This post authored by John Coonrod, Technical Marketing Manager, and team originally appeared on the ROG Blog hosted by Microwave Journal.

Circuit materials are evaluated by a number of different parameters, including dielectric constant (Dk) and dissipation factor (Df). Those two parameters also have temperature-based variants that provide insight into the expected behavior of a circuit material with changes in temperature, notably the thermal coefficient of dielectric constant (TCDk) and the thermal coefficient of dissipation factor (TCDf). The parameters detail the amounts of change in a material’s Dk and Df, respectively, as a function of temperature, with less change representing a material that is more stable with temperature.

A circuit material with an ideal TCDk, with a Dk that would remain at a fixed value even as the temperature changes, would have a TCDk of 0 ppm/°C. In the real world, however, circuit materials exhibit some change in Dk with changing temperature. A circuit material considered to have stable Dk with temperature will have a very low value of TCDk, typically less than 50 ppm/°C. When an application requires that a circuit board be subjected to a wide operating-temperature range and deliver stable performance all the while, a circuit board’s TCDk parameter is one of the key specifications to consider after determining the required Dk for that application’s circuits.

Although circuit temperature stability requirements for military and aerospace applications are well documented, due to the typically hostile operating conditions of military circuits and systems in the field, commercial applications can also endure conditions of changing temperatures that can require better-than-average circuit-board TCDk performance. Power amplifiers for wireless base stations or outdoor-mounted microcells may not experience the wide temperature swings of military environments, but they depend on circuit Dk stability with temperature to maintain stable gain and output power. Their active devices are impedance matched to a typical 50-Ω circuit/system environment for optimum gain, output power, and power-added efficiency, and changes in Dk will cause variations in amplifier performance. In addition to environmental temperatures, the self-heating effects of amplifiers can further complicate changes in a material’s Dk characteristics with temperature, especially for materials with high values of TCDk.

Similarly, passive components, such as filters, can suffer from unwanted frequency changes with changes in Dk, such as shifts in passbands and stopbands. Ideally, wireless base stations would be maintained in temperature-controlled environments but, again in the real world, this is often not the case, and circuit-board TCDk is then a key performance parameter of interest.

When performance with temperature is important, specifying a circuit-board material should be done deliberately and by taking a close look at available data sheets. Circuit designers should never assume that two materials from the same manufacturer or the same product line will have the same TCDk characteristics. Different materials based on PTFE, for example, can have widely different values of TCDk. PTFE, of course, is the basis for many excellent, low-loss, high-frequency circuit materials, but the TCDk characteristics of these materials can vary widely. Some PTFE-based circuit materials can suffer large changes in Dk with temperature, evidenced by TCDk values of 200 ppm/°C and even higher.

At the same time, some PTFE-based circuit based materials can provide near-ideal TCDk characteristics. PTFE-based RO3003™ circuit material from Rogers Corp. has outstanding TCDk of -3 ppm/°C, making it a strong candidate for temperature-sensitive circuit designs facing wide operating temperature ranges. The family of circuit laminates includes materials with Dk from 3.00 to just over 10.00 when measured through the z-axis (thickness) of the material at 10 GHz. The laminates are popular circuit choices for military and commercial applications through millimeter-wave frequencies of 77 GHz and higher, including in automotive radar systems which much maintain stable performance over wide temperature extremes.

Just as suppliers of circuit materials may test and specify the Dk values of their materials in different ways, such as at different test frequencies, any valid comparison of circuit materials for their TCDk performance levels calls for an understanding of the measurement methods used to determine TCDk for a particular material. Material measurements are often based on industry-standard IPC test methods for agreement of values among different material suppliers.

