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

Solder mask is an often-overlooked component of an RF/microwave printed-circuit board (PCB). It provides protection for a circuit but can also have an effect on final performance, especially at higher frequencies. Solder mask may not always be used in RF/microwave circuits but, when it is part of a circuit, it should be accounted for electrically as well, for the most accurate modeling and simulation. Knowing more about the material properties of solder mask can help boost an understanding of how these circuit layers can impact performance at RF/microwave frequencies.

A solder mask is essentially a thin polymer layer that protects copper conductors from oxidation and helps to minimize the creation of short circuits by means of bridges formed by excess solder.

Solder mask provides protection for areas of a PCB that do not require any final plating finish. Traditionally, solder masks have provided their green color to PCBs but solder masks are now available in many other colors as well as in lead-free versions. The requirements for a particular solder mask will be determined by PCB conductor thickness, circuit density, viaholes, and types of components to be attached to the PCB, such as surface-mount-technology (SMT) components.

Solder mask can be applied in dry (film) or liquid forms. Because of the more complete coverage they provide on practical, three-dimensional (3D) surfaces, most solder mask is applied in liquid form. Liquid solder mask is typically formed from a two-component mixture of liquid photoimageable (LPI) polymers and solvents. The liquid components are mixed and used to form a thin coating that will adhere to the different surfaces and materials of a PCB, notably the conductive traces. An LPI solder mask provides protection for the PCB’s circuitry and dielectric materials, and the solder mask must be patterned to form openings (solder mask voids) for electrical connections of circuit components. When using LPI solder-resist inks, openings and other patterns can be formed in the solder mask by exposure to an ultraviolet (UV) light source, using either photolithography or direct laser imaging to create desired openings and patterns.

Most LPI solder mask is based on either an epoxy or acrylic formulation. Each solder mask approach brings its own contributions to a PCB, such as high dissipation factor and poor moisture absorption, that do not necessarily add to excellent high-frequency performance. For example, in contrast to circuit laminates intended for RF/microwave applications and often characterized at 10 GHz, the typical dissipation factor (Df) of an LPI solder mask is 0.025 at 1 GHz, with a dielectric constant (Dk) of around 3.3 to 3.8 at the same frequency, depending upon formulation.

The moisture absorption of an LPI solder mask also depends on the solder mask formulation, and is typically around 1% to 2%. Compare this to moisture absorption of less than 0.3% for most RF/microwave circuit laminates, and it is clear that both parameters of a PCB laminate will be affected by the contributions of an LPI solder mask. Because of the negative impact on RF/microwave performance, solder mask is often omitted from the RF/microwave portion of a PCB even if it can provide protection that enhances reliability.

In high-frequency circuits with microstrip or grounded-coplanar-waveguide (GCPW) transmission lines fabricated on low-loss circuit laminates, the addition of solder mask will increase the dielectric losses and the effective Dk of the circuit compared to a circuit without solder mask. Whether in double-copper-layer or multilayer designs, the characteristics of the solder mask must be included in any computer-aided-engineering (CAE) modeling and simulation performed to predict circuit performance, especially if a design goal is concerned with minimizing circuit losses.

Often, small patches of solder mask are used in RF/microwave circuit designs as “dams” for those areas where solder will be applied and SMT components assembled. In contrast to full circuit solder mask, these smaller patches or dams tends to have negligible impact on electrical performance. In general, if a solder mask patch is less than one-tenth wavelength of the operating frequency, it will not have a significant impact on the performance at that frequency. Provided that such solder mask patches are sufficiently small, they will have negligible effects on a high-frequency PCB. However, the use of multiple solder mask patches within a relatively small section of a PCB can result in a change in the material properties within that area that can have resulting electrical effects, such as higher loss.

