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

High-temperature processing is a routine part of manufacturing high-frequency circuit materials. From first forming dielectric prepreg materials to laminating the dielectric materials with conductive metals and eventually adding circuit elements, heat is an ally in producing printed-circuit-board (PCB) materials. Two types of composite materials—thermoplastic and thermoset materials—are commonly used for the dielectric layers in PCBs or as adhesives in manufacturing circuit laminates and they each have their own traits and characteristics. But how do they differ? What are the strengths and weaknesses of each type of material and why choose one over the other for an application?

Thermoplastic and thermoset materials are both processed at elevated temperatures. Thermoplastics are normally in a rigid or hardened state and soften as temperature is increased toward a material’s melting point. Thermoplastic materials can be reinforced with fillers, such as woven glass or ceramic materials. One of the best-known thermoplastic materials used in high-frequency PCBs is polytetrafluoroethylene (PTFE), which is often reinforced with some form of filler.

Thermoset materials harden as a result of a thermochemical reaction, such as the reaction that hardens the two components of an epoxy when mixed together. Because they start out in a soft or liquid state, it can be a simple matter to reinforce thermoset materials with fillers. Once hardened or “cured,” thermoset materials are typically harder than thermoplastic materials. Unlike thermoplastic materials, thermoset materials go through the thermochemical reaction to a hardened state once, and cannot be re-melted like a thermoplastic. Prior to the cure of thermoset materials, they have a limited shelf life compared to thermoplastic materials, which are stable at room temperature.

Thermoplastic materials such as early PTFE-based circuit boards were considered difficult to process, with relatively high coefficient of thermal expansion (CTE) compared to copper, contributing to the challenges of forming reliable plated through holes. Special chemical treatments of through hole walls were required to form sufficiently strong bonds between the plating metal and the thermoplastic PTFE. In contrast, thermoset materials tend to have CTEs that are much closer in value to the CTE of copper, allowing the use of standard processing methods when preparing through hole walls for plating.

Perhaps the simplest way to describe the differences between thermoplastic and thermoset materials for PCBs is that thermoplastic materials tend to provide better electrical performance but can require more elaborate manufacturing processes, while thermoset PCBs are easier to manufacture but traditionally have offered lower performance.

Thermoplastic materials typically have less electrical loss than thermoset materials, with less change in electrical performance over time and at elevated temperatures than thermoset materials. Unlike thermoplastic materials, thermoset materials can oxidize over time. The oxidation process can cause changes in a PCB material’s dielectric constant (Dk) and dissipation factor (Df) resulting in a potential for performance change at RF/microwave frequencies.

Through research and refinement, however, scientists at Rogers Corp. have improved upon the characteristics of both thermoplastic and thermoset materials for PCBs, with the choice of filler material having a great impact on electrical and mechanical performance levels. As an example, RO3000® circuit material is a thermoplastic material, a ceramic-filled PTFE composite that is available with Dk values from 3.0 to 10.2. As a thermoplastic material, it is very stable electrically and mechanically over time and temperature, with low temperature coefficient of dielectric constant (TCDk). It represents a dramatic refinement of early PTFE-based thermoplastic circuit materials, which exhibited CTE values in the z-axis of 300 ppm/°C or higher.

Even though RO3003™ circuit material is based on PTFE for low loss at microwave frequencies (dissipation factor of 0.0010 at 10 GHz), it is not plagued by the typically high CTE of PTFE-based circuit materials. It incorporates a special ceramic-based filler material to significantly reduce the CTE to 24 ppm/C, closely matched to copper at 17 ppm/C.  In addition, although most thermoplastic circuit materials require special chemical treatments to prepare the walls of through holes for plating with conductive metals, RO3003 thermoplastic circuit material can fabricate reliable plated through holes using a straightforward plasma process.

In contrast, RO4835™ thermoset-based circuit material was also developed and refined through experimentation. The ceramic-filled circuit material features much higher oxidation resistance than conventional thermoset materials. It is a high-performance material for high-frequency applications but, because it is not based on PTFE, it does not require special preparation (such as a sodium etch) to enable the formation of reliable plated through holes. The material is compatible with RoHS-compliant, lead-free processing. It supports low-cost fabrication processes comparable to those used for FR-4 circuit materials, but achieves outstanding RF/microwave electrical performance.