For example, measurements of circuit laminate TCDk at Rogers Corp. are performed by means of the clamped stripline test detailed in IPC test method IPC-TM-650 2.5.5.5c. Prior to testing a circuit laminate, all of the copper is etched from the substrate. The substrate is then clamped into a fixture, which behaves like a loosely coupled stripline resonator. The fixture and its material to be tested are placed into a laboratory temperature-controlled environment, such as an oven. The temperature is changed in steps and the fixture and material are allowed to reach thermal equilibrium with each change in temperature before the Dk is measured. Many measurements are performed to cover a wide operating temperature range and to measure the Dk at different points across that temperature range. The end result is a curve of Dk versus temperature for that material, which is the TCDk of that material.

Temperature-Dependent Loss

Just as TCDk is a barometer of how a material’s Dk changes with temperature, TCDf is a measure of a circuit material’s dissipation factor (Df) or loss tangent and how it changes with changing temperature, typically with loss increasing as temperature increases. As with TCDk, the temperature effects of TCDf can impact the performance of both active and passive circuits. At higher temperatures, a circuit material with high value of TCDf can compromise the gain and output power of an amplifier, and increase losses in passive circuits, such as filters or passive antennas.

The TCDf values of circuit materials from Rogers Corp. are characterized with a measurement method that is the same as that used for testing TCDk. The measurements are complicated by the variations in the loss properties of the resonator circuit, which is integral to the clamped stripline fixture and the fact that copper conductivity changes with temperature. Rather than attempting to measure and report Df versus temperature to derive a TCDf value for the material, loss versus temperature is measured, where it is the loss of the resonator in the form of its inverse quality factor (1/Q). As with TCDk, an ideal value of TCDf would be close to zero, to indicate little or no change in dissipation factor with temperature. In the real world, circuit materials with the lowest possible values of TCDf are to be preferred for temperature-sensitive circuit applications, whether for active or passive circuits.

All of the information shown above is in regards to the effect of temperature on the Dk and Df properties (TCDk and TCDf) of material when considering a short-term thermal event.  How a laminate responds to long-term thermal exposure is a different subject than TCDk and TCDf, even though these properties can be involved with long-term thermal aging evaluations.  These aging evaluations are critical to understand if the circuit material is the proper choice for the conditions it will be exposed to in the end-use environment and that it will meet the needs of the intended application over the life of the product.  More information on long-term thermal aging can be found a in two-part series blog, Picking A PCB For High Reliability and PCB Formulated for Reliability.

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Download the ROG Mobile app to access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

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antennaMany antennas depend on the durability and repeatability of the printed-circuit-board (PCB) materials on which they are formed. The type of material has a strong impact on the size and performance possible from an antenna. A printed-circuit antenna is typically fabricated on a PCB material composed of a dielectric layer with copper on one or both sides. For reliable electrical performance, that combination of copper and dielectric must be consistent, over time and over temperature. Understanding some of the key PCB material parameters and how they relate to antenna performance can pay large dividends for those working with PCB-based antennas. Here are some of our newest products and technical resources for antenna designers.

PIM and PCB Antennas: Guide to Circuit Materials for Low PIM Antennas

Passive intermodulation (PIM) can result from many factors in a circuit or system. Even the thickness and dielectric constant (Dk) of a laminate can affect PIM by contributing to the physical dimensions of transmission lines. This can lead to higher current densities in densely spaced circuits. To address the need for low-PIM antennas, the Advanced Connectivity Solutions group developed the RO4500™ & RO4700JXR™ series thermoset laminates and the AD series™ woven-glass,

polytetrafluoroethylene (PTFE)/ceramic antenna circuit materials. With these circuit laminates as starting points, antenna designers can be assured of an opportunity to create antenna designs with the lowest PIM possible. Download the PIM and PCB Antennas Guide.

VIDEO: AD300C™ Antenna Grade Laminates

Our AD300C high frequency circuit materials are featured in this video that highlights topics from datasheet properties and measured results showing insertion loss, dielectric constant, and PIM performance. The information in this video explains why AD300C antenna grade laminates are a popular choice for PCB based antenna applications.

