposed to bring broadband speeds to mobile devices. But there are so many varieties of 3G that you could get speeds from 400Kbps to more than 10X that. 4G phones are supposed to be even faster, but that’s not always the case. According to PC Magazine,
The International Telecommunications Union, a standards body, tried to issue requirements to call a network 4G but they were ignored by carriers, and eventually the ITU backed down. 4G technologies include HSPA+ 21/42, WiMAX, and LTE (although some consider LTE the only true 4G of that bunch, and some people say none of them are fast enough to qualify.)
With all this variability, PC Magazine runs an annual “Fastest Mobile Networks” test across the US. The results for 2014 show different carriers ranking at the top in different regions.
But what type of mobile traffic is actually being used? How many 4G devices are out there? Are faster 4G connections consuming more bandwidth?
With networking equipment installed around the world, Cisco is in a unique position to measure data traffic. According to the “Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2013-2018,”
In 2013, a fourth-generation (4G) connection generated 14.5 times more traffic on average than a non-4G connection. Although 4G connections represent only 2.9% of mobile connections today, they already account for 30% of mobile data traffic.
The Mobile Network in 2013
More results from the Cisco report include…
- Global mobile data traffic reached 1.5 exabytes per month at the end of 2013, up from 820 petabytes per month at the end of 2012.
- Last year’s mobile data traffic was nearly 18 times the size of the entire global Internet in 2000. One exabyte of traffic traversed the global Internet in 2000, and in 2013 mobile networks carried nearly 18 exabytes of traffic.
- Mobile video traffic exceeded 50% for the first time in 2012. Mobile video traffic was 53 percent of traffic by the end of 2013.
- Over half a billion (526 million) mobile devices and connections were added in 2013. Global mobile devices and connections in 2013 grew to 7 billion, up from 6.5 billion in 2012. Smartphones accounted for 77% of traffic.
- In 2013, on an average, a smart device generated 29 times more traffic than a non-smart device.
- Mobile network connection speeds more than doubled in 2013. Globally, the average mobile network downstream speed in 2013 was 1,387 kilobits per second (Kbps), up from 526 Kbps in 2012.
- The top 1% of mobile data subscribers generated 10 percent of mobile data traffic, down from 52% at the beginning of 2010.
- Average smartphone usage grew 50% in 2013. The average amount of traffic per smartphone in 2013 was 529 MB per month, up from 353 MB per month in 2012.
- Smartphones represented only 27% of total global handsets in use in 2013, but represented 95% of total global handset traffic. In 2013, the typical smartphone generated 48 times more mobile data traffic (529 MB per month) than the typical basic-feature cell phone (which generated only 11 MB per month of mobile data traffic).
The Mobile Network Through 2018
Cisco projects mobile data traffic will reach the following milestones within the next five years.
- Monthly global mobile data traffic will surpass 15 exabytes by 2018.
- The number of mobile-connected devices will exceed the world’s population by 2014.
- The average mobile connection speed will surpass 2 Mbps by 2016.
- Due to increased usage on smartphones, smartphones will reach 66 percent of mobile data traffic by 2018.
- Monthly mobile tablet traffic will surpass 2.5 exabyte per month by 2018.
- Tablets will exceed 15 percent of global mobile data traffic by 2016.
- 4G traffic will be more than half of the total mobile traffic by 2018.
- There will be more traffic offloaded from cellular networks (on to Wi-Fi) than remain on cellular networks by 2018.
As users demand smarter, lighter, higher performance devices, designers need to balance weight, size, and radiation characteristics, such as gain, beamwidth, side-lobe levels, and polarization. The choice of PCB substrate material has a major impact. For instance, the high concentration of electrical energy in dielectric materials with high dielectric constants (Dk), degrades radiation efficiency.
Rogers Corporation provides a broad selection of high frequency circuit materials that are designed with these considerations in mind. Some materials have high dielectric constant to aid in the size reduction of antennas, for instance, while others are made with easy to process resins to assist in the reduction of cost for high frequency printed circuit boards.
There are a number of test methods to determine the dielectric constant of circuit materials used in the microwave or high frequency industry.
In this video, “Common Test Methods for Measuring Dielectric Constant,” you will learn about the most common test methods like Clamped Stripline Resonator Test, Split Post Dielectric Resonator, Full Sheet Resonance (FSR), and Microstrip Differential Phase Length Method.
For additional information and technical tools, join us at rogerscorp.com/techub
There is a fundamental reengineering of the electrical services industry underway. A lot of that change is centered on technical infrastructure and “smart grids.” A smart grid adds intelligence to the electric grid in the form of sensors, smart meters, communications technology, and advanced control methods. The goal is to gather information about the behavior of consumers and suppliers in order to improve the efficiency, reliability, and sustainability of the production and distribution of electricity.
Communications networks enable a two-way flow of data. Understanding how data flows most effectively to and from a control center to devices on the grid will dictate the design parameters for the networks.
Handling Critical Data
Backhaul, high volume data flow with high bandwidth requirements, is the most critical data for power utilities. This includes customer use and billing info, as well as data flow from grid devices to the control center.
Data varies in terms of speed, bandwidth, and throughput. This results in a hybrid communications network to handle the various data flows. Regardless of the speed, backhaul needs the most security, whether wired (fiber optics) or wireless (microwave).
In the upper grid, closest to the control center, point-to-point technology such as fiber optic cable typically is used to connect the control center with the substations. The number of locations of transmission-level substations that need to be connected with the control center is small, but the density of each is high. High bandwidth is needed for reliability.
Further out on the system, point-to-point technology yields to point-to-multipoint systems. The further you go from the control center, the number of substations increases but the criticality of the data falls off. Typically, the largest number of substations exists as points scattered over a broad area where the network relies on licensed and unlicensed wireless spectrum to provide high throughput at relatively modest cost.
Public wi-fi networks are a good option as they can blanket wide areas cost-effectively. At the edge of the grid, unlicensed spread-spectrum technology is most effective; because these are commonly found in rural areas, little interference is expected.
Preventing thermal and voltage fluctuations is a must for these high performance communication systems. It is also critical to balance demanding performance parameters: signal integrity, dielectric constant (Dk), dissipation factor (Df), and thermal conductivity.
Rogers’ high frequency laminates and circuit materials are designed for the demands of high reliability electronics. The RO4000® Series High Frequency Circuit Materials combine high frequency performance with low cost fabrication methods. The RO3000 High Frequency Laminates (PTFE/Ceramic) deliver improved temperature stability at a fraction of the cost of traditional military-grade counterparts.
Everything changes with time and printed circuit boards are no different! Watch this video to learn about four items that contribute to the aging of high frequency PCBs and the impact on electrical performance.
Once you’ve watched the video, send us your thoughts on Twitter at @Rogers_ACM. We love hearing from you!