Although thermoset materials are not noted for low electrical loss compared to thermoplastic materials, the RO4835 thermoset material achieves low dielectric loss enabling it to be used in low-cost circuit applications above 500 MHz. It has a dielectric constant of 3.48 in the z axis at 10 GHz, held to a tolerance of ±0.05. Plated through holes can be fabricated on RO4835 laminate using standard processing methods, since the material achieves a z-axis CTE of 31 ppm/°C which is close to the 17 ppm/°C of copper commonly used for plating through holes.

RO4835 circuit materials build upon the success of another thermoset material from Rogers Corp., ROHS-compliant RO4350B™ circuit materials which have become a popular starting point for designers of high-power, high-frequency amplifiers. RO4350B laminates are rigid thermoset materials that do not use PTFE but can achieve excellent RF/microwave performance over time and even at elevated temperatures. They feature excellent thermal conductivity and mechanical thermal stability for stable and reliable use in power amplifier and other higher-power RF circuits. RO4835 and RO4350B thermoset materials remain rigid and stable at room temperature and share the benefit of ease of processing, using manufacturing methods typically applied to FR-4 materials.

Screen shot 2014-08-08 at 1.33.54 PMROG Mobile App

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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Wearable wireless devices are rapidly moving into the mainstream of society, led by the health and fitness markets. Advances in mobile communications and battery technology, miniaturized electronics, high performance materials, and app software have turned bulky wearable computing gear designed for the military into lightweight, energy-efficient, and fashionable wearable devices.

As more functional and fashionable devices hit the market, the focus is shifting from fashion to usability. It will soon be critical to deliver wire-equivalent connectivity for wearable devices, sometimes at very high data rates.

High Speed Wireless

With an avalanche of new wearables, it is important to find ways to supply low-latency high speed data connections to enable truly demanding use cases such as augmented reality. This is particularly true for high-density wearable computing scenarios, such as public transportation, where existing wireless technology may have difficulty supporting stringent application requirements wearable devices, sometimes at very high data rates.

For instance, if 25 out of 50 passengers in a conventional bus want to use Bluetooth communications for their headsets at the same time, there is barely enough bandwidth. That is for an average of 0.5 wearable devices per passenger. In the near future, we could be seeing five devices per passenger.

commuter train and wireless

The reality is that existing wireless protocols are a compromise between simplicity, efficiency, and flexibility. For wearables, this is a problem as data needs to get from the device to a place where it can be used. And the volume of sensor-based devices is increasing at an incredible rate. A panel of experts assembled by ECN cited forecasts of 10 trillion sensors by 2027, not including RFID devices. “To handle that much information, the industry needs to miniaturize antennas and examine how much bandwidth sending data away from a device will require. Or, data processing can be done on the device itself, if the battery life and a way to extract the data can be integrated.”

Design Limitations

The design and fabrication of wearable electronics require embedding electronic components inside clothes or wearable accessories that do not compromise the appearance and usability of the product. Per standards in the fashion industry, wearable technology must be useful, comfortable, and must not be intrusive to the user, who must be able to carry out daily activities without any movement limitation or additional burdens.

short range radio technologies

According to a team of researchers at the University of Salento,

Considering these requirements, either near-field or far-field wireless technologies may successfully serve the purpose for both data and power transmission. To guarantee a seamless integration of electronic devices and antennas in wearable and portable accessories, it is crucial to select appropriate materials and fabrication techniques. To this purpose, the use of nonconventional materials such as textile materials, conductive threads, electrotextile fabrics, and nonwoven conductive fabrics should be preferred.

Some wearable antennas for far-field Wireless Power Transmission (WPT) links have been proposed, such as a multifrequency rectifying antenna (rectenna) where the multiband behavior is obtained by using a slotted annular-ring microstrip antenna. The overall system is a multilayer structure using two layers of pile and a thermoadhesive layer at each interface between pile and conductive fabric. In addition, two textile logo antennas fabricated by means of a self-adhesive nonwoven conductive fabric have been presented. For near-field WPT links for wearable applications, a system using two resonators on a layer of leather has been proposed.