In selecting solder mask for PCBs, a number of characteristics should be considered. These include long shelf life, high adhesion, high electrical insulation, good heat resistance, high plating resistance to all forms of plating, including with electroless nickel and gold, and compliant with halogen-free requirements. For applications where performance is critical, the choice of solder mask color can influence the material’s Df and Dk characteristics, where a difference in color can mean a difference, although slight, in both parameters. Proper cleaning and preparation of a PCB surface can also go a long way in ensuring that solder mask adheres strongly to the laminate surface when applied.

Additional information on solder mask is available from the global trade association, Association Connecting Electronics Industries, and its Institute of Printed Circuits (IPC), in the form of publication IPC-SM-840C, which reviews material types, adhesion characteristics, and other parameters of interest. By the nature of the LPI solder mask materials, and the need to form precise openings and other features typically by photolithography, they will be limited in terms of electrical performance compared to modern low-loss, high-performance circuit laminates. But when the protection of solder mask is needed for a design, and through proper planning and simulation with modern CAE circuit modeling software, solder mask can be added to RF/microwave double-copper-layer and multilayer circuits with minimal or no penalties in performance.

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PC_logoRogers Elastomeric Material Solutions sells PORON® and BISCO® materials through an exclusive network of Preferred Converters, a select group of partners who deliver an exceptional level of service and support for Rogers products in industrial, automotive, and mass transit applications. This global customer network utilizes Rogers’ products and manufactures them into final parts for end customers. The converters offer broad adhesive, lamination, cutting, and processing capabilities. They also create innovative solutions by combining materials or developing new manufacturing techniques to help their customers solve design challenges and create differentiation.

Preferred Converters maintain high standards of quality, technical capability, and customer service ensuring the best possible experience for our shared end users. Each partner gets involved in an interesting array of end use challenges. Here is a behind-the-scenes look at some of our partners and their customer projects.

Manufacturing a Display Mount Cushion for a Marine Application

CGR Products

CGR_blogAn OEM of marine industry products needed a gasketing and cushioning system for an industrial-duty touch screen in a humid and wet environment. The primary challenge was the diverse set of functions needed: weather vibrations, physical shocks, temperature fluctuations, and provide a high degree of conformability. The end product required two levels of compression deflection resistance and durometer combined into one part, and four different gasket components be manufactured and assembled. CGR Products designed the assembly process and used highly resilient PORON® 40 and 50 Series polyurethane foams. Here’s how.

Providing Gaskets and Seals for HEV Battery System

Marian Inc.

Marian_blogA global designer and manufacturer of electronic components and integrated systems for the automotive industry designed an HEV battery system. It included an enclosure the needed a group of seals and gaskets to protect the sensitive components inside from moisture, dust, and debris. The designed also required the seals and gaskets protect the system from failure due to vibration and/or impact. Marian engineers created a stable and aggressive bond between the foam and the enclosure housing using extra soft, slow rebound PORON® 4790-92 polyurethane materials. Here’s how.

Protecting Workers by Improving Gas Detector Reliability

Standard Rubber Products (SRP)

SRPCO_blogWorkers who use portable gas detectors risk their lives and need durable and effective protection. A safety device OEM discovered that the closed-cell neoprene blend and PVC sponge gasket materials being used to seal out dust and moisture were failing. SRP’s design skill and customized processing made it possible to use PORON® 92 foam to replace the failing materials. The PORON 92 formulation combines the softness, sealing performance, and impact absorption properties needed to help the gas detectors withstand impacts from up to a 3.7 meter (12 foot) drop. Here’s how.

 

From talking refrigerators to implanted heart monitors to trains that report seat availability, sensors are a vital part of the Internet of Things (IoT). In fact, you could make a good argument for calling it the “Internet of Sensors.” Most IoT devices include a sensor array, a microcontroller (MCU), Bluetooth or WiFi radio, and power management. Some devices also have a display or push-button inputs.