New UL 94 V-0 Circuit Material for Cost Effective, High Performance PCB Antennas and Active Antenna Arrays for 4G, 5G, and IoT

The RO4730G3™ UL 94 V-0 antenna-grade laminates are designed to meet present and future performance requirements in active antenna arrays and small cells, notably in 4G base transceiver stations (BTS) and Internet of Things (IoT) applications as well as emerging 5G wireless systems. These flame-retardant (per UL 94V-0), thermoset laminate materials are an extension of Rogers’ dependable RO4700™ circuit materials, which are a popular choice for base station antennas. RO4730G3 laminates provide the low dielectric constant (Dk) of 3.0 favored by antenna designers, held to a tolerance of ±0.05 when measured at 10 GHz. Download the Antenna Grade Laminates data sheet.

 

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Millimeter-wave circuits were once considered exotic and only used for specialized applications, typically in the military space. For one thing, frequencies with such small wavelengths, from about 30 to 300 GHz, required special components and circuits scaled to those diminutive wavelengths. But lower-frequency bands are being consumed by a growing number of wireless applications, and millimeter-wave frequency bands are looking more and more attractive for communications systems of the future. Such high frequencies have even been proposed as part of an emerging fifth-generation (5G) wireless standard that will be challenged to connect billions of global Internet of Things (IoT) devices by means of available wireless bandwidths. Millimeter-wave bandwidths have long been employed for military radar systems and are increasingly being used in commercial automotive collision-avoidance radar systems. Achieving millimeter-wave circuit designs on reliable printed-circuit-board (PCB) materials in a practical manner will be the challenge in making these higher frequencies affordable. Substrate-Integrated-Waveguide (SIW) circuit technology may just be the solution.

As noted in an earlier blog (“Make Waveguide in Planar PCB Form”), SIW structures are essentially waveguide in planar form, with the capability to support millimeter-wave signals with relatively low loss even at those higher frequencies. SIW technology offers improved performance at millimeter-wave frequencies compared to traditional transmission-line technologies, such as microstrip, stripline, and even grounded coplanar-waveguide (GCPW) approaches, with limitations at millimeter-wave frequencies.

SIW has often been described as a form of transition between microstrip and dielectric filled waveguide (DFW). SIW can be fabricated with many of the same methods as microstrip. At millimeter-wave frequencies, however, microstrip circuits require small features and extremely tight machined tolerances to support the transmission of such high frequencies. In addition, at millimeter-wave frequencies, SIW circuits do not exhibit radiation losses suffered by microstrip. In fact, SIW circuits in general do not have the potential problems with electromagnetic interference (EMI) of the other transmission-line formats. SIW technology provides the means to realize extremely compact components; it is suitable for passive components, such as filters, but has also been used as active components, such as oscillators, at microwave through millimeter-wave frequencies. Commercial EM simulation software is most often used to aid in the design, simulation, and optimization of SIW circuitry, and such software programs can effectively model the effects of the dielectric substrates used as the foundations for SIW circuitry.

In forming SIW transmission lines, a rectangular waveguide is created within a substrate, usually on circuit-board material such as RO4350B™ LoPro® laminates from Rogers Corp. which has a low relative dielectric constant of 3.48 in the z-axis (through the thickness) measured at 10 GHz. This low-loss circuit material, which is widely used as the foundation for wireless base-station power amplifiers, features properties well suited to SIW circuits. It can be fabricated with the methods used for FR-4 circuit materials, to maintain low production costs.

SIW circuits and their dielectric-filled waveguide transmission lines are formed on a circuit material such as RO4350B LoPro laminate by adding a top metal plane over a laminate with a ground plane, then fabricating rows of conductive plated viaholes on both sides along the length of the substrate material. These plated-through-hole (PTH) viaholes are used to make the sidewalls of the rectangular waveguide structure formed on the PCB material. In forming the SIW embedded waveguide structure, more conductive metal is actually used than in stripline or microstrip transmission lines for similar wavelengths, resulting in less conduction loss at microwave and millimeter-wave frequencies.

