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

High reliability is a goal and desire for all designers and end-users of high-frequency printed-circuit boards (PCBs). Since all of the components mounted on the PCB depend on it, it is expected to deliver dependable and consistent performance over time. But, depending on the operating conditions, it can sometimes be difficult to achieve. A high-temperature environment or conditions of high humidity can challenge the stability and reliability of the best-engineered PCB materials.

Of course, some PCB materials hold up better than others under challenging conditions and some deliver outstanding reliability for a wide range of circuits. It can be helpful to know what to look for in a PCB material when high reliability is critical, and what types of operating conditions can put the reliability of a PCB material to the test. When considering different PCB material choices, it can be useful to know which material parameters and characteristics can provide insight into the expected reliability. In an attempt to help, we will explore PCB material reliability: this post, Part 1, will review some of the general obstacles for a PCB material to achieve good long-term reliability. The next post, Part 2, will take a close look at how the characteristics of one particular PCB material add up to good long-term reliability.

Nothing lasts forever, not even a PCB. But a well-engineered PCB material can provide reliable and consistent performance for a long time and across a wide breadth of operating conditions. For many applications, such as for circuits and systems in medical, military, and space applications, PCB material reliability is critical. Anything that can cause a change in the PCB material’s performance or behavior, such as a change in dielectric constant, can be considered a threat or challenge to the long-term reliability of the PCB material.

Elevated temperatures, and the high power levels that can produce them in a high-frequency circuit, may pose the greatest challenges to the long-term reliability of any PCB material. Any PCB laminate consists of different materials with different properties, including dielectric materials and conductive metals, such as copper. These materials expand and contract at different rates with temperature, respectively, resulting in thermally induced stresses on junctions between different materials.

Thermal expansion occurs at different rates through the different dimensions of a PCB material, described by a parameter known as coefficient of thermal expansion (CTE). It is not unusual for a PCB material to have different CTE values through its x and y directions—the width and length—compared to its z dimension or thickness. To improve reliability, PCB materials are often formulated for a dielectric with CTE in the x and y directions matched to that of copper (about 17 ppm/°C), so that the conductive metal and dielectric materials expand and contract similarly with temperature, avoiding stress from thermal junctions.

But this may not be the same in the z direction, where plated through holes (PTHs) are drilled and metalized through the thickness of the material to form signal and ground interconnections as well as to connect different layers of multilayer circuits. The metal/dielectric junction for these structures must also endure thermal stresses, but may not enjoy the same matched CTE values as in the other two PCB dimensions. High reliability depends on the durability of these different metal/dielectric junctions over time and temperature. For that reason, a PCB material engineered for reliability usually features a CTE in the z direction for minimal dimensional changes in PTHs with time and temperature.

Design engineers typically sort through potential PCB material choices by their performance levels, such as the dielectric constant or loss at a target frequency. But when picking a PCB material for high reliability, it is necessary to compare another set of parameters. In its simplest terms, a PCB material built for good long-term reliability should exhibit stable mechanical and electrical characteristics with temperature. A PCB material’s parameters with close ties to reliability include CTE, dissipation factor (Df), and glass transition temperature (Tg).

The Df, which is the loss attributed to the dielectric material, can impact reliability at high operating temperatures and power levels, since high loss will contribute to high material temperatures and added thermal stress. The glass transition temperature, Tg, is something of a warning point for reliability, since it represents the temperature at which dramatic changes can occur in a PCB material’s CTE behavior. Above this temperature, a material can become mechanical and electrically unstable. Although it may be necessary to exceed the temperature for short-term processing steps, it is a temperature that should not be exceeded for any length of time to ensure good long-term reliability.

Another concern for the long-term reliability of some PCB materials is the effect of oxidation, which can cause a small increase in the dielectric constant of a PCB material over time, especially for thermoset PCB materials. The dielectric material covered by copper is protected against oxidation, but the uncovered dielectric material is subject to oxidation and “darkening” in color over time, especially at elevated temperatures. Some thermoplastic PCB materials, such as polytetrafluoroethylene (PTFE), are relatively immune to oxidation, but can be impacted by CTE and other reliability concerns. Oxidation, which is accelerated at higher temperatures, can also cause a small increase in dissipation factor and the loss of transmission lines.

Oxidation can also cause an increase in the loss of transmission lines, although this change is relatively small over time and at elevated temperatures. It has also caused small increases in dielectric constant over time and temperature of a few percent or less, which can be a concern for some circuits where dielectric constant is critical, such as resonators and filters. Of course, oxidation will not occur where oxygen is not present, such as in space in satellite-communications (satcom) applications. When oxidation is a concern, a circuit can be stored in an oxygen-free environment, such as under nitrogen or in a vacuum.

Numerous standards laboratories, including Underwriters’ Laboratory (UL), have studied PCB reliability. UL’s long-term thermal rating for PCB materials is known as the relative thermal index (RTI). It refers to the maximum temperature that a material can handle indefinitely, without compromising performance or critical material properties. It is backed by another UL rating, the maximum operating temperature (MOT), which refers to a particular PCB construction. In that particular PCB configuration, it is the highest temperature that the circuit can withstand without enduring changes in performance or material properties. The MOT can never exceed the RTI for a given material.

Design engineers must review the requirements of an application when sorting through their PCB material choices. Some applications may not have the environmental/performance demands of others and not require the same PCB materials to ensure high reliability. Because PCB material is a complex issue, the next post, Part 2 on PCB reliability, will explore how these general guidelines for reliability in Part 1 can be met by a specific PCB material, Rogers  RO4835™ laminate. This laminate was formulated specifically to improve the long-term aging characteristics of the company’s popular RO4350B™ laminate.

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.