Battery Technology for Wearables

Batteries for wearables need to be small, thin, lightweight, and rarely charged. But battery technology has been slow to change. Li-Ion dominates thus far. Lithium Polymer is also an attractive choice for lightweight batteries.

According to battery expert Dr. Manfred Leimkühler,Li-Ion coin cells may be fine for sensors and other very low power wearable devices, but they struggle to keep up with the demands of more capable wearables, such as fitness bands and smartwatches. Extending the battery life is critical for these types of devices to gain market acceptance and for the wearables market.”

Advances in wireless charging, battery management, ultra-low power conversion, low power solutions such as Bluetooth and microcontrollers, even energy harvested from the body are in development to extend the battery life of wearable designs.

For flexible wearables, a new design strategy has emerged. Instead of changing the chemistry or thinning the device, a special arrangement of cells makes the existing batteries flexible or even foldable.

Regardless of the specific wearable application, batteries need to be packaged to absorb internal impact energy. PORON® polyurethane materials and BISCO® silicone foams withstand collapse that can happen due to the stresses of compression and temperature in battery packs over time. This Compression Set Resistance (C-set resistance) Resistance can help extend the life of the battery by continuing to seal and absorb shock. These unique materials from Rogers Corporation also have a unique ability to act as a spring by retaining a very consistent level of force across a range of compressions. This allows the designer more flexibility and reliability in packaging of the battery pack due to the ability to predict the cushioning material’s behavior across varied dimensional tolerances.

Open-Cell-vs-Closed-Cell-photosTangible improvements in wireless and battery technologies are surfacing. This, in turn, is having a significant impact on advances in, and use of, wearable devices.

 

“When you think about what determines the success of a company, it’s not just about money,” said Tim Gauthier, Global Director of Corporate EHS at Rogers Corp. “For Rogers, success involves listening to customers, investors, employees, channel partners, and regulatory agencies to refine our definition of corporate responsibility.”


At Rogers Corp., corporate responsibility is a commitment to manage our activities in a responsible way: business ethics, health and safety, environmental practices, employee engagement community activities, and human rights.

Recently, customers made it clear they wanted to learn more about Rogers’ corporate responsibility. With this feedback, we assembled a project team and determined that we needed one place on our web site to highlight all the great things we do every day.

This month we launched the Corporate Responsibility hub. It provides an inside look at the conscience of the company and how Rogers operates around the world.

Corporate Responsibility

We organize our responsibilities around seven key pillars:

  • Employee Health and Safety
  • Community Care
  • Environmental
  • Code of Business Ethics
  • Supply Management
  • Energy
  • Quality

The core of our Corporate Responsibility program is Health and Safety. We promote a workplace free of occupational injuries and illness by emphasizing individual responsibility for safety from all employees. This is supported at all levels of management. Since Bruce Hoechner, CEO, joined Rogers, we’ve put a huge effort into health and safety. We’ve significantly reduced injuries and lost time, as the data below shows. We’re especially proud of our employees; they are 100% engaged in the company’s safety programs.

Rogers Corp Injury Case Rate 2016

The Community Care principle demonstrates the fact that we respect and value the diversity reflected in our various backgrounds, experiences, and ideas. Together, we provide each other with an inclusive work environment that fosters respect for our employees and those with whom we do business.

Our Environmental program works to proactively achieve environmental excellence globally.

“Companies who do well in these areas perform better in the market,” said Gauthier. “For Rogers, showcasing our beliefs and our actions that support these beliefs allows us to recruit the best employees, provide the highest quality products, and establish long-term, mutually beneficial relationships with customers and suppliers.”

Our Code of Business Ethics is public information that we invite all to review. We believe that how we conduct our business is just as important as what we achieve.

“Our key stakeholders don’t want to work with a company that’s going to create a bad reputation for them,” stated Gauthier. “They want to know Rogers is a good company, that we take business seriously, and that we treat people well.”

Our Supply Management program is based on our belief that in our interactions with suppliers, employees are to conduct themselves with honesty and integrity.