What is critical in the fabrication of SIW circuits is the formation and spacing of the viaholes. Close spacing yields less conduction loss through the use of more conductive metal to form the waveguide structures, but also results in longer and more complex production times in fabricating the SIW PCBs. Wider spacing can save production time, but can also raise conduction losses and can result in higher EM leakage losses because of the wide spacing. SIW circuits will also suffer dielectric losses (as will all high-frequency circuit formats), which are dependent upon the choice of circuit laminate material. For SIW circuits, whether at microwave or millimeter-wave frequencies, and really with all high-frequency circuits, PCB materials should be chosen wisely for optimum balance between performance and cost.

One PCB material parameter that is critical for SIW reliability is coefficient of thermal expansion (CTE) which gauges the expansion of a circuit material with elevated temperatures. The SIW viaholes are plated through holes (PTHs) through the dielectric PCB material, and high values of CTE, which denote excessive expansion with temperature, will result in undue stress on the sidewalls of the PTHs. Circuit materials, such as RO4350B LoPro noted previously, and RO4835™ LoPro material from Rogers Corp., with stable CTE characteristics, are ideal candidates for high-reliability SIW circuits. RO4835 LoPro circuit materials have been used for years in the fabrication of multilayer circuits with high layer counts, relying on PTHs for interconnection of those many layers. As an added benefit for creating cost-effective SIW circuits, both RO4835B LoPro and RO4835 LoPro materials can be fabricated with standard FR-4 epoxy/glass processes to help minimize production costs.

At millimeter-wave frequencies, SIW circuits exhibit low loss similar to their larger mechanical waveguide descendants, and considerably less than the other, more conventional transmission-line formats, such as microstrip, stripline, and GCPW. But SIW circuits also share other traits of larger mechanical waveguide, including a lower-frequency cutoff point. As with mechanical waveguide, SIW circuits are designed for particular operating frequencies and bandwidths depending upon the circuit dimensions, and designers must be aware that they will be working with a lower-frequency (and upper-frequency) cutoff point and a target low-loss passband. But SIW circuits can also work quite well with traditional transmission-line formats, with microstrip and GCPW transmission lines serving as fairly simple and effective feedlines from other parts of a circuit to the SIW circuitry.

With the expected growth in the demand for millimeter-wave circuits, especially with the expansion of IoT and higher-frequency automotive applications, SIW technology appears quite attractive as an effective and practical solution for designing and fabricating affordable millimeter-wave circuits. In particular, SIW technology provides the means to realize miniaturized planar antennas as millimeter-wScreen shot 2014-08-08 at 1.33.54 PMave frequencies, perhaps in volumes reaching millions of components as needed for these many emerging wireless applications.

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Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

PrintDelay lines are useful component building-block functions for adjusting signals in both analog and digital circuits on printed-circuit boards (PCBs). High-frequency and high-speed delay lines are characterized by their bandwidths and delay times, as well as their insertion loss across their operating frequency range, their return loss, VSWR, rise time, and their delay stability. Delay lines can be realized with a number of different circuit elements, including coaxial cable assemblies, bulk-acoustic-wave (BAW) devices, and surface-acoustic-wave (SAW) devices, but the choice of PCB material can also play a major role in the final performance of a delay-line design. For example, the consistency of the dielectric constant (Dk) across a PCB and the consistency of the PCB’s thicknesses are critical for consistent and predictable delay-line performance. Quite simply: the better behaved a PCB’s Dk characteristics, and the more consistent the thickness of the material, the better the stability of the delay lines fabricated on that PCB, whether working with stripline or microstrip circuit technologies.

How does a delay line work? It is a function of the propagation medium for electromagnetic (EM) signals. When that medium is air, EM signals travel through air at the speed of light, or 186,280 miles/s. In practical terms for designers working in PCB dimensions, the speed of light is equivalent to 11.8 in./ns. When those signals travel through some other medium, such as a PCB, they slow down as a function of the material’s properties, such as a PCB’s dielectric constant (Dk). All circuit materials have a Dk value greater than 1, with higher values representing greater capacity to store charge and slower travel of an EM wave through that material.