IoT_graphicIoT devices are focused on physical interfaces that use sensors and actuators, as opposed to the human interfaces of most computing platforms. IoT devices receive input through sensors that are typically miniaturized through MEMS technology. They send out information through wired or wireless interfaces to mobile devices and cloud-based servers. Sensor types include pressure, chemical, strain, and temperature, each with varying measurement frequencies and triggers. Communication connectivity involves varying intervals, distances, data rates, packet sizes, etc.

Most industrial devices are now smart, connected products with built-in sensing, processing, and communications capabilities. Much of the equipment that is remotely located includes embedded sensors that identify equipment problems and report them back to manufacturers through cloud software over the Internet, thus making them IoT devices. Sensors generate data, data produces knowledge, and knowledge drives action. Therefore, making sense of the data that those devices generate creates productivity improvements through equipment maintenance, inventory optimization, energy savings, and labor efficiencies.

According to ABI Research, the increasing adoption of IoT within industrial settings will result in a substantial growth of the number of connected devices, in particular control devices like programmable controllers (PLCs). It is expected that the number of connected industrial controllers will triple by 2020. This will produce an enormous strain on the existing infrastructure, both wired and wireless.

IoT Constraints: Power and Miniaturization

Regardless of the application, IoT devices are often located without easy access to power. This makes low power consumption one of the most universal constraints across the IoT space. IoT devices also require long lifetimes, further constraining power consumption. Bigger systems can afford µW – mW average powers, but millimeter-size devices need to make do with nW powers.

According to a team of researchers at the University of Michigan, “In low duty cycle systems, the sensor will be mostly inactive, reducing active power consumption and making standby power the dominant component. On the other hand, systems with high bandwidth requirements will have significantly higher power budgets with the radio as the dominant factor.”

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Figure 1: IoT Sensor Properties and Power Constraints.

IoT device packaging requirements typically include miniaturization in the form of low-cost, good power dissipation (low power for the silicon portion), good RF shielding, and support for multiple RF standards, such as WiFi, ZigBee, or BTLE (Bluetooth low energy, aka Bluetooth Smart). As discussed in EDN, “Cavity-based solutions are popular when sensors are involved, especially when there are stimulus delivery requirements such as port holes in microphones. IoT packages must also be production ready, since waiting for a new custom package is often not an option due to time-to-market constraints. Finally, regardless of whether the solution is discrete or integrated, the footprint must be small.”

MEMS_packaging

Figure 2. Common MEMS packages and die fabrication techniques that could be adopted for IoT devices.

IoT Constraints: Bandwidth

The IoT is also constrained by the Internet’s available bandwidth. The demand for WiFi and the transmission of mass quantities of mobile data is predicted to grow exponentially. By 2019, more than ten billion mobile devices will exchange 35 quintillion bytes of information each month. Add to that the countless desktop computers and computer-based equipment located in households, schools, government facilities, and throughout industry.

McKinsey reports that the IoT has a total potential economic impact of $3.9 trillion to $11.1 trillion a year by 2025. At the top end, that level of value—including the consumer surplus—would be equivalent to about 11 percent of the world economy.

IoT_economic_impactTo achieve this kind of impact, companies need to work with each other and with government agencies to overcome technical, organizational, and regulatory hurdles.

Let’s start with the glut of IoT data, much of which is not used:

  • On an oil rig with 30,000 sensors, only 1 percent of the data is examined. That’s because this information is used mostly to detect and control anomalies—not for optimization and prediction, which provide the greater value.
  • A fleet of 100 modern rail cars produces between 100 to 200 billion data points annually. Siemens is working with railways to create data teams to monitor the data that a train sends out, including information about what parts of the train have broken, what spare parts have been used, what spare parts are still available, what geographic regions it has traveled through (is there a hill that is notorious for causing problems?), and whether it’s near a service depot that has capacity to provide maintenance.
  • According to Deloitte, the primary challenge in chronic care is linking the devices so they can communicate reliably and securely. While in-home blood-glucose and heart-rate sensors, for instance, are widely available, they are rarely set up to export their data to a system that aggregates and shares information with all involved parties.