Reducing our Energy usage and emissions is a critical part of our business.

Through our Quality program, we demonstrate that we are committed to providing products and services that exceed customer expectations.

Rogers will continue to develop the Corporate Responsibility program. Our quality teams are adding processes. Our health and safety teams are evolving programs. And our customers are working with us to advance our products and support procedures.

This is only the beginning of an evolving commitment to demonstrate what we believe in at Rogers Corp.

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

VIDEO: High Frequency Circuits Which Bend and Flex

Flexibility can be an important feature for printed-circuit boards (PCBs). Not all circuits are planar; some may need to be bent once to fit a particular product design while some might need to undergo continuous flexing as part of an application. Not all circuit materials are created equal, and some are more mechanically flexibile than others and can survive a certain amount of bending and flexing without damage. Understanding what makes a circuit material capable of bending and flexing, and what happens to it when it is bent or flexed, helps when specifying circuit materials for such uses.

Circuit boards are composites of different materials, such as conductive metals and dielectric materials, each with its own mechanical properties. The material stackup will depend on the type of circuit and the number of circuit layers. As more different materials are combined to form a PCB, especially in multilayer PCBs, the task of predicting the effects of bending and flexing becomes more complex. A key material parameter in determining how well a particular material will bend and flex is the modulus or stiffness of the material, with some of the composite materials of a PCB significantly stiffer, or with much higher modulus values, than others.

For example, the metallization in an RF/microwave PCB, primarily copper, will essentially determine the limits of flexibility in a circuit board since it has the highest modulus value of the material stackup, at 17,000 kpsi. Compare this to the much lower modulus values of dielectric materials, such as polytetrafluoroethylene (PTFE) with ceramic filler, at 300 kpsi, or PTFE with microfiber glass filling, at 175 kpsi. In a typical microstrip circuit, with conductor layer, dielectric, and ground-plane layer, the dielectric layer offers great flexibility but the top and bottom metal layers will set the limits of bending and flexibility for the composite structure.

Since high-frequency circuit boards are composite structures, the differences in flexibility of the component materials must be considered to determine how much bending and flexing a circuit board can withstand without damage to the stiffest of its material components, the metallization layers. This can be done by treating a PCB as if it were a beam being bent, with a certain bend radius depending upon the stiffness of the beam. A rubber beam will bend much more easily than a higher modulus metal beam, and be capable of enduring a much smaller bend radius without cracking. A PCB considered as a beam will also have a certain bend radius depending upon the overall stiffness of the composite group of materials, with the metallization layers setting the limits on the flexibility and minimum bend radius of the circuit board.

As with a beam, when a PCB is bent into a section of an imaginary circle, with a bend radius for that circuit, strain is placed on different parts of the beam and the PCB, with tension on the outer side and compression on the inner side of the bend radius. Between the areas of tension and compression lies an almost infinitely thin transition area or neutral axis with no strain. The strain increases as the distance from the neutral axis to the tension or compression plane increases. In a balanced circuit board, the neutral axis would lie at the geometrically center of the circuit board.

Stress from tension and compression works in different ways on a PCB’s materials, with tension pulling materials apart and compression squeezing them together. For a PCB with microstrip circuitry and copper conductors on the outer bend radius, this means that the stiffest or highest-modulus material in the composite PCB is being subjected to a certain amount of tension that will increase as the bend radius is made smaller. At the same time, the bottom ground plane is also being stressed and subjected to compression. Both forms of stress, if excessive, can lead to cracks in a microstrip circuit’s metallization layers. In addition, stress occurs at the interfaces of materials with different modulus values, such as the intersection of the copper conductor layer and the dielectric layer. Cracks from stress can start at the interface and work through the copper layer. To minimize damage to the metallization layers and ensure reliability in bent and flexed circuit boards, the key is to determine the amount of stress that a particular PCB can endure without cracking the metal layers.

The amount of stress on a PCB from bending and flexing is not simply a matter of knowing the modulus of the stiffest material component but in knowing how the PCB is constructed. For example, in a multilayer circuit board, differences in the thicknesses of the dielectric layers can cause increased amounts of strain when the circuit is bent. Each layer of a multilayer circuit structure will have its own modulus, and the structure will have a modulus as a whole. Since copper is the stiffest material component of most microwave circuits, the thickness of the copper and the percentage of copper in the entire PCB material stackup will contribute a great deal to the overall modulus and flexibility of the PCB as a whole.