On a PCB trace, EM signals move at a speed equivalent to the speed of light (c) divided by the square root of Dk, or c/(Dk)0.5. The Dk of a vacuum (and approximately of air) is 1, so when the propagation medium is air, it essentially has no effect on the EM propagation speed. For a circuit material like FR-4, with a Dk of 4, the speed of the signals traveling through that PCB is divided by the square root of the material’s Dk value, or 2. As a result, the speed of signals traveling through an FR-4 circuit board is about one-half the speed of light through air or through a vacuum.

For a delay line in an RF/microwave microstrip circuit, the EM field moves through a metal conductor and a combination of dielectric materials, including the PCB dielectric material below the conductive circuit trace and the air above the circuit trace. In an RF/microwave stripline delay line, the EM field moves through PCB dielectric material above and below the circuit traces, typically in multilayer circuit designs with plated through holes (PTHs) connecting the multiple circuit layers. Coplanar-waveguide (CPW) PCB techniques are also applied to the fabrication of RF/microwave delay lines, and variations in the PCB material properties, such as dielectric thickness and even the tolerance of the plated copper conductor thickness, can impact delay line performance.

Of course, circuit fabrication processes and assembly techniques can have a great deal to do with achieving consistent delay-line performance from a particular PCB material. Ideally, the PCB material exhibits consistent thickness within a fairly tight tolerance and consistent Dk value across the material, also within a fairly tight tolerance; variations in these PCB material properties can translate into variations in delay-line performance. Unwanted capacitances, such as circuit junctions, should be minimized since added capacitance also means added delays (above a design target). For good electrical stability, any PCB-based delay-line circuit will have a large ground plane.

For practical delay-line circuits, finding a suitable PCB material starting point will inevitably involve some tradeoffs. For example, in terms of pure performance, RT/duroid® 5880 circuit materials from Rogers Corp. are materials based on polytetrafluoroethylene (PTFE) and reinforced with glass microfibers. The RT/duroid 5880 materials feature an extremely low Dk of 2.20 and impressive Dk tolerance of ±0.02, with low dissipation factor for low loss. They are available in a variety of sheet sizes and thicknesses (as thin as 0.005 in.) with tight thickness control to minimize variations in delay time when fabricating delay lines. But performance generally comes at a price and, with their low Dk value and extremely tight Dk tolerance, these materials are somewhat higher in cost than many PCB materials. They are designed for use in the most challenging circuit applications, including in military electronic systems.

Accepting some tradeoffs in performance and material parameters for a lower cost, the same company’s RO3003™ PCB materials are also based on PTFE but filled with ceramic materials for stability. The RO3003 materials exhibit a Dk of 3.00 with Dk tolerance that is still good, at ±0.04, and also with low dissipation factor and excellent thickness control to minimize delay-line variations. A PCB material that offers a good blend of cost and performance for delay lines is the RO4835™ laminate, with a Dk of 3.48 through the z-axis at 10 GHz and a Dk tolerance that is still quite tight, at ±0.05.  In addition to being compatible with lead-free processes (RoHS-compatible), this material offers good thickness tolerance and it can be fabricated using standard FR-4 material processes to minimize production costs. This material is available in a wide range of thicknesses (as thin as 0.0066 in. thick) and different weights of copper cladding to accommodate different design requirements.

Screen shot 2014-08-08 at 1.33.54 PMAchieving design goals in delay lines often involves more than just the choice of PCB material, and every interface in an RF/microwave circuit design is a potential addition to the delay time of a delay line. For PCBs using coaxial connectors to launch signals, the interfaces between the circuit board and the connectors can introduce variations in the delay time and these interfaces or signal launch points should be as consistent as possible to minimize delay-time variations in the circuit.  A circuit material such as RO4835 laminate can provide the tight Dk tolerance, excellent material thickness control, and low-loss performance levels required for consistent delay-line performance.