To handle the data, a network is needed that balances conflicting requirements, such as IoT device range, battery life, bandwidth, density, endpoint cost, and operational cost.

According to Gartner:

Low-power, short-range networks will dominate wireless IoT connectivity through 2025, far outnumbering connections using wide-area IoT networks. However, trade-offs mean that in many cases, network types will coexist.

For IoT applications that need wide-area coverage combined with relatively low bandwidth, good battery life, low hardware and operating costs, and high connection density, cellular networks don’t deliver. Wide-area IoT networks need to to deliver data rates from hundreds of bits per second (bps) to tens of kilobits per second (kbps) with nationwide coverage, a battery life of up to 10 years, an endpoint hardware cost of around $5, and support for hundreds of thousands of devices connected to a base station.

The first low-power wide-area networks (LPWANs) were based on proprietary technologies, but in the long term emerging standards such as Narrowband IoT (NB-IoT) will likely dominate this space.

Industrial IoT applications present an additional challenge in that they often require high data rates in order to transmit and analyze data in real time. Systems creating tens of thousands of events per second are common, and millions of events per second can occur in some telecom and telemetry situations. To address such requirements, distributed stream computing platforms (DSCPs) have emerged.

Other new technologies are being created to help the IoT become a self-sustaining network of sensors and devices that deliver valuable insight obtained from massive amounts of data:

  • Laminated multilayer busbars provide efficient and compact connections for propulsion, auxiliary, and other IGBT based converters in connected car and connected rail systems.
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A significant gap exists between what U.S. companies want to achieve in terms of innovation vs what they are able to do, said Adi Alon of Accenture Strategy. A recent survey by Accenture of U.S. company executives showed that 60% said their organizations do not learn from past mistakes, nearly double the 36% who admitted to this three years ago. Many of these companies (72%) also miss opportunities to exploit underdeveloped areas or markets, up from 53% three years ago. In addition, 67% of those surveyed believe their companies are risk averse, a large increase from 46% in the previous survey.

Screen Shot 2016-07-29 at 2.22.07 PMBrands looking for innovation and inspiration can start by scanning other industries for new technologies. A group that knows this best and uses it to great advantage is Rogers’ Preferred Converter Network. Most often active in our general industrial, automotive, or mass transit segments, this global customer network utilizes Rogers’ products and manufactures them into final parts for end customers. The converters offer broad adhesive, lamination, cutting, and processing capabilities. They also create innovative solutions by combining materials or developing new manufacturing techniques to help their customers solve design challenges and create differentiation. Being a member of the Rogers’ Preferred Converter Network is not a prerequisite to purchasing material from Rogers, but the group is integral to feeding the ideas and needs of the markets and segments back to Rogers.

The Rogers Global Preferred Converter Conference is an event that greatly facilitates the collaboration between Rogers and the Preferred Converter Network. Held at Foxwoods Resort in Mashantucket Connecticut on June 6, 2016, the event drew Preferred Converters from around the world, many of whom have worked with Rogers Corporation for over 40 years. The Conference provides an opportunity to exchange knowledge and generate ideas for growth, and provides another mechanism for Rogers to learn about customer needs, trends, and new opportunities.

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This year’s Conference was our best-attended to date, with over 90 PCs gathering from around the globe: Italy, United Kingdom, Australia, Hungary, Denmark, Netherlands, Israel, Germany, China, Canada, and United States.

The days were jam-packed with learning — multiple sessions on new product introductions and new application areas…

PC Conference 63

…and fun, including a “Shark Tank” pitch competition!

Football

Continuing in the spirit of friendly competition, an application contest was also held. Congratulations to the winners:

And last, but certainly not least, kudos to the top foursome at the Patrick J. Fahey Memorial Golf Tournament:

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.

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.

 
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