Even the type of copper can determine the flexibility of a microwave circuit. Due to differences in the grain structures of rolled copper and electrodeposited (ED) copper, rolled copper is typically better than ED copper for PCBs that must be bent or flexed. For applications that may call for ED copper, some special types of ED copper are available for better bending and flexing than standard ED copper.  In addition, finishes added to copper conductors, such as electroless nickel/immersion gold (ENIG) plating, can add a high modulus to the overall PCB modulus, limiting the amount of bending and flexing that a PCB can safely endure.

Different microwave circuit constructions will present different bending and flexing capabilities. Stripline, with copper conductors sandwiched between upper and lower dielectric layers, is inherently better equipped for bending and flexing than microstrip. The signal conductor layer in a typical stripline construction is at or close to the neutral axis for minimum stress; however, the outer ground planes will typically have high stress.

General guidelines to avoid damage when bending or flexing circuit materials pertain to single-bend and dynamic flexing situations. When a single bend is required, the bend radius should be at least 10 times the thickness of the circuit so that the strain on the circuit layer is 2% or less. For dynamic flexing, strain should be held to less than 0.2% for more than 1 million flex cycles and less than 0.4% for 1 million or less flex cycles.

Readers wishing to learn more about how to model stresses placed on PCBs from bending and flexing are invited to view John Coonrod’s MicroApps presentation, “High Frequency Circuits Which Bend and Flex,” from the 2016 IEEE International Microwave Symposium (IMS). The presentation provides circuit bending prediction models and includes a microstrip example using ½-oz. rolled copper on 5-mil-thick RO3003™ laminate material from Rogers Corp.

ROG Mobile App

Download the ROG Mobile app to access Rogers’ calculators, including the popular Microwave Impedance simulation tool, literature, technical papers, and the ability to oScreen shot 2014-08-08 at 1.33.54 PMrder samples of the company’s high performance printed circuit board materials.

Ask an Engineer

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.

When reliability, efficiency, and performance are critical, design engineers partner with Rogers to develop and deliver the material technologies they require. Here are some of our newest product power electronics product introductions and RF/microwave design tutorials.

POWER ELECTRONICS SOLUTIONS

Power electronics concepts for the world of tomorrow…today.

VIDEO: Welcome to the World of Rogers’ Power Electronics Solutions

At PCIM Europe 2016, Rogers introduced ROLINX® CapEasy and ROLINX® CapPerformance capacitors busbar assemblies. These new products combine the unique capacitor technology developed by SBE Inc and Rogers Corporation laminated busbars. These assemblies offers a significant reduction in equivalent series inductance (ESL) and equivalent series resistance (ESR) compared with traditional designs. The result is lower total system cost and increased power density, due to lower overshoot voltages and less micro F/kW of required total capacitance. Integrated capacitor-busbar assemblies were developed for critical DC link applications in traction drive inverters for HEV/EV and inverter systems for solar and wind power.

VIDEO: curamik® Ceramic Strategies

curamik® high temperature/high voltage substrates consist of pure copper bonded to a ceramic substrate. They provide great heat conductivity and temperature resistance for high performance and high temperature applications, high insulation voltage, and high heat spreading.

ADVANCED CONNECTIVITY SOLUTIONS

On-demand webinars to help optimize RF/microwave circuit design for fabricators and OEMs.

WEBINAR: Bonding Layer Material Selection for Use in High Performance Multilayer Circuit Board Design

In this webinar, we discuss bonding layer material properties commonly used in high frequency/high reliability applications and how the material selection and fabrication process affect the electrical and mechanical performance of the finished board.

WEBINAR: High Frequency Materials and Characterization up to Millimeter Wave Frequencies

This webinar provides an overview of common test methods used to determine the dielectric constant (Dk) for high frequency materials, followed by a discussion of methods that are best for characterizing material properties for microwave modeling and design.

 
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