Download the ROG Mobile appto access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to order samples of the company’s high performance printed circuit board materials.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

 

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This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

Choosing a high frequency circuit board material often requires weighing several factors, including cost and performance. A key starting point in sorting through printed circuit board (PCB) materials is usually the dielectric constant, or Dk, one of the essential characteristics of a laminate material and one that is subject to much comparison among different suppliers of PCB materials.

ROG_DkApp_finalThe dielectric constant of a laminate refers to a measure of the capacitance or energy between a pair of conductors in the vicinity of the laminate compared to that pair of conductors in a vacuum. The value for a vacuum is 1.0, with all other materials having a value somewhat higher than that. A laminate with higher values of Dk can store more energy than materials with lower Dk values. But at higher Dk values, electromagnetic energy will flow at a slower rate through the conductors (lower frequency).

Several earlier blog posts addressed different approaches available to measure the Dk of PCB materials. These methods involve different test fixtures and circuit configurations, such as the clamped stripline resonator test method and the full sheet resonance (FSR) test. Unfortunately, depending upon the laminate being measured and the frequency, these methods can reveal very different values of Dk for the same material under test.

For that reason, Rogers Corp. has developed alternative sets of dielectric-constant values, Design Dk values, to represent the company’s laminates during design and engineering stages. These are Dk values that can be used reliably and accurately within commercial computer-aided-engineering (CAE) software tools. The Design Dk values are measured by yet other measurement techniques, the differential phase length method. The approach is based on fabricating two microstrip circuits of significantly different length on the same laminate and in close proximity, identical in every way except for length. The test method measures the transmission characteristics of a quasi-transverse-electromagnetic (quasi-TEM) wave propagation and its phase response for a pair of microstrip transmission line circuits. By comparing the expected phase of the lines for a given frequency with the measured results, it can be possible to calculate the Dk for the laminate. In this approach, a large difference in length, such as 1:3, is recommended to simplify the measurements; the shorter circuit will limit the low-frequency accuracy.

But rather than just take Rogers’ word for it, it is also possible to apply the differential phase length method to a laminate of choice to determine its Dk firsthand. For those interested, Rogers Corp. now offers free downloadable software, Rogers’ Microstrip Dk Calculator Software, to determine PCB Dk values. The software works with the aid of associated test equipment, such as a microwave vector network analyzer (VNA). A high-quality test fixture should be used with the same signal launch for both circuits under test. The software can gather data from the measurements and produce a plot of Dk versus frequency, of particular value to designers of broadband circuits wishing to know the relative dielectric constant of the laminate beyond a certain operating frequency range. The range of frequencies across which this method can test depends on the lengths of the circuits, the return loss between the test fixture and the analyzer, and a number of different network analyzer parameters. The accuracy of the measurements depends on these different parameters and the length ratio between the two transmission lines. In addition to the software, an operator’s manual for performing the measurements can also be downloaded for free. The user’s manual provides details about the test method and why it tends to provide reliable results for Dk values.

These Design Dk values are generated for all of Rogers’ commercial laminates, based on this measurement method. The Rogers Microstrip Dk Calculator Software is available online for free download from the Rogers Technology Support Hub, which also includes technical papers and videos and several calculators, including the latest version of the MWI Microwave Impedance and loss calculator, MWI-2013. This free downloadable software tool features an improved grounded coplanar model, added capability to plot insertion loss as a function of frequency, and can compare as many as five models at one frequency or as many as five models over a range of frequencies.

Visitors to the Rogers Technology Support Hub can also download a copy of the ROG Converter software, a web-based application designed for a tablet or smartphone. It can provide simple conversions of dimensions from metric to English units and back, for temperature, for copper thickness, for CTE, and for thermal coefficient of dielectric constant (TCDk). Recently added conversions include for return loss: as VSWR, mismatch loss, and reflection coefficient. Coming soon is a free software tool that will predict minimum bend radius for a PCB without fracturing the copper traces. Based on Rogers’ materials, it can also help when planning multilayer material stackups.

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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