Congratulations to the ROG Blog team on their 50th post! This post authored by John Coonrod originally appeared on the ROG Blog hosted by Microwave Journal.

This ROG Blog series on printed-circuit-board (PCB) materials has reached the half-century mark, already covering a wide range of topics on circuit materials with this, the 50th ROG Blog. For example, this series has recommended materials for amplifiers, for antennas, for filters, and for different types of transmission lines. It has even detailed the effects of different PCB material thicknesses on circuit performance, and described the influence of conductor roughness on circuit performance.

While it would be difficult to pick out the top 10 Blogs from the first 49 Blogs appearing since August 2010, at least 10 of these ROG Blogs deserve mention for how they have attempted to help readers with their different uses of PCB materials.

From the very first ROG Blog, in August 2010, which compared low-cost FR-4 circuit substrates with higher-frequency PCB materials such as the Rogers substrates, to the latest ROG Blogs, which examine circuit material requirements for emerging millimeter-wave wireless applications through 300 GHz and higher, the ROG Blogs have attempted to provide clear and honest information on the use of circuit materials. The next 50 ROG Blogs will pursue the same ambitious goals, in hopes of providing readers with greater benefits for their uses of high-performance circuit materials.

While it would be difficult to name the “Top Ten” ROG Blogs from the series so far (see the list below), it is not surprising to find that one of the most popular (in terms of viewers/readers) would be one that also refers to something for free: the January 2011 ROG Blog on Rogers’ free transmission-line modeling tool, the MWI-2010 Microwave Impedance Calculator. This easy-to-use modeling tool, which has also been reviewed in many of the leading RF/microwave trade publications, calculates key parameters for most common microwave transmission lines, including microstrip, stripline, and coplanar-waveguide transmission lines. The executable (.exe) file is available for free download from the Rogers’ website and runs on Windows-based personal computers (PCs), including those with Windows XP, Windows Vista, and Windows 7 operating systems. The free software is even backed by a 22-page operator’s manual in PDF file format, also available for free from the Rogers website.

In many ways, the ROG Blog series is like a book on circuit materials, unfolding online before its readers, with each Blog adding a new chapter to the book. Each chapter shares what Rogers’ engineers have learned over the years about making and using circuit materials, and this first set of 50 Blogs has covered some areas of interest to a large number of readers. In line with the ROG Blog on free software, the ROG Blog “Comparing RF Circuit Material Processing Costs & Performance” also offers advice meant to help readers save money without sacrificing their performance goals. Although first appearing on “April Fool’s Day” (April 1) in 2011, this ROG Blog takes a serious look at the total costs of circuit materials, and how some circuit materials may have lower material costs than other materials, but pay for it later with higher processing costs and lower yields. It also explains how some performance parameters, such as passive intermodulation (PIM) in wireless circuits and signal integrity in digital circuits, require a careful consideration of tradeoffs in material and processing costs when choosing a circuit material.

These first 50 ROG Blogs have drawn readers for familiar themes as well as for some not-so-familiar topics. For example, the ROG Blog appearing on November 19, 2010, “What Is Outgassing and When Does It Matter,” addresses a subject that may be unknown to some readers but quite significant to others. Outgassing, which refers to the release of gas inside a solid such as a circuit material, especially when it is placed in a vacuum, can greatly impact the performance of circuits used in satellite-communications systems in space, or in medical electronics systems. This ROG Blog introduced many readers to a material term known as total mass loss (TML), and how the parameter could be used to help guide the selection of a circuit material for space-based or other applications where outgassing was a critical concern.

On the other hand, some of the more popular ROG Blogs covered the roles that circuit materials play in the design of some basic RF/microwave components, such as amplifiers, couplers, and filters, and how the choice of a circuit material can affect transmission-line losses in high-frequency circuits. One of the more popular ROG Blogs, “When Digital Signals Reach Microwave Frequencies,” covered an area of  interest to many microwave circuit designers, how to deal with digital circuits operating at microwave frequencies. This ROG Blog, appearing on February 23, 2011, reviews some of the important concerns for selecting a circuit material when circuits cross over from the digital area into the microwave realm. These high-speed digital signals will behave much like analog microwave signals, affected by PCB loss and even conductor surface roughness. To guide those in need of circuit materials for high-speed digital designs or even multilayer circuits that may combine fast digital and microwave circuits, this ROG Blog points out how different circuit material characteristics, such as dielectric constant and even coefficient of thermal expansion (CTE), can impact high-speed digital circuit performance.

At times, readers of the ROG Blog series shared their areas of interest and applications for circuit materials, and these applications are many and diverse, from lower-frequency analog and power circuits to high-speed digital and even microwave/millimeter-wave circuits. The ROG Blog series is written to serve its readers with new information on circuit materials as that information is needed, much like new chapters to an on-going, online book about circuit materials. Do you have a suggestion for future ROG blogs? We’d love to get your input. Let us know what you are interested in reading about.

Top 10 Popular ROG Blogs (based on reader feedback)

  1. Transmission-Line Modeling Tool: Free Downloadable Software” (1/27/11)
  2. What Is Outgassing And When Does It Matter” (11/19/10)
  3. Comparing RF Circuit Material Processing Costs & Performance” (4/1/11)
  4. Controlling Conductor Losses In Coplanar Transmission Lines” (3/14/11)
  5. When Digital Signals Reach Microwave Frequencies” (2/23/11)
  6. Do You Have An Award Winning Application?” (11/11/11)
  7. The Role of PCB Materials In Impedance Matching” (12/3/12)
  8. Choose Circuit Materials For Bandpass Filters” (1/16/13)
  9. Make Waveguide In Planar PCB Form” (10/18/12)
  10. Celebrating ROG Award Contest Winners at IMS 2012” (7/17/12)

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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