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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The Advanced Circuit Materials Division has announced RO4360G2™, next generation PCB laminates with improved thermal reliability for higher UL maximum operating temperatures (MOT’s).

In 2010, Rogers Corporation introduced its groundbreaking product, RO4360™ laminate, the first high Dk RF thermoset laminate. We have now launched the next generation, RO4360G2 laminate, with improved thermal reliability that will help fabricators achieve higher UL MOT’s.

With a tailored high Dk of 6.15 @ 10 GHz, this material allows next generation power amplifier designers to meet size and cost reduction targets. Specifically, the laminate’s higher Dk allows for a significant reduction in finished circuit board size (20-30%). RO4360G2 laminates process similar to FR-4, are automated assembly compatible, and offer the same reliability and repeatability that customers have come to expect from Rogers RO4350B™ material.

RO4360G2 laminates are UL 94V-0 flame rated (pending) and fully lead-free process capable. They possess excellent thermal conductivity of .81 W/m/K for improved reliability, a low Z-axis CTE for reliable plated through holes, and drill performance as good as or better than RO4350B laminates.

Typical applications for RO4360G2 laminates are power amplifiers, LNAs, RF components (combiners/splitters), patch antennas, and as a replacement material for designs previously employing LTCC (low temperature, co-fired ceramic). RO4360G2 laminates are the material solution designers working on 4G and next generation defense/aerospace platforms have been looking for!

This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal. You can read part 1 of the series here.

Achieving high reliability for a high-frequency circuit or system starts with the printed circuit board (PCB). The PCB material must deliver consistent performance over time and changing conditions, such as temperature. As explained in the previous blog post, it is possible to spot PCB materials that are “built to last” by assessing a number of their key performance parameters, such as coefficient of thermal expansion (CTE). In fact, PCB materials such as Rogers RO4835™ laminates can be engineered for high reliability through a careful combination of material components resulting in specific performance characteristics.

RO4835 laminates are thermoset materials like FR-4. They are part of Rogers RO4000® family of PCB materials and can be processed with the standard epoxy/glass methods used with low-cost FR-4 materials. RO4835 laminates exhibit a typical dielectric constant of 3.48 in the z direction at 10 GHz with low dissipation factor of 0.0037 in the z direction at 10 GHz. They offer x- and y-direction CTEs of 11 and 9 ppm/°C, respectively, that are relatively compatible with the 17 ppm/°C CTE of copper; the CTE is typically 26 ppm/°C in the z direction. RO4835 laminates have a glass transition temperature (Tg) of greater than +280°C to handle effects of high-temperature circuit processing.

As detailed in the previous blog, a number of material parameters can point to potential reliability issues, including a material’s CTE, its resistance to oxidation, and its heat- and power-handling capabilities. The CTE characteristics of RO4835 laminates represent stable mechanical and electrical behavior at higher power levels and across wide temperature ranges. In addition, the material has been engineered to be resistant to the effects of oxidation. In general, the material has been formulated for demanding applications where long-term reliability is a concern.

Oxidation can impact all thermoset laminate materials over time and at elevated temperatures. It is essentially caused by the absorption of oxygen atoms to form a carbonyl group within the material, leading to small increases in its dielectric constant and dissipation factor which are not reversible. The electrical impact of oxidation can also be affected by elevated temperatures. Physically, oxidation can also result in a “darkening” effect on the exposed dielectric surfaces of the laminate. The oxidation begins on the surface and slowly penetrates into the dielectric as the oxygen diffuses through the material. Copper metallization on a laminate greatly reduces the effect of oxidation on the dielectric material beneath the copper.

Where oxidation may be a concern, it might be necessary to store a circuit in an oxygen-free environment or enclosure, such as in a vacuum or nitrogen environment. Where such an option may not be available, RO4835 laminates are less affected by oxidation than most high-frequency circuit materials. RO4835 laminates were developed to combat the effects of oxidation and, in so doing, to promote better long-term reliability. They are composites formed of fused silica and woven glass fabric. They are bound with a highly cross-linked hydrocarbon polymer matrix and include an anti-oxidant additive, to be more oxidant resistant than traditional thermoset PCB materials. The RO4835 laminates provide electrical and mechanical properties quite similar to those of Rogers RO4350B™ laminates, with heightened resistance to oxidation because of the anti-oxidant additive.

Elevated temperatures are a threat to any PCB’s long-term reliability, especially when coupled with the need to handle high RF/microwave power levels. When subjected to the combination of high temperatures and high RF/microwave power levels, it is not just the amount of material expansion (as characterized by the CTE) but the rates of expansion (and contraction) of the different materials comprising a PCB that can result in stress junctions, such as between copper conductors and dielectric materials. Ideally, manufacturing processes support optimum thermal management of a PCB, such as proper implementation of plated through holes (PTHs). A through hole in a PCB with poor quality copper plating, for example, can result in undue stress on that portion of the circuit at elevated temperatures. Similarly, manufacturing flaws such as starved thermal viaholes can lead to hot spots and stress points on a PCB.

Proper thermal management of a PCB can also help control the effects of temperature swings on a laminate’s electrical performance. For example, a laminate’s variations in dielectric constant as a function of temperature are defined by a parameter called the thermal coefficient of dielectric constant, and typically evidenced as variations in the impedance of transmission lines. The value of the parameter is different for each laminate, but the amount of change in the dielectric constant due to this effect can be minimized by properly dissipating heat from a PCB.

Of course, starting with a circuit material that is designed for wide temperature ranges can help overcome even manufacturing/production shortcomings such as these. For applications where it may be necessary to handle both higher power levels and operating temperatures, the RO4835 laminates are based on dielectric material with CTE values in the x and y dimensions that are very closely matched to that of copper, to minimize stress junctions at elevated operating temperatures and power levels. In addition, the CTE through the thickness of the material (the z axis) is engineered for stable and reliable PTH quality, even when subjected to elevated thermal conditions.

In fact, the RO4000 family of materials, including RO4350B laminates, is formulated to deliver consistent performance even under more challenging operating conditions, such as high temperatures and power levels. The RO4000 series circuit materials feature low dielectric losses as well as high Tg, to maintain stable mechanical and electrical characteristics over a wide range of material processing temperatures. They are also characterized by excellent thermal conductivity, a parameter which indicates a circuit material’s effectiveness in dissipating heat.

The RO4000 series laminates are affected by oxidation, like all thermoset materials and unlike PTFE materials. But RO4000 materials, such as RO4835 laminates, are RoHS compliant and do not require special viahole preparation like PTFE materials. The RO4000 circuit materials can be processed using standard FR-4 production techniques and, in the case of RO4835 laminates, were formulated for minimal effects of oxidation and with thermal and mechanical properties which support excellent long-term reliability